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Risks of Non-Compliant Rodenticides to Nontarget Wildlife

Background Paper for Science Advisory Panel on 

Notice of Intent to Cancel Non-RMD compliant Rodenticide Products

November 1, 2011 

United States Environmental Protection Agency

Office of Chemical Safety and Pollution Prevention

Office of Pesticides Programs

Environmental Fate and Effect Division

Prepared by: 

Brian Anderson, Shannon Borges, Kristina Garber, Christine Hartless, Justin Housenger,

Nicholas Mastrota, Edward Odenkirchen, Elizabeth Riley, Michael Wagman
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Table of Contents

i. Executive Summary................................................................................................................. 4

1 Background............................................................................................................................ 14

2 Analysis Plan ......................................................................................................................... 15

2.1 Active Ingredients Assessed........................................................................................... 15

2.2 Product Labels under Evaluation ................................................................................... 18

2.3 Modes of Action............................................................................................................. 27

2.4 Conceptual Model of Exposure...................................................................................... 28

2.5 Taxonomic Groups Potentially at Risk .......................................................................... 29

2.6 Measures of Exposure .................................................................................................... 30

2.7 Measures of Toxicity...................................................................................................... 31

2.8 Measures of Risk............................................................................................................ 32

3 Toxicity, Accumulation, and Elimination of the Evaluated Rodenticides............................ 34

3.1 Avian and Mammalian Toxicity .................................................................................... 34

3.2 Elimination of Rodenticides........................................................................................... 50

3.3 Estimated Accumulation of Dose over Time ................................................................. 61

3.4 Measured Rodenticide Residues .................................................................................... 67

4 Primary Exposure and Risk ................................................................................................... 73

4.1 Risks based on the Acute Oral LD50 .............................................................................. 73

4.2 Risks based on the acute dietary LC50............................................................................ 83

4.3 Uncertainties................................................................................................................... 85

4.4 Summary of Rodenticide Risks to Bait-consuming Birds and Mammals...................... 85

5 Secondary Exposure and Risk ............................................................................................... 87

5.1 Rodenticide Ingestion through Consumption of Contaminated Carcasses- Theoretical 

Analysis..................................................................................................................................... 88

5.2 Rodenticide Ingestion through Consumption of Contaminated Carcasses- Residue 

Analysis..................................................................................................................................... 98

5.3 Secondary Exposure Studies........................................................................................ 102

5.4 Uncertainties................................................................................................................. 115

5.5 Discussion of Risks of Rodenticides to Secondary Birds and Mammals .................... 117

6 Wildlife Incidents................................................................................................................ 120
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6.1 Mortality Incidents....................................................................................................... 120

6.2 Open Literature Evaluations......................................................................................... 136

6.3 Exposure Incidents....................................................................................................... 140

7 Synthesis of Conclusions..................................................................................................... 153

7.1 Primary Exposure Risk Conclusions............................................................................ 161

7.2 Secondary Exposure Risk Conclusions........................................................................ 165

8 References ........................................................................................................................... 171
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i. Executive Summary

A. Background 

EPA has updated its previous risk assessment findings conducted in support of the May 2008 

RMD through the application of additional effects and exposure data, use of additional exposure 

modeling, and quantitative risk assessment techniques for the four rodenticides subject to the 

NOIC (brodifacoum, difethialone, warfarin and bromethalin) and for two rodenticides that are 

likely alternatives (i.e., active ingredients included in products with RMD-compliant 

registrations for commensal rodent control, specifically, chlorophacinone and diphacinone; 

bromethalin has both compliant and non-compliant registrations). 

Rodenticide baits are intended to be lethal to rodents and a few other small mammals, but they 

are not selective to target species. Mammal and bird species are vulnerable to adverse effects 

from rodenticides if exposed. Exposure to non-target wildlife has been documented in urban, 

suburban, and rural areas, exposure to does not appear to be limited to isolated incidents, and 

rodenticide exposures and mortalities have been documented for mammals and birds that are 

primary and secondary consumers. 

For the purpose of this assessment, primary exposure is defined as non-target organism 

consumption of rodenticide treated bait. Use of rodenticide baits around structures is likely to 

result in primary exposures among non-target wildlife. Many factors influence which nontarget 

animals might be exposed directly to baits. For instance, birds and mammals that are attracted to 

seeds and grains may consume grain-based rodenticide baits and baits in forms similar to seeds 

and grains. Some non-target animals will likely readily consume rodenticide baits that have a

block form. Exposure has been reported for a number of species that are primary consumers of 

bait, including various species of birds and squirrels, opossums, raccoons, skunks, and deer. 

Secondary consumers are defined as those animals that prey upon or scavenge primary 

consumers of bait. Rodenticide baits also poses potential secondary risks, because predators and 

scavengers are likely to be attracted to dead or dying rats, mice, and poisoned non-target animals. 

In addition, secondary consumers may also consume commensal rodent baits that contain 

flavorings that might appeal to carnivores and omnivores. Exposure has been documented in a 

number of secondary consumer species including larger mammals such as bobcats and foxes, and 

numerous bird species such as hawks, eagles and owls.

EPA is concerned with both the primary and secondary risks to birds and non-target mammals 

from exposure to the commensal rodent control products subject to this notice. This concern is 

based upon consideration of several lines of evidence, including (1) an assessment of the risks to 

non-target animals associated with primary exposure to rodenticides; (2) an assessment of the 
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risks of non-target animals through secondary exposure to rodenticides; (3) an evaluation of 

available feeding studies as they relate to secondary exposure risks; and (4) an evaluation of 

reported wildlife incidents as they relate to primary and secondary mortality events for nontarget species in a variety of land use settings, including urban, suburban, and rural settings. 

The risk assessment involved calculation of RQs based on estimates of exposure and toxicity. 

RQs were compared to the acute level of concern (LOC), which is 0.5. An RQ that exceeds the 

LOC of 0.5 is considered to pose a risk of concern for non-listed species. The RQ analysis was 

supplemented with additional characterization that evaluates the potential for effects to occur,

including an estimate of the amount of bait (and duration of feeding) required to consume the 

median lethal dose, number of prey items required to be consumed to reach the median lethal 

dose, and a characterization of the uptake and elimination properties of the rodenticides included 

in this assessment. 

B. Primary Exposure Methods

Risk of rodenticide exposures to primary consumers is influenced by several factors, including 

toxicity, toxicokinetics (chemical absorption, distribution and elimination in the body), 

concentration of active ingredient in the bait, and availability of bait for consumption. For each 

of the evaluated chemicals, risk is assessed in several ways:

• single day dose-based exposure to bait based on allometric equations for food intake that 

allow for differentiation of dose according to body size is compared to weight adjusted 

acute oral LD50 toxicity values;

• multiple day dose-based exposure to bait based on food intake allometric equations and 

metabolism that allow for differentiation of dose according to body size and elimination 

of the chemical over time is compared to weight adjusted acute oral LD50 toxicity values; 

and

• Dietary exposure (concentration of chemical active ingredient in bait) is compared to 

acute dietary LC50 values.

Further lines of evidence include an analysis of the number of days of bait consumption required 

to accumulate a median lethal dose of rodenticide and the grams of bait individuals would need 

to consume in order to accumulate that median lethal dose. An evaluation of required feeding 

period and the required amount of treated material necessary for consumption to result in 

exposures of lethal concern is a useful comparative tool among chemicals. Chemicals that 

require little feeding time or small amounts of treated material to reach the median lethal dose

likely represent a greater opportunity for lethal incidents (and thus pose a greater risk) as 

compared to compounds that would need consistent feeding or large amounts of consumption 

over protracted periods of time.
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C. Secondary Exposure Methods

Secondary risk potential was assessed using several lines of evidence, including: 

• RQ analysis based on theoretical and empirical residue levels in prey;

o As with the primary exposure analysis, an analysis was conducted assuming target 

mammals fed on rodenticide bait for single or multiple days. 

• Accumulation and elimination characteristics;

• Results from secondary feeding studies;

• Estimation of the number of organisms required to consume the equivalent of the median 

lethal dose for a predator or scavenger;

• Incident data

Time to death also influences the potential for exposure, since compounds that kill rapidly would 

prevent the primary consumer from continuing to eat bait, thereby consuming a dose that would 

be toxic to a secondary consumer. Rodenticides that are slower to cause death also prolong the 

period when contaminated (and potentially more vulnerable) prey are available to predators. The 

toxicity of the rodenticide to the secondary consumer would also affect the risk of adverse effects 

that would result from exposure. Based on these characteristics, rodenticides that are eliminated 

more slowly and have greater potential to accumulate within body tissues, take longer to kill the 

primary consumer, and have greater toxicity to secondary consumers are expected to present 

greater secondary risk. 

D. Incident Findings Related to Primary & Secondary Risk Analyses

The quantitative risk assessment assumes that exposure is occurring under a set of conservative 

assumptions. It does not evaluate the actual potential for wildlife to come into contact with 

treated bait; it simply evaluates the consequences of exposure. EPA uses available incident and 

exposure information from a variety of sources, including state and local governments and 

pesticide registrants. The majority of available reported incidents for the rodenticides assessed

are included in EPA’s Environmental Incident Information System (Version 2.1) EIIS. EPA 

believes these data offer strong support for the conclusion that residential and commercial use of 

rodenticides in urban/suburban and rural areas provide complete exposure pathways to a variety 

of wildlife and that primary and secondary exposure lethal incidents involving non-target 

wildlife can occur. 
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E. Summary of Results

Results of the risk assessment are summarized in the following tables. Conclusions of the 

assessment are detailed in Section 7 of this assessment and include the following:

• Primary Risks

o Primary exposure risk to mammals is above concern levels and is similar across 

the assessed rodenticides, which is consistent with their use as rodenticides;

o Primary exposure risk to birds is above concern levels for brodifacoum, 

difethialone, and bromethalin, and less than a single day of feeding could result in 

a median lethal dose;

o Primary risk to birds is above concern levels for chlorophacinone and warfarin; 

however, consumption of bait is generally required to occur for greater durations 

to result in median lethal doses compared with the apformentioned brodifacoum, 

difethialone, and bromethalin;

o No LOCs were exceeded for diphacinone for primary exposure risks to birds.

• Secondary Risks

o Secondary exposure risks to mammals exceeded concern levels for all assessed 

chemicals under some exposure scenarios. However, a greater opportunity for 

secondary exposure to result in median lethal doses was estimated to be greater 

for brodifacoum and difethialone relative to the other assessed rodenticides. 

o Secondary exposure risk to birds is above concern levels for brodifacoum, and 

difethialone under all assessed scenarios, and consumption of less than a single 

contaminated prey animal could result in a median lethal dose;

o Secondary exposure risk to birds was lower than concern levels for 

chlorophacinone and diphacinone under all scenarios evaluated, and less than 

concern levels for warfarin under all scenarios evaluated except for small 

scavengers consuming contaminated mice that accumulated 0.054% warfarin bait 

for 6 days. Multiple contaminated prey items were estimated to be required to 

result in a median lethal dose for these chemicals. For bromethalin, concern 

levels were exceeded for secondary exposure. However, the duration for primary 

consumers to feed on bromethalin bait and accumulate the rodenticide is limited 

due to this chemical's rapid mode of action and observations that target animals 

stop feeding after a lethal dose is reached. This in turn limits the potential 

exposure for secondary consumers.
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Table 2-1. Avian RQ analysis: primary exposure of passerine birds to bait and secondary exposure of carnivore/scavenger birds to residues 

in consumed rodents. 

Body 

weights (g)

BrodifacoumDifethIaloneWarfarin 

(0.025%)

Warfarin 

(0.054%)

ChlorophacinoneDiphacinoneBromethalin

Primary exposure

Single day dose RQ

20 19.61* 29.80* 0.23 0.50* 0.07 0.01 7.76*

100 22.67* 18.39* 0.14 0.31 0.04 0.01 4.76*

1000 27.89* 9.22* 0.07 0.16 0.02 <0.01 2.40*

Six day dose RQ

20 117* 168* 1.36* 2.94* 0.40 0.05 34*

100 135* 104* 0.84* 1.81* 0.25 0.03 21*

1000 166* 52* 0.42 0.91* 0.12 0.01 11*

Dietary RQ NA 38* 45* 0.40 0.86* 0.89* 0.06 0.48

Theoretical secondary exposure

House mouse consumption: 

1 day accumulation in prey 

RQ

100 6.60* 5.84* 0.05 0.10 0.01 <0.01 1.52*

1000 5.15* 1.85* 0.01 0.03 <0.01 <0.01 0.48

5000 4.33* 0.83* 0.01 0.01 <0.01 <0.01 0.22

Norway rat consumption: 1 

day accumulation in prey 

RQ

100 1.75* 1.54* 0.01 0.03 <0.01 <0.01 0.40

1000 1.36* 0.49 <0.01 0.01 <0.01 <0.01 0.13

5000 1.15* 0.22 <0.01 <0.01 <0.01 <0.01 0.06

House mouse 

consumption: 3 day 

accumulation in prey RQ

100 19.77* 17.09* 0.14 0.30 0.04 0.01 4.06*

1000 15.42* 5.42* 0.04 0.09 0.01 <0.01 1.29*

5000 12.96* 2.43* 0.02 0.04 0.01 <0.01 0.58*

Norway rat consumption: 

3 day accumulation in 

prey RQ

100 5.25* 4.52* 0.04 0.08 0.01 <0.01 1.07*

1000 4.08* 1.43* 0.01 0.02 <0.01 <0.01 0.34

5000 3.43* 0.64* 0.01 0.01 <0.01 <0.01 0.15

House mouse 

consumption: 6 day 

accumulation in prey RQ

100 39.40* 32.98* 0.27 0.58* 0.08 0.01 NA

1000 30.74* 10.45* 0.09 0.19 0.02 <0.01 NA

5000 25.84* 4.68* 0.04 0.08 0.01 <0.01 NA

Norway rat consumption: 

6 day accumulation in 

prey RQ

100 10.43* 8.73* 0.07 0.15 0.02 <0.01 NA

1000 8.14* 2.77* 0.02 0.05 0.01 <0.01 NA

5000 6.84* 1.24* 0.01 0.02 <0.01 <0.01 NA
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House mouse 

consumption: dietary RQ NA 17.81* 2069* 0.19 0.41 0.32 0.02 0.20

Norway rat consumption: 

dietary RQ NA 4.71* 5.48* 0.05 0.11 0.09 <0.01 0.05

Empirical

Secondary 

Exposure

Single day dose RQ

100 4.42* 4.64* <0.01 <0.01 <0.01 ND

1000 5.01* 1.47* <0.01 <0.01 <0.01 ND

5000 4.21* 0.66* <0.01 <0.01 <0.01 ND

Dietary RQ NA 5.25* 5.54* <0.01 0.02 <0.01 ND

NA = not applicable; ND = no data; * = RQ exceeds the acute LOC of 0.5
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Table 2-2. Avian Risk Characterization: Additional lines of evidence

Body 

weights (g)

BrodifacoumDifethIaloneWarfarin 

(0.025%)

Warfarin 

(0.054%)

ChlorophacinoneDiphacinoneBromethalin

Primary exposure

Days to consume sufficient bait 

to reach the LD50

20 <1 <1 4-5 1-2 16-17 >365 <1

100 <1 <1 7-8 3-4 30-31 >365 <1

1000 <1 <1 14-15 6-7 117-118 >365 <1

Grams bait consumed to reach 

the LD50

20 0.3 0.2 25 10 86 >1800 0.7

100 0.9 1.1 160 80 620 >7300 4.2

1000 5 15 2100 990 17000 >52000 59

Secondary exposure

Number of house mice 

consumed to reach the LD50: 1 

day accum. in prey

100 <1 <1 38 18 130 821 1

1000 2 4 539 249 1836 11602 16

5000 5 27 3429 1587 11690 73852 104

Number of Norway rats to 

consume to reach the LD50: 1

day accum. in prey

100 <1 <1 7 3 23 147 <1

1000 <1 <1 97 45 329 2079 3

5000 <1 5 614 284 2094 13232 17

Secondary feeding study 

summary counts

(#mortalities/ #tested)

NA 63/149 ND 2/23 0/112 3/34 ND

Incident data 

summary 

counts

primary exposure NA 3 0 1 3 1 0

secondary exposure NA 164 3 5 3 5 0

unknown exposure NA 21 0 0 1 0 0

NA = not applicable 

ND = no data
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Table 2-3. Mammalian RQ analysis: primary exposure of rodents to bait and secondary exposure of 

carnivore/scavenger mammals to residues in consumed rodents.

Body weights 

(g)

BrodifacoumDifethIaloneWarfarin 

(0.025%)

Warfarin 

(0.054%)

ChlorophacinoneDiphacinoneBromethalin

Primary exposure

Single day dose RQ

15 9.93* 4.30* 8.36* 18.05* 0.82* 1.97* 5.1*

35 8.48* 3.67* 7.14* 15.42* 0.70* 1.68* 4.4*

1000 4.55* 1.97* 3.83* 8.27* 0.38 0.90* 2.4*

Six day dose RQ

15 59* 24* 49* 106* 4.7* 11.6* 23*

35 51* 21* 42* 90* 4.0* 9.9* 19*

1000 27* 11* 22* 48* 2.1* 5.3* 10*

Dietary RQ NA 94* ND 57* 123* 44* 22* 18*

Theoretical secondary exposure

House mouse consumption: 1 day 

accumulation in prey RQ

50 3.71* 1.62* 2.97* 6.41* 0.28 0.94* 1.62*

1000 4.60* 2.01* 3.68* 7.95* 0.35 1.16* 2.01*

3000 4.98* 2.17* 3.99* 8.61* 0.38 1.26* 2.17*

Norway rat consumption: 1 day 

accumulation in prey RQ

50 0.98* 0.43 0.79* 1.70* 0.08 0.25 0.43

1000 1.22* 0.53* 0.97* 2.11* 0.09 0.31 0.53*

3000 1.32* 0.58* 1.05* 2.28* 0.10 0.33 0.58*

House mouse consumption: 3 day 

accumulation in prey RQ

50 11.10* 4.74* 8.81* 19.03* 0.84* 2.49* 4.31*

1000 13.78* 5.88* 10.93* 23.61* 1.04* 3.09* 3.53*

3000 14.91* 6.37* 11.83* 25.56* 1.12* 3.35* 5.79*

Norway rat consumption: 3 day 

accumulation in prey RQ

50 2.94* 1.25* 2.33* 5.04* 0.22 0.66* 1.14*

1000 3.65* 1.56* 2.89* 6.25* 0.27 0.82* 1.42*

3000 3.95* 1.68* 3.13* 6.76* 0.30 0.89* 1.53*

House mouse consumption: 6 day 

accumulation in prey RQ

50 22.13* 9.15* 17.35* 37.48* 1.63* 4.20* NA

1000 27.46* 11.35* 21.53* 46.51* 2.02* 5.21* NA

3000 29.72* 12.28* 23.30* 50.33* 2.18* 5.64* NA

Norway rat consumption: 6 day 

accumulation in prey RQ

50 5.86* 2.42* 4.59* 9.92* 0.43 1.11* NA

1000 7.27* 3.00* 5.70* 12.31* 0.53* 1.38* NA

3000 7.87* 3.25* 6.17* 13.32* 0.58* 1.49* NA
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House mouse consumption: dietary 

RQ NA 44.69* ND 26.64* 57.54* 20.42* 10.11* ND

Norway rat consumption: dietary 

RQ NA 11.23* ND 7.05* 15.23* 5.41* 2.68* ND

Empirical 

secondary 

exposure

Single Day RQ

50 3.59* 1.41* 0.26 0.07 0.19 ND

1000 4.46* 1.75* 0.32 0.08 0.23 ND

3000 4.83* 1.89* 0.34 0.08 0.25 ND

Dietary RQ NA 13.17* NA 0.67* 1.39* 0.61* ND

NA = not applicable; ND = no data; * = RQ exceeds the acute LOC of 0.5
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Table 2-4. Mammalian Risk Characterization: Additional lines of evidence from the mammal risk assessment.

Body 

weights (g)

BrodifacoumDifethIaloneWarfarin 

(0.025%)

Warfarin 

(0.054%)

ChlorophacinoneDiphacinoneBromethalin

Primary exposure

Days to consume sufficient bait 

to reach the LD50

15 <1 <1 <1 <1 1-2 <1 <1

35 <1 <1 <1 <1 1-2 <1 <1

1000 <1 <1 <1 <1 1-2 <1 <1

Grams bait consumed to reach 

the LD50

15 0.3 0.7 0.3 0.2 6 1.1 0.6

35 0.5 1.3 0.7 0.3 9 2.1 1.1

1000 6.7 16 8.0 3.7 61 25 13

Secondary exposure

Number of house mice 

consumed to reach the LD50: 1 

day accum. in prey

50 <1 <1 <1 <1 3 <1 <1

1000 2 5 3 1 27 8 5

3000 5 11 6 3 60 18 11

Number of Norway rats to 

consume to reach the LD50: 1 

day accum. in prey

50 <1 <1 <1 <1 <1 <1 <1

1000 <1 <1 <1 <1 5 1 <1

3000 <1 2 1 <1 11 3 2

Secondary feeding study summary counts

(#mortalities/ #tested) NA 8/19 ND 9/100 32/55 19/33 0/4

Incident data 

summary 

counts

primary exposure NA 41 1 2 3 9 1

secondary exposure NA 22 1 0 1 4 0

unknown exposure NA 16 0 0 0 0 0

NA = not applicable 

ND = no data
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1 Background

In order to develop mitigations for risks associated with rodenticides, the Environmental Fate 

and Effects Division (EFED) of the Office of Pesticide Programs (OPP) completed an ecological 

risk assessment in 2004 evaluating primary and secondary exposure of birds and mammals to

rodenticides. In this context, primary exposure relates to animals that directly consume 

rodenticide bait and secondary exposure relates to animals that prey upon or scavenge animals 

that have consumed rodenticide bait. The 2004 assessment ranked the potential primary and 

secondary risk for nine rodenticides and determined that brodifacoum and difethialone posed the 

greatest risks to non-target birds and mammals, followed by bromadiolone and diphacinone. 

Zinc phosphide also posed high primary risk to non-target animals. 

Based on the results of the 2004 analysis completed by EFED as well as an assessment 

completed by OPP’s Health Effects Division (HED), the Agency issued a Risk Mitigation 

Decision (RMD) for Rodenticides (EPA, 2008). The RMD issued the final reregistration 

eligibility status of rodenticide products that involved commensal rodent control (house mouse, 

roof rat, and Norway rat) uses in and around buildings and underground manual application to 

pocket gopher and mole burrows. The two major mitigation components of the RMD were to 

minimize children’s exposure to rodenticide products used in homes and to reduce wildlife 

exposure and ecological risks of these products. Mitigation measures that may affect ecological 

risks include requirements to place baits in bait boxes and limiting applications to within 50 feet 

of a structure. In addition, registrants must control distribution of the second generation 

anticoagulant products so that they are distributed or sold only to agricultural, farm, and tractor 

stores or directly to Pest Control Operators (PCO), and the products are not sold or distributed to 

hardware and home improvement stores, grocery stores, convenience stores, drug stores, club 

stores, big box stores, and other general retailers.

To date, some products containing the following rodenticide active ingredients have not 

voluntarily conformed with the provisions of the RMD: brodifacoum, difethialone, warfarin, and 

bromethalin. Brodifacoum and difethialone products do not conform with the RMD because 

packaging requirements intended to reduce wide spread homeowner applications have not been 

implemented. Warfarin and bromethalin non-conforming products do not comply with the risk 

mitigation decision because the form of bait has not been changed to forms of bait that are 

reasonably expected to remain in a bait station (such as blocks). Lack of conformance with the 

provisions in the RMD has led the Agency to publish a notice of intent to cancel (NOIC) the 

registrations of the non-conforming products. It should be noted that not all products containing 

the above active ingredients are non-conforming with the RMD (see Table 2-2 and 2-3). 
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The purpose of this document is to provide information on the ecological risks associated with 

the rodenticide active ingredients in products that are the subject of the NOIC. This document 

updates previous risk assessment findings through the application of additional effects and 

exposure data, use of additional exposure modeling, and quantitative risk assessment techniques 

for those chemicals/products subject to the NOIC and for rodenticides that are likely 

replacements for commensal rodent uses (i.e., those with current conforming registrations for the 

commensal homeowner uses). 

The risk assessment focuses on a number of lines of evidence of ecological risk which are similar 

to previous assessments, including: (1) an assessment of the risks of non-target animals 

associated with primary exposure to rodenticides; (2) an assessment of risks of non-target 

animals through secondary exposure to rodenticides; (3) an evaluation of available feeding 

studies as they relate to secondary exposure risks; (4) an evaluation of reported wildlife incidents 

as they relate to primary and secondary mortality events for non-target species in a variety of 

land use settings, including urban, suburban, and rural settings; and (5) a discussion of how the 

above lines of evidence inform the RMD.

2 Analysis Plan

2.1 Active Ingredients Assessed

This assessment evaluates the potential risks to non-target wildlife from exposure to products 

containing four rodenticide active ingredients, specifically brodifacoum, difethialone, warfarin, 

and bromethalin, that do not currently conform to the RMD. In addition, active ingredients with 

RMD-conforming labels for control of commensal rodents, i.e., chlorophacinone and 

diphacinone, were also evaluated because they are assumed to serve as likely replacements of 

products with proposed cancellations that have overlapping uses. 

All of the assessed chemicals, except bromethalin, are anticoagulant rodenticides. Bromethalin 

is a neurotoxic agent. Brodifacoum and difethialone are commonly referred to as second 

generation anticoagulants whereas warfarin, chlorophacinone, and diphacinone are commonly 

referred to as first generation anticoagulants. Second generation anticoagulants are generally 

more toxic and more persistent within an animal. However, as discussed in subsequent sections 

of this assessment, some properties of some of the first generation anticoagulants are similar to 

the second generation anticoagulants. Mode of action of each of the assessed chemicals is 

described later in Section 2. 

The chemical structures of these rodenticides and some of their physical/chemical properties are 

presented in Table 2-1. 
Table 2-1. Structures and Selected Physical and Chemical Properties of Assessed Rodenticides

Chemical Name Structure Selected Properties

Brodifacoum Class: coumarin anticoagulant

Molecular formula: C31H23BrO3

Molecular weight: 523.4 g/mol

Physical state: solid

Melting point: 228-232o C

Solubility in water: <10 ppm at 20 o C, pH 7

Difethialone Class: coumarin anticoagulant

Molecular formula: C31H24BrO2S

Molecular weight: 539.5 g/mol

Physical state: solid

Melting point: 230 o C

Solubility in water: 0.39 ppm at 25 o C

Chlorophacinone Class: indandione anticoagulant

Molecular formula: C23H14O3Cl

Molecular weight: 373.8 g/mol

Physical state: solid

Melting point: 140 o C

Solubility: 20-34 ppm
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Chemical Name Structure Selected Properties

Diphacinone Class: indandione anticoagulant

Molecular formula: C23H16O3

Molecular weight: 340.4 g/mol

Physical state: solid

Melting point: 141-145 o C

Solubility: 17-30 ppm in water

Warfarin Class: coumarin anticoagulant

Molecular formula: C19H16O4; C19H15NaO4 (sodium salt)

Molecular weight: 308.4 g/mol; 330.1 g/mol (sodium salt)

Physical state: solid

Melting point: 159-165o C

Solubility: 0.196 ppm in water at 25 o C

Bromethalin Class: diphenylamine

Molecular formula: C13H7Br3F3N3O4

Molecular weight: 578.0 g/mol

Physical state: solid

Melting point: 148-152 o C

Solubility in water: 0.0038 ppm at 25°C
2.2 Product Labels under Evaluation

Table 2-2 summarizes label and application information for currently registered consumer use 

rodenticide labels that do not conform to the 2008 RMD. Table 2-3 summarizes label and 

application information for all currently registered consumer use rodenticide labels that do 

conform to the RMD. The products listed in Table 2-2 are the subject of EPA’s proposed 

cancellation. In total, there are four brodifacoum labels, seven difethialone labels, five warfarin 

labels, and four bromethalin labels that do not conform to the RMD. There are four 

chlorophacinone labels, 12 diphacinone labels, and 15 bromethalin labels that do conform to the 

RMD. 

As indicated in Tables 2-2 and 2-3, for each chemical, there is variation in the amount of bait to 

be used at each placement, target species, and description of placement location. Bait form for 

the non-conforming labels is described as powder (to be mixed with bait by consumer), bait bits, 

pellets, meal, crushed pellets, block, and paste. All conforming products are composed of a bait 

block(s) and packaged with a bait station. Chlorophacinone labels under consideration are 

restricted to indoor uses; therefore, primary consumption of chlorophacinone bait by nondomesticated animals is unlikely. Distance between placements varies depending on the target 

pest. For house mice, placements are usually prescribed to be 8-10 feet apart. For Norway and 

roof rats, placements are typically 15-30 feet apart. The amount of bait and active ingredient to 

be placed at each location varies for the different labels (Tables 2-4 and 2-5). This variation in 

placement size will affect the quantity of a.i. at each placement and may affect the likelihood of a 

non-target organism finding and consuming a lethal quantity of the rodenticide in a single 

location at a single feeding. 
Page 19 of 183

Table 2-2. Non-conforming consumer use labels for rodenticides: summary of label specifications.

 Reg. No Name Form %a.i Target 

species

Max app rates Location of placements

Brodifacoum

3282-65 D-CON MOUSE 

PRUFE II

Pellets 0.005 House 

mouse

Open tab, Place wedges (1.5 or 3.0 

oz each) at 8-12 ft intervals if 

activity not highly localized (does 

not specify spacing when activity is 

localized)

Indoor only (homes, industrial, commercial, 

agricultural, public buildings)

3282-66 D- CON PELLETS 

GENERATION II

Pellets 0.005 Norway rat, 

roof rat, 

house mouse

Rats: 4 bait trays (12 oz. bait) per 

placement maintain for at least 10 

days, placements at 15-20 ft intervals

In and around homes, industrial, commercial, 

agricultural, public buildings. In transport vehicles 

(ships, trains, aircraft) and in and around related port 

or terminal buildings

3282-74 D- CON BAIT 

PELLETS II

Pellets 

(Place 

pack)

0.005 Norway rat, 

roof rat, 

house mouse 

Rat: 16 bait packs (16 oz. bait) per 

placement 15-30 ft intervals, 

maintain for at least 10 days

In and around homes, industrial, commercial, 

agricultural, public buildings. In transport vehicles 

(ships, trains, aircraft) and in and around related port 

or terminal buildings

3282-81 D-CON READY 

MIXED 

GENERATION II

Crushed 

Pellet

0.005 Norway rat, 

roof rat, 

house mouse

Rats: 4 bait trays (12 oz. bait) per 

placement maintain for at least 10 

days, placements at 15-20 ft intervals

In and around homes, industrial, commercial, 

agricultural, public buildings. In transport vehicles 

(ships, trains, aircraft) and in and around related port 

or terminal buildings

Difethialone

3282-85 D-CON MOUSEPRUFE IIIPellet 0.0025 House 

mouse

Open tab, Place wedges (1.5 or 3.0 

oz) 8-12 ft intervals if activity not 

highly localized (Label does not 

specify rate if activity is localized)

Indoor only (homes, industrial, commercial, 

agricultural, public buildings)

3282-86 D- CON BAIT 

PELLETS III

Pellet 0.0025 Norway rat, 

roof rat, 

house mouse

Rats: 4 trays (12 oz. bait) per 

placement, placements at 15-30 ft. 

intervals. Maintain at least 10 days. 

URBAN - In and around homes, industrial, 

commercial, agricultural, public buildings. In 

transport vehicles (ships, trains, aircraft) and in and 

around related port or terminal buildings

NON-URBAN – inside home and agricultural 

buildings

3282-87 D- CON II 

READY MIX 

BAITBITS III

Crushed 

pellet

0.0025 Norway rat, 

roof rat, 

house mouse

Rats: 4 trays (12 oz. bait) per 

placement, placements at 15-30 ft. 

intervals. Maintain at least 10 days. 

URBAN - In and around homes, industrial, 

commercial, agricultural, public buildings. In 

transport vehicles (ships, trains, aircraft) and in and 

around related port or terminal buildings

NON-URBAN – inside home and agricultural 

buildings
Page 20 of 183

Table 2-2. Non-conforming consumer use labels for rodenticides: summary of label specifications.

 Reg. No Name Form %a.i Target 

species

Max app rates Location of placements

3282-88 D- CON BAIT 

PACKS III

Pellet 

(Place 

pack)

0.0025 Norway rat, 

roof rat, 

house mouse

Rat: 16 bait packs (16 oz. bait) per 

placement, 15-30 ft intervals, 

maintain for at least 10 days

URBAN - In and around homes, industrial, 

commercial, agricultural, public buildings. In 

transport vehicles (ships, trains, aircraft) and in and 

around related port or terminal buildings

NON-URBAN – inside home and agricultural 

buildings

7173-247 GENERATION 

MEAL BAIT 

PACKS

meal 0.0025 Norway rat, 

roof rat, 

house mouse

Rat: 16 bait packs (14 oz bait) per 

placement, 15-30 ft intervals, 

maintain for at least 10 days

URBAN - In and around homes, industrial, 

commercial, agricultural, public buildings, food 

processing facilities and similar man-made structures. 

In transport vehicles (ships, trains, aircraft) and in and 

around related port or terminal buildings. Also use in 

alleys.

NON-URBAN – inside home, agricultural, industrial, 

and commercial buildings

7173-283 DIFETHIALONE 

BAIT STATION

Block 0.0025 House 

mouse

Place 0.7 oz. bait stations 8 – 12 ft 

apart.

Indoor only – homes, industrial, commercial, 

agricultural, and public buildings

7173-285 DIFETHIALONE 

6G PASTE PL 

PKS

paste 0.0025 Norway rat, 

roof rat, 

house mouse

Rats: 75 packs (16 oz. bait) per 

placement, 15-30 ft apart. Maintain 

at least 10 days.

URBAN - In and around homes, industrial, 

commercial, agricultural, public buildings, food 

processing facilities and similar man-made structures. 

In transport vehicles (ships, trains, aircraft) and in and 

around related port or terminal buildings. Also use in 

alleys.

NON-URBAN – inside home, agricultural, industrial, 

and commercial buildings

Warfarin

3282-3 D-CON 

CONCENTRATE 

KILLS RATS & 

MICE

Powder 0.3 (Mix 1 

part 

concentrate 

with 11 parts 

bait) results in 

0.025% a.i in 

bait

Norway rat, 

roof rat, 

house mouse

Rats: 16 oz bait per placement, 

maintain for at least 10 days, no 

restriction on how much per area

In and around homes, industrial buildings, and similar 

man-made structures

3282-4 D-CON READY 

MIXED KILLS 

RATS & MICE

Bait bits 0.025 Norway rat, 

roof rat, 

house mouse

Rats: 4 bait trays (16 oz. bait) per 

placement maintain for at least 10 

days, no restriction on how much per 

area

In and around homes, industrial buildings, and similar 

man-made structures
Page 21 of 183

Table 2-2. Non-conforming consumer use labels for rodenticides: summary of label specifications.

 Reg. No Name Form %a.i Target 

species

Max app rates Location of placements

3282-9 D-CON MOUSEPRUFE KILLS 

MICE

Pellets 0.054 House 

mouse

Apply one 2oz box at 8-12 ft 

intervals, maintain for at least 15 

days

In and around homes, industrial buildings, and similar 

man-made structures

3282-15 D- CON PELLETS 

KILLS RATS & 

MICE

Pellets 

(Place 

pack)

0.025 Norway rat, 

roof rat, 

house mouse

Rats: 4 bait trays (16 oz. bait) per 

placement maintain for at least 10 

days, no restriction on how much per 

area

In and around homes, industrial buildings, and similar 

man-made structures

8845-39 RID-A-RAT RAT 

& MOUSE 

KILLER

meal 0.025 Norway rat, 

roof rat, 

house mouse

Rats: 4 bait packets (16 oz. bait) per 

placement maintain for at least 10 

days, no restriction on how much per 

area

In and around homes, industrial buildings, and similar 

man-made structures

Bromethalin

8845-125 HOT SHOT 

SUDDEN DEATH 

BRAND MOUSE 

KILLER

Pellet 

(Place 

pack)

0.01 House 

mouse

2 place packs (1.5 oz bait) if high 

activity or 1 place pack (0.75 oz bait) 

per location (8-12 ft apart), maintain 

for 7 days or till all feeding ceases 

(whichever longer)

In and around homes, industrial and agricultural 

buildings, and similar man-made structures. Alleys in 

urban areas, inside transport vehicles (ships, trains, 

aircraft), and in and around related port or terminal 

buildings.

8845-126 HOT SHOT 

SUDDEN DEATH 

BRAND RAT 

KILLER 1 

Pellet 

(Place 

pack)

0.01 Norway rat, 

roof rat

4 place packs (6.0 oz bait) per 

placement, placements 15-30 ft 

apart. maintain for 7 days or till all 

feeding ceases (whichever longer)

In and around homes, industrial and agricultural 

buildings, and similar man-made structures. Alleys in 

urban areas, inside transport vehicles (ships, trains, 

aircraft), and in and around related port or terminal 

buildings.

8845-127 HOT SHOT 

SUDDEN DEATH 

BRAND RAT & 

MOUSE KILLER

Pellet (PI 

pack)

0.01 Norway rat, 

roof rat, 

house mouse

8 place packs (6.0 oz bait) per 

placement, placements 15-30 ft 

apart. maintain for 7 days or till all 

feeding ceases (whichever longer)

In and around homes, industrial and agricultural 

buildings, and similar man-made structures. Alleys in 

urban areas, inside transport vehicles (ships, trains, 

aircraft), and in and around related port or terminal 

buildings.

8845-128 HOT SHOT 

SUDDEN DEATH 

BRAND MOUSE 

KILLER BAIT 

STATION 

Pellet 

(Place 

pack)

0.01 House 

mouse

2 units (1.5 oz bait) per placement, 

placements are 8-12 ft apart, 

continue placements till activity 

ceases. If unit untouched after 3 

days, remove or place elsewhere.

INDOORS ONLY – homes, industrial, and 

agricultural buildings
Page 22 of 183

Table 2-3. Conforming consumer use labels for anticoagulants: summary of label specifications.

Reg. No Name Form and 

Station Tier1

%a.i Target species Max app rates Location

Chlorophacinone

7173-287 Chlorophacinone 

Bait Station

Bait block, Tier 

4

0.005 House mouse One 0.7 oz bait station 

every 8 feet

Indoors only.

7173-289 Chlorophacinone 

Block 0803 Bait 

Station

Bait block, Tier 

4

0.005 House mouse One 0.7 oz bait station 

every 8 feet

Indoors only.

7173-293 Chlorophacinone 

Refillable Bait 

Station

Bait block, Tier 

3

0.005 House mouse One 0.42 oz bait station 

every 8 feet

Indoors only.

7173-295 Chlorophacinone 

Block 0803 Bait 

Station II

Bait block, Tier 

4

0.005 House mouse One 0.7 oz bait station 

every 8 feet

Indoors only.

Diphacinone

3282-98 D Con Bait Station X Bait block, Tier 

3

0.005 House mouse 1 oz per placement, every 8 

feet.

Indoors only

3282-99 D Con Bait Station 

IX

Bait block, Tier 

3

0.005 House mouse 1 oz per placement, every 8 

feet.

Indoors only

3282-100 D Con Bait Station 

XIV

Bait block, 

Tier1

0.005 House mouse 7 oz bait per placement, 

every 8 feet.

Indoors and Outdoors (within 50 feet of buildings)

3282-101 D Con Bait Station 

XIII

Bait block, Tier 

1

0.005 House mouse Place one 1-oz bait inside 

bait station every 8 feet.

Indoors and Outdoors (within 50 feet of buildings)

3282-102 D Con Bait Station 

XI

Bait block, Tier 

1

0.005 House mouse Place one 1-oz bait inside 

bait station every 8 feet.

Indoors and Outdoors (within 50 feet of buildings)

3282-103 D Con Bait Station 

XII

Bait block, Tier 

1

0.005 House mouse Place one 1-oz bait inside 

bait station every 8 feet.

Indoors and Outdoors (within 50 feet of buildings)

12455-119 Tomcat Mouse Killer 

VI

Bait block, Tier 

1

0.005 House mouse Place one 1-oz bait inside 

bait station every 8 feet.

Indoors and Outdoors (within 50 feet of buildings)

12455-126 Tomcat Mouse Killer 

V

Bait block, Tier 

1

0.005 House mouse Place one 1-oz bait inside 

bait station every 8 feet.

Indoors and Outdoors (within 50 feet of buildings)

12455-127 Tomcat Mouse Killer 

VII

Bait block, Tier 

3

0.005 House mouse Place one 1-oz bait inside 

bait station every 8 feet.

Indoors only
Page 23 of 183

Table 2-3. Conforming consumer use labels for anticoagulants: summary of label specifications.

Reg. No Name Form and 

Station Tier1

%a.i Target species Max app rates Location

12455-128 Tomcat Mouse Killer 

IX

Bait block, Tier 

4

0.005 House mouse Place one 1-oz bait in bait 

station every 8 feet.

Indoors only

12455-130 Tomcat Mouse Killer 

VIII

Bait block, Tier 

3

0.005 House mouse Place one 1-oz bait inside 

bait station every 8 feet.

Indoors only

61282-19 Prozap Mouse Maze 

Kills Mice

Bait block, Tier 

4

0.005 House mouse Place one 1.0 oz bait 

station every 8 feet.

Indoors only

Bromethalin

3282-96 D-con Hideaway 

Bait Shield II

Bait block, Tier 

3

0.01 House mouse Use one (0.5 oz) bait 

station every 8 feet. 

Indoor only

3282-97 D-con Corner Fit 

Bait Shield VIII

Bait block, Tier 

3

0.01 House mouse Use one (0.5 oz) bait 

station every 8 feet. 

Indoor only

9688-285 Chemisco 

Disposable Bait 

Station

Bait block, Tier 

3

0.01 House mouse Use one (0.5 oz) bait 

station every 8 feet.

Indoor only.

9688-291 Chemisco Refillable 

Bait Station

Bait block, Tier 

3

0.01 House mouse Use one (0.5 oz) bait 

station every 8 feet.

Indoor only.

12455-120 Tomcat Mouse Killer 

I

Bait block, Tier 

1

0.01 House mouse Use one bait station (1 oz 

bait) per placement every 8 

feet.

Indoors and Outdoors (within 50 feet of buildings)

12455-121 Tomcat Mouse Killer Bait block, Tier 

1

0.01 House mouse Use one bait station (1 oz 

bait) per placement every 8 

feet.

Indoors and Outdoors (within 50 feet of buildings)

12455-122 Tomcat Rat Killer Bait block, Tier 

1

0.01 House mouse, Norway 

and roof rat

Use one bait station (4 oz) 

every 15 feet

Indoors and outdoors (within 50 feet of buildings)

12455-123 Tomcat Mouse Killer 

II

Bait block, Tier 

3

0.01 House mouse Use one bait station (1 oz) 

every 8 feet.

Indoor only.

12455-124 Tomcat Mouse Killer 

IV

Bait block, Tier 

4

0.01 House mouse Use one bait station (1 oz) 

every 8 feet.

Indoor only.

12455-125 Tomcat Rat Killer I Bait block, Tier 

4

0.01 House mouse, Norway 

and roof rat

Use one bait station (4 oz) 

every 8 feet for mice; every 

15 feet for rats.

Indoor only.

12455-129 Tomcat Mouse Killer 

III

Bait block, Tier 

3

0.01 House mouse Use one bait station (1 oz) 

every 8 feet.

Indoor only.
Page 24 of 183

Table 2-3. Conforming consumer use labels for anticoagulants: summary of label specifications.

Reg. No Name Form and 

Station Tier1

%a.i Target species Max app rates Location

36488-51 Bromethalin Mouse 

Bait Station

Bait block, Tier 

4

0.01 House mouse Use one bait station (0.75 

oz) every 8 feet.

Indoor only.

67517-89 Assault Mouse Bait 

Station 

Bait block, Tier 

4

0.01 House mouse Use one bait station (0.5 

oz) every 8 feet.

Indoor only.

67517-90 Assault Mouse Bait 

Station II

Bait block, Tier 

2

0.01 House mouse Use one bait station with 1 

oz (2 bait blocks) every 8 

feet.

Indoor only.

67517-92 Assault Mouse Bait 

Station III

Bait block, Tier 

3

0.01 House mouse Use one bait station with 

one bait block (0.5 oz) 

every 8 feet.

Indoor only.

1 Bait station tier descriptions:

Tier I Bait Stations: Tamper-Resistant and Weather-Resistant. These bait stations are resistant to weather and to tampering by children and dogs. To be used indoors and outdoors

(within 50 feet of buildings, defined as structures that possess walls and a roof).

Tier II Bait Stations: Tamper-Resistant (but not weather resistant). These bait stations are resistant to tampering by children and dogs. To be used indoors only.

Tier III Bait Stations: Tamper-Resistant for Children Only. These bait stations are resistant to tampering by children. To be used indoors only.

Tier IV Bait Stations: Tamper-Resistance Unknown. These bait stations must not claim to be tamper resistant. To be used indoors only and only in areas inaccessible to children 

and pets.
Page 25 of 183

Table 2-4. Maximum weight of rodenticide bait and active ingredient 

to be used at each placement for non-conforming labels.

Reg. Num. %a.i. oz bait/

placement

gms bait/

placement

mg a.i/

placement

Brodifacoum

3282-65 0.005 3 85 4.3

3282-66 0.005 12 341 17.0

3282-74 0.005 16 454 22.7

3282-81 0.005 12 341 17.0

Difethialone 

3282-85 0.0025 3 85 2.1

3282-86 0.0025 12 341 8.5

3282-87 0.0025 12 341 8.5

3282-88 0.0025 16 454 11.4

7173-247 0.0025 14 397 9.9

7173-283 0.0025 0.7 20 0.5

7173-285 0.0025 16 454 11.4

Warfarin

3282-3 0.025 16 454 113.5

3282-4 0.025 16 454 113.5

3282-9 0.054 2 57 30.6

3282-15 0.025 16 454 113.5

3282-39 0.025 16 454 113.5

Bromethalin

8845-125 0.01 1.5 43 4.3

8845-126 0.01 6 170 17.0

8845-127 0.01 6 170 17.0

8845-128 0.01 1.5 43 4.3
Page 26 of 183

Table 2-5. Maximum weight of rodenticide bait and active ingredient 

to be used at each placement for conforming labels.

Reg. Num. %a.i. oz bait/

placement

gms bait/

placement

mg a.i/

placement

Chlorophacinone

7173-287 0.005 0.7 20 1.0

7173-289 0.005 0.7 20 1.0

7173-293 0.005 0.42 12 0.6

7173-295 0.005 0.7 20 1.0

Diphacinone

3282-98 0.005 1 28 1.4

3282-99 0.005 1 28 1.4

3282-100 0.005 7 199 9.9

3282-101 0.005 1 28 1.4

3282-102 0.005 1 28 1.4

3282-103 0.005 1 28 1.4

12455-119 0.005 1 28 1.4

12455-126 0.005 1 28 1.4

12455-127 0.005 1 28 1.4

12455-128 0.005 1 28 1.4

12455-130 0.005 1 28 1.4

61282-19 0.005 1 28 1.4

Bromethalin

3282-96 0.01 0.5 14 1.4

3282-97 0.01 0.5 14 1.4

9688-285 0.01 0.5 14 1.4

9688-291 0.01 0.5 14 1.4

12455-120 0.01 1 28 2.8

12455-121 0.01 1 28 2.8

12455-122 0.01 4 114 11.4

12455-123 0.01 1 28 2.8

12455-124 0.01 1 28 2.8

12455-125 0.01 4 114 11.4

12455-129 0.01 1 28 2.8

36488-51 0.01 0.75 21 2.1

67517-89 0.01 0.5 14 1.4

67517-90 0.01 1 28 2.8

67517-92 0.01 0.5 14 1.4
Page 27 of 183

2.3 Modes of Action

The anticoagulant rodenticides (warfarin, difethialone, chlorophacinone, and diphacinone are 

anticoagulants that are included in this assessment) are vitamin-K antagonists that disrupt normal 

blood-clotting mechanisms and induce capillary damage (Pelfrene 1991). Hadler and Buckle 

(1992), in their review of the development of anticoagulant rodenticides provide a summary of 

the mechanism of action of these compounds:

Within the liver, Vitamin K is utilized to drive the production of blood clotting 

factors II, VII, IX & X by the gamma-carboxylation of glutamic acid residues of 

precursor proteins. Concomitant with this reaction, the active form of vitamin K, a 

hydroquinone (KHj), is converted to an inactive 2,3-epoxide. Some of this is 

excreted but the majority is reactivated by an epoxide reductase enzyme...All 

available anticoagulants seem to share the same site of action; they block the 

epoxide reductase enzyme. The recycling of activated vitamin K is therefore 

stopped and there is insufficient incoming vitamin K to maintain the 

carboxylation reaction. The production of clotting factors is critically reduced and 

eventually, when the supply of factors already present has degraded, the clotting 

mechanism fails and hemorrhaging begins.

The anticoagulant rodenticides have been grouped in previous assessments into "first-generation" 

(warfarin, chlorophacinone, diphacinone) and "second-generation" (brodifacoum, bromadiolone, 

difethialone) compounds. Although there is overlap in their metabolic and toxicological 

properties, second-generation anticoagulants tend to be more acutely toxic than are the firstgeneration anticoagulants, and they are retained longer in body tissues of primary consumers.

The greater potency and duration of action of long-acting anticoagulant rodenticides is attributed 

to their: (i) greater affinity for vitamin K(1)-2,3-epoxide reductase; (ii) ability to disrupt the 

vitamin K(1)-epoxide cycle at more than one point; (iii) hepatic accumulation; and (iv) unusually 

long biological half-lives due to high lipid solubility and enterohepatic circulation (Watt et al. 

2005). The terms “first generation” and “second generation” are not used in this risk assessment 

as generic grouping terms to differentiate anticoagulant rodenticides in any qualitative or 

quantitative manner. Rather, the assessment evaluates risks to wildlife based on specific toxicity, 

exposure, body burden, metabolism, and incident information available for each rodenticide. 

However, of the chemicals evaluated in this assessment, warfarin, chlorophacinone, and 

diphacinone are commonly referred to first generation anticoagulant rodenticides, and 

brodifacoum and difethialone are commonly referred to second generation anticoagulants. 

Bromethalin is a neurotoxicant that causes adverse effects and histological changes to the central 

nervous system. Van Lier and Cherry (1988) determined that acute toxicity is caused by the 
Page 28 of 183

uncoupling of mitochondrial oxidative phosphorylation. This leads to fluid build-up and 

demeyelination inside the central nervous system, eventually leading to respiratory failure 

because of lack of nerve impulse transmission to the lungs. While both bromethalin and the 

primary metabolite desmethylbromethalin are active in uncoupling oxidative phosphorylation, 

the desmethylbromalin metabolite is much more potent than the parent compound. Therefore, 

the rapid transformation of bromethalin into desmethylbromethalin in mammals (other than the 

guinea pig) is largely responsible for the high acute toxicity of this compound in most mammals 

(Van Lier and Cherry, 1988). A single feeding of bromethalin may be lethal, with death being 

delayed for two or three days (Spaulding et al, 1985), although doses in excess of the LD50 may 

cause death within 8-12 hr (Van Lier and Cherry, 1988). Further feeding is inhibited after 

ingestion of a lethal dose, and death typically occurs rapidly, often within 2 days (Spaulding and 

Spanning 1988, Hyngstrom et al. 1994). In acute oral mammalian studies submitted to the EPA, 

death was observed 3-7 days after dose administration.

2.4 Conceptual Model of Exposure

The chemicals evaluated in this assessment are used to control commensal rodents. Commensal 

rodent control in urban and suburban areas may result in exposure to wildlife as demonstrated by 

the presence of wildlife mortality incidents and survey data indicating that exposure occurs in 

urban and suburban areas in addition to agricultural areas. In general terms, the exposure 

pathways for rodenticides are from direct consumption of rodenticide bait (primary exposure) or 

consumption of animals that have consumed bait by predators or scavengers (secondary 

exposure). 

For primary exposure, the bait itself is the potential food item of concern. Thus, the amount of 

active ingredient in the formulated bait represents the exposure concentration. This information 

is used to estimate the amount of bait that birds and mammals of various sizes need to consume 

to obtain a dose expected to be lethal to 50% of the individuals in the population (i.e., LD50). 

Estimates of daily food-ingestion rates are determined from established allometric equations 

presented in the Wildlife Exposure Factors Handbook (USEPA 1993) for birds and mammals. 

Potential exposure pathways evaluated in this assessment are illustrated in Figure 2-1. 

Additional details on the exposure analysis can be found below. Exposure pathways illustrated 

with dotted lines were not evaluated as part of this assessment because exposure was expected to 

be negligible relative to direct consumption of bait or consumption of contaminated prey or 

because methods used to quantify such exposures are not available. 
Page 29 of 183

Stressor

Source

Attribute

Change

Rodenticide

Placement of Bait

Terrestrial

Invertebrates

Individual organisms

Reduced survival

Reduced growth

Reduced reproduction

Habitat Integrity

Reduced availability

of cover and nest sites

Food Chain

Reduction in prey 

availability

Terrestrial

Vertebrates

Exposure Media 

& Receptors

Exposure to Bait Ingestion

Ingestion

Secondary 

exposure

Primary exposure

Plants

Uptake

Figure 2-1. Conceptual Model Diagram of Exposure and Effects in Nontarget Species.

2.5 Taxonomic Groups Potentially at Risk

The taxonomic groups that are potentially at risk due to the rodenticides considered in this 

assessment include mammals and birds that are primary (e.g., granivores) and secondary (e.g., 

predators) consumers of those rodenticides. The Agency’s concern about risks to birds and nontarget 

mammals is based on several factors, including:

(1) High acute toxicity of these rodenticide baits, which are designed to be lethal to mammals; 

(2) Risk quotients (RQs) from past assessments that were based on exposure and toxicity data, 

that exceed Agency levels of concern (LOCs); 

(3) Empirical data involving mortality of birds and non-target mammals exposed to rodenticide 

baits or poisoned prey in laboratory, pen, and field settings; 

(4) Retention time of residues in body tissues of primary consumers; and 

(5) Numerous reported incidents that indicate exposure and mortalities of numerous non-target 

species, including avian and mammalian predators and scavengers. 

In addition, primary and secondary consumers may be exposed to rodenticides. Baits containing 

rodenticides are formulated to be lethal to rodents and other mammals, and they are not selective to 

target species. Many factors influence which non-target animals might be exposed to baits, but many 

birds and mammals are attracted to seeds and grains and are likely to consume grain-based baits. A 

few commensal baits also contain flavorizers such as fish, molasses/peanut butter, or apple that might 
Page 30 of 183

appeal to carnivores and omnivores. Rodenticide baits also pose potential secondary risks, because 

predators and scavengers are likely to be attracted to dead or dying rats, mice, and poisoned 

nontarget animals.

As will be discussed elsewhere in this document, the records of incidents involving the rodenticides 

considered in this assessment indicate that they have been detected in numerous wildlife species 

collected from a variety of land use areas including urban, suburban and rural locations. These data 

indicate that complete exposure pathways exist where wildlife were exposed to rodenticides in areas 

where commensal rodent control is expected to occur and mortalities resulted.

This assessment does not evaluate potential risks to aquatic organisms or to plants. Exposure to 

aquatic organisms is assumed to be negligible relative to potential exposures to terrestrial animals for 

commensal rodent control given that (1) direct application to water is unlikely to occur; (2) spray 

drift of baits is not expected; (3) large, contiguous areas are not expected to be treated; and (4) 

leaching of rodenticides from baits is limited. 

2.6 Measures of Exposure

EPA’s exposure assessment for rodenticides differs from that used for most other pesticides, 

which are often administered to treatment sites via sprays, granules, and other diffuse application 

methods. For pesticides applied via sprays, exposure is assessed as a concentration of the 

pesticide on plants and insects located on the treated field. For a rodenticide, the bait itself is the 

potential food item of concern. Thus, the amount of active ingredient in the formulated bait is 

used to represent exposure, which can then be compared directly to dietary toxicity values such 

as LC50s or NOAECs expressed as mg/kg-food. The concentration of active ingredient in bait is 

also used along with estimates of food consumption rates to estimate the amount of grain bait 

that birds and mammals of various sizes need to consume to obtain a dose expected to be lethal 

to 50% of the individuals in the population (i.e., LD50). Estimates of food-ingestion rates (g dry 

matter per day) are determined from established allometric equations presented in EPA’s 

Wildlife Exposure Factors Handbook (EPA 1993). 

Secondary exposure analysis requires consideration of residues in tissues of target and non-target 

organisms that are commonly consumed by predators and scavengers. Exposures of predators to 

individual rodenticides were calculated in two ways. First, the theoretical amount of rodenticide 

within prey animals was estimated using allometric equations for food intake estimation (U.S. 

EPA, 1993) assuming prey consumed either a single day’s feeding or repeated feedings for 

several days. These two values represent low-end and high-end theoretical estimations of 

rodenticide exposures for secondary consumers. In addition, empirical residue concentrations 

from prey used in secondary feeding studies were used to derive exposure values for the 

secondary consumers. Although these values do not likely represent high-end exposure values 
Page 31 of 183

because the prey animals used in these studies may have fed on items other than rodenticide bait, 

they represent concentrations of rodenticides that have been measured in primary consumers. 

The theoretical and empirical concentrations of the rodenticides in primary consumers were both 

used to estimate potential risks to predators and scavengers. Additional details are presented in 

the secondary exposure analysis section of this assessment.

2.7 Measures of Toxicity

Toxicity endpoints are established based on data generated from guideline studies submitted by 

the registrant, and from open literature studies that meet the criteria for inclusion into the 

ECOTOX database. A number of non-target animal species have been tested for some chemicals 

included in this assessment, but the suite of available species tested is not uniform across all the 

evaluated rodenticides. In order to allow for risk estimations to be made across the assessed 

chemicals without a bias associated with differential sensitivities of tested species that vary 

across the assessed chemicals (i.e., the number and identity of species tested varies for each 

chemical assessed), the toxicity data used for RQ calculations were based on OPP’s standard test 

species for which data were available across all the chemicals evaluated (i.e., Norway rat (Rattus 

norvegicus) for mammals and the Northern bobwhite quail (Colinus virginianus) or mallard duck 

(Anas platyrhynchos) for birds). Use of standard species allows for a comparable analysis across 

the assessed chemicals; however, it may also result in an underestimation of risk in cases where 

data are available for more sensitive test species. In cases where more sensitive species have 

been tested for a particular chemical (e.g., American kestrels (Falco sparverius) for 

diphacinone), the potential impact of using the more sensitive data is evaluated. For rodenticides

where particularly sensitive species are not tested, their absence may represent a source of 

uncertainty that results in a less conservative risk assessment. It should be noted that in cases 

where more sensitive toxicity data are available for a chemical (besides the standard test 

species), assessments conducted by the Agency to support registration, registration review, or 

litigation for the assessed chemicals will likely utilize the more sensitive data for risk estimation, 

if available, than was used here. The data for the standard test species were used here to allow 

for comparisons of risk conclusions among the chemicals. Last, the assessment only evaluated 

potential acute exposures and effects. Risk of reproductive effects was not evaluated due to 

several factors, including: paucity of reproduction toxicity data and the desire to evaluate an 

endpoint that can be compared with available wildlife incidents (i.e., mortality). It is possible 

that reproductive effects may occur at lower exposure levels than exposures associated with 

mortality. Details on the toxicity data used for this assessment are included in Sections 3 and 4. 
Page 32 of 183

2.8 Measures of Risk 

Risk was evaluated using several lines of evidence. These include, Risk Quotients (RQs) and 

data used to further characterize risks including pharmacokinetic data, estimation of the amount 

of bait required to reach the median lethal dose, estimation of the number of primary consumers 

that need to be consumed to reach the median lethal dose of secondary consumers, results of 

feeding studies, and incident reports. These are described below.

2.8.1 Risk Quotients (RQs)

Risk from primary exposure (direct consumption of bait) was evaluated by calculating dietary 

based and dose-based risk quotients (RQs). The RQ is a unitless value that is the ratio of 

exposure to the toxicity endpoint. An RQ of 1 means that the exposure (dose or concentration 

based) is equal to the selected toxicity value (i.e., LD50 or LC50). For example, in the context of 

acute avian risk estimates (e.g., mortality), an RQ of 1 would mean that non-target bird species 

may be exposed in the environment to an amount of the pesticide that results in mortality 

(specifically, the dose at which 50% of the test species died). EPA compares RQ values to the 

Agency’s “levels of concern” (LOCs) for non-target species. For this risk assessment, the nonlisted species LOC for acute exposures (LOC = 0.5) was used to evaluate RQs. Additional levels 

of concern may also be used by the Agency in other assessment to evaluate potential risks to 

listed species (LOC = 0.1); however, for simplicity, this analysis only used the LOC for nonlisted species. Therefore, conclusions made with respect to the LOC of 0.5 may not apply to 

listed species. 

For primary consumers of bait, dietary based RQs were calculated by comparing concentration 

of active ingredient in bait (exposure value) to LC50 toxicity values (RQ = a.i. concentration in 

bait / LC50). Dose-based RQs were calculated by converting dietary exposure values to an 

internal dose for various sized animals using allometric equations that estimate food 

consumption as a function of body weight as described further in Section 3. The resulting dosebased exposure values were then compared to LD50s that were also adjusted for body weight of 

the assessed animal. (RQ = Dose-based EEC / Adjusted LD50). 

Potential risks from secondary exposure (i.e., exposures of predators and scavengers through 

consumption of contaminated prey) were also evaluated on a dose and concentration basis. Risk 

was estimated assuming that predator/scavenger that consumed prey that fed on bait for 1, 3, or 6 

days. During this time period, the animals were assumed to eliminate the chemical. Elimination 

rates in target animals (i.e., Norway rat and house mouse) were based on empirical data 

describing elimination half-lives in the liver or plasma as described in Section 3. 
Page 33 of 183

2.8.2 Additional Risk Characterization

Because this deterministic assessment is intended to serve as a screening tool for identifying a 

potential for adverse effects, it is somewhat limited and conservative by design. As a result, 

EPA does not believe that the RQs derived in this assessment can be used as a precise measure of 

the magnitude of effects that will occur, but rather serve as tools for addressing whether or not a 

chemical poses a risk to assessed animals. Because of their design, RQs should not be directly 

compared among chemicals. In this assessment, the Agency uses other lines of evidence to 

characterize the relative risks of the rodenticides that are the subject of the NOIC and the 

alternatives considered here. These lines of evidence include factors intended to describe the 

opportunity for exposure to occur at median lethal doses: 

• Amount of bait required to reach median lethal dose in primary consumers

o Amount of bait required to reach the median lethal dose was estimated using 

allometric equations for food intake assuming an animal consumed 100% of their 

diet as dry bait. In addition, the time needed to consume a median lethal dose was 

also calculated. These factors were used to compare the opportunity for primary 

exposure to occur at lethal doses across the assessed chemicals. 

• Number of prey required to reach median lethal dose in secondary consumers

o The number of prey animals required to achieve a median lethal dose was used as 

a comparative metric for the opportunity to achieve a lethal dose across the 

assessed rodenticides. This analysis assumed that the prey animal consumed bait 

for 1 to 6 days, then were consumed by a predator or scavenger. The opportunity 

for a lethal dose to be consumed by a predator or scavenger was assumed to be 

greater as the number of animals required to be consumed to reach a lethal dose 

decreased. 

• Secondary feeding studies 

o Toxicity studies are available where small mammals were fed diets containing 

one of the rodenticides, and these animals died or were sacrificed. These 

contaminated carcasses were fed to the birds and mammals (i.e., secondary 

consumers) being evaluated in the studies. These types of secondary exposure 

studies provide information on the potential effects from rodenticide exposure that 

may occur under field conditions; however, it is often not possible to quantify the 

level of exposure (rodenticide residues in primary consumers or food 

consumption by secondary consumers) or the sensitivity of the exposed animals to 

the rodenticide. 
Page 34 of 183

• Pharmacokinetic data

o The potential for secondary exposure of scavengers and predators to rodenticides

was evaluated by considering pharmacokinetic data. Specifically, the relative 

elimination half-lives of the rodenticides considered in this assessment were used 

to characterize the relative risks based on the persistence of the rodenticides in 

exposed primary consumers. Although not a quantitative measure of risk, it was 

assumed that the potential for secondary exposure to a rodenticide increases as the 

bio-elimination half life increases. 

2.8.3 Incident Reports

Many incidents of wildlife mortality have been reported to the Agency for the rodenticides 

considered in this assessment. These incidents were evaluated in order to confirm or refute the 

conclusions that were based on the deterministic assessment. Incidents were also used to 

determine the spatial overlap of reported incidents with urban, agricultural, or suburban areas. 

3 Toxicity, Accumulation, and Elimination of the Evaluated 

Rodenticides

3.1 Avian and Mammalian Toxicity 

A summary of the available acute ecotoxicity information for the evaluated chemicals 

(brodifacoum, difethialone, warfarin, chlorophacinone, diphacinone, and bromethalin) is 

provided in Sections 3.1.1 through 3.1.4. Acute toxicity to terrestrial animals is categorized 

using the classification system shown in Table 3-1 (USEPA, 2004a). Toxicity categories for all 

six chemicals are provided in Table 3-2. For consistency when comparing across chemicals, 

avian toxicity categories were based on the most sensitive bobwhite quail or mallard duck studies 

and mammalian toxicity categories were based on the most sensitive laboratory rat studies.

When evaluated across chemicals (Table 3-2), there is more variability in toxicity to birds than 

there is in toxicity to mammals. For avian acute oral toxicity, warfarin and diphacinone were 

considered slightly toxic, while chlorophacinone was considered moderately toxic and 

brodifacoum, difethialone, and bromethalin were considered very highly toxic. For avian dietary 

toxicity, the ordering of chemicals based on toxicity category was very similar. Warfarin and 

diphacinone were considered moderately toxic, chlorophacinone and bromethalin were 

considered highly toxic, and brodifacoum and difethialone were classified as very highly toxic to 

birds. For mammals, there was much less variation in toxicity categories. All evaluated 

rodenticides were classified as very highly toxic to mammals based on the acute oral toxicity 
Page 35 of 183

test. The four chemicals with quantitative toxicity data (brodifacoum, warfarin, chlorophacinone, 

and diphacinone), were all classified as very highly toxic to mammals on a dietary basis.

Table 3-1. Categories of Acute Toxicity for Avian and Mammalian Studies

Toxicity Category Oral LD50 Dietary LC50

Very highly toxic < 10 mg/kg < 50 mg/kg-diet

Highly toxic 10 - 50 mg/kg 50 - 500 mg/kg-diet

Moderately toxic 51 - 500 mg/kg 501 - 1000 mg/kg-diet

Slightly toxic 501 - 2000 mg/kg 1001 - 5000 mg/kg-diet

Practically non-toxic > 2000 mg/kg > 5000 mg/kg-diet
Page 36 of 183

Table 3-2. Acute oral and dietary toxicity values and categories for the evaluated 

rodenticides1

Chemical

Avian Mammalian

LD50

(mg a.i./kg-bwt)

LC50

(mg a.i./kg-diet)

LD50

(mg a.i./kg-bwt)

LC50

(mg a.i./kg-diet)

Brodifacoum 0.25 1.33 0.48 0.53 

Difethialone 0.30 0.56 0.56 Not available

Warfarin 525 625 3.0 5.4 

Chlorophacinone 258 72 6.26 7.2 

Diphacinone 1630 906 1.9 4.2 

Bromethalin 4.6 210 2.2 Not available

Avian Mammalian

Oral Dietary Oral Dietary

Brodifacoum Very highly toxic Very highly toxic

Very highly toxic

Very highly toxic

Difethialone Very highly toxic Very highly toxic Not available

Warfarin Slightly toxic Moderately toxic Very highly toxic

Chlorophacinone Moderately toxic Highly toxic Very highly toxic

Diphacinone Slightly toxic Moderately toxic Very highly toxic

Bromethalin Very highly toxic Highly toxic Not available 1 For consistency when comparing across chemicals, avian toxicity values and categories were based on 

the most sensitive bobwhite quail or mallard duck studies and mammalian toxicity values and categories 

were based on the most sensitive laboratory rat studies. 

3.1.1 Avian acute oral toxicity 

Table 3-3 summarizes the most sensitive avian acute oral (gavage or capsule) toxicity endpoints 

for either Northern bobwhite quail (Colinus virginianus) or mallard duck (Anas platyrhynchos), 

based on an evaluation of both the registrant-submitted studies and the open literature. When 

considering these data, brodifacoum and difethialone have the lowest LD50 values (i.e., 0.25 and 

0.30 mg a.i./kg-bwt, respectively) for the standard test species. The LD50 for bromethalin is an 

order of magnitude higher (i.e., LD50 = 4.6 mg a.i./kg-bwt) and the LD50 values for 

chlorophacinone, warfarin, and diphacinone (LD50 values = 258, 525, and 1630 mg a.i./kg-bwt, 

respectively) are three to four orders of magnitude higher than those of brodifacoum and 

difethialone. All evaluated chemicals have multiple avian acute oral studies available resulting in 

a wide range of reported toxicity (Appendix A). When considering the two standard avian test 

species, neither the bobwhite quail nor the mallard duck was consistently more sensitive. In most 

cases, either the bobwhite quail or the mallard duck represented the most sensitive test species. 
Page 37 of 183

Table 3-3. Avian acute oral studies used for calculating RQs (restricted to mallard duck 

and bobwhite quail).

Chemical/ 

Species

% 

a.i.

LD50, 

mg a.i/kgbwt

(95% CI)

Probit 

slope 

(95% CI)

Average 

body 

weight, g 

(day=0)

Study 

length

(days)

Time to 

mortality

(days)

MRID and 

classification

Brodifacoum

Mallard duck

97.6 0.25 

(0.11, 0.40)

3.0 

(1.3, 4.7)

1054 28 3-23 days; 

majority of 

mortality from 

7-10 days

41563303

Acceptable

Difethialone

Northern 

bobwhite quail 

96 0.30 

(0.16, 0.38)2

5.9 

(1.9, 9.9)

196 30 3-19 days 406969-01

Acceptable

Warfarin

Mallard duck 

Tech. 525 

(336, 689)1

3.1 

(1.6, 4.6)

NA, 21-

weeks old

14 < 1 day Accession#

00248782

acceptable

Chlorophacinone

Northern 

bobwhite quail 

100 258 

(168, 360)

2.9 

(1.3, 4.4)

203 30 2-5 days 41513101 

Acceptable

Diphacinone

Northern 

bobwhite quail

96.9 16303 NA 190 21 5-9 days 42245201

Supplemental

Bromethalin

Northern 

bobwhite quail

96.3 4.6 

(3.3-6.0)

3.6 

(1.6, 5.7)

194 14 1-7 days 00086742, 

acceptable

1 Previous assessments reported the LD50 = 621 mg a.i/kg-bwt based on the moving average estimate. Data were 

re-evaluated and the probit model was selected as providing the best fit to the data. 2 Previous assessments reported the LD50 = 0.26 mg a.i/kg-bwt. Data were re-evaluated and the probit model was 

selected as providing the best fit to the data. 

3 Previous assessments reported the LD50 ≥ 400 mg a.i/kg-bwt. Data were re-evaluated and the binomial model 

provided an adequate fit to the data. 

For brodifacoum, the study with the most sensitive endpoint was conducted using mallard duck

and resulted in an LD50 = 0.25 mg a.i./kg-bwt (MRID 41563303). Additional toxicity tests 

conducted on ring-necked pheasants (Phasianus colchicus) and Japanese quail (Coturnix c. 

japonica) resulted in LD50s of 0.59 and 11.6 mg a.i./kg-bwt (MRIDs 6351303 and 46351304). 

For difethialone, the study with the most sensitive endpoint was conducted using bobwhite quail 

and resulted in an LD50 = 0.30 mg a.i./kg-bwt (MRID 40696901). One additional avian gavage 

study was identified; an LD50 of 23.4 mg a.i./kg-bwt was reported for Japanese quail (MRID 

42687702, 40268913).
Page 38 of 183

For warfarin, the study with the most sensitive endpoint was conducted using mallard ducks and 

resulted in an LD50 = 525 mg a.i./kg-bwt (MRID 00248782). In addition, toxicity to domestic 

chickens (Gallus domesticus) was estimated as LD50 = 942 mg a.i./kg-bwt (Bai and 

Krishnakumari 1986). Toxicity of a material containing 10% warfarin to mallards was reported 

as LD50 = 120 mg a.i./kg-bwt (MRID 5005716). Two studies using bobwhite quail resulted in 

non-definitive LD50s of >2000 and >2150 mg a.i./kg-bwt (MRIDs 156284 and 153369).

For chlorophacinone, the study with the most sensitive endpoint was conducted using bobwhite 

quail and resulted in an LD50 = 258 mg a.i./kg-bwt (MRID 41513101). One additional avian oral

study using bobwhite quail was identified; an LD50 of 495 mg a.i./kg-bwt was reported (MRID 

0039233).

For diphacinone, the most sensitive endpoint that corresponded to a standard test species 

(bobwhite quail or mallard duck) was an LD50 = 1630 mg a.i./kg-bwt from a bobwhite quail 

study (MRID 42245201). A second bobwhite quail study was available in the open literature 

(Rattner et al., 2010), which reported an LD50 of 2014 mg a.i./kg-bwt. In addition, an LD50 = 

3158 mg a.i./kg-bwt was determined for mallard ducks (MRID 00128411). 

Also for diphacinone, an LD50 of 96.8 mg a.i./kg-bwt is available for one raptor species 

(American kestrel, Falco sparverius) (Rattner et al. 2011). This study suggests that American 

kestrels may be more sensitive than the standard test species (i.e., based on the available studies, 

17 to 21 times more sensitive than bobwhite quail and 33 times more sensitive than mallard 

ducks). None of the other evaluated rodenticides have data for which an LD50 for raptors, or, 

more generally, predatory birds is available. If the sensitivity of the American kestrel to 

diphacinone is representative of other species of raptors, this information suggests that raptors 

may be more sensitive to diphacinone than either bobwhite quail or mallard duck. The potential 

for increased sensitivity of raptors to warfarin was observed by Watanabe et al. (2010), who 

demonstrated that hydroxylation rates of warfarin in the great horned owl (Bubo virginianus) and 

snowy owl (Bubo scandiacus) were much less than that observed in mallards, chickens (Gallus 

gallus), and laboratory rats. This study suggests that raptors may metabolize and detoxify 

warfarin at slower rates compared to mallard ducks. Taken together, these data may indicate that 

raptors, which represent secondary consumers of rodenticides, are more sensitive to some 

rodenticides when compared to the standard test species used to derive the RQs calculated in this 

assessment. This information will be considered later in the assessment when characterizing the 

risks of anticoagulant rodenticides to secondary consumers.

The most sensitive results for bromethalin were obtained from a bobwhite quail study (MRID 

00086742) that provided an LD50 = 4.6 mg a.i./kg-bwt. Two additional studies were conducted 

using bromethalin on bobwhite quail. One study resulted in a definitive LD50 = 11.0 mg a.i./kg-
Page 39 of 183

bwt (MRID 00086741), and one resulted in a non-definitive endpoint of LD50 < 160 mg a.i./kgbwt as all birds died by day two of the study (van Lier et al. 1980).

Figure 3-1 allows for visual comparison in toxicity sensitivities across chemicals and among 

studies within a chemical. For avian acute oral toxicity, the most sensitive LD50 values were 

obtained from either Northern bobwhite quail or mallard duck studies (850.2100 recommended 

guideline species) for all chemicals except diphacinone. 

Figure 3-1. Avian acute oral LD50 (mg a.i./kg-bwt) values for all evaluated chemicals. LD50

values represented as a range were plotted using a mid-point value; no other non-definitive values were included. 

Only toxicity values obtained using the technical material were included. Circles represent toxicity values used for 

RQ calculation. 

Although mortality is the endpoint of interest for this assessment, sublethal effects observed in 

the available toxicity studies are worth noting. Sublethal effects observed in the anticoagulant 

acute oral studies included lethargy, subcutaneous, intramuscular, and internal hemorrhaging, 

piloerection, diarrhea, bloody diarrhea, and anorexia. Sublethal effects noted in the bromethalin 

acute oral studies included clonic convulsions, prostration, salivation, crouched body posture, 

ataxia, lethargy, diarrhea, and loose feces. 

3.1.2 Avian dietary toxicity

Table 3-4 summarizes the most sensitive avian dietary toxicity endpoints for either bobwhite 

quail or mallard duck, based on an evaluation of both the registrant-submitted studies and the 

open literature. Mallard ducks and northern bobwhite quail were the only species for which the 

0.1 1 10 100 1000 10000 LD50 (mg a.i./kg-bwt)

Warfarin

Chlorophacinone

Difethialone

Diphacinone

Bromethalin

Brodifacoum
Page 40 of 183

acute avian dietary studies were available. As with the acute avian toxicity data discussed above, 

difethialone and brodifacoum have the most sensitive acute dietary toxicity endpoints for birds 

(i.e., LC50 values = 0.56 and 1.33 mg/kg-diet, respectively). The LC50 value for chlorophacinone 

is 1-2 orders of magnitude higher (LC50 = 72 mg/kg-diet). Bromethalin, warfarin and 

diphacinone have LC50 values (i.e., 210, 625 and 906 mg/kg-diet, respectively) that are 2-3 

orders of magnitude less sensitive compared to difethialone and brodifacoum. All six chemicals 

have multiple avian dietary studies available resulting in a range of reported toxicity values 

(Appendix A). 

Table 3-4. Avian acute dietary studies used for calculating RQs (restricted to mallard 

duck and bobwhite quail).

Chemical/ 

Species

% a.i. LC50, 

mg 

a.i/kgdiet

(95% CI)

Probit 

slope 

(95% CI)

Average 

body 

weight, g 

(day=0)

Study 

length 

(days)

Time to 

mortality

(days)

MRID and 

classification

Brodifacoum

Northern 

bobwhite quail

94 1.33 

(0, 2.96)1

NA 22 40 most from days 

3-10

00124477

acceptable

Difethialone

Northern 

bobwhite quail 

96 0.56 

(0.16, 1.9)

NA 30 30 2-23 days 406969-02

Acceptable

Warfarin

Northern 

bobwhite quail

100 625 NA 33 15 most from 

days 3-7, one 

on day 10

00153365, 

acceptable,

Chlorophacinone

Northern 

bobwhite quail 

100 72 

(32, 129)

1.5 

(0.8, 2.2)

22 30 2-9 days 41513102

Acceptable

Diphacinone

Mallard duck

96.9 906 

(187, 

35110)

0.5 

(0.2, 0.8)

185 25 most from 3-8 

days, all less 

than 16 days

424088-02

Acceptable

Bromethalin

Northern 

bobwhite quail

96.3 210 

(150, 310)

3.7 

(1.8, 5.6)

20.7 8 1-5 days 00086744, 

Acceptable

1 Moving average; previous assessments used an LC50=0.8 based on the probit model. Probit does not fit the data 

well and predicts the LC50 will be less than the lowest test concentration (1.0 mg a.i/kg-diet) which experienced 

only 20% mortality. Also noted that 12% control mortality in study due to toe and nostril picking.

NA – not available
Page 41 of 183

For brodifacoum, one bobwhite quail and one mallard duck dietary toxicity study were 

identified. The resulting LC50s of 1.33 and 2.75 mg a.i./kg-diet were calculated (MRIDs 

00124477 and 00124476).

For difethialone, one bobwhite quail and one mallard duck dietary toxicity study were identified. 

The resulting LC50s of 0.56 and 1.4 mg a.i/kg-diet were calculated, respectively (MRIDs 

406969-01; 40268912 and 42687703).

For warfarin, the study with the most sensitive endpoint was conducted using bobwhite quail and 

resulted in an LC50 = 625 mg a.i./kg-bwt (MRID 00153365). Three additional bobwhite quail 

dietary studies were conducted, resulting in LC50s ranging from 3000 to 6690 mg a.i./kg-diet 

(MRIDs 117979, 161724, 156283). Three mallard duck studies were reviewed, resulting in 

definitive LC50 values of 890 and 1726 mg a.i./kg-diet and a non-definitive LC50 > 5000 mg 

a.i./kg-diet (MRIDs 153366, 157812, 156285). Some of the variability in the warfarin toxicity 

endpoints may be due to impurities in technical material and variability between batches 

(USEPA, 1982).

For chlorophacinone, the study with the most sensitive endpoint was conducted using bobwhite 

quail and resulted in an LC50 = 56 mg a.i./kg-bwt (MRID 41513102). One additional bobwhite 

quail study had an LC50 = 242 mg a.i./kg-diet (MRID 29144), and two mallard duck studies had 

LC50s of 172 and 426 mg a.i./kg-diet (MRIDs 41513103, 29144). A Japanese quail study 

conducted using a 0.25% oil concentrate of chlorophacinone resulted in an LC50 = 60 mg a.i./kgdiet (MRID 47323201).

For diphacinone, one mallard duck and one bobwhite quail dietary toxicity study were identified. 

The resulting LC50s were 906 and >5000 mg a.i/kg-diet (MRIDs 424088-02 and 424088-01).

For bromethalin, the study with the most sensitive endpoint was conducted using bobwhite quail 

and resulted in an LC50 = 210 mg a.i./kg-bwt (MRID 00086744). Two additional bobwhite quail 

studies were available [LC50s > 50 mg a.i/kg-diet (MRID 00086745) and >200 mg a.i/kg-diet

(Ecoref No. 150784)]. A dietary study was also conducted on mallard ducks resulting in an LC50

= 620 mg a.i/kg-diet (MRID 00086746).

Figure 3-2 allows for visual comparison in toxicity sensitivities across chemicals and among 

studies within a chemical. For avian acute dietary toxicity, the most sensitive LD50’s were 

obtained from either Northern bobwhite quail or mallard duck studies (850.2100 recommended 

guideline species) for all chemicals. 
Page 42 of 183

Figure 3-2. Avian acute oral LC50 (mg a.i./kg-diet) values for all evaluated chemicals. LC50

values represented as a range were plotted using a mid-point value; no other non-definitive values were included. 

Only toxicity values obtained using the technical material were included. Circles represent toxicity values used for 

RQ calculation.

3.1.3 Mammalian acute oral toxicity 

Table 3-5 summarizes the most sensitive laboratory rat acute oral toxicity endpoints, based on an 

evaluation of both the registrant-submitted studies and the open literature. Based on these data, 

all of the rodenticides considered in this assessment are very highly toxic to the laboratory rat; 

however brodifacoum and difethialone are an order of magnitude more toxic when compared to 

the other four chemicals. As summarized below and in Appendix A, all six chemicals have 

multiple acute oral studies available with different species of mammals. These data indicate a 

wide range in mammalian sensitivities to acute exposures to these rodenticides. Although the 

standard mammalian test species (i.e., the laboratory rat) is among the most sensitive test species, 

more conservative endpoints from other species are available for almost all of the rodenticides 

considered in this assessment.

0.1 1 10 100 1000 10000

Chlorophacinone

LC50 (mg a.i./kg-diet)

Warfarin

Difethialone

Diphacinone

Bromethalin

Brodifacoum
Page 43 of 183

Table 3-5. Mammalian acute gavage studies used for calculating RQs (restricted to 

laboratory rat, Rattus norvegicus).

Chemical % a.i. LD50, 

mg a.i/kgbwt

(95% CI)

Probit 

slope 

(95% CI)

Average 

body 

weight, g 

(day=0)

Study 

length 

(days)

Time to 

mortality

(days)

MRID and 

classification

Brodifacoum 96.1 0.482 NA 240 15 4-9 42687501

Acceptable3

Difethialone 98.9 0.56 

(0.53,0.59)

15 

(10, 20)

230 21 4-12 40268903 

Acceptable3

Warfarin1 100 3.0 

(2.0, 3.8)

NA >175 14 1-13 05002272 

(Gaines 1960)

Supplemental

Chlorophacinone

100 6.26 

(4.68, 8.30)

2.0

(1.4, 2.9)

175 21 4 - 13 41875301

Acceptable3

Diphacinone1 100 1.9

(1.4, 2.3)

NA >175 14 2-9 05002272 

(Gaines 1960)

Supplemental

Bromethalin Tech 2.2 

(2.0, 2.5)

7.2 

(4.7, 9.7)

110-135 14 1-5 00026524 and 

00026523

Minimum3

1 Only male rats were used in this study. LD50 was estimated by the study author using the Litchfield and 

Wilcoxon method. Initial rat weights provided as a minimum.

2 Estimated using binomial method. Confidence intervals could not be calculated.

3 Study classifications by Health Effects Division.

For brodifacoum, one study conducted on laboratory rats resulted in an LD50 = 0.48 mg a.i/kgbwt (MRID 42687501). Toxicity tests conducted on pine voles and meadow voles resulted in 

LD50s of 0.42 and 0.82 mg a.i/kg-bwt (Byers, 1978). The LD50 for Richardson’s ground squirrel 

(Urocitellus richardsonii), was 0.13 mg a.i/kg-bwt (Baril and Pallister 1981, as cited in 

Matschke et al. 1983). Mink (Neovison vison) had an LD50 = 9.2 mg a.i/kg-bwt (MRID 

00080248). LD50s reported for domesticated animals included beagle dogs (Canis lupus 

familiaris) with an LD50 between 0.25 and 1.0 mg a.i/kg-bwt (MRID 00087134), Dutch rabbit 

(Oryctolagus cuniculus) with an LD50 = 0.29 mg a.i/kg-bwt (MRID 00128426), pigs (Sus scrofa 

domesticus) with an LD50 between 0.5 and 2.0 mg a.i/kg/diet (MRID 00080246), guinea pig 

(Cavia porcellus), at 2.8 mg a.i/kg-bwt (00087133), cat (Felis catus) at LD50 = 25 mg a.i/kg-bwt 

(MRID 00087134), and sheep (Ovis aries) at LD50 >25 mg a.i/kg-bwt (MRID 00080247).

For difethialone, one study conducted on laboratory rats resulted in an LD50 = 0.56 mg a.i/kg-bwt 

(MRID 40268903). LD50s for laboratory mice, house mouse, roof rats, and Norway (wild) rats 
Page 44 of 183

ranged from 0.29 to 1.29 mg a.i./kg-bwt (MRID 40268904 and Lorgue 1984, as cited in 

Lechevin and Poché 1988). LD50s for the hare (Lepus spp.) and pig were 0.75 and 2-3 mg a.i/kgbwt, respectively (Lorgue 1984 as cited in Lechevin and Poché 1988). Reported LD50s for dogs 

were 4 and 11.8 mg a.i/kg-bwt (Harling et al, 1986 as cited in LiphaTech, Inc, 1997, and MRID 

41442201).

For warfarin, the most sensitive laboratory rat data was an LD50 = 3.0 mg a.i/kg-bwt (MRID 

05002272). Additional laboratory rat studies resulted in a range of LD50 values from low values 

of 8.7 and <10 mg a.i/kg-bwt to a high value of 512 mg a.i/kg-bwt. The LD50 value for the cat

was <2.5 mg a.i/kg-bwt. Some of the variability in the warfarin toxicity endpoints may be due to 

impurities in technical material and variability between batches of technical material (USEPA, 

1982). Poché and Mach (2001) also suggest that the degradation rate of warfarin in the 

gastrointestinal tract of rats probably depends on the variation of bacterial species present and 

their abundance.

For chlorophacinone, one study conducted on laboratory rats resulted in an LD50 = 6.26 mg 

a.i/kg-bwt (MRID 41875301). The LD50 for pine vole (Microtus subterraneus) was 17.3 mg 

a.i/kg-bwt (Byers, 1978) and the LD50 for deer mouse was 1.0-3.75 mg a.i/kg-bwt (Byers, 1978). 

A toxicity test conducted on black-tailed prairie dogs (Cynomys ludovicianus) resulted in an 

LD50 of 1.94 mg a.i/kg-bwt (MRID 47333601).

For diphacinone, the most sensitive laboratory rat data reported an LD50 = 1.9 mg a.i/kg-bwt

(MRID 05002272). A second laboratory rat study resulted in an LD50 = 2.46 mg a.i/kg-bwt 

(MRID 00060605). Toxicity tests conducted on pine voles and meadow voles (Microtus 

pennsylvanicus) resulted in LD50s of 67.7 and 11.7 mg a.i/kg-bwt (Byers, 1978). LD50 values for 

dogs and cats ranged from 0.88 to 15 mg a.i/kg-bwt (LiphaTech, Inc, 1997). LD50s for the coyote 

(Canus latrans) and mongoose (Herpestes spp) were 0.6 and 0.18 mg a.i/kg-bwt, respectively 

(Savarie et al. 1979 and MRID 405655-01). These data suggest that diphacinone may be more 

toxic to carnivores than those animals experiencing primary exposure to diphacinone. This 

increased sensitivity of secondary exposure animals relative to primary exposure animals was 

also observed in the avian acute oral toxicity studies. 

For bromethalin, the study conducted on laboratory rats used for risk calculation resulted in an 

LD50 = 2.2 mg a.i/kg-bwt (MRIDs 00026524 and 00026523). Other laboratory rat studies 

resulted in LD50 = 2.59 mg a.i/kg-bwt (MRID 44775101) and LD50 = 9.1 and 10.7 for females 

and males, respectively (MRIDs 00026524 and 00026523). An LD50 = 6.6 mg a.i/kg-bwt was 

identified for roof rats (Jackson et al. 1982). Laboratory mouse studies consisted of bromethalin 

administered in a carrier, PEG-200, (LD50 = 8.1 and 5.3 for females and males, respectively) and

bromethalin administered in a carrier, acacia, (LD50 = 28.9 and 35.9 for females and males, 
Page 45 of 183

respectively) (MRID 00026523). LD50 values for rabbits, dogs, and cats ranged from 1.9 to 100 

mg a.i/kg-bwt (MRID 00026523).

Figure 3-3 allows for visual comparison in toxicity sensitivities across chemicals and among 

studies within a chemical. For mammalian acute oral toxicity, the most sensitive LD50 values 

were from a variety of species; however, risk estimation was conducted using the most sensitive 

data from laboratory rats for all chemicals. 

Figure 3-3. Mammalian acute oral LD50 (mg a.i./kg-bwt) values for all evaluated chemicals.

LD50 values represented as a range were plotted using a mid-point value; no other non-definitive values were 

included. Only toxicity values obtained using the technical material were included. Circles represent toxicity values 

used for RQ calculation.

As discussed previously, although mortality is the endpoint of interest for this assessment, 

sublethal effects observed in the available toxicity studies are worth noting. Sublethal effects 

noted in the acute oral anticoagulant studies included lethargy, subcutaneous, intramuscular, and 

internal hemorrhaging, diarrhea, bloody diarrhea, and anorexia. Sublethal effects noted in the 

bromethalin studies included hypoactivity, tremors, loss of righting reflex, dyspnea, ptosis, 

diarrhea, and dieresis. Some of these sublethal effects, in particular lethargy and hypoactivity 

could decrease the fitness of the target animals, thereby increasing the likelihood that the target 

animals may become prey to secondary predators, resulting in an exposure of secondary 

predators to rodenticides.

3.1.4 Mammalian dietary toxicity

Table 3-6 summarizes the most sensitive laboratory rat dietary toxicity endpoints, based on an 

evaluation of both the registrant-submitted studies and the open literature. Reliable acute dietary 

0.1 1 10 100 1000

Brodifacoum

Difethialone

Warfarin

Chlorophacinone

Diphacinone

Bromethalin

LD50 (mg a.i./kg-bwt)
Page 46 of 183

toxicity studies are not available to derive RQs for difethialone and bromethalin; however, there 

are data available to characterize the dietary toxicity of bromethalin to mammals. When 

considering available data for brodifacoum, warfarin, chlorophacinone and diphacinone, all of 

these chemicals are very highly toxic. As with the acute oral toxicity data, brodifacoum is an 

order of magnitude more toxic to mammals on an acute dietary basis. Several chemicals have 

multiple acute dietary studies available resulting in a range of reported toxicity (Appendix A). 

Table 3-6. Laboratory rat acute dietary studies used for calculating RQs.

Chemical % a.i. LC50, 

mg a.i/kgdiet

(95% CI)

Probit 

slope 

(95% CI)

Average 

body 

weight, g 

(day=0)

Study 

length 

(days)

Time to 

mortality

(days)

MRID and 

classification

Brodifacoum1 98 0.53 

(0.45-0.68)

NA 98 14 4-14 Teeters 1981 

(TNM 110) 

Supplemental

Difethialone No data -- -- -- -- --

Warfarin 98 4.41 

(3.43-5.64)

5.4 

(2.8, 8.0)

98 14 3-9 Teeters 1981 

(TNM 126) 

Supplemental

Chlorophacinone

100 1.14 

(0.98-1.35)

7.2 

(3.8, 10.6)

65 14 4-9 Teeters 1981 

(TNM 117)

Supplemental

Diphacinone 99.8 2.08 

(1.57, 2.76)

4.2 

(2.3, 6.1)

136 14 4-12 Teeters 1981 

(TNM 75)

Supplemental

Bromethalin No 

quantitative 

data

-- -- -- -- --

1 LC50 values estimated via the binomial method. Slopes and confidence intervals could not be calculated.

For brodifacoum, the dietary LC50 values for laboratory rat range from 0.53 to 0.84 mg a.i/kgdiet (Teeters 1981). 

For difethialone, no mammalian dietary toxicity data were identified. Since no dietary toxicity 

data were available, risk estimation based on dietary toxicity for difethialone cannot be 

conducted for mammals.

For warfarin, the dietary LC50 values for laboratory rat range from 4.41 to 6.03 mg a.i/kg-diet 

(Teeters 1981). Mink (Neovison vison) had an LC50 = 11.7 mg a.i/kg-diet (Aulerich et al., 1987) 

and black-tailed prairie dogs had an LC50 = 97 mg a.i/kg-diet (Mach et al., 2002). 

For chlorophacinone, the dietary LC50 values for laboratory rat range from 1.14 to 1.26 mg 

a.i/kg-diet (Teeters 1981).
Page 47 of 183

For diphacinone, the dietary LC50 values for laboratory rat range from 2.08 to 2.55 mg a.i/kg-diet 

(Teeters 1981).

For bromethalin, the available LC50 value for laboratory rat was 5.5 mg a.i/kg-diet (Jackson et 

al., 1982). Jackson et al. (1982) also evaluated dietary toxicity to roof rats and house mice, 

resulting in LC50s of 7.2 and 14.1 mg a.i/kg-diet, respectively. This open literature paper was 

classified as qualitative (no control animals and no low dose with no mortality, lack of reported 

methods and summary data) and cannot be used for risk estimation but can be used for risk 

characterization.

Figure 3-4 allows for visual comparison in toxicity sensitivities across chemicals and among

studies within a chemical. For mammalian acute dietary toxicity, the most sensitive LC50 values 

were obtained from laboratory rat studies. Only two of the evaluated chemicals had data for 

additional species. Dietary toxicity data were available for mink and black-footed ferrets on 

diphacinone and for house mice and roof rats for bromethalin. 

Figure 3-4. Mammalian acute dietary LC50 (mg a.i./kg-diet) values for all evaluated 

chemicals. No data were available for difethialone. LC50 values represented as a range were plotted using a midpoint value; no other non-definitive values were included. Only toxicity values obtained using the technical material 

were included. Circles represent toxicity values used for RQ calculation. No data were available for RQ calculation 

for difethialone or bromethalin.

0.1 1 10 100 1000

Brodifacoum

Difethialone

Warfarin

Chlorophacinone

Diphacinone

Bromethalin

LC50 (mg a.i./kg-diet)
Page 48 of 183

3.1.5 Uncertainties in Toxicity Studies

A description of assumptions, uncertainties, strengths, and limitations of the basic ecological risk 

assessment as conducted by EFED is described in Chapter 6 of the Agency’s Overview 

Document (USEPA 2004) and include those related to the effects assessment. Additional 

uncertainties specific to this toxicity endpoints used in this assessment are discussed below.

All available toxicity endpoints are based on administered dose, not the bioavailable dose. For 

the acute oral studies, the administered dose is the amount of material enters into the test 

organism’s body (e.g., mg a.i./kg-bwt). The bioavailable portion of this dose refers to the fraction 

of the total amount of material in contact with a port-of-entry (e.g., lung, gut, skin) that actually 

enters the blood. The bioavailability depends on the absorption, distribution, metabolism, and 

excretion rates of the chemical in the test organism. These factors can depend on factors such as 

species, maturity of test animals, dietary matrix, animal health, and properties of the test 

chemical. For the evaluated rodenticides it is unknown what percentage of the administered dose 

is bioavailable to the test organism. It is important to note this uncertainty and to ensure that 

exposure as calculated for risk estimation is assumed to also be an administered dose. 

For many of the toxicity studies, there is uncertainty in the estimated LD50 or LC50 value, as the 

slope of the dose-response curve appears to be very flat. For example, the mortality responses 

obtained in a dietary warfarin study on mallard ducks were 40, 40, 60, 60, 70% for diet 

concentrations of 312, 625, 12500, 2500, and 5000 mg a.i./kg-diet (MRID 153366). Based on the 

mortality percentages, it would be reasonable to conclude that the LC50 could lie anywhere 

between 312 and 2500 mg a.i./kg-diet and possibly as high as 5000 mg a.i/kg-diet. An LC50 = 

879 mg a.i./kg-diet and a slope of 0.7 was estimated based on the probit regression model. This 

indicates a lack of sensitivity to increasing concentrations of the chemical that may be due to 

individual variation in sensitivity to warfarin or individual variation in the likelihood that an 

incident causing internal bleeding (aggressive behavior, bumping into cage, etc.) would occur. In

addition to the flat dose-response curves, some studies resulted in very steep dose-response 

curves, indicating a rapid change in sensitivity to the chemical as dose increased. 

Mammalian dietary LC50 endpoints were not available for risk estimation for difethialone and 

bromethalin. No acute dietary mammalian studies were identified for difethialone. One 

bromethalin paper in the open literature (Jackson et al., 1982) did contain dietary LC50 data for 

three mammalian species (laboratory rat, roof rat, and house mouse). Limitations identified in the 

published article (no reporting of control data, minimal methods and summary data reported) 

preclude its use for risk estimation; however, these data were of adequate quality for use in the 

risk characterization. Although the acute oral data allow for an estimate of toxicity based on a 

precise one-time administered dose, the dietary toxicity data are important in characterizing 

toxicity based on a longer exposure period when the test material is administered in a dietary 
Page 49 of 183

matrix. A complete set of toxicity endpoints for both acute oral (LD50) and acute dietary (LC50) 

toxicity tests would enable comparisons across chemicals and across exposure scenarios.

For some of the assessed chemicals, there was a large range in LD50 or LC50 estimates such that 

the smallest and largest values spanned several orders of magnitude. Review of the available 

toxicity data suggests the variability occurs within species, across species, across body sizes, and 

feeding strategies. Based on the large variation in toxic response seen in the laboratory studies, it 

can be expected that individual response to rodenticide exposure in real product application 

situations would also vary greatly. 

An additional limitation to the toxicity data sets is that predator/scavenger toxicity data are not 

represented for all chemicals for both birds and mammals. For birds, the available information 

suggests that raptors may be more sensitive to anticoagulants than the standard toxicity test 

species. For diphacinone, an LD50 of 96.8 mg a.i./kg-bwt is available for the American kestrel 

(Rattner et al. 2011). This study suggests that American kestrels may be more sensitive to 

diphacinone than the standard test species (i.e., based on the available studies, 17 to 21 times 

more sensitive than bobwhite quail and 33 times more sensitive than mallard ducks). In addition, 

the potential for increased sensitivity of raptors to warfarin was observed by Watanabe et al.

(2010), who demonstrated that hydroxylation rates of warfarin in the great horned owl (Bubo 

virginianus) and snowy owl (Bubo scandiacus) were much less than that observed in mallards, 

chickens (Gallus gallus), and laboratory rats. This study suggests that raptors may metabolize 

and detoxify warfarin at slower rates compared to mallard ducks. Taken together, these data may 

indicate that raptors, which represent avian secondary consumers of rodenticides, are more 

sensitive to rodenticides when compared to the standard test species used to derive the RQs 

calculated in this assessment. No avian dietary toxicity data (LC50) were available for 

predatory/scavenger species. 

For mammals, laboratory dog, cat, or pig toxicity data are available for several of the evaluated 

chemicals; however, the confidence in these LD50s is limited as the number of individuals and 

the number of dose levels tested was often very small. These LD50s vary in sensitivity to the 

tested chemical relative to the most sensitive rat data used for risk estimation, suggesting large 

individual or species variation in sensitivity to the tested rodenticides. For diphacinone, both 

coyote and mongoose (LD50 = 0.6 and 0.18 mg a.i./kg-bwt, respectively) were more sensitive 

than the most sensitive laboratory rats (LD50 = 1.9 mg a.i./kg-bwt). For brodifacoum, mink (LD50

= 9.2 mg a.i./kg-bwt) were less sensitive than the most sensitive laboratory rats (LD50 = 0.48 mg 

a.i./kg-bwt); however, Ringer and Aulerich (1978) noted that mink have a very rapid transit rate 

(approximately 2 hours) of food through the intestinal tract possibly allowing for excretion of 

test material before adequate absorption occurs. A mink dietary toxicity study was available for 

warfarin (LC50 = 11.7 mg a.i./kg-diet); this study also resulted in a lower toxicity to the tested 

anticoagulant than the most sensitive rat dietary study (LC50 = 4.4 mg a.i./kg-diet).
Page 50 of 183

3.2 Elimination of Rodenticides 

Considerable differences exist in absorption, metabolism and excretion of the anticoagulant 

rodenticides, which may have important consequences for both primary and secondary risk. A 

compound that is rapidly metabolized or excreted from a primary consumer may result in a lesser 

risk of mortality than one that bioaccumulates with repeated sublethal exposure, even if repeated 

exposure occurs weeks or even months after initial exposure (Eason and Murphy 2000). Those 

compounds more rapidly cleared from the body are less likely to pose such long-term risk. The 

available information indicates that brodifacoum and difethialone are more persistent in animal 

tissue than warfarin, chlorophacinone and diphacinone. Few data exist for the nonanticoagulants, but based on lack of toxicity in secondary tests, apparently this rodenticide is not 

retained in toxicologically significant amounts in animal tissues. 

Elimination of anticoagulants from the body is sometimes described as rapid (e.g., Poche 1986, 

Kaukeinen et al. 2000); however, such characterizations usually refer to the rapid excretion of 

unbound or unabsorbed material being excreted principally in feces during the first few days 

after administration. Alternatively, it may refer to the clearance from the blood as compared with 

tissue retention. Rather than concentrating solely on the amount of anticoagulant excreted, this 

risk assessment includes a discussion on the material retained in the body after single and 

multiple exposures. The studies summarized below indicate the differences among these 

compounds and their potential to bioaccumulate with repeat exposure. 

3.2.1 Elimination of Brodifacoum

Available data indicate that brodifacoum is not readily metabolized, and the major route of 

excretion of unbound compound is through the feces. After absorption, high concentrations 

circulate in the blood and are rapidly established in the liver and in other tissues. The half-life of 

brodifacoum in ranges from 128 to 307.4 days in liver and 6 to 91.7 days in blood plasma in 

mammals (Table 3-6). Elimination from liver is much slower and biphasic, with a very 

prolonged terminal phase. It is apparent from the studies discussed below that a proportion of 

any ingested dose of brodifacoum bound in the liver, kidney, or pancreas remains in a stable 

form for some time and is only very slowly excreted. 

Table 3-6. Retention of Brodifacoum in Blood and Liver

Rodenticide Species Dose (mg 

a.i./kg) # doses Blood t1/2a(days)

Liver 

retentiona,b

(days)

Reference 

(MRID)

Brodifacoum Mouse 6.44 μg/mouse 1 91.7 307.4 (t1/2) Vandenbroucke et al. 

2008
Page 51 of 183

Brodifacoum Rat 0.02 or 0.15

0.35

1

1 NA 350c (t1/2)128d (t1/2)

Batten and Bratt 1990 

(42007502)

Brodifacoum Rat 0.2 1 NA 282 (t1/2) Hawkins et al. 1991 

(42596801)

Brodifacoum Rat 0.25 1 NA 150-200 

(t1/2)

Bratt and Hudson 1979 

(00080235)

Brodifacoum Rat 0.06

4

(at weekly 

intervals)

NA 136 (t1/2) Belleville 1991 

(42065009)

Brodifacoum Rat 0.35 1 NA 130 (t1/2) Parmar et al. 1987e

Brodifacoum Rat unknown unknown 6.5 >80 Backmann and Sullivan 

1983f

Brodifacoum Rat 0.1 1 NA 113.5 (t1/2) Fischer et al. 2003 

(48190801)

Brodifacoum Possum 0.1 1 20-30 >252 Eason et al. 1996

Brodifacoum Rabbit unknown unknown 2.5 NA Brechenridge et al. 

1985f

Brodifacoum Sheep 0.2 or 2.0 1 NA >128 Laas et al. 1985e

Brodifacoum Dog unknown unknown 6 NA Woody et al. 1992f

Brodifacoum Dog unknown unknown 0.9-4.7 NA Robben et al. 1998f

a t1/2 for plasma and liver is the elimination half-life b liver retention is expressed as either the time period for which residues persist or as the elimination half-life c the elimination half-life of 350 days is for a single oral dose of 0.02 or 0.15 mg a.i./kg; elimination was not biphasic d the elimination half-life of 128 days is the terminal phase for a single oral dose of 0.35 mg a.i./kg; elimination was 

biphasic

e Cited in US EPA (2004) Potential Risks of Nine Rodenticides to Birds and Nontarget Mammals: a Comparative 

Approach

f cited in Eason et al. 2001

Hawkins et al. (1991) administered brodifacoum to rats in a single oral dose of 0.2 mg a.i./kg. 

Elimination was biphasic, with half-lives of 63 days for brodifacoum in the initial 28 days and 

282 in the terminal phase. These differences are not statistically significant, but mean liver 

concentrations of brodifacoum were significantly higher throughout the study. 

Bratt and Hudson (1979) found that radiolabeled brodifacoum was rapidly and almost 

completely absorbed when administered to rats in a single oral dose (0.25 mg a.i./kg). After 10 

days, 11 to 14% of the dose had been eliminated in urine and feces, but 74.6% of the dose was 

still retained in body tissues. Almost half the dose administered was detected in the carcass and 

skin, with lesser amounts in the liver (22.8%), pancreas (2.3%), kidney (0.8%), spleen (0.2%), 

and heart (0.1%). The estimated half-life of brodifacoum in rat tissue was 150-200 days. 

Fisher et al. (2003) administered single oral doses of 0.1 mg a.i./kg brodifacoum to 20 rats. Rats 

were monitored for 5 days after dosing for symptoms of anticoagulant poisoning. Groups of six 
Page 52 of 183

or seven rats were euthanized at weeks 1, 18, and 24. The livers were removed post-mortem for 

analysis of brodifacoum residue by HPLC. Brodifacoum elimination half-life in rat liver tissue 

was estimated to be 113.5 days. 

Batten and Bratt (1990) orally dosed male rats with a single dose of radiolabeled brodifacoum at 

0.02, 0.15, or 0.35 mg a.i./kg. The highest concentration of radioactivity in the liver was found 1

day after dosing, but 21 to 34% of the dose was still detected after 13 weeks and >11% after 104 

weeks. The elimination half-life for the 2 lowest doses was 350 days. For rats dosed at 0.35 mg 

a.i./kg, a near-lethal dose (LD50 = 0.39 to 0.56 mg a.i./kg), elimination from the liver was 

biphasic and consisted of a rapid phase (days 1 to 4) in which the half-life was approximately 4 

days and a slower phase (days 28 to 84) in which the half-life was 128 days. Two rats dosed at 

that level died during the study. Signs of brodifacoum toxicosis were observed in some 

survivors. Some dosed rats also had gained less body-weight and displayed signs of internal 

hemorrhage when dissected. 

Based on those findings, the authors concluded that the existence of biphasic kinetics in the liver 

for brodifacoum has two important consequences. First, half-lives determined based on the 

initial, or fast, phase of elimination may underestimate the potential persistence of the chemical. 

The second consequence of biphasic liver kinetics is that the chemical concentration in the liver 

at the beginning of the terminal, or slow phase, is independent of the initial dose.

Parmar et al. (1987) also reported biphasic elimination of radio-labeled brodifacoum from rat 

liver. The initial phase occurred from days 2 to 8 after dosing, followed by a prolonged terminal 

phase when the elimination half-lives were 130 and 170 days. However, the results were 

presented only in an abstract with too few details provided to adequately evaluate the results 

reported. 

Belleville (1991) orally dosed rats with 0.06 mg a.i./kg brodifacoum on 4 occasions at weekly 

intervals. After 6 months, 21% of the total brodifacoum dose was retained in hepatic tissue 

(Table 3-7). The hepatic half-life calculated for the 158 days after the final dose (days 22-180) 

was 136 days for brodifacoum. 
Page 53 of 183

Table 3-7. Hepatic Concentrations in Rats Dosed with Brodifacoum at 0.06 mg a.i./kg on 

Days 0, 7, 14, and 21 (adapted from Belleville 1991, MRID# 42065009)

Time after initial dose Brodifacoum

(mg a.i./kg)

22 days 2.01 ± 0.15

49 days 1.50 ± 0.48

77 days 0.98 ± 0.32

4 months 0.85 ± 0.15

6 months 0.87 ± 0.16

Studies in species other than rats also indicate that brodifacoum can be retained in animal tissue 

for a prolonged period of time. Eason et al. (1996) detected brodifacoum residue 9 months after 

administration of a sublethal dose of 0.1 mg a.i./kg in possums. Laas et al. (1885) examined 

retention of brodifacoum in sheep tissues and its excretion via feces after a single, sublethal oral 

dose of either 0.2 or 2.0 mg a.i./kg in 14 sheep. Sheep were sacrificed at several intervals over a 

128 day period after dosing and liver, carcass, and fat tissues analyzed for residue. Brodifacoum 

was detected in the liver after 128 days, at concentrations of 0.64 and 1.07 mg a.i./kg dry weight 

(equivalent to 0.22 and 0.36 mg a.i./kg wet weight), respectively, for the 2 doses. Residue was 

also detected for up to 8 days in fat and up to 15 days in the carcass. Breckenridge et al. (1985) 

reported a plasma elimination half-life of about 2.5 days for rabbits dosed with brodifacoum, and 

Woody et al. (1992) observed an elimination half-life for brodifacoum in serum of 6 ± 4 days in 

four dogs. 

Vandenbroucke et al. (2008) orally dosed groups of mice with 6.44 μg/mouse, which is 

approximately one-half the LD50 of brodifacoum. Brodifacoum was then assayed in liver tissue 

and blood plasma from euthanized mice at various times up to 21 days after dose administration. 

The study authors estimated the elimination half-life of brodifacoum in liver as 307.4 days and 

the half-life in plasma as 91.7 days.

3.2.2 Elimination of Difethialone

Data available for difethialone suggest that this rodenticide is not as persistent as brodifacoum 

(Table 3-8). Available elimination half-lives in blood and liver tissues of mammals range 2.2-

38.9 days and 28.5-108 days, respectively. 
Page 54 of 183

Table 3-8. Retention of Difethialone in Blood and Liver

Rodenticide Species Dose (mg 

a.i./kg) # doses Blood t1/2a(days)

Liver 

retentiona,b

(days)

Reference 

(MRID)

Difethialone Mouse 20.77 

μg/mouse 1 38.9 28.5 (t1/2) Vandenbroucke et al. 2008

Difethialone Rat 0.5 1 2.3 108 (t1/2)

Belleville 1986 

(42065010)

Lechevin and Poche 

1988

Difethialone Rat 0.06

4

(at weekly 

intervals)

NA 74 (t1/2) Belleville 1991 

(42065009)

Difethialone Dog unknown unknown 2.2-3.2 NA Robben et al 1998c

a t1/2 for plasma and liver is the elimination half-life b liver retention is expressed as either the time period for which residues persist or as the elimination half-life c cited in Eason et al. 2001

Vandenbroucke et al. (2008) orally dosed mice with approximately one-half the LD50 of 

difethialone, 20.77 μg/mouse. Difethialone was then assayed in liver tissue and blood plasma 

from euthanized mice at various times up to 21 days after dose administration. The study authors 

estimated the elimination half-life in liver as 28.5 days and the half-life in plasma as 38.9 days. 

Studies submitted to EPA (Belleville 1986 and 1991, MRID#s 42065010 and 42065009 

respectively) indicate that the half-life of difethialone may be several times greater than 

Vendenbroucke et al (2008). Belleville (1991) orally dosed rats with 0.06 mg a.i./kg difethialone 

on 4 occasions at weekly intervals. After 6 months, 7% of the total difethialone dose was 

retained in hepatic tissue. The hepatic half-life calculated for the 158 days after the final dose 

(days 22-180) was 74 days for difethialone. 

3.2.3 Elimination of Warfarin

The available information for warfarin indicates it is generally less persistent in the blood and 

body tissues than brodifacoum (Table 3-9). Available elimination half-lives in blood and liver 

tissues of mammals range from 0.2 to 14.9 days and 26.2 to 66.8 days, respectively. 

Table 3-9. Retention of Warfarin in Blood and Liver

Rodenticide Species Dose (mg 

a.i./kg) # doses Blood t1/2a(days)

Liver 

retentiona,b

(days)

Reference 

(MRID)

Warfarin Mouse 5985 

μg/mouse 1 14.9 66.8 (t1/2) Vandenbroucke et al. 2008

Warfarin Rat unknown unknown

0.7 (male)

1.2 

(female)

NA Pyrola 1968d
Page 55 of 183

Warfarin Rat 1.0 1 NA 26.2 (t1/2) Fisher et al. 2003 

(48190801)

Warfarin Rabbit unknown unknown 0.2 NA Breckenridge et al 

1985d

Warfarin Possum unknown unknown 0.5 NA Eason et al. 1999c

Warfarin Pig unknown unknown NA 30-40 O’Brien et al 1987d

a t1/2 for plasma and liver is the elimination half-life b liver retention is expressed as either the time period for which residues persist or as the elimination half-life c Cited in US EPA (2004) Potential Risks of Nine Rodenticides to Birds and Nontarget Mammals: a Comparative 

Approach

d cited in Eason et al. 2001

In contrast to the other anticoagulants, warfarin is extensively metabolized and the major route of 

excretion is in the urine. Meehan (1984) states that approximately half the warfarin consumed by 

a rat remains in the body after 6 hours, suggesting that up to 50% may be eliminated in this time 

period. EPA (1982) noted that only 7.6% of the warfarin consumed in bait by 11 rats remained 

in the carcass after a 5-day feeding period. Ford (1993; cited in Poché and Mach 2001) reported a 

half-life of 42 hours in the gastro-intestinal tract, which supports the conclusion that a portion of 

the consumed warfarin dose is rapidly eliminated from the body. However, there is evidence that 

warfarin residues that are not excreted though urine and fecal matter may accumulate in various 

body tissues, especially in the liver. Though warfarin concentrates in the liver, measurable 

amounts have also been found in the adrenal glands, lungs, bone marrow, kidneys, and lymph 

nodes (Machlin 1984). 

Vandenbroucke et al. (2008) orally dosed groups of mice with 5.985 mg warfarin/mouse, which 

is approximately one-half of the LD50 dose. Warfarin was then assayed in liver tissue and blood 

plasma from euthanized mice at various times up to 21 days after dose administrations. The 

study authors estimated the elimination half-life of warfarin in liver tissue to be 66.8 days and 

the half-life in blood plasma to be 14.9 days. 

Fisher et al. (2003) administered a single dose of 1.0 mg a.i./kg to 20 rats. Rats were monitored 

for 5 days after dosing for symptoms of anticoagulant poisoning. Groups of six or seven rats 

were euthanized at weeks 1, 18, and 24. The livers were removed post-mortem for analysis of 

warfarin residue by HPLC. Warfarin elimination half-life in rat liver tissue was estimated to be 

26.2 days. 

The discrepancies between the half-lives estimated in Vandenbroucke et al. (2008) and Fisher et 

al. (2003) may be partially a result of the different test organisms as there is some indication that 

elimination times for warfarin may vary among species. For example, O’Brien et al. (1987; cited 

in Eason et al. 2001) found comparatively rapid clearance of warfarin in pigs, with 

concentrations declining to very low levels after approximately 30 days. In addition, Wantabe et 
Page 56 of 183

al. 2010 examined the differences in metabolism of warfarin in birds in order to study differences 

in species sensitivities to this rodenticide. Liver samples from domestic chickens (Gallus gallus), 

ostriches (Struthio camelus), mallard ducks (Anas platyrhunchos), snowy owls (Bubo 

scandiacus) and a great horned owl (B. virginianus) were used to measure the metabolism of 

warfarin to its hydroxylated degradates. Results of the study indicated that owls had the lowest 

metabolism of warfarin. Therefore, relative to the other species tested, owls may have a limited 

ability to detoxify warfarin. This could potentially lead to increased warfarin sensitivity in owls, 

relative to OPP's typical test species (e.g., mallard ducks). 

Half-life values for blood plasma also vary greatly among species. Breckenridge et al. (1985) 

intravenously dosed male New Zealand white rabbits with 20 μmol/kg warfarin. Blood samples 

were taken at regular intervals and analyzed for warfarin concentration. The study authors 

reported a plasma elimination half-life for warfarin of 5.6 hours in rabbits. 

3.2.4 Elimination of Chlorophacinone

Available data on the retention times for chlorophacinone are provided in Table 3-10. Data on 

elimination from blood are available from 2 studies (half-life ranges from 0.4 to 11.7 days). One 

available liver elimination half-life is available (35.4 days).

Table 3-10. Retention of Chlorophacinone in Blood and Liver

Rodenticide Species Dose (mg 

a.i./kg) # doses Blood t1/2a(days)

Liver 

retentiona,b

(days)

Reference

(MRID)

Chlorophacinone Mouse 336 

μg/mouse 1 11.7 35.4 (t1/2) Vandenbroucke et al. 2008

Chlorophacinone Rat 4-5 1 0.4 NA Belleville 1981 

(00155540) a t1/2 for plasma and liver is the elimination half-life b liver retention is expressed as either the time period for which residues persist or as the elimination half-life

Belleville (1981) orally administered radio-labeled chlorophacinone to rats with either a single 

dose of 1 to 1.26 mg a.i./rat (~4 to 6 mg a.i./kg) or 3 daily doses of 1.43 a.i/rat (~6 to 7 mg 

a.i./kg). The compound was rapidly absorbed and metabolized; 90% was excreted within 48 

hours and 100% with 4 days. Elimination was almost totally via the feces; < 1% was via urine 

and CO2. The half life in blood was 9.8 hours, with the maximum concentration attained after 4 

to 8 hours. The maximum blood concentration in rats that received 3 doses was 1.8 to 3.7 times 

higher than that from rats receiving a single dose, indicating that chlorophacinone can 

accumulate in the body. Concentrations in body tissues after 4 hours and 48 hours were highest 

in the liver, but chlorophacinone was also present in kidneys, lungs, heart, muscle, fat, and other 

parts of the carcass (Table 3-11).
Page 57 of 183

Table 3-11. Chlorophacinone Residue in Rats 4 and 48 hours After Oral Dose of 1.26 mg 

a.i. per Rat (adapted from Belleville 1981, MRID# 00155540)

Organ

Residue (μg a.i./g) 

48 h after 

dosing

48 h after 

dosing

Liver 31.1 2.9

Kidney 6.6 1.2

Lung 4.5 0.4

Heart 3.1 0.2

Muscle (thigh) 2.0 0.1

Fat 1.2 0.7

Carcass 5.2 0.3

3.2.5 Elimination of Diphacinone

Available data on the retention times for diphacinone are provided in Table 3-12. Yu et al. 

(1982) studied the metabolism and disposition of diphacinone in rats and mice. Rats given a 

single oral dose of radiolabeled diphacinone at either 0.18 or 0.4 mg a.i./kg, about 70% of the 

dose was eliminated in feces and 10% in urine within 8 days, whereas about 20% of the dose was 

retained in body tissues. Mice given a single dose of 0.6 mg a.i./kg eliminated most diphacinone 

within 4 days, and only 7% was retained in body tissues. In both rats and mice, most 

radioactivity (59 to 69%) was detected in the liver and the kidneys (9 to 12%). Radioactivity was 

also detected in the brain, heart, spleen, lungs, blood, muscle, fat, and gonads. Several major 

metabolites were identified, and parent diphacinone in excreta and liver accounted for only about 

20% of the dose. In another study, cattle that received a single injection of 1 mg a.i./kg had 

almost constant residue concentrations in liver and kidney at 30, 60, and 90 days after dosing 

(Bullard et al. 1979). 

Table 3-12. Retention of Diphacinone in Blood and Liver

Rodenticide Species Dose (mg 

a.i./kg) # doses Blood t1/2a(days)

Liver 

retentiona,b

(days)

Reference

(MRID)

Diphacinone Pig 12.5 3 or 5 NA 5.43 (t1/2) Fisher 2006

Diphacinone Rat 1.5 1 NA 3 (t1/2) Fisher et al. 2003 

(48190801)

Diphacinone Cattle 1.0 1 NA > 90 Bullard et al. 1976 a t1/2 for plasma and liver is the elimination half-life b liver retention is expressed as either the time period for which residues persist or as the elimination half-life

Diaz and Whitacre (1976) orally dosed rats with diphacinone (0.32 mg a.i./kg/day) for 1 or 2 

days. Rats dosed for 2 days were sacrificed 72 hours after the second dose and those dosed for 1 
Page 58 of 183

day were sacrificed after 48 hours. In rats dosed for 2 days, about 45% of the total dose 

administered was excreted (86% in feces, 14% in urine) and 25% was retained in body tissues 72 

hours after the last dose. The remaining 30% of the dose was not recovered. The body tissues 

retaining the most diphacinone at 96 hours were the hide and tail, liver, intestine, blood, and the 

carcass (Table 3-13). In rats dosed for 1 day and sacrificed after 48 hours, about 5% of the dose 

was excreted and 61% retained; the remainder was not recovered. 

Table 3-13. Percentage of Diphacinone Retained by Rats Dosed for 1 or 2 Days with 0.32 

mg a.i./kg (adapted from Diaz and Whitacre 1976)

Organ

% of dose retained per groupa

48 h after 1 

dose

72 h after 2 

doses

Intestine 22.1 4.1

Liver 19.4 5.4

Hide and tail 10.9 6.5

Carcass 3.9 3.8

Blood 1.8 4.0

Muscle 0.8 0.4

Kidney 0.7 0.3

Testis not reported 0.8

Lung 0.5 0.2

Fat 0.2 0.4

Heart 0.1 0.2

Spleen 0.1 0.1

Brain < 0.1 < 0.1

a because only 66-70% of the total dose was recovered, 

percentages in tissues are likely to be higher than the 

values tabulated

3.2.6 Elimination of Bromethalin

Available data on the non-anticoagulant rodenticide bromethalin indicate that it is rapidly

eliminated from the blood (Table 3-14). While there are no data on the elimination half-life of 

bromethalin in liver, a metabolism study was conducted in Fischer 344 rats following oral 

administration of 14C-bromethalin at 1 mg/kg to assess the half-life of bromethalin in blood 

plasma (MRID# 0004724). Blood samples were taken from the orbital sinus at 0.25, 0.5, 1, 2, 4, 

and 24 hours, and at 2, 3, 4, 6, 8, 11, 14, 17, and 21 days after dosing. Based on radiolabeled 

material, the plasma half-life was 134 hours (5.6 days). 
Page 59 of 183

Table 3-14. Retention of Bromethalin in Blood and Liver

Rodenticide Species Dose

(mg a.i./kg) No. doses Blood t1/2a(days)

Liver 

retentiona,b

(days)

Reference

(MRID)

Bromethalin Rat 1 1 5.6 NA MRID 0004724

a t1/2 for plasma and liver is the elimination half-life b liver retention is expressed as either the time period for which residues persist or as the elimination half-life

3.2.7 Discussion of Elimination Half-life Data

Toxicokinetic data presented in this section indicate that, of the anticoagulants, brodifacoum is 

more persistent in animal tissue than all other chemicals evaluated. For example, brodifacoum 

was detected in rat livers up to 200 days post exposure to a single oral dose of 0.2 mg a.i./kg 

(Hawkins et al. 1991), and up to 11.7% was detected in livers of rats after 104 weeks following a 

single dose of 0.15 mg a.i./kg (Batten and Bratt 1990). Other studies discussed in section 3.2 

provide further evidence of the long-term retention of brodifacoum in body tissues, and half-lives 

range up to 307.4 days in liver tissue and 91.7 days in plasma (Vandenbroucke et al. 2008). The 

study by Vandenbroucke et al. (2008) suggests that difethialone may be much less persistent in 

the body than brodifacoum, with a liver half-life of 28.5 days and a plasma half-life of 38.9 days. 

However, a half-life for difethialone of up to approximately 100 days has been also been 

reported (Belleville 1986).

Persistence data available for warfarin show variation among the species tested; however, most 

of the information on tissue retention indicates that it is generally less persistent in the body than 

brodifacoum. A liver half-life of up to 66.8 days has been calculated for warfarin in rats 

(Vandenbroucke et al. 2008), though other estimates are less than this value. Most data indicate 

that the plasma half-lives are <1 day. Available data also show that chlorophacinone and 

diphacinone may be less persistent than brodifacoum, difethialone and warfarin. Liver half-life 

for chlorophacinone is 35.4 days (Vandenbroucke et al. 2008). Half-lives for diphacinone have 

been estimated to be shorter in some species (ranging from 3 – 5.43 days), but liver retention 

times in cattle were determined to be >90 days (Fisher 2006, Fisher et al. 2003, Bullard et al. 

1976).

Available data on the non-anticoagulant rodenticide bromethalin indicate that it is rapidly 

eliminated by the body. While there are no data on the elimination half-life of bromethalin in 

liver, a metabolism study was conducted in rats indicated a plasma half-life of 5.6 days.

Brodifacoum may persist in animal tissues for several months after exposure compared to days to 

several weeks for the other rodenticides evaluated. These data suggest that brodifacoum is longer 

lived in animal tissues relative to the other rodenticides evaluated. This could potentially result in 

greater opportunity for secondary exposure to occur. Figures 3-5 and 3-6 allow for visual 
Page 60 of 183

comparison of the liver and blood plasma half-lives for the assessed rodenticides, respectively. 

Due to the lack of whole-body elimination data, liver half-lives selected from the range of 

available data were used as a conservative representation of whole body elimination in animals 

consuming bait. If liver half-lives were not available, as in the case of bromethalin, the blood 

plasma half-life was used. Calculations that estimated accumulation of rodenticides in target 

animals and non-target wildlife used the most conservative half-life value available, except in the 

case of difethialone, where variation associated with differing study designs was reduced by 

utilizing the half-life value from Vandenbroucke et al. (2008). 

Figure 3-5. Liver half-life values for all evaluated chemicals. Half-life values represented as a 

range were plotted using the reported value. Circles represent values used to calculate dose 

accumulation in animals for primary and secondary exposure. No liver half-life data were 

reported for bromethalin. A plasma half-life of 5.6 days has been reported for bromethalin.

1 10 100 1000

Brodifacoum

Difethialone

Warfarin

Chlorophacinone

Diphacinone

Bromethalin

T1/2 (half-life days)
Page 61 of 183

Figure 3-6. Blood plasma half-life values for all evaluated chemicals. Half-life values 

represented as a range were plotted using the reported value. Circles represent values used to 

calculate dose accumulation of bromethalin for primary and secondary exposure. 

3.3 Estimated Accumulation of Dose over Time

As indicated previously, the rodenticides considered in this white paper typically cause mortality 

several days after exposure, which may allow target animals to feed for several days upon 

rodenticide bait before dying. This can lead to doses that far exceed the lethal dose. A compound 

that accumulates in body tissues may pose greater risk to primary and secondary consumers, 

especially if repeated sublethal exposure results in the accumulation of a lethal dose or if 

additional exposures occur before the lethal dose has taken its effect (i.e., leading to a dose that is 

beyond what is sufficient to cause mortality). Considerable differences exist in accumulation of 

the anticoagulants considered in this assessment, which may have important consequences for 

both primary and secondary risk. The purpose of this section is to quantify the extent to which an 

accumulated dose may exceed the median lethal dose of target animals. 

To do this, EFED simulated rodenticide doses (Dt, Equation 1) in target animals (i.e., house 

mouse and Norway rat) over time. This analysis assumed that 100% of the daily diet of the target 

animals is represented by the rodenticide bait and that they eliminate the rodenticide according to 

half-lives presented in Table 3-15. In Equation, the variable “C” represents the concentration of 

the pesticide in the bait. Elimination rate constants (k) are calculated according to Equation 2. 

Food intake (FI) was calculated according to the allometric equation for rodents provided by 

USEPA 1993. In this approach, it was assumed that bait consumed by the target animals was dry. 

0.1 1 10 100 1000

Brodifacoum

Difethialone

Warfarin

Chlorophacinone

Diphacinone

Bromethalin

T1/2 (half-life days)
Page 62 of 183

In order to allow for comparisons of half-lives among chemicals, liver half-lives for 

brodifacoum, difethialone, warfarin, and chlorophacinone from the same study by 

Vandenbroucke et al. (2011) were selected. Although this approach reduces variability due to 

differences attributed to laboratories, it is not necessarily the most conservative approach that 

could be undertaken. For instance, the liver half-life value for difethialone that was reported by 

Vandenbroucke (28 days) is lower than provided in other studies (74 days from Belleville, 1991). 

Though a liver half-life value is available for diphacinone, it is based on pig data, which differs 

from the organisms used in the other studies. There is some indication from the available halflife studies that species react to anticoagulant rodenticides differently. Therefore, this pig value 

may overestimate or underestimate the liver half-life of rats for this chemical. For bromethalin, 

there is no available data on the liver half-life for bromethalin, so a blood plasma value is used 

instead. It is uncertain how a blood plasma half-live would differ from the liver half-life for this 

chemical. 

This analysis assumes that the liver half-life adequately represents the elimination from the entire 

body during this time. As discussed in section 3.2.2, rapid elimination of unbound or 

unabsorbed rodenticide is initially eliminated in feces. However, 100% absorption was assumed, 

which is expected to account for long term accumulation not represented by the use of the firstorder elimination constant. 

Target animals were assumed to be a house mouse (assumed weight of 23 g) or a Norway rat 

(assumed weight of 485 g), since products relevant to the NOIC are intended for use on these 

pests. These body weights are from Whitaker (1996), and they are consistent with theoretical 

analyses of this type in previous risk assessments (e.g., see EPA 2011). 

Equation 1. D D e FI C k

t t * * = ( 1) + −

−

Equation 2. ( ) 1/ 2 k = ln 2 t

Equation 3. FI = 0.621 * Wt0.564
Page 63 of 183

Table 3-15. Elimination half-lives for rodenticides. Values based on elimination rates from 

liver.

Rodenticide Elimination half-life (days)

Brodifacoum 307.4

Difethialone 28.5

Warfarin 66.8

Chlorophacinone 35.4

Diphacinone 5.5

Bromethalin 5.6*

*Value is based on elimination from blood plasma.

Toxicity studies discussed in section 3.1 provide information on the time to death for mammals 

after dosing with or feeding on the rodenticides considered in this white paper. This information 

is important for putting this dose accumulation analysis in context for the various rodenticides. A 

rodenticide that is fast acting and causes death to occur after only a few days will greatly reduce 

the opportunity for dose accumulation. Bromethalin caused death within a day of presentation of 

feed for and mammals; all test animals were dead within 1-5 days of dosing or beginning to feed. 

However, a recent study by Pitt et al. (2011) found that time to death for house mice, Polynesian

rats (Rattus exulans), and black rats (Rattus rattus) offered bromethalin may be more rapid than 

suggested in registrant submitted data for the Norway rat, which may reduce the potential for 

bromethalin to accumulate in prey organisms. Data from this study also suggest that house mice 

and Polynesian rats exposed to bromethalin bait may stop feeding on the bait after reaching a 

lethal dose, further reducing the potential for accumulation. Warfarin also caused death to occur 

within a day of dosing and within three days of presentation of feed for mammals. All mortality 

for mammals occurred within 9-13 days in the studies available. In toxicity studies with the 

remaining rodenticides (brodifacoum, difethialone, chlorophacinone, and diphacinone), death 

first occurred within 3-4 days of the dose, and death was last observed within 9-13 days. No 

clear trend related to dosing or feeding was apparent. Observed time to death for each of the 

rodenticides assessed is summarized in Table 3-16 below. In summary, onset of death was most 

rapid for bromethalin and warfarin, though observed times where death occurred was more 

prolonged for mammals dosed with or fed warfarin. Time to death was approximately the same 

for brodifacoum, difethialone, chlorophacinone, and diphacinone. 
Page 64 of 183

Table 3-16. Observed times where mortalities were observed in acute oral toxicity studies 

with rats exposed to rodenticides.

Rodenticide Time after initial dose where 

mortality was observed 

(days)

Brodifacoum 4-9 

Difethialone 4-12

Warfarin 1-13 

Chlorophacinone 4-13

Diphacinone 3-9

Bromethalin 1-5

Tables 3-17 and 3-18 show the accumulation of rodenticide residues in the house mouse and 

Norway rat, respectively. Rodenticide residues in the house mouse and Norway rat with a diet of 

rodenticide bait are above the LD50 (adjusted) within one day of feeding for the majority of the 

rodenticides considered here. 

Table 3-17. Estimated Body Burden Over Time for House Mouse (mg/kg-bw)*

Day Brodifacoum Difethialone

Warfarin 

(0.025%)

Warfarin 

(0.054%) Chlorophacinone Diphacinone Bromethalin

1 7.9 4.0 39.6 85.5 7.9 7.9 15.8

2 15.8 7. 8 78.7 170.0 15.7 14.9 29.8

3 23.7 11.6 117.5 253.7 23.3 21.0 42.2

4 31.5 15.3 155.8 336.6 30.7 26.5 53.1

5 39.4 18.9 193.8 418.6 38.1 31.2 62.7

6 47.2 22.4 231.4 499.7 45.2 35.5 71.3

7 55.0 25.8 268.5 580.0 52.3 39.2 78.8

8 62.8 29.1 305.3 659.5 59.2 42.4 85.4

9 70.6 32.4 341.7 738.2 65.9 45.3 91.3

10 78.3 35.6 377.8 816.0 72.6 47.9 96.5

11 86.1 38.7 413.4 893.0 79.1 50.1 101.1

12 93.8 41.7 448.7 969.3 85.5 52.1 105.2

13 101.5 44.6 483.7 1044.7 91.7 53.8 108.7

14 109.2 47.5 518.2 1119.4 97.8 55.4 111.9

* Highlighted cells indicate range of days where mortalities were observed in acute oral toxicity studies with rats.
Page 65 of 183

Table 3-18. Estimated Body Burden Over Time for Norway Rat (mg/kg-bw)*

Day Brodifacoum Difethialone

Warfarin 

(0.025%)

Warfarin 

(0.054%) Chlorophacinone Diphacinone Bromethalin

1 2.1 1.0 10.5 22.6 2.1 2.1 4.2

2 4.2 2.1 20.8 45.0 4.1 3.9 7.9

3 6.3 3.1 31.1 67.2 6.2 5.6 11.2

4 8.3 4.0 41.2 89.1 8.1 7.0 14.1

5 10.4 5.0 51.3 110.8 10.1 8.3 16.6

6 12.5 5.9 61.2 132.3 12.0 9.4 18.9

7 14.6 6.8 71.1 153.5 13.8 10.4 20.9

8 16.6 7.7 80.8 174.6 15.7 11.2 22.6

9 18.7 8.6 90.5 195.4 17.5 12.0 24.2

10 20.7 9.4 100.0 216.0 19.2 12.7 25.5

11 22.8 10.2 109.4 236.4 20.9 13.3 26.8

12 24.8 11.0 118.8 256.6 22.6 13.8 27.8

13 26.9 11.8 128.0 276.5 24.3 14.3 28.8

14 28.9 12.6 137.2 296.3 25.9 14.7 29.6

* Highlighted cells indicate range of days where mortalities were observed in acute oral toxicity studies with rats.

Tables 3-19 and 3-20 provide daily ratios of the body burden to the adjusted LD50 for the house 

mouse and the Norway rat consuming the rodenticides considered in this white paper. 

Highlighted cells indicate the range of days were mortalities were observed in acute oral toxicity 

studies with rats. For warfarin, estimated doses in target animals are 4x to 204x the LD50 at days 

when mortality was observed in the acute oral toxicity study with the rat assuming that bait is 

continuously being consumed over the duration of the simulation. For brodifacoum, difethialone 

and bromethalin, estimated doses in target animals ranged from 2x to 82x the LD50. For 

chlorophacinone and diphacinone, estimated doses ranged from 3x to 13x the LD50. This analysis 

indicates that target animals continually consuming rodenticide bait may contain doses that are 

orders of magnitude above the levels that are sufficient to cause mortality. This is of particular 

concern for secondary consumers that may prey upon target mammals because they could

potentially receive elevated doses of rodenticides. 
Page 66 of 183

Table 3-19. Ratio of estimated body burden to LD50 for House Mouse*

Day Brodifacoum Difethialone

Warfarin 

(0.025%)

Warfarin 

(0.054%) Chlorophacinone Diphacinone Bromethalin

1 9 4 8 17 1 2 4

2 18 8 15 33 2 4 7

3 27 12 23 50 2 6 10

4 37 15 30 66 3 8 13

5 46 19 38 82 4 9 16

6 55 22 45 97 4 11 18

7 64 26 52 113 5 12 19

8 73 29 60 129 6 13 21

9 82 33 67 144 6 13 23

10 91 36 74 159 7 14 24

11 100 39 81 174 8 15 25

12 109 42 88 189 8 16 26

13 118 45 94 204 9 16 27

14 127 48 101 218 9 16 28

* Highlighted cells indicate range of days where mortalities were observed in acute oral toxicity studies with rats.

Table 3-20. Ratio of estimated body burden to LD50 for Norway rat*

Day Brodifacoum Difethialone

Warfarin 

(0.025%)

Warfarin 

(0.054%) Chlorophacinone Diphacinone Bromethalin

1 5 2 4 9 0.4 1 2

2 10 4 9 19 0.9 3 4

3 16 7 13 28 1 4 6

4 21 9 17 37 2 4 7

5 26 11 21 46 2 5 9

6 31 13 26 55 2 6 10

7 36 15 30 64 3 7 11

8 41 17 34 73 3 7 12

9 46 18 38 82 4 8 13

10 52 20 42 90 4 8 14

11 57 22 46 99 4 8 14

12 62 24 50 107 5 9 15

13 67 25 54 116 5 9 15

14 72 27 57 124 5 9 16

*Highlighted cells indicate range of days where mortalities were observed in acute oral toxicity studies with rats.
Page 67 of 183

Available residue data for the anticoagulant rodenticides indicate that, regardless of the 

rodenticide used, they accumulate in animal tissues when continuously exposed to rodenticide 

bait. While warfarin has the potential to accumulate 4-17X the LD50 for rats within one day of 

feeding on bait, onset of death was also shown to occur within one day in the gavage study, 

which may limit the ability of prey organisms to accumulate a dose lethal to avian and 

mammalian predators/scavengers. The remaining rodenticides caused death within 2-3 days 

after dosing in the acute oral studies. Of these, chlorophacinone is expected to accumulate the 

least, followed by diphacinone, bromethalin, difethialone, and brodifacoum. Difethialone is 

expected to accumulate at least 9-42X the LD50 for mammals, while brodifacoum is expected to 

accumulate at least 21-82X the LD50 for mammals before death. 

The information available from the accumulation of dose in primary consumers calculated in this 

section indicate that, of the anticoagulants, brodifacoum and difethialone accumulate in body 

tissues to a greater extent than warfarin, chlorophacinone, and diphacinone. For bromethalin, 

dose accumulation calculations using a blood half-life of 5.6 days indicate that this chemical has 

potential to accumulate in body tissues 4-16X the LD50; however, death is expected to occur 

within five days of exposure, reducing the availability of contaminated animals for secondary 

consumers. In addition, a recent study by Pitt et al. (2011) found that time to death for house 

mice, Polynesian rats (Rattus exulans) and black rats (Rattus rattus) offered bromethalin may be 

more rapid than suggested in registrant submitted data for the Norway rat, which may reduce the 

potential for bromethalin to accumulate in some prey organisms. Data from this study also 

suggest that house mice and Polynesian rats exposed to bromethalin bait may stop feeding on the 

bait after reaching a lethal dose, further reducing the potential for accumulation.

3.4 Measured Rodenticide Residues

As discussed in the estimated accumulation of dose section above, the potential to accumulate 

rodenticides within body tissues may contribute substantially to the risks of these compounds. 

Measured residue values from various field and laboratory studies provide useful information on 

the potential for each of these rodenticides to accumulate in body tissues under field conditions. 

In addition, these data allow a means to test the assumptions employed in the theoretical analysis 

by comparing estimated and empirical concentrations of the rodenticides in mammals. 

Most of the available measured data on concentration of anticoagulants in rodents is from studies 

that involved a single, sublethal oral dose. One uncertainty in using these data is that they may 

under represent the uptake of an anticoagulant by a rodent because rats and mice entering a 

baiting station will not die for several days or more after ingesting a lethal dose and may 

continue consuming bait. For example, a wild Norway rat feeding only on brodifacoum bait may 

ingest a dose that is equivalent to 80x the LD50 (ICI Americas, Inc. 1978b). In a study where 

treated and untreated baits were available to rats, they still ingested doses of brodifacoum that 
Page 68 of 183

were 40x the LD50 (ICI Americas, Inc. 1978b). In a situation of repeat exposure for several days 

or more, anticoagulants may circulate in the blood at higher levels and for a longer time than 

suggested by studies in which a single, sublethal dose was administered (Belleville 1981). 

Data are available for a variety of small and medium sized mammalian granivores and omnivores 

exposed to brodifacoum, difethialone, and warfarin. No data were identified for bromethalin or 

other exposed taxonomic groups (e.g., birds, reptiles, and invertebrates). For all studies, it was 

assumed by the Agency the concentrations were reported by the study authors as wet weights of 

the mammals. Information on whole-body residues in target species exposed to the rodenticides 

considered in this assessment is provided in Table 3-21. 

In studies with brodifacoum, mean concentrations ranged from 0.35 to 8.63 mg a.i./kg-bw, while 

individual brodifacoum concentrations ranged from 0.1 to 25.97 mg a.i./kg-bw. Two residue 

studies indicate that the amount of whole-body residue in the target species may be related to the 

amount of active ingredient in the bait. Kaukeinen (1982) provides mean tissue residue levels in 

voles exposed to brodifacoum bait in the laboratory. Separate groups of males and females were 

exposed for 4 days to 50 mg a.i./kg bait or 10 mg a.i./kg bait. Residues were 5.21 mg a.i./kg and 

2.17 mg a.i./kg for males and females, respectively, exposed to 50 mg a.i./kg bait but only 0.53 

mg a.i./kg and 0.40 mg a.i./kg for males and females exposed to 10 mg a.i./kg bait. In field trials 

for vole control in orchards, Merson et al. (1984) collected voles 1 to 7 days after bait 

application. Two collections of voles exposed to 50 mg a.i./kg bait had mean whole-body 

residues of 2.07 mg a.i./kg and 4.07 mg a.i./kg, whereas those exposed to 10 mg a.i./kg bait had a 

mean residue level of 0.35 mg a.i./kg. For those studies that used bait currently registered bait 

concentrations, measured residue values are similar to the estimated body burden in rats after 

feeding on brodifacoum bait for 1 to 14 days (2.1 to 28.9 mg a.i/kg-bw; Table 3-18).

When considering two studies available for difethialone, mean concentrations ranged from 2.0 to 

3.1 mg a.i./kg-bw, while individual difethialone concentrations ranged from 0.77 to 2.67 mg 

a.i./kg-bw. Savorie (2005) assessed the accumulation of formulated difethialone pellets in male 

albino rat carcasses using two different feeding studies. In the first scenario, animals were 

offered bait for 3 days and then euthanized. In the second scenario, animals were offered bait 

until poisoning death occurred (usually between 4 and 7 days). Rats that were euthanized on day 

3 had a mean residue of difethialone in the body of 2.8 mg a.i./kg- carcass. Rats that were fed 

continuously with the same diet had a mean time of death of 5.2 days (range of 4-7 days) and the 

mean difethialone residue was 3.1 mg a.i./kg-carcass. These residue values are similar to the 

estimated body burden in rats after feeding on difethialone bait for 3 to 5 days (3.1 to 5.0 mg 

a.i./kg-bw). 

Mean warfarin concentration from six studies ranged from 0.038 to 104 mg a.i./kg-bw, 

individual concentrations were not reported. The highest mean residue values in this range come 
Page 69 of 183

from a study by Aulerich et al. (1987) where rabbits were fed low concentration warfarin bait (52 

or 67 mg a.i./kg bait) for 35 days, resulting in residue values of 82 and 104 mg a.i./kg-bw, 

respectively. This may indicate that mammals consuming small amounts of warfarin bait over an 

extended period of time may survive poisoning. This study may suggest that mammals have the 

potential to accumulate many times the lethal dose of warfarin. If these low bait concentrations 

were used to estimate accumulation of dose as above, the calculated doses in Norway rats after 

35 days are similar to the measured values in rabbits (64.3 and 82.8 mg a.i./kg-bw for the 52 and 

67 mg a.i./kg-bait, respectively); however, the weight of the rabbits used in this study were 

roughly an order of magnitude larger than the Norway rat. Since registered warfarin baits contain 

0.025% a.i. (250 mg a.i/kg-bait) or 0.054% a.i. (540 mg a.i./kg-bait), the low dose values found 

in the majority of the residue studies may not reflect true field conditions. 

Mean chlorophacinone concentrations ranged from 0.122 to 1.58 mg a.i./kg-bw, with individual 

concentrations ranging from the limit of detection to 4.1 mg a.i./kg-bw. Most of these studies 

assessed the efficacy of field application of chlorophacinone bait at the registered concentrations 

of either 0.005% a.i. (50 mg a.i./kg-bait) or 0.01% (100 mg a.i./kg-bait) to treat ground squirrel 

or vole populations in California. Since the majority of these studies are field based, the number 

of days that prey organisms were exposed to the bait is unknown; however, estimated residue 

values for the Norway rat, which approximates the weight of the ground squirrels tested, are 

similar to measured residues (2.1 mg a.i./kg-bw for 1 day accumulation and 4.1 mg a.i./kg-bw 

for 2 day accumulation). 

Mean diphacinone concentration ranged from 0.9 to 1.4 mg a.i./kg-bw. Individual concentrations 

ranged from 0.48 to 3.4 mg a.i./kg-bw. Similar to chlorophacinone, most of these studies 

assessed the efficacy of field application of chlorophacinone bait at the registered concentrations 

of either 0.005% a.i. (50 mg a.i./kg-bait) or 0.01% (100 mg a.i./kg-bait) to treat ground squirrel 

populations in California. Since the majority of these studies are field based, the number of days 

that prey organisms were exposed to the bait is unknown; however, estimated residue values for 

the Norway rat, which approximates the weight of the ground squirrels tested, are similar to 

measured residues (2.1 mg a.i./kg-bw for 1 day accumulation and 3.9 mg a.i./kg-bw for 2 day 

accumulation).

Comparisons of measured and calculated residues in rodents showed general agreement for the 

rodenticides considered in the assessment, with the exception of warfarin. This indicates that the 

conservative assumptions used in the accumulation of dose analyses did not impact overall risk 

characterization. 
Page 70 of 183

Table 3-21. Residue Levels of Anticoagulant Rodenticides in Primary Consumers

Rodenticide Mg a.i./kg bait Target species Site Sample size Days exposed Mean whole-carcass residue in mg 

a.i /kg-carcass (range) Reference

Brodifacoum 50 Rat Field 50 Unknown Most < 7

Some up to 11-13 Kaukeinen 1993a

Brodifacoum 50 Rat Field 6 Unknown 2.7 (0.1-6.6) ICI America 1979c

Brodifacoum 50 Rat Field

5 male

4 female

3 juvenile

Unknown

7.08 (3.92-9.17)

5.61 (1.39-12.19)

8.63 (1.77-25.97)

Howald 1997

Brodifacoum 50 Vole Field 74 1-7 4.07 ± 0.20 (SE) Merson et al. 1984

Brodifacoum 50 Vole Field 62 1-7 2.07 ± 0.17 (SE) Merson et al. 1984

Brodifacoum 50 Vole Lab 15 male

15 female

4

4

5.21 ± 2.06 (sd)

2.17 ± 1.17 (sd) Kaukeinen 1982

Brodifacoum 25b Deer mouse Field 10 4-9 2.71 (0.68-4.25) Howald et al. 2001

Brodifacoum 20b Mouse Lab nr 3 2.21 Anonymous 1981a

Brodifacoum 20b Mouse Lab 10 1 0.44 Newton et al. 1990

Brodifacoum 10b Vole Lab 15 male

15 female

4

4

0.53 ± 0.24 (sd)

0.40 ± 0.20 (sd) Kaukeinen 1982

Brodifacoum 10b Vole Field 43 1-7 0.35 ± 0.03 (SE) Merson et al. 1984

Difethialone 25 Rat Lab 20 3 2.0 ± 0.51 (sd)

(0.77-2.67) Goldade et al. 2001

Difethialone 23.94 Rat Lab 10

10

3

4-7

2.8 (2.1-3.3)

3.1 (1.9-4.3)

Savarie 2005

MRID# 46656501

Warfarin 200c Mouse Lab 17 3 2.95 ± 0.26 (SE) Townsend et al. 1984

Warfarin 67c Rabbit Lab nr 35 104 Aulerich et al. 1987

(E39689)

Warfarin 50c Mouse Lab 62 3 1.63 ± 0.1 (SE) Townsend et al. 1981

(J004518/E54919)

Warfarin 50c Mouse Lab 18 3 1.58 ± 0.1 (SE) Townsend et al. 1984
Page 71 of 183

Warfarin 52

c Rabbit Lab nr 35 82 Aulerich et al. 1987

(E39689)

Warfarin 10

c Mouse Lab 15

3 0.42 ± 0.04 (SE) Townsend et al. 1984

Chlorophacin

one

100 Ground squirrel Field 10 unknown 1.27 ± 0.56 (sd)

(0.237 – 2.24) Baroch 1996 

Chlorophacin MRID# 43922201

one

50 Ground squirrel Field 10 unknown 0.52 ± 0.31(sd)

(0.244 – 1.29)

Chlorophacin

one

50 Ground squirrel Field 10 unknown 0.57 ± 0.27 (sd)

(0.123 – 0.896)

Baroch 1996 

MRID# 43922202

Chlorophacin

one

100 Ground squirrel Field 38 unknown

6 carcasses: <LOD=0.025

32 carcasses:

0.159 ± 0.141(sd)

(0.025 – 0.546)

Ramey et al. 2007

Primus et al. 2001

(E0165)

Chlorophacin

one

50 Ground squirrel Field 161-15 0.122

(<LOD, 0.265) Primus et al. 2001

(E0165)

Matschke, 1999 Chlorophacin

one

50 Microtus sp. 

vole Field

31-15 1.58

(0.26-4.1)

Chlorophacin

one

50 and 100 Valley pocket 

gopher Field

81-13 0.357

(<LOD, 1.21)

Primus et al. 2001

(E0165)

Stewart et al 2000

Chlorophacin

one

50 Ground squirrel Field 212-15 0.28

(0.02 – 0.71) Salmon et al 2002

MRID# 45855101

Goodall et al. 2002

MRID# 46117303

Chlorophacin

one

100 Ground squirrel Field 162-15 1.41

(0.30 – 2.72)

Chlorophacin

one

50 Ground squirrel Field 162-15 0.26

(<LOD, 1.01)

Chlorophacin

one

100 Ground squirrel Field 202-15 0.51

(<LOD – 1.31)

Chlorophacin

one

50

Black tailed 

prairie dogs (8) 

and one 

cottontail rabbit

Field92-15 0.58

(0.01, 1.25)

MRIDs 47333602, 

47333603 and 

48294401 
Page 72 of 183

Chlorophacin

one

NA

Black-tailed 

prairie dog

Field 8 unknown 1.48

(0.85, 2.24)

Chlorophacin

one

50 Laboratory rat Field 5 5 0.47

(0.21-0.93)

Baroch 1997

MRID# 44631402

Chlorophacin

one

50 Laboratory rat Lab 4 5 0.45

(0.18-0.81)

Ahmed et al. 1996 

MRID# 44631401

Diphacinone 50 Ground squirrel Field 10 unknown 1.4 

(0.6-3.4)

Baroch 1994a

MRID# 43534601

Diphacinone 50 Ground squirrel Field 7 unknown 0.9

(0.48-1.89)

Baroch 1994b

MRID# 43534602

Diphacinone 100 Ground squirrel Field 10 unknown 1.4

(0.6-2.6)

Baroch 1994a

MRID# 43534601

Diphacinone 50 and 100d Ground squirrel Field 76 unknown 0.98 Goodall et al. 2002

MRID# 46117303

nr = not reported

NA = not available

a Cited in US EPA (2004) Potential Risks of Nine Rodenticides to Birds and Nontarget Mammals: a Comparative Approach 

b baits registered in the U.S. are 0.005% a.i.

c baits registered in the U.S. are 0.025% a.i.

d the study did not allow for distinguishing among squirrels exposed to 50 mg a.i./kg and 100 mg a.i./kg baits
Page 73 of 183

4 Primary Exposure and Risk

For the purpose of this assessment, primary exposure is defined as exposure to primary 

consumers (i.e., those species likely to directly eat bait). Primary risk is influenced by factors 

including toxicity, toxicokinetics (chemical accumulation and elimination in the body), 

concentration of active ingredient in the bait, and availability of bait for consumption. For each 

of the evaluated chemicals, risk is assessed in several ways:

• single day dose-based exposure to bait based on allometric equations that allow for 

differentiation among taxa, feeding rates, and body size is compared to acute oral LD50

toxicity values;

• six day dose-based exposure to bait based on metabolism rate in the body and allometric 

equations that allow for differentiation among taxa, feeding guild, and body size is 

compared to acute oral LD50 toxicity values; and

• Dietary exposure (concentration of chemical active ingredient in bait) is compared to 

acute dietary LC50 values.

Further lines of evidence include an analysis of the number of days of bait consumption required 

to reach the median lethal dose of rodenticide and the grams of bait individuals would need to 

consume in order to accumulate the median lethal dose. 

4.1 Risks based on the Acute Oral LD50

4.1.1 Calculation of Dose-based Ingestion Rates for Birds and Mammals

For the dose-based exposure method, exposure is calculated as mg a.i./kg-bwt, where kg-bwt is 

the kilograms of the individual consuming bait for three standard weight classes of birds and 

mammals. Exposure (food dry weight consumption) estimates were derived using allometric 

equations based on body weight (USEPA 1993). For these primary consumers of rodenticide 

bait, the default weight classes of birds are small (20 g), medium (100 g), and large (1000 g), and 

the default weight classes for mammals are small (15 g), medium (35 g), and large (1000 g). 

These are the standard bird and mammal body weights used in EFED risk assessments (T-REX 

documentation, version 1.4.1, December 2008). Allometric equations were used for the generic 

bird and mammal. In addition, allometric equations for passeriform birds and rodent mammals 

were also used as these would best approximate those individuals with high potential for 

consuming bait and they would give the most conservative exposure estimates. 

Animal exposure was determined based on daily food intake rates (dry weight), assuming 100% 

of their diet consisted of bait, and the bait contained no water. With the exception of one 
Page 74 of 183

warfarin label (EPA Reg. No. 3282-3), all baits are waxy-type blocks, pellets, bait bits, paste, or 

meal; water content in these bait types was assumed to be very low. For EPA Reg. No. 3282-3, 

the product is in powder form and is to be mixed into the users own bait matrix. Water content 

for end mixture to which animals are exposed will vary, but for consistency in this analysis it 

was assumed that the water content was insignificant. Also, it was assumed that the form of the 

bait (block, pellets, bits, etc.) would not influence rate of intake or total intake. Formulas for 

calculation of dose estimates are provided in Box 4-1, and the resulting oral consumption (dose) 

estimates based on one day of feeding exclusively on bait are listed in Table 4-1. Intake rates 

based on the generic bird and mammal allometric equations can be found in Appendix B. All 

three evaluated size classes of Passeriformes will consume more food, and, thus, consume more 

rodenticide than the equivalent weight class of the generic bird based on the allometric 

equations. Based on the allometric equations, small rodents (15 g) have higher food intake rates 

than small generic mammals, medium sized rodents (35 g) have food intake rates similar to 

medium-sized generic mammals, and large rodents (1000 g) have lower food intake rates than 

large generic mammals. 

Risk estimations based on passerines and generic birds are similar; therefore; only the numeric 

quantities associated with passerines will be presented in the text. All intake rates and RQ 

calculations for both passerines and generic birds are presented in Appendix B. Similarities and 

differences in risk conclusions will be discussed in the main text. A similar approach is used for 

the rodents and generic mammal risk estimation. 

Box 4-1. Formulas for Calculation of Rodenticide Intake (mg a.i./kg-bwt) based on 

Consumption of Bait.

Passeriform bird food intake (g, dry weight): FI (g dry-wt/day) = 0.398 * Wt(g)0.850

Generic bird food intake (g, dry weight): FI (g dry-wt/day) = 0.648 * Wt(g)0.651

Rodent mammal food intake (g, dry weight): FI (g dry-wt/day) = 0.621 * Wt(g)0.564

Generic mammal food intake (g, dry weight): FI (g dry-wt/day) = 0.235 * Wt(g)0.822

Active ingredient intake (mg a.i./kg-bwt/day) = 

 FI (g dry-wt/day) * C mg a.i./kg-bait / (Wt(g)/1000)

Where: Wt(g) = weight (in grams) of the bird or mammal consumer

 C(mg a.i./kg-bait) = concentration of rodenticide in bait 
Page 75 of 183

Table 4-1 Estimated rodenticide daily intake rates for passeriforms and rodents (mg 

a.i./kg-bwt) 

Passeriforms Rodents

20 g 100 g 1000 g 15 g 35 g 1000 g

Brodifacoum 13 10 7.1 10 6.6 1.5

Difethialone 6.4 5.0 3.5 4.8 3.3 0.8

Warfarin (0.025%) 63 50 35 48 33 7.6

Warfarin (0.054%) 137 108 76 103 71 17

Chlorophacinone 13 10 7.1 10 6.6 1.5

Diphacinone 13 10 7.1 10 6.6 1.5

Bromethalin 25 20 14 19 13 3.1

4.1.2 Calculation of Weight-Adjusted LD50s for Birds and Mammals

Weight-adjusted LD50 values for birds and mammals were used to calculate risk quotients. 

Scaling factors derived from Mineau et al. (1996) for birds are used in this assessment, just as is 

done in T-REX v.1.4.1. A chemical-specific scaling factor was available for brodifacoum (0.76) 

from Mineau et al. (1996), and for all other chemicals, the default scaling factor (1.15) was used 

to adjust the avian LD50s. Using additional data, Mineau et al. (2001) provided an alternative 

scaling factor for brodifacoum (0.88) and a chemical-specific scaling factor for chlorophacinone 

(-0.53). Mammalian weight-adjusted LD50 values were calculated using the “body weight 3⁄4” 

adjustment (USEPA 1995, 2011). Formulas for calculating the adjusted LD50s for birds and 

mammals are presented in Box 4-2. Avian LD50s adjusted using the scaling factors presented in 

Mineau et al. (1996) and mammalian LD50s adjusted using the standard scaling factor are 

provided in Table 4-3. All adjusted LD50s (including those calculated using scaling factors 

presented in Mineau et al. (2001)) are provided in Appendix B. 
Page 76 of 183

Box 4-2. Formulas for Calculation of Weight-adjusted Avian and Mammalian LD50s.

Adjusted avian LD50: Adj LD LD

AW

TW

x

.

( )

50 50

1

= 



 





−

 where:

Adj. LD50 = adjusted LD50 (mg/kg-bw) 

LD50 = endpoint reported from bird study (mg/kg-bw)

TW = body weight of tested animal *

AW = body weight of assessed animal (20g, 100g, and 1000g)

x = scaling factor for birds**

Adjusted mammalian LD50: 

(0.25)

50 50 . 



 



 = AW

TW Adj LD LD where:

Adj. LD50 = adjusted LD50 (mg/kg-bw)

LD50 = endpoint reported from mammal study (mg/kg-bw)

TW = body weight of tested animal ***

AW = body weight of assessed animal (15g, 35g, 1000g)

*Average body weight of birds on day 0 of study, if not available, the default values of 1580g (mallard) or 178g 

(bobwhite) are used. 

** Scaling factors: EFED default 1.15 (Mineau et al., 1996), Brodifacoum 0.76 (Mineau et al., 1996), 

Brodifacoum 0.88 (Mineau et al., 2001), Chlorophacinone -0.53 (Mineau et al., 2001)

***Average body weight of rats on day 0 of study, if not available, the default value of 350 g is used. 

Table 4-2. LD50 values (mg a.i./kg-bwt): unadjusted and adjusted based on assessed 

animal weight

Birds1 Mammals2

Unadjusted 20 g 100 g 1000 g Unadjusted 15 g 35 g 1000 g

Brodifacoum 0.25 0.65 0.44 0.25 0.48 0.96 0.78 0.34

Difethialone 0.30 0.21 0.27 0.38 0.56 1.11 0.90 0.39

Warfarin 525 273 347 490 3.0 5.7 4.6 2.0

Chlorophacinone 258 182 232 328 6.26 11.6 9.4 4.1

Diphacinone 1630 1163 1480 2091 1.9 4.8 3.9 1.7

Bromethalin 4.6 3.27 4.16 5.88 2.2 3.7 3.0 1.3 1 LD50 values were adjusted using scaling factors presented in Mineau et al. (1996). 2 LD50 values were adjusted using the default mammalian scaling factor.

4.1.3 Avian risks based on acute oral LD50 toxicity 

RQs are calculated by dividing the daily rodenticide a.i. intake (Table 4-1) from each weight 

class by the weight-adjusted LD50 (Table 4-2). Details of these calculations are presented in 
Page 77 of 183

Appendix B. RQs for three weight classes of passerines are provided in Table 4-3. For 

brodifacoum and difethialone, all RQs exceeded the acute risk LOC (RQs range from 9 to 30).

For warfarin bait containing 0.025% a.i, the acute risk LOC was not exceeded for any weight 

class. For warfarin bait containing 0.054% a.i, the acute risk LOC was exceeded for small 

passerines (RQ = 0.50). For chlorophacinone and diphacinone, no RQs exceeded the acute risk 

LOC (RQs range from <0.01 to 0.07). All bromethalin RQs exceeded the acute risk LOC (RQs 

range from 2.4 to 7.8).

As indicated previously, mortality due to rodenticide consumption may not occur until several 

days after initial exposure. A useful characterization for comparing risks among the chemicals 

considered in this white paper is to calculate how long it would take an individual to consume 

the quantity of bait needed to reach the LD50 based on the estimated daily food intake (Box 4-1

and Table 4-1) and chemical-specific elimination rates (Section 3.3.2). Exposure was 

determined based on daily food intake rates (dry weight) for passerines, assuming 100% of their 

diet consisted of dry bait. It was assumed that the form of the bait would not influence intake. 

Based on the assumed weights, the percent of a.i. in the bait, and daily food intake, the 

concentration within the animal at the end of a day’s feeding was determined. As discussed in 

Section 3.2, a first-order exponential decay model was used to estimate daily elimination of the 

evaluated chemicals from the body. For all anticoagulants, daily residual body burdens were 

estimated using liver half-lives. For bromethalin, daily residual body burden was estimated using 

blood plasma half-life, as no liver half-life data were available. 

The day (or 24-hr interval) during which the LD50 (mg a.i./kg-bwt) was reached is provided for 

each chemical in Table 4-3. 

For brodifacoum, difethialone, and bromethalin, less than one day of feeding on bait would 

enable the individual passerine to consume sufficient material to reach the LD50. For warfarin, 

days to reach the LD50 depended on body weight and bait concentration. The range was from 1-2 

days for small passerines consuming 0.054% warfarin bait to 14-16 days for large passerines 

consuming 0.025% warfarin bait. As chlorophacinone is less toxic than many of the other 

evaluated chemicals, more consumption days were needed to reach the LD50 (from 16-17 days 

for small passerines to 117-118 days for large passerines). For diphacinone, the LD50 was not 

reached when body burden was calculated after 365 days of bait consumption, as diphacinone is 

less toxic and has faster elimination rates than the other evaluated chemicals. Furthermore, after 

approximately one month, there was little change in the total body burden (mg diphacinone/kgbwt) as the asymptote for body burden was reached (Table 4-3).

A second characterization useful for comparing risks among rodenticides considered in this 

white paper is mass of bait required to be consumed for an individual to reach the LD50 as a body 

burden concentration (Table 4-3). For chemicals estimated to require less than one day of bait 

consumption to reach the LD50, bait consumption was estimated as:
Page 78 of 183

Weight-adjusted LD50 (mg a.i/kg-bwt) * weight of assessed bird (g) / a.i. concentration (mg a.i./kg-bait).

For chemicals estimated to require more than one day of bait consumption to reach the LD50, bait 

consumption was estimated based on the upper bound of the estimated days of consumption:

Days of consumption * 0.398 * weight of assessed bird (g) 0.85.

For brodifacoum, difethialone, and bromethalin, a small amount of bait relative to body weight 

of the individual would be required to reach the LD50. Across all labels, the maximum grams of 

bait per placement that can be used are 454 g bait/placement for brodifacoum and difethialone 

and 170 g bait/placement for bromethalin (Table 2-4). Therefore, it is possible for passerines to 

consume sufficient bait at one placement to reach the median lethal dose for these chemicals.

For small and medium sized passerines consuming warfarin bait, sufficient bait would be 

available at one placement (some labels allow for a maximum of 454 g warfarin bait/placement) 

for an individual to consume the median lethal dose. The largest passerines would require 

multiple days of bait from multiple stations to reach the median lethal dose.

For passerines consuming chlorophacinone or diphacinone bait, consumption would need to 

occur at multiple placements for multiple days for the median lethal dose to be reached. 

Maximum placement size for chlorophacinone is 20 g bait/placement, and maximum placement 

size for diphacinone is 199 g bait/placement. 

Table 4-3 RQs for Passeriforms Based on Consumption of Bait and Days to Consume an 

LD50-dose

RQ1 Days to consume sufficient bait to 

reach the LD50

Bait (g) to consume to 

reach the LD50

20 g 100 g 1000 g 20 g 100 g 1000 g 20 g 100 g 1000 g

Brodifacoum 19.61* 22.67* 27.89* <1 <1 <1 0.3 0.9 5.1

Difethialone 29.80* 18.39* 9.22* <1 <1 <1 0.2 1.1 15

Warfarin (0.025%) 0.23 0.14 0.07 4-5 7-8 14-15 25 160 2100

Warfarin (0.054%) 0.50* 0.31 0.16 1-2 3-4 6-7 10 80 990

Chlorophacinone 0.07 0.04 0.02 16-17 30-31 117-118 86 620 17000

Diphacinone 0.01 0.01 <0.01 >365 >365 >365 >1800 >7300 >52000

Bromethalin 7.76* 4.76* 2.40* <1 <1 <1 0.7 4.2 59 1

* RQ exceeds Acute Risk LOC.

Finally, body burden and associated RQs were calculated with the assumption that bait was 

consumed exclusively for six days. Estimated daily food intake (Box 4-1 and Table 4-1) and 

chemical-specific elimination rates (Section 3.3.2) were used to determine 6-day accumulation 

concentrations (mg a.i./kg-bwt) (Table 4-4). RQs are calculated by dividing the cumulative 

rodenticide a.i. from each weight class by the weight-adjusted LD50 (Table 4-2). Details of these 
Page 79 of 183

calculations are presented in Appendix B. RQs for three weight classes of passerines are 

provided in Table 4-4. 

For brodifacoum and difethialone, all RQs exceeded the acute risk LOC (RQs range from 20 to 

76). For warfarin baits containing 0.025% a.i, the acute risk LOC was exceeded for small and 

medium sized passerines (RQ = 1.36 and 0.84, respectively). For warfarin baits containing 

0.054% a.i, the acute risk LOC was exceeded for all size classes of passerines (RQs range from 

0.91 to 2.94). For chlorophacinone and diphacinone, no RQs exceeded the acute risk LOC (RQs 

range from 0.01 to 0.40). RQs assuming six days of feeding on bait were not calculated for 

bromethalin as exposed animals are not expected to continue feeding on bait for six days after 

initial exposure. 

After adjusting exposure to six days of exclusive feeding and accounting for metabolism through 

the use of estimated half-lives, LOC exceedance conclusions are the same for all evaluated 

anticoagulants except warfarin. Assuming a one-day or a six-day exposure period, acute risk 

LOCs were exceeded for brodifacoum and difethialone, and acute risk LOCs were not exceeded 

for chlorophacinone and diphacinone. When accumulation of warfarin over a 6-day exposure 

interval is calculated, RQs for all bait concentrations and bird sizes except for large passerines 

consuming 0.025% a.i. bait exceeded the acute risk LOC. This indicates a change in the risk 

picture for warfarin when bait exposure and consumption is extended over time.

Table 4-4 Accumulation of rodenticide (mg a.i./kg-bwt) after 6 days of 

consumption and associated RQs for passeriforms 

Six day accumulation 

(mg a.i./kg-bwt)

RQ1

20 g 100 g 1000 g 20 g 100 g 1000 g

Brodifacoum 76 60 42 117* 135* 166*

Difethialone 36 28 20 168* 104* 52*

Warfarin (0.025%) 371 292 206 1.36* 0.84* 0.42

Warfarin (0.054%) 802 630 446 2.94* 1.81* 0.91*

Chlorophacinone 73 57 40 0.40 0.25 0.12

Diphacinone 56 44 31 0.05 0.03 0.01

Bromethalin 112 88 62 NA2 NA NA 1

* RQ exceeds Acute Risk LOC. 2 NA- not applicable. Animals consuming bromethalin are not expected to continue feeding on 

bromethalin bait for six days.

The one-day-exposure RQs, days of bait consumption to reach the LD50, and six-day-exposure 

RQs for generic birds are listed in Appendix B. In addition to the RQs and bait consumption 

rates for the generic birds; alternative RQ and bait consumption analyses were conducted using 

the unadjusted LD50s and the brodifacoum and chlorophacinone LD50s adjusted following 

Mineau et al. (2001) for both passerine and generic birds (Appendix B). Although the resulting 
Page 80 of 183

RQs values were different, these alternative analyses yielded the same LOC exceedances as for 

the passeriform birds consuming bait using the adjusted LD50s following Mineau et al. (1996) 

with a few exceptions. For the 0.054% warfarin bait, the one-day-exposure RQ for small 

passerines using the adjusted LD50 exceeded the LOC (RQ=0.50); however, the RQ for small 

generic birds using the adjusted LD50 and the RQs for both small passerines and small generic 

birds using the unadjusted LD50 did not exceed the LOC (RQs = 0.26, 0.16, and 0.15, 

respectively).

With the exception of chlorophacinone and diphacinone, the following conclusions can be made 

for birds in a generic class and for alternative LD50 adjustments. When days of bait consumption 

to reach the LD50 were compared between passerine and generic birds of the same weight class 

or between adjusted and unadjusted LD50s of the same weight class, most differences were less 

than three days. When grams of bait consumed to reach the LD50 were compared between 

passerine and generic birds of the same weight class or between adjusted and unadjusted LD50s 

of the same weight class, most differences were small relative to the total amount of bait that 

needed to be consumed. 

Chlorophacinone is less acutely toxic to birds and the concentration of the a.i. in the bait is low; 

therefore, more time is needed for individual birds to consume a median lethal dose. In addition, 

the elimination half-life is relatively short (35 days), thus further increasing the time to reach the 

median lethal dose. The chlorophacinone-specific scaling factor (Mineau et al. 2001) was based 

on data from six avian species with a median LD50 of 25.45, indicating the species used for 

development of the scaling factor were more sensitive to chlorophacinone than the bobwhite 

quail data available to the USEPA (LD50 = 258 mg a.i./kg-bwt, MRID 41513101). In addition, 

the data used in Mineau et al. (2001) indicated that chlorophacinone sensitivity increased as bird 

weight increased, which is counter to the sensitivity trend across all chemicals as presented by 

Mineau et al. (2001). 

As with chlorophacinone, diphacinone is also less acutely toxic to birds and the concentration of 

the a.i. in the bait is low. The elimination half-life was very short (5.5 days). Based on estimated 

feeding rates, even if birds were to exclusively feed on diphacinone bait for a year, the median 

lethal dose would still not be attained.

4.1.4 Mammalian risks based on acute oral LD50 toxicity 

RQs were calculated by dividing the rodenticide a.i. intake (Table 4-1) from each weight class 

by the weight-adjusted LD50 (Table 4-2). Details of these calculations are presented in 

Appendix B. RQs for three weight classes of rodents are provided in Table 4-5. For all 

evaluated chemicals except chlorophacinone, the acute risk LOC was exceeded (exceeding RQs 

ranged from 0.70 to 18.05) for all three weight classes. For chlorophacinone, RQs for small and 
Page 81 of 183

medium rodents (0.82 and 0.70, respectively) exceeded the acute risk LOC; however, the RQ for 

large rodents (0.38) did not exceed the acute risk LOC. 

As indicated previously, mortality due to rodenticide consumption may not occur until several 

days after initial exposure. A characterization useful for comparing risks among the chemicals is 

to calculate how long (days) it would take an individual to consume the quantity of bait needed 

to reach the LD50 based on the estimated daily food intake (Box 4-1 and Table 4-1) and 

chemical-specific elimination rates (Section 4.3.2). Exposure was determined based on daily 

food intake rates (dry weight) for rodents, assuming 100% of their diet consisted of bait and the 

bait contained no water. It was assumed that the form of the bait would not influence intake. 

Based on the assumed weights, the percent of a.i. in the bait, and daily food intake, the 

concentration within the animal at the end of a day’s feeding was determined. As discussed in 

Section 3.2.2, a first-order exponential decay model was used to estimate daily elimination of the 

evaluated chemicals from the body. For all anticoagulants, daily residual body burdens were 

estimated using liver half-lives. For bromethalin, daily residual body burden was estimated using 

blood plasma half-life, as no liver half-life data were available. The day (or interval) during 

which the LD50 (mg a.i./kg-bwt) was reached is provided for each chemical in Table 4-5. 

For all evaluated chemicals except for chlorophacinone, less than one day of feeding on bait 

would enable the individual to consume sufficient material to reach the LD50. For 

chlorophacinone, days to reach the LD50 ranged from one to three days depending on body 

weight. Comparing consumption time across chemicals and within weight class for the rodents 

provides an indication of the relative risk based on differing acute toxicities and concentration of 

a.i. in the bait. For all evaluated rodenticides, there was little difference in days to reach the LD50

via bait consumption.

A second characterization useful for comparing risks among rodenticides considered in this 

white paper is mass of bait required to be consumed for an individual to reach the LD50 as a body 

burden concentration (Table 4-5). For chemicals estimated to require less than one day of bait 

consumption to reach the LD50, bait consumption was estimated as:

Weight-adjusted LD50 (mg a.i/kg-bwt) * assessed mammal weight (g) / a.i. concentration (mg a.i./kg-bait).

For chemicals estimated to require more than one day of bait consumption to reach the LD50, bait 

consumption was estimated based on the upper bound of the estimated days of consumption:

Days of consumption * 0.621 * weight of assessed mammal (g) 0.564.

For all chemicals except chlorophacinone, maximum placement sizes (Table 2-4) are sufficient 

to enable rodents to consume sufficient bait to reach the median lethal dose from one placement. 

For chlorophacinone, small and medium sized rodents could consume sufficient bait to reach the 
Page 82 of 183

median lethal dose from one placement, while large rodents would require feedings from 

multiple placements.

Table 4-5 RQs for Rodents Based on Consumption of Bait and Days to Consume an LD50-

dose

RQ1 Days to consume sufficient bait 

to reach the LD50

Bait (g) to consume to 

reach the LD50

15 g 35 g 1000 g 15 g 35 g 1000 g 15 g 35 g 1000 g

Brodifacoum 9.93* 8.48* 4.55* <1 <1 <1 0.3 0.5 6.7

Difethialone 4.30* 3.67* 1.97* <1 <1 <1 0.7 1.3 16

Warfarin (0.025%) 8.36* 7.14* 3.83* <1 <1 <1 0.3 0.7 8.0

Warfarin (0.054%) 18.05* 15.42* 8.27* <1 <1 <1 0.2 0.3 3.7

Chlorophacinone 0.82* 0.70* 0.38 1-2 1-2 2-3 6 9 61

Diphacinone 2.63* 2.24* 1.20* <1 <1 <1 1.1 2.1 25

Bromethalin 5.12* 4.38* 2.35* <1 <1 <1 0.6 1.1 13

1

* RQ exceeds Acute Risk LOC.

Finally, body burden and associated RQs were calculated with the assumption that bait was 

consumed exclusively for six days. Estimated daily food intake (Box 4-1 and Table 4-1) and 

chemical-specific elimination rates (Section 4.3.2) were used to determine 6-day accumulation 

concentrations (mg a.i./kg-bwt) (Table 4-4). RQs are calculated by dividing the cumulative 

rodenticide a.i. from each weight class by the weight-adjusted LD50 (Table 4-2). Details of these 

calculations are presented in Appendix B. RQs for three weight classes of rodents are provided 

in Table 4-6. 

For brodifacoum and difethialone, all RQs exceeded the acute risk LOC (RQs range from 20 to 

76). For warfarin baits containing 0.025% a.i, the acute risk LOC was exceeded for small and 

medium sized passerines (RQ = 1.36 and 0.84, respectively). For warfarin baits containing 

0.054% a.i, the acute risk LOC was exceeded for all size classes of passerines (RQs range from 

0.91 to 2.94). For chlorophacinone and diphacinone, no RQs exceeded the acute risk LOC (RQs 

range from 0.01 to 0.40). RQs assuming six days of feeding on bait were not calculated for 

bromethalin as exposed animals are not expected to continue feeding on bait for six days after 

initial exposure. 

When a six day feeding exposure is assumed, all RQs (range from 2.1 to 106) exceed the acute 

risk LOC for all evaluated anticoagulants. These conclusions are the same the conclusions when 

a single day exposure was assumed (Table 4-5) with the exception that under a single day 

exposure, the acute risk LOC for large rodents consuming chlorophacinone was not exceeded. 
Page 83 of 183

Table 4-6 Accumulation of Rodenticide (mg a.i./kg-bwt) after 6 days of 

Consumption and Associated RQs for Rodents

Six day accumulation 

(mg a.i./kg-bwt)

RQ1

15 g 35 g 1000 g 15 g 35 g 1000 g

Brodifacoum 57 39 9 59* 51* 27*

Difethialone 27 19 4 24* 21* 11*

Warfarin (0.025%) 279 193 45 49* 42* 22*

Warfarin (0.054%) 602 416 96 106* 90* 48*

Chlorophacinone 54 38 9 4.7* 4.0* 2.1*

Diphacinone 42 29 7 11.6* 9.9* 5.3*

Bromethalin 84 58 14 NA2 NA NA 1

* RQ exceeds Acute Risk LOC. 2 NA- not applicable. Animals consuming bromethalin are not expected to continue feeding on 

bromethalin bait for six days.

The one-day-exposure RQs, days of bait consumption to reach the LD50, and six-day-exposure 

RQs for generic mammals are listed in Appendix B. In addition to the RQs and bait 

consumption rates for the generic mammals; alternative RQ and bait consumption analyses were 

conducted using the unadjusted LD50s. These alternative analyses yielded the same LOC 

exceedances as for the rodents consuming bait using the adjusted LD50s. In addition, estimated 

days to consume the LD50 and grams of bait required to consume the LD50 were also similar. 

4.2 Risks based on the acute dietary LC50

A second risk estimation method for primary consumption of bait depends on the concentration 

of a.i. in the bait and the dietary LC50. The LC50 (mg a.i./kg-diet) is obtained from 5-day 

exposure dietary toxicity studies. RQs are calculated as a ratio of a.i. concentration in the bait 

and the LC50.

4.2.1 Avian risks based on the acute dietary LC50

Concentration of a.i. in the baits, avian LC50s, and RQs are provided in Table 4-7. Based on 5-

day dietary exposure, the acute risk LOC was exceeded for brodifacoum, difethialone, warfarin 

baits with 0.054% a.i., and chlorophacinone. Acute risk LOCs were not exceeded for warfarin 

baits with 0.025% a.i., diphacinone, and bromethalin. 

With the exception of chlorophacinone and bromethalin, these results generally concur with the 

conclusions from the bird acute oral RQ results. For some anticoagulants, multiple exposures to 

the active ingredient for several days appear to be more effective and require less total chemical 

ingestion in order to attain mortality (Ashton et al. 1987). The Ashton et al (1987) study 
Page 84 of 183

concludes that in order to reach the median lethal dose for chlorophacinone, the total dose of 

ingested chlorophacinone over a 5-day period can be smaller if it is administered in several small 

daily amounts rather than in one single large amount. 

It is not expected that individuals feeding on bromethalin bait will continue to feed on it over 

multiple days. When bromethalin was introduced into the dietary matrix in the dietary toxicity 

test, birds tended to reduce food consumption over the course of the study; thereby, reducing 

their exposure to bromethalin. As such, the chemical appears to have less risk potential as the 

LOC when considering the dietary toxicity study was not exceeded. 

Table 4-7. Avian RQs based on dietary toxicity LC50s

Chemical Bait concentration 

(mg a.i./kg-bait)

LC50 

(mg a.i./kg-diet) RQ1

Brodifacoum 50 1.33 38*

Difethialone 25 0.56 45*

Warfarin (0.025%) 250 625 0.40

Warfarin (0.054%) 540 625 0.86*

Chlorophacinone 50 56 0.89*

Diphacinone 50 906 0.06

Bromethalin 100 210 0.48 1

* RQ exceeds Acute Risk LOC.

4.2.2 Mammalian risks based on dietary toxicity

Concentration of a.i. in the baits, LC50s, and RQs are provided in Table 4-8. Mammalian LC50

data was not available for difethialone. Based on 5-day dietary exposure, the acute risk LOC was 

exceeded for brodifacoum, warfarin baits with 0.025% a.i. and 0.054% a.i., chlorophacinone, and 

diphacinone. Toxicity data for RQ calculation based on acute dietary tests were not available for 

difethialone and bromethalin. These results generally concur with the conclusions from the 

mammal acute oral RQ results. 

Table 4-8. Mammalian RQs based on dietary toxicity LC50s

Chemical Concentration in bait 

(mg a.i./kg-bait)

LC50

(mg a.i./kg-diet) RQ1

Brodifacoum 50 0.53 94*

Difethialone 25 ND ND

Warfarin (0.025%) 250 4.41 57*

Warfarin (0.054%) 540 4.41 123*

Chlorophacinone 50 1.14 44*

Diphacinone 50 2.31 22*
Page 85 of 183

Table 4-8. Mammalian RQs based on dietary toxicity LC50s

Chemical Concentration in bait 

(mg a.i./kg-bait)

LC50

(mg a.i./kg-diet) RQ1

Bromethalin 100 ND ND 1

* RQ exceeds Acute Risk LOC.

ND – no quantitative mammalian dietary toxicity data, RQ could not be calculated

4.3 Uncertainties

A description of assumptions, uncertainties, strengths, and limitations of the basic ecological risk 

assessment as conducted by EFED is described in Chapter 6 of the Agency’s Overview 

Document (USEPA 2004) and include those related to the effects assessment. Uncertainties 

relating to toxicity estimates were discussed in Section 3.1.5, and uncertainties relating to 

accumulation and elimination of the evaluated rodenticides were discussed in Sections 3.2 and 

3.3. 

An additional uncertainty is the use of scaling factors to adjust the LD50s to account for weight 

differences between the tested animals and the assessed animals. For birds, weight-adjusted 

LD50s were calculated based on equations presented in Mineau et al. (1996) and Mineau et al. 

(2001). RQs, days to consume sufficient bait to reach the median lethal dose, and grams bait 

required for consumption to reach the median lethal dose were calculated based on the 

unadjusted LD50s, as well as LD50s based on the non-chemical specific adjustment in Mineau et 

al. (1996), chemical specific adjustment for brodifacoum in Mineau et al. (1996), and the 

chemical specific adjustments for brodifacoum and chlorophacinone in Mineau et al. (2001). For 

mammals, the unadjusted LD50s and LD50s based on the “body weight 3⁄4” adjustment (USEPA 

1995, 2011) were used to calculate RQs, days to consume sufficient bait to reach the median 

lethal dose, and grams bait required for consumption to reach the median lethal dose. Although 

there were differences in the calculated RQs, days to consume sufficient bait to reach the median 

lethal dose, and grams bait required for consumption to reach the median lethal dose, none of 

these differences were large enough to alter the risk conclusions.

4.4 Summary of Rodenticide Risks to Bait-consuming Birds and Mammals

The assessment of primary exposure risks for wildlife involved a series of assessment 

methodologies that encompassed toxicity, toxicokinetics, concentration of active ingredient in 

the bait, and availability of bait for consumption. 
Page 86 of 183

Based on the analysis of number of days to consume sufficient bait to reach the LD50 and the 

quantity (grams) of bait to consume to reach the LD50, the following general conclusions can be 

made:

• smaller primary consumers are at a greater risk than larger primary consumers;

• brodifacoum, difethialone, and bromethalin pose greater primary risk to birds than 

warfarin, chlorophacinone, and diphacinone;

• risks to non-target mammals are similar across all evaluated rodenticides,

• non-target mammals are generally more at risk from consumption of bait than birds; and

• Longer exposure to bait increases the likelihood that a non-target mammal or bird will 

consume sufficient bait to reach the LD50.

As with all rodenticides, brodifacoum and difethialone represent a lethal primary risk to nontarget small mammals that may eat either loose treated bait or utilize a bait station. In addition, 

the quantitative assessment also demonstrates that bait containing these two rodenticides 

constitutes a primary exposure risk concern for larger mammals as well as all avian size classes. 

This conclusion is consistent across all primary exposure risk analyses performed (single oral 

dose, six-day accumulated dose, and dietary concentration). EPA’s analysis indicates birds and 

mammals could attain a median lethal dose (LD50) of either brodifacoum or difethialone upon 

feeding for less than day on treated bait. The brevity of feeding required to reach a potentially 

lethal exposure represents an increased opportunity (relative to all other rodenticides assessed) 

for non-target birds and mammals to chance upon loose treated bait and be lethally exposed.

Warfarin also demonstrates a primary exposure risk concern to small mammals. The assessment 

of warfarin indicates a risk to large mammals for all assessment methods employed as well. 

EPA’s analysis indicates that feeding on treated bait for less than a day is sufficient to attain a 

median lethal dose in all evaluated size classes of mammals, suggesting a similar opportunity, 

relative to brodifacoum and difethialone, for non-target mammals to chance upon treated bait and 

consume a lethal dose. In contrast, warfarin’s primary exposure risk for birds is limited to small 

passeriforms and other medium sized birds. These results are dependent on the assessment 

method EPA considered. While a single day dose risk assessment suggests only a risk to small 

passeriforms from the highest concentration formulation, accounting for the potential for 

multiple day accumulation on a dose basis expands the concerns to include passeriforms and 

other small and medium sized birds at all formulation concentrations assessed. The dietary 

assessment does not consider body weight as a determinant factor. However, this assessment 

method still predicts a risk concern at the highest formulation concentration. EPA’s analysis 

indicates that attainment of a lethal warfarin dose can occur after 1 to 7 days of feeding (0.054% 

warfarin bait) or after 4 to 15 days of feeding (0.025% warfarin bait), depending on body weight 

of the passerine in question. This finding suggests that, for birds there is less of an opportunity 

for chance lethal encounters with warfarin when compared with brodifacoum and difethialone, 

since it would require multiple days of feeding on warfarin treated bait.
Page 87 of 183

For bromethalin, as can be expected for a relatively fast acting compound (target mortality 1 to 7 

days, Pitt et al. 2011, and registrant submitted studies), acute risk concern levels are exceeded for 

all mammalian size classes feeding on treated bait based on a single day of exposure. Primary 

risk to birds from bromethalin exceeds risk concern levels in all assessment methodologies with 

the exception of the dietary exposure methodology, which returns a risk quotient that closely 

approaches but does not exceed the acute risk LOC. EPA’s analysis indicates that less than a 

single day’s feeding of bromethalin bait is sufficient to attain a median lethal dose in birds and 

mammals, suggesting that like brodifacoum and difethialone, adverse effects to non-target 

wildlife are likely from primary exposure events with treated bait. 

Two anticoagulant alternatives, chlorophacinone and diphacinone, also present primary exposure 

risk concerns for nearly all rodent and mammal sizes assessed using all primary exposure 

assessment methods (large rodents consuming chlorophacinone bait for one day are not expected 

to experience risk concerns). EPA’s analysis indicates that <1 day (diphacinone) and <1 to 3 

days (chlorophacinone) of feeding on bait are sufficient to attain a median lethal dose in 

mammals. This is not materially different from the other rodenticides with respect to the 

opportunity for non-target mammals to chance upon and feed lethally on treated bait. In contrast 

to mammals, avian risk is below concern levels for these two compounds. This may be a 

limitation in the analysis of primary exposure models terminating at six days (i.e., longer 

exposure periods might provide sufficient exposure to bait for adverse accumulation to occur); 

however, the analysis indicates that, unlike brodifacoum and difethialone, a very consistent and 

protracted feeding period for either diphacinone or chlorophacinone is required to attain a lethal 

dose in birds (i.e., daily feedings for the former versus weeks to months for the latter). 

5 Secondary Exposure and Risk

Secondary exposure is defined for the purpose of this assessment as exposure to secondary 

consumers (i.e., predators and scavengers). Secondary risk is influenced by factors, including: 

toxicity to primary and secondary consumers, elimination rates of each of the rodenticides in 

both primary and secondary consumers, and time to death. 

As noted above, a compound that is rapidly metabolized or excreted from a primary consumer 

may result in a lesser secondary exposure compared to compounds that accumulate with repeated 

sublethal exposures, even if repeated exposure occurs weeks or months after initial exposure 

(Eason and Murphy 2000). When considering rodenticides with similar toxicity, the rodenticides 

that are more rapidly cleared from the body are less likely to pose secondary risk compared to 

those that accumulate over time. This is because the rapidly cleared rodenticide is less likely to 
Page 88 of 183

accumulate to a level that would cause mortality to secondary consumers. Rodenticides that are 

slower to cause death are expected to prolong the availability of contaminated prey items and 

increase the dose that a secondary consumer may receive from eating target animals that have 

consumed a dose of rodenticide that is beyond the lethal dose (Section 3.3.2). The toxicity of the 

rodenticide to the secondary consumer would also affect the risk of mortality that would result 

from exposure. In summary, rodenticides that are eliminated more slowly and have greater 

potential to accumulate within body tissues, take longer to kill the primary consumer, and have 

greater toxicity to secondary consumers are expected to present greater secondary risk. 

Risk quotients for secondary consumers were calculated in three ways, including two theoretical 

methods and one empirical:

1) Dose-based exposure of secondary consumers to contaminated prey was calculated by the 

following 3 steps:

(a) Estimate rodenticide concentration in primary consumers; (b) convert concentrations 

in primary consumers to doses in secondary consumers using food intake allometric 

equations; and (c) compare the estimated doses in secondary consumers to weight 

adjusted LD50 toxicity values. 

2) Dietary exposure that compared estimated active ingredient concentrations in primary 

consumers to LC50 values, and 

3) Empirical residue data from carcass analyses were also used to characterize potential exposure 

to secondary consumers. 

Further lines of evidence that are considered in this assessment include an analysis of the number 

of target species secondary consumer birds and mammals would need to consume in order to 

accumulate a lethal dose of rodenticide and available data on secondary toxicity from secondary 

feeding studies. These analyses can be used to directly compare the risks of the rodenticides 

considered in this assessment. 

5.1 Rodenticide Ingestion through Consumption of Contaminated 

Carcasses- Theoretical Analysis

Secondary risk was quantitatively examined using a theoretical approach to calculate exposure. 

Similar to the analysis of primary risks, exposure in primary and secondary consumers is 

estimated using allometric equations of daily food intake as presented in the Wildlife Exposure 

Factors Handbook (EPA 1993). 
Page 89 of 183

Rodenticide intake based on consumption of bait by the primary consumers (prey) was 

calculated as described in Section 3.3.1. For this theoretical analysis, rodenticide accumulation 

in prey (house mouse and Norway rat) was calculated for a one day period, a three day period 

and a six day period (assuming consumption of 100% bait as diet over this time period) in order 

to bracket the available feeding durations from residue studies submitted to the agency and 

occurring in open literature. For bromethalin, a recent study by Pitt et al. (2011) found that time 

to death for house mice, Polynesian rats (Rattus exulans), and black rats (Rattus rattus) offered 

bromethalin may be more rapid than suggested in registrant submitted data for the Norway rat, 

which may reduce the potential for bromethalin to accumulate in prey organisms. Data from this 

study also suggest that house mice and Polynesian rats exposed to bromethalin bait may stop 

feeding on the bait after reaching a lethal dose, further reducing the potential for accumulation. 

Based on this information, rodenticide accumulation in prey was calculated for a one day period 

and a three day period only for this chemical. See Tables 3-21 and 3-22 for rodenticide body 

burden over time for the house mouse and Norway rat for the rodenticides considered here. 

The calculated body burden in the prey was then used to estimate rodenticide exposure in the 

secondary consumer using the general food intake allometric equations described in Section 3.1

above. Secondary consumers were assumed to be mammals weighing 50g, 1000g, or 3000g, or 

birds weighing 100g, 1000g, or 5000g. For example, American kestrels range from 100-150g, 

red-tailed hawks range from 900-1000g and bald eagles range from 4000-5000g (Dunning 1984). 

These weights were assumed based on the range of secondary consumers identified in 

rodenticide incidents in the Environmental Incident Information System (EIIS) and in the 

secondary feeding studies discussed in Section 5.3 below. The rodenticide dose expected to be 

consumed by secondary consumer birds feeding daily on house mice and Norway rats are shown 

in Tables 5-1 and 5-2, respectively. The daily rodenticide intake for secondary consumer 

mammals feeding on house mice and Norway rats are shown in Tables 5-3 and 5-4, respectively. 

Table 5-1. Calculated Rodenticide Daily Intake for Secondary Consumer Birds Based on 

Consumption of House Mouse (mg a.i./kg-bwt/day)

Chemical

1 Day Accumulation in Prey 

by Secondary Consumer 

Weight Class

3 Day Accumulation in Prey 

by Secondary Consumer 

Weight Class

6 Day Accumulation in Prey 

by Secondary Consumer 

Weight Class

100 g 1000 g 5000 g 100 g 1000 g 5000 g 100 g 1000 g 5000 g

Brodifacoum 3.21 1.44 0.82 9.61 4.30 2.45 19.16 8.58 4.89

Difethialone 1.61 0.72 0.41 4.70 2.11 1.20 9.08 4.06 2.32

Warfarin (0.025%) 16.1 7.19 4.10 47.7 21.4 12.2 93.9 42.0 24.0

Warfarin (0.054%) 34.7 15.5 8.86 103 46.1 26.3 203 90.8 57.8

Chlorophacinone 3.21 1.44 0.82 9.45 4.23 2.41 18.4 8.22 4.69

Diphacinone 3.21 1.44 0.82 8.54 3.82 2.18 14.4 6.44 3.67

Bromethalina 6.42 2.88 1.64 17.1 7.66 4.37 -- -- -- a

Given bromethalin’s rapid mode of action, RQs for this chemical were calculated based on a one day and three day 

accumulation scenario only.
Page 90 of 183

Table 5-2. Calculated Rodenticide Daily Intake for Secondary Consumer Birds Based on 

Consumption of Norway Rat (mg a.i/kg-bwt/day)*

Chemical

1 Day Accumulation in Prey 

by Secondary Consumer 

Weight Class

3 Day Accumulation in Prey 

by Secondary Consumer 

Weight Class

6 Day Accumulation in Prey by 

Secondary Consumer Weight 

Class

100 g 1000 g 5000 g 100 g 1000 g 5000 g 100 g 1000 g 5000 g

Brodifacoum 0.85 0.38 0.22 2.54 1.14 0.65 5.07 2.27 1.29

Difethialone 0.43 0.19 0.11 1.24 0.56 0.32 2.40 1.08 0.61

Warfarin (0.025%) 4.25 1.90 1.09 12.6 5.65 3.22 24.9 11.1 6.35

Warfarin (0.054%) 9.18 4.11 2.34 27.3 12.2 6.96 53.7 24.0 13.7

Chlorophacinone 0.85 0.38 0.22 2.50 1.12 0.64 4.86 2.18 1.24

Diphacinone 0.85 0.38 0.22 2.26 1.01 0.58 3.81 1.71 0.97

Bromethalina 1.70 0.76 0.43 4.53 2.03 1.16 -- -- --

*Currently registered chlorophacinone and diphacinone products for home-owner use are not labeled for use on rats. a

Given bromethalin’s rapid mode of action, RQs for this chemical were calculated based on a one day and three day accumulation 

scenario only.

Table 5-3. Calculated Rodenticide Daily Intake for Secondary Consumer Mammals Based on 

Consumption of House Mouse (mg a.i./kg-bwt/day)

Chemical

1 Day Accumulation in Prey 

by Secondary Consumer 

Weight Class

3 Day Accumulation in Prey 

by Secondary Consumer 

Weight Class

6 Day Accumulation in Prey 

by Secondary Consumer 

Weight Class

50 g 1000 g 3000 g 50 g 1000 g 3000 g 50 g 1000 g 3000 g

Brodifacoum 2.90 1.70 1.40 8.67 5.09 4.18 17.28 10.14 8.34

Difethialone 1.45 0.85 0.70 4.24 2.49 2.05 8.18 4.80 3.95

Warfarin (0.025%) 14.5 8.50 6.99 43.0 25.2 20.8 84.7 49.7 40.9

Warfarin (0.054%) 31.2 18.4 15.1 92.9 54.5 44.8 183 107 88.3

Chlorophacinone 2.90 1.70 1.40 8.52 5.00 4.11 16.6 9.71 7.99

Diphacinone 2.90 1.70 1.40 7.70 4.52 3.72 13.0 7.61 6.26

Bromethalina 5.79 3.40 2.79 15.4 9.05 7.45 -- -- -- a

Given bromethalin’s rapid mode of action, RQs for this chemical were calculated based on a one day and three day accumulation

scenario only.

Table 5-4. Calculated Rodenticide Daily Intake for Secondary Consumer Mammals Based on 

Consumption of Norway Rat (mg a.i/kg-bwt/day)

Chemical

1 Day Accumulation in Prey 

by Secondary Consumer 

Weight Class

3 Day Accumulation in Prey 

by Secondary Consumer 

Weight Class

6 Day Accumulation in Prey by 

Secondary Consumer Weight 

Class

50 g 1000 g 3000 g 50 g 1000 g 3000 g 50 g 1000 g 3000 g

Brodifacoum 0.77 0.45 0.37 2.29 1.35 1.11 4.57 2.68 2.21

Difethialone 0.38 0.22 0.18 1.12 0.66 0.54 2.17 1.27 1.05

Warfarin (0.025%) 3.83 2.25 1.85 11.4 6.68 5.49 22.4 13.2 10.8

Warfarin (0.054%) 8.28 4.86 3.99 24.6 14.4 11.9 48.4 28.4 23.4

Chlorophacinone 0.77 0.45 0.37 2.26 1.32 1.09 4.38 2.57 2.11
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Diphacinone 0.77 0.45 0.37 2.04 1.20 0.98 3.43 2.02 1.66

Bromethalina 1.53 0.90 0.74 4.08 2.40 1.97 -- -- --

*Currently registered chlorophacinone and diphacinone products for home-owner use are not labeled for use on rats. a

Given bromethalin’s rapid mode of action, RQs for this chemical were calculated based on a one day and three day 

accumulation scenario only.

5.1.1 Risk Calculation and Characterization

Weight-adjusted LD50 values for birds and mammals were used to calculate risk quotients. In 

keeping with EFED policy regarding scaling factors, weight-adjusted LD50 values were 

calculated using the standard adjustment for mammals discussed above, and scaling factors 

derived from Mineau et al. (1996) for birds. A chemical-specific scaling factor (slope = 0.7589) 

was only available for brodifacoum from Mineau et al. (1996), so this scaling factor was used to 

adjust the avian LD50 for that rodenticide. Avian LD50s for the other rodenticides were adjusted 

using the default scaling factor of 1.15. Mineau et al. (2001) contains additional data, with a 

recalculated avian scaling factor for brodifacoum (slope = -0.12) and an additional scaling factor 

for chlorophacinone (slope = -1.53). Avian LD50s for these rodenticides using the chemical 

specific scaling factors were included for consideration as part of Appendix C. Weight-adjusted 

LD50 values for birds and mammals were calculated as described in Section 3.1. Risk quotients 

were also calculated using unadjusted LD50 values for secondary mammals and birds. Results of 

these alternate calculations did not change the overall risk conclusions (Appendix C). 

Therefore, RQs were calculated by dividing the exposure estimates of rodenticide intake from 

Tables 5-1 to 5-4 for birds and mammals consuming house mice and Norway rats for each 

weight class by the most sensitive LD50 adjusted to the weights of the secondary animals as 

presented in Section 3. 

5.1.1.1 Acute Dose-based RQs for Secondary Birds

RQs for secondary birds consuming contaminated house mice are provided in Table 5-5. Based 

on this theoretical analysis, RQs calculated for secondary birds consuming house mice exposed 

to brodifacoum and difethialone exceed acute LOCs. No LOCs are exceeded for RQs calculated 

for warfarin, diphacinone, and chlorophacinone, except for small secondary birds consuming 

prey exposed to the higher concentration of warfarin (0.054%) bait for six days. Bromethalin 

shows varying results, where RQs exceeded acute LOC for small secondary birds consuming 

small prey exposed to bromethalin bait for one day, and all weight classes assessed for small 

prey exposed to bromethalin bait for three days. Given that prey exposed to bromethalin may 

stop feeding on the bait after consuming a lethal dose, the one day accumulation scenario is more 

likely than the three day accumulation scenario for this chemical. 
Page 92 of 183

Table 5-5. Secondary Bird Acute RQs Based on Consumption of Contaminated House Mouse

Chemical

1 Day Accumulation in Prey by 

Secondary Consumer Weight 

Class

3 Day Accumulation in Prey 

by Secondary Consumer 

Weight Class

6 Day Accumulation in Prey 

by Secondary Consumer 

Weight Class

100 g 1000 g 5000 g 100 g 1000 g 5000 g 100 g 1000 g 5000 g

Brodifacoum 6.60* 5.15* 4.33* 19.77* 15.42* 12.96 39.40* 30.74* 25.84*

Difethialone 5.48* 1.85* 0.83* 17.09* 5.42* 2.43* 32.98* 10.45* 4.68*

Warfarin (0.025%) 0.05 0.01 0.01 0.14 0.04 0.02 0.27 0.09 0.04

Warfarin (0.054%) 0.10 0.03 0.01 0.30 0.09 0.04 0.58* 0.19 0.08

Chlorophacinonea 0.01 <0.01 <0.01 0.04 0.01 0.01 0.08 0.02 0.01

Diphacinonea <0.01 <0.01 <0.01 0.01 < 0.01 < 0.01 0.01 < 0.01 < 0.01

Bromethalinb 1.52* 0.48 0.22 4.06* 1.29* 0.58* -- -- --

Bolded (*) RQs exceed acute risk LOC (0.5).

a

Current registered baits in the U.S. are for use on House Mouse only.

b

Given bromethalin’s rapid mode of action, RQs for this chemical were calculated based on a one day and three day accumulation 

scenario only.

RQs for secondary birds consuming contaminated Norway rats are provided in Table 5-6. For 

secondary birds consuming Norway rats exposed to brodifacoum, RQs exceed acute LOC for all 

scenarios. RQs for difethialone exceed for all scenarios, except medium and large birds

consuming rats exposed to difethialone bait for one day. For bromethalin, RQs only exceeded 

acute LOC for small predators/scavengers consuming prey animals exposed to bromethalin bait 

for three days. Again, available data suggest that secondary exposure to bromethalin is limited 

due to reduced feeding consumption that has been reported to occur after consumption of a lethal 

dose by primary consumers (Pitt et al., 2011). None of the RQs exceed the LOC for warfarin, 

chlorophacinone or diphacinone. 
Page 93 of 183

Table 5-6. Secondary Bird Acute RQs Based on Consumption of Contaminated Norway Rat

Chemical

1 Day Accumulation in Prey by 

Secondary Consumer Weight 

Class

3 Day Accumulation in Prey by 

Secondary Consumer Weight 

Class

6 Day Accumulation in Prey by 

Secondary Consumer Weight 

Class

100 g 1000 g 5000 g 100 g 1000 g 5000 g 100 g 1000 g 5000 g

Brodifacoum 1.75* 1.36* 1.15* 5.25* 4.08* 3.43* 10.43* 8.14* 6.84*

Difethialone 1.54* 0.49 0.22 4.52* 1.43* 0.64* 8.73* 2.77* 1.24*

Warfarin (0.025%) 0.01 <0.01 <0.01 0.04 0.01 0.01 0.07 0.02 0.01

Warfarin (0.054%) 0.03 0.01 <0.01 0.08 0.02 0.01 0.15 0.05 0.02

Chlorophacinonea <0.01 <0.01 <0.01 0.01 <0.01 <0.01 0.02 0.01 <0.01

Diphacinonea <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01

Bromethalinb 0.40 0.13 0.06 1.07* 0.34 0.15 -- -- --

Bolded (*) RQs exceed acute risk LOC (0.5).

a

Current registered baits in the U.S. are for use on House Mouse only.

b

Given bromethalin’s rapid mode of action, RQs for this chemical were calculated based on a one day and three day accumulation

scenario only.

5.1.1.2 Acute Dose-based RQs for Secondary Mammal

RQs for secondary mammals consuming contaminated house mice are provided in Table 5-7. 

For mammals consuming house mice exposed to rodenticide bait for three days, RQs for all 

rodenticides exceed the, except for mammals in all weight classes assessed consuming house 

mice exposed to chlorophacinone bait for one day. RQs for secondary mammals consuming 

contaminated Norway rats are provided in Table 5-8. RQs for brodifacoum, difethialone, and 

warfarin exceed the acute LOC in all scenarios, except for small mammals consuming rats 

exposed to difethialone bait for one day. This may be a result of the relatively small 

concentration of difethialone in bait (25 mg a.i./kg-bait) compared to the other chemicals (50 to 

540 mg a.i./kg-bait). Chlorophacinone shows varying results, where RQs only exceeded the 

acute LOC for medium and large predators/scavengers consuming Norway rats exposed to 

chlorophacinone bait for 6 days. RQs for diphacinone exceed acute LOC for mammals 

consuming rats exposed to diphacinone bait for three or six days, but not for rats exposed to 

diphacinone bait for one day. Bromethalin RQs exceed acute LOC in all assessed scenarios 

except for small mammals consuming rats exposed to bromethalin bait for one day.

Table 5-7. Secondary Mammal Acute RQs Based on Consumption of Contaminated House Mouse

Chemical

1 Day Accumulation in Prey 

by Secondary Consumer 

Weight Class

3 Day Accumulation in Prey 

by Secondary Consumer 

Weight Class

6 Day Accumulation in Prey 

by Secondary Consumer 

Weight Class

50 g 1000 g 3000 g 50 g 1000 g 3000 g 50 g 1000 g 3000 g

Brodifacoum 3.71* 4.60* 4.98* 11.10* 13.78* 14.91* 22.13* 27.46* 29.72*
Page 94 of 183

Difethialone 1.62* 2.01* 2.17* 4.74* 5.88* 6.37* 9.15* 11.35* 12.28*

Warfarin (0.025%) 2.97* 3.68* 3.99* 8.81* 10.93 11.83* 17.35* 21.53* 23.30*

Warfarin (0.054%) 6.41* 7.95* 8.61* 19.03* 23.61* 25.56* 37.48* 46.51* 50.33*

Chlorophacinonea 0.28 0.35 0.38 0.84* 1.04* 1.12* 1.63* 2.02* 2.18*

Diphacinonea 0.94* 1.16* 1.26* 2.49* 3.09* 3.35* 4.20* 5.21* 5.64*

Bromethalinb 1.62* 2.01* 2.17* 4.31* 5.35* 5.79* -- -- --

Bolded (*) RQs exceed acute risk LOC (0.5).

a

Current registered baits in the U.S. are for use on House Mouse only.

b

Given bromethalin’s rapid mode of action, RQs for this chemical were calculated based on a one day and three day 

accumulation scenario only.

Table 5-8. Secondary Mammal Acute RQs Based on Consumption of Contaminated Norway Rat

Chemical

1 Day Accumulation in Prey by 

Secondary Consumer Weight 

Class

3 Day Accumulation in Prey 

by Secondary Consumer 

Weight Class

6 Day Accumulation in Prey 

by Secondary Consumer 

Weight Class

50 g 1000 g 3000 g 50 g 1000 g 3000 g 50 g 1000 g 3000 g

Brodifacoum 0.98* 1.22* 1.32* 2.94* 3.65* 3.95* 5.86* 7.27* 7.87*

Difethialone 0.43 0.53* 0.58* 1.25* 1.56* 1.68* 2.42* 3.00* 3.25*

Warfarin (0.025%) 0.79* 0.97* 1.05* 2.33* 2.89* 3.13* 4.59* 5.70* 6.17

Warfarin (0.054%) 1.70* 2.11* 2.28* 5.04* 6.25* 6.76* 9.92* 12.31* 13.32*

Chlorophacinonea 0.08 0.09 0.10 0.22 0.27 0.30 0.43 0.53* 0.58*

Diphacinonea 0.25 0.31 0.33 0.66* 0.82* 0.89* 1.11* 1.38* 1.49*

Bromethalinb 0.43 0.53* 0.58* 1.14* 1.42* 1.53* -- -- --

Bolded (*) RQs exceed acute risk LOC (0.5).

a

Current registered baits in the U.S. are for use on House Mouse only.

b

Given bromethalin’s rapid mode of action, RQs for this chemical were calculated based on a one day and three day accumulation 

scenario only.

5.1.1.3 Acute Dietary-based RQs for Secondary Birds and Mammals

RQs were also calculated based on subacute dietary LC50 values and expected concentration of 

the rodenticides in prey items. Daily rodenticide intake values (mg a.i./kg-bw/day) for primary 

consumers based on their consumption of bait, as shown above in Section 3.3.1, were used as an 

estimate of the a.i. concentration in prey items after one day of feeding. This value can be 

considered as the dietary concentration (mg a.i./kg diet) for secondary consumers. RQs were 

calculated by dividing this value by the LC50s for birds and mammals, as listed above in Section 

3. RQs for secondary birds and mammals are presented in Tables 5-9 for birds and mammals. 

Based on a subacute dietary exposure of secondary birds exposed to the house mouse or Norway 

rat, RQs exceed the acute LOC for brodifacoum and difethialone, while RQs for warfarin, 

chlorophacinone, diphacinone, and bromethalin do not exceed the acute LOC. For secondary 
Page 95 of 183

mammals exposed to the house mouse or Norway rat, RQs exceed the acute LOC for all assessed 

rodenticides. Dietary data is not available for difethialone and bromethalin so these chemicals 

could not be included in this analysis. 

Table 5-9. Subacute Dietary RQs for Birds and Mammals Consuming Target 

Mammals Containing Rodenticides

Chemical

Birds Mammals

Consuming 

House Mouse

Consuming 

Norway Rat

Consuming 

House Mouse

Consuming 

Norway Rat

Brodifacoum 17.8* 4.71* 44.7* 11.2*

Difethialone 20.7* 5.48* ND ND

Warfarin (0.025%) 0.19 0.05 26.6* 7.05*

Warfarin (0.054%) 0.41 0.11 57.5* 15.2*

Chlorophacinone 0.32 0.09 20.42* 5.41*

Diphacinone 0.02 <0.01 10.1* 2.68*

Bromethalin 0.20 0.05 ND ND

ND = no data

Bolded (*) RQs exceed acute risk LOC (0.5).

5.1.1.4 Number of Target Mammals Sufficient to Reach LD50 of Secondary Consumers

In a final analysis, the number of individuals of the target species (i.e., house mouse and Norway 

rat) necessary for consumption of an LD50 dose in secondary birds and mammals was calculated. 

Two scenarios were developed to represent the low and high end of potential risk. In the first 

scenario, secondary birds and mammals consumed house mice and Norway rats exposed to 

rodenticide bait for one day. This scenario represents the low end of exposure and can be 

applied to all assessed rodenticides. In the second scenario, secondary birds and mammals 

consumed house mice and Norway rats exposed to rodenticide bait for six days. Since six days of 

accumulation is not likely for prey organisms feeding on bromethalin, the analysis for this 

chemical was based on a three day accumulation scenario. For this analysis the dose of the 

rodenticide in the secondary consumer was calculated after having consumed the prey exposed to 

bait for one, three or six days. This value was then compared to the weight-adjusted LD50 value 

for the secondary animals. 

Table 5-10 shows the number of house mice a secondary bird would need to consume in order to 

reach a median lethal dose for each assessed rodenticide. Depending on body weight and 

exposure scenario, secondary mammals could receive the LD50 dose of brodifacoum from 

consuming less than 1 up to 5 mice, while difethialone would require less than 1 up to 27 mice. 

Secondary birds exposed to bromethalin through contaminated prey would need to consume 
Page 96 of 183

anywhere from less than 1 up to 104 mice, depending on body weight and accumulation 

scenario, in order to reach the LD50. The range of warfarin contaminated mice needed to reach 

the LD50 dose in secondary birds is quite large, ranging from 3 to over 3400 depending on weight 

class assessed, warfarin concentration considered, and accumulation scenario. For 

chlorophacinone and diphacinone, secondary birds would need to consumer 23 to 12,000 and 

183 to 74,000 mice, respectively, in order to reach their respective LD50 values. 

Table 5-10. Number of House Mice Consumed to Reach a LD50 in Secondary Birds 

Chemical

1 Day Accumulation in Prey by Secondary 

Consumer Weight Class

6 Day Accumulation in Prey by 

Secondary Consumer Weight Class**

100 g 1000 g 5000 g 100 g 1000 g 5000 g

Brodifacoum < 1 2 5 < 1 < 1 < 1

Difethialone < 1 4 27 < 1 < 1 5

Warfarin (0.025%) 38 539 3429 7 92 586

Warfarin (0.054%) 18 249 1587 3 43 271

Chlorophacinone 130 1836 11690 23 321 2045

Diphacinone 821 11602 73852 183 2590 16483

Bromethalin 1 16 104 < 1 6 39

**For bromethalin, numbers are based on 3 day accumulation in prey.

Table 5-11 shows the number of Norway rats a secondary bird would need to consume in order 

to reach the LD50 value for each rodenticide. Rats were exposed to the various rodenticide baits 

for one, three or six days as described above. For brodifacoum, a secondary bird could receive a 

median lethal dose from consuming less than 1 rat, while difethialone would require 1 to 5 rats. 

Secondary birds exposed to bromethalin through contaminated prey would need to consume 

anywhere from less than 1 to 17 rats, depending on body weight and accumulation scenario, in 

order to reach the LD50. The range of warfarin contaminated rats needed to reach the LD50 in 

secondary birds is quite large, ranging from less than 1 to over 600 depending on weight class 

assessed, warfarin concentration considered, and accumulation scenario. For chlorophacinone, 

secondary birds would need to consume 4 to 2094 rats in order to reach the LD50. The range of 

diphacinone contaminated rats needed to reach the LD50 in secondary birds is quite large, 

ranging from 33 to over 13,000. 
Page 97 of 183

Table 5-11. Number of Norway Rats Consumed to Reach a LD50 in Secondary Birds

Chemical

1 Day Accumulation in Prey by Secondary 

Consumer Weight Class

6 Day Accumulation in Prey by 

Secondary Consumer Weight Class**

100 g 1000 g 5000 g 100 g 1000 g 5000 g

Brodifacoum < 1 < 1 < 1 < 1 < 1 < 1

Difethialone < 1 < 1 5 < 1 < 1 < 1

Warfarin (0.025%) 7 97 614 1 17 105

Warfarin (0.054%) 3 45 284 < 1 8 49

Chlorophacinone 23 329 2094 4 58 366

Diphacinone 147 2079 13232 33 464 2953

Bromethalin < 1 3 17 < 1 1 7

**For bromethalin, numbers are based on 3 day accumulation in prey.

Table 5-12 shows the number of house mice a secondary mammal would need to consume in 

order to reach the LD50. Mice were exposed to the various rodenticide baits for one, three or six 

days as described above. Depending on body weight and exposure scenario, secondary mammals 

could receive an LD50 dose of brodifacoum and warfarin from consuming less than 1 to 5 mice, 

while difethialone would require up to eleven mice. Predators/scavengers exposed to bromethalin 

through contaminated prey would need to consume anywhere from 1 to 11 mice, depending on 

body weight and accumulation scenario, in order to reach the LD50 dose. For diphacinone, the 

range of contaminated mice needed to reach the LD50 in secondary mammals is anywhere from 

less than 1 to 13, while chlorophacinone would require up to 60 mice depending on weight class 

assessed and accumulation scenario. 

Table 5-12. Number House Mice Consumed to Reach a LD50 in Secondary Mammals

Chemical

1 Day Accumulation in Prey by Secondary 

Consumer Weight Class

6 Day Accumulation in Prey by 

Secondary Consumer Weight Class**

50 g 1000 g 3000 g 50 g 1000 g 3000 g

Brodifacoum < 1 2 5 < 1 < 1 < 1

Difethialone < 1 5 11 < 1 < 1 2

Warfarin (0.025%) < 1 3 6 < 1 < 1 < 1

Warfarin (0.054%) < 1 1 3 < 1 < 1 < 1

Chlorophacinone 3 27 60 < 1 5 11

Diphacinone < 1 8 18 < 1 2 4

Bromethalin < 1 5 11 < 1 2 4

**For bromethalin, numbers are based on 3 day accumulation in prey.

Table 5-13 shows the number of Norway rats a secondary mammal would need to consume in 

order to reach the LD50. Rats were exposed to the various rodenticide baits for one, three or six 

days as described above. Based on this analysis, secondary mammals would be exposed to an 
Page 98 of 183

LD50 dose of brodifacoum, difethialone, warfarin, and bromethalin after consuming less than 1 

Norway rat, expect for large mammals exposed to 1 day of accumulation, which would require 

up to 2 rats. For diphacinone, the range contaminated rats needed to reach the LD50 in secondary 

mammals is anywhere from less than 1 to 3, while chlorophacinone would require up to 11 rats 

depending on weight class assessed and accumulation scenario. 

Table 5-13. Number Norway Rats Consumed to Reach a LD50 in Secondary Mammals

Chemical

1 Day Accumulation in Prey by Secondary 

Consumer Weight Class

6 Day Accumulation in Prey by 

Secondary Consumer Weight Class**

50 g 1000 g 3000 g 50 g 1000 g 3000 g

Brodifacoum < 1 < 1 < 1 < 1 < 1 < 1

Difethialone < 1 < 1 2 < 1 < 1 < 1

Warfarin (0.025%) < 1 < 1 1 < 1 < 1 < 1

Warfarin (0.054%) < 1 < 1 < 1 < 1 < 1 < 1

Chlorophacinone < 1 5 11 < 1 < 1 2

Diphacinone < 1 1 3 < 1 < 1 < 1

Bromethalin < 1 < 1 2 < 1 < 1 < 1

**For bromethalin, numbers are based on 3 day accumulation in prey.

5.2 Rodenticide Ingestion through Consumption of Contaminated 

Carcasses- Residue Analysis

The determination of rodenticide intake for individuals consuming rodenticide poisoned animals 

or carcasses can be calculated in a manner similar to the approach for individuals consuming bait 

directly (Section 4.1) when empirical residue data are available. Rodenticide whole-carcass 

residue data is available from open-literature studies for all chemicals considered in this white

paper (See Section 3.2).

In field and laboratory studies, rodenticide whole-carcass residue was determined in mammals 

after exposure to bait. Data are available for a variety of small and medium sized mammalian 

granivores and omnivores exposed to brodifacoum, difethialone, and warfarin. No data were 

identified for bromethalin or other exposed taxonomic groups (e.g., birds, reptiles, and 

invertebrates). For all studies, it was assumed the concentrations were reported using wet 

weights of the mammals. 

5.2.1 Measured Residues of Rodenticides in Contaminated Carcasses

The maximum mean residue values in rats for brodifacoum and difethialone, 6.98 mg a.i./kg-bw 

(calculated as the weighted average of means reported in Howald, 1997) and 3.1 mg a.i./kg-bw 
Page 99 of 183

(Savarie, 2005) respectively, are used for this assessment as they represents conservative 

measured values based on the available data (see Section 3.2 for more information on available 

residue data). These residue values are greater than the calculated residue for rats exposed to 

brodifacoum or difethialone bait for one day (2.1 mg a.i./kg-bw and 1.0 mg a.i/kg-bw, 

respectively). In fact, these values are almost the same as the calculated residue for rats exposed 

to brodifacoum or difethialone for three days (6.3 mg a.i/kg-bw and 3.1 mg a.i./kg-bw). 

For warfarin, the mean residue value in mice of 2.95 mg a.i./kg-carcass is used because the bait 

concentration used in the study (Townsend et al., 1984) approximates the concentration found in 

registered warfarin baits for commensal uses. This value is an order of magnitude less than the 

calculated residue for mice exposed to the high or low concentration warfarin bait for one day 

(85.5 mg high concentration a.i./kg-bw and 39.6 mg low concentration a.i./kg-bw). This 

difference may be a function of the variable elimination half-life for this chemical based on 

species discussed previously. 

The maximum reported mean value for chlorophacinone, 1.58 mg a.i./kg-bw (Primus et al. 

2001), is for the California vole. This value is used for this assessment as it represents the 

conservative measured value based on the available data. Although this value is less than the 

calculated residue for mice exposed to chlorophacinone bait for one day (7.9 mg a.i./kg-bw), it is 

within a factor of 6 of the calculated value. This difference may be a function of the relatively 

rapid elimination half-life for this chemical, uncertainty in exposures that occurred in the field 

study relative to assumptions in the theoretical calculations, or species differences (field study 

for vole versus theoretical calculations mice). 

The maximum reported mean residue value for diphacinone, 1.4 mg a.i./kg-bw (Primus et al. 

2001), is for the California ground squirrel. This value, which is similar to the calculated residue 

for rats exposed to diphacinone bait for one day (2.1 mg a.i./kg-bw) is used for this assessment 

because it represents the conservative measured value based on available data. The slight 

differences in residue values may be a function of the relatively rapid elimination half-life for 

this chemical, uncertainly in exposures that occurred in the field study relative to assumptions in 

the theoretical calculations, or species differences (field study for ground squirrel versus 

theoretical calculations for rats.)

Table 5-14. Measured Mean Whole-Carcass Residue and Estimated Body Burden After 1 

Day Rodenticide Accumulation in Primary Consumers 

Chemical

Empirical Residue Levels Theoretical Residue Levels

Mg a.i./kg 

bait Species

Mean wholecarcass residue 

(mg a.i./kg-bw)

Mg a.i./kg 

bait Species

Estimated Body 

Burden (mg 

a.i./kg-bw)

Brodifacoum 50 Rat 6.98b 50 Norway Rat 2.1
Page 100 of 183

Difethialone 25 Rat 3.1 25 Norway Rat 1.0

Warfarin 200a Mouse 2.95 250 House 

Mouse 36.9

Chlorophacinone 50 Vole 1.58 50 House 

Mouse 7.9

Diphacinone 50 Ground 

Squirrel 1.4 50 Norway Rat 2.1

a baits registered in the U.S. are 0.025% a.i.

b Value calculated as the weighted average of the means reported in Howald 1997

Rodenticide exposure through carcass consumption is calculated as mg a.i./kg-bw, where kg-bw 

is the kilograms of the consuming individual for three weight classes of birds and mammals. This 

analysis used the same weight classes for secondary birds and mammals, calculated food intake, 

and weight-adjusted LD50 values as the theoretical analysis described in Section 5.1 above. 

However, the calculation for daily rodenticide intake used the residue values selected for this 

analysis (concentration in whole prey animal) instead of the concentration of bait. 

5.2.2 Risk Calculation and Characterization for Consumption of Contaminated Carcasses

The acute LOC was exceeded for secondary birds for brodifacoum and difethialone; however, 

RQs for secondary birds exposed to warfarin did not exceed the acute LOC. RQs for secondary 

birds exposed to chlorophacinone or diphacinone on an acute basis did not exceeded the acute 

LOC (Table 5-15). If the maximum individual residue value for each rodenticide was used to 

calculate risk quotients instead of the mean value, LOC exceedances for secondary birds would 

not change.

Carnivore/scavenger mammalian RQs for brodifacoum and difethialone exceeded the acute 

LOC. For warfarin, chlorophacinone, and diphacinone, RQs did not exceed the acute LOC 

(Table 5-15). Although the use of the maximum individual residue value would increase RQs, 

LOC exceedances for secondary mammals would not change with the exception of small 

mammals exposed to chlorophacinone and diphacinone.
Page 101 of 183

Table 5-15. Secondary Acute RQs based on a Single-dose of Rodenticide through 

Consumption of Contaminated Carcasses

Predator Birds Mammals

Predator 

Weight Class

100 g 1000 g 5000 g 50 g 1000 g 3000 g

Brodifacoum 6.42* 5.01* 4.21* 3.59* 4.46* 4.83*

Difethialone 4.64* 1.47* 0.66* 1.41* 1.75* 1.89*

Warfarin < 0.01 < 0.01 < 0.01 0.26 0.32 0.34

Chlorophacinone < 0.01 < 0.01 < 0.01 0.07 0.08 0.08

Diphacinone < 0.01 < 0.01 < 0.01 0.19 0.23 0.25

Bolded (*) RQs exceed acute risk LOC (0.5).

For toxic response elicited from the dietary exposure route extended over several days, exposure 

is measured as mg a.i./kg-diet. Toxicity is measured by the LC50 obtained from the dietary 

studies (5 days on treated diet) for birds and mammals. Avian RQs based on dietary studies are 

provided in Table 5-16.Carnivore/scavenger bird RQs for brodifacoum and difethialone 

exceeded Acute Risk LOCs. Warfarin RQs for carnivore/scavenger birds did not exceed any 

LOCs. RQs for birds exposed to chlorophacinone and diphacinone in their diet do not exceed 

Acute Risk LOCs. 

Mammalian RQs based on dietary studies are provided in Table 5-16. RQs for brodifacoum and 

warfarin exceed the acute risk LOC. There are no available mammalian dietary data for 

difethialone or bromethalin. For the alternative rodenticides with conforming labels, mammalian 

RQs for chlorophacinone and diphacinone exceeded the acute LOC. 

Table 5-16. Secondary Dietary RQs based on a 5-day Exposure of 

Rodenticide through Consumption of Contaminated Carcasses

Birds Mammals

Brodifacoum 5.25* 13.17*

Difethialone 5.54* ND

Warfarin < 0.01 0.67*

Chlorophacinone 0.02 1.39*

Diphacinone < 0.01 ND

ND = no data

Bolded (*) RQs exceed acute risk LOC (0.5).
Page 102 of 183

5.3 Secondary Exposure Studies

Toxicity studies are available for several species of birds and mammals that represent potential 

secondary consumers of rodenticides. In these studies, small mammals were fed diets containing 

one of the rodenticides, and these animals died or were sacrificed. These contaminated carcasses 

were fed to the birds and mammals (i.e., secondary consumers) being evaluated in the studies. 

Though these types of secondary exposure studies provide information on the potential effects 

from rodenticide exposure that may occur under field conditions, it is often not possible to 

quantify the level of exposure (rodenticide residues in primary consumers or food consumption 

by secondary consumers) or the sensitivity of the exposed animals to the rodenticide. 

Birds

In the studies cited, raptors or avian scavengers were exposed to rodenticide in whole or ground 

carcasses, which were usually those of rats or mice, or in fortified meat. Most prey animals were 

fed treated bait, although some were dosed orally. Most studies involved only 1 rodenticide but 

often more than 1 raptor or scavenger species was tested. Mortality was included as a 

measurement endpoint in all studies. Some studies also reported signs of toxicosis (e.g.,

bleeding, prolonged blood-coagulation time, abnormal behavior, or regurgitation) in surviving 

test animals. Although exposure scenarios, test species, and the number of test animals varied 

among the studies, collectively these studies provide useful information to characterize 

secondary hazards from short-term exposure. The studies are summarized and tabulated below. 

Brodifacoum was tested in 12 studies involving 8 species of birds. Of 149 individuals exposed to 

brodifacoum-poisoned prey (at various doses), 63 (42%) individuals died (Table 5-17). 

Mortality occurred in 11 of 20 barn owls (Tyto alba), 6 of 6 red-tailed hawks (Buteo jamaicensis) 

and red-shouldered hawks (Buteo lineatus), 13 of 65 American kestrels (Falco sparverius), 1 of 

4 Eurasian harriers (Circus pygargus), and 32 of 50 laughing gulls (Larus atricilla). No deaths 

occurred in 4 golden eagles (Aquila chrysactos) tested by Marsh and Howard (1978), but 3 bled 

externally. Some studies did not report whether signs of toxicosis were observed in surviving 

birds. In those studies that examined survivors for signs of toxicosis, such as external bleeding, 

internal hemorrhaging, and/or prolonged blood-coagulation time, about one-third of the survivors 

visually examined or necropsied exhibited symptoms of toxicity. 

There are no secondary studies involving exposures of birds to difethialone or bromethalin. 

The four studies available for warfarin indicate that this chemical causes low mortality rates in 

secondary birds exposed to mammals that consumed 50 or 500 mg a.i. kg-bait. In the warfarin 

studies, 2 (5%) of 23 individuals birds died (Table 5-18); no adverse signs were reported in the 

survivors. 
Page 103 of 183

No mortality occurred in 8 chlorophacinone studies that, in total, included 112 individual birds 

from 9 species (Table 5-19). Birds tested included 28 carrion crows (Corvus corone), 20 

Eurasian buzzards, 20 American kestrels, 20 black-billed magpies, 10 red-tailed hawks, 6 white 

storks (Ciconia ciconia), 4 tawny owls (Strix aluco), 2 barn owls, and 2 great horned owls. Some 

survivors showed signs of intoxications, mostly prolonged blood-coagulation time. 

About 9% mortality was recorded in 3 diphacinone studies that involved 34 individual birds from 

5 species (Table 5-20). Test species were barn owls, great horned owls, saw-whet owls 

(Aegolius acadicus), golden eagles, and American crows (Corvus brachyrhynchos). Thirteen 

(42%) of the survivors displayed some signs of toxicity. 
Page 104 of 183

Table 5-17. Observed Effects of Brodifacoum on Birds Consuming Contaminated Carcasses

Secondary 

species Prey offered to birds No. prey offered daily per bird

No. days 

birds 

exposed

No. birds 

exposed

No. birds 

dead

No. survivors with 

signs of brodifacoum 

toxicitya

Reference

(MRID)

Barn owl

Rats fed choice of 0.002% 

baitb and untreated food for 

5 days

1-2

1

3

6

10

1

2

1

2

0

2

1

2

0

No survivors

No survivors

No survivors

Mendenhall and Pank 

1980

(MRID 46750931

40077202

00094694)

Barn owl Mice fed 0.002% baitb for 

1 day

3

2

2

1

3

6

6

2c

2c

4

0

0

nr

nr

2 (eb/ct)

Newton et al. 1990

and Wyllie 1995e

Barn owl Mice fed 0.005% bait for 

1-2 days

Enough to provide 

50-220 μg a.i. per 

day

15 4 1 3 (eb/ih) Gray et al. 1994e

Barn owl Rats fed 0.005% bait 4 total 5-7 4 1 0 Lee 1994e

Red-tailed 

hawk

Rats fed 0.005% bait for 3 

days limitedd 4 4 4 No survivors

Marsh and Howard 

1978

(MRID # 00080243)

Redshouldered 

hawk

Mice fed 0.005% bait for 3 

days limitedd 4 2 2 No survivors

Golden eagle Rats fed 0.005% bait for 3 

days limitedd 4 4 0 3 (eb)

American 

kestrel

Voles fed 0.005% bait for 3 

days

1

1

2

6

10

10

0

4 (ct)

Savarie and LaVoie 

1979

(MRID 80244)

American 

kestrel

Ground 

vole tissue 

0.3 mg a.i./kg

0.8 mg a.i./kg

1.6 mg a.i./kg

3.2 mg a.i./kg

6.0 mg a.i./kg

ad libitum

5

5

5

5

5

8

8

8

8

8

0

1

0

0

4

nr

nr

nr

nr

nr

LaVoie 1990e

Eurasian 

buzzard Mice fed 0.005% bait 5 6 5 4 1 (bl) Lutz 1987e
Page 105 of 183

Australasian 

harrier

Rabbit dosed at 6.5 mg 

a.i./kg 1 1 4 1 nr Godfrey 1985e

Laughing gull

Ground, 

spiked rat 

tissues at 

0.72 mg a.i./kg

1.62 mg a.i./kg

3.41 mg a.i./kg

7.26 mg a.i./kg

14.0 mg a.i./kg

ad libitum

5

5

5

5

5

5

5

5

5

5

3

5

5

5

5

0

No survivors

No survivors

No survivors

No survivors

ICI Americas, Inc 

1979a

(MRID 80236)

Laughing gull

Ground, 

spiked rat 

tissues at 

0.13 mg a.i./kg

0.34 mg a.i./kg

0.84 mg a.i./kg

2.10 mg a.i./kg

5.26 mg a.i./kg

ad libitum

5

5

5

5

5

5

5

5

5

5

0

1

0

4

4

0

0

1 (eb)

0

0

ICI Americas, Inc 

1979b

(MRID 80236)

a eb = external bleeding; ih = internal hematoma; bl = bleeding (unspecified); ct = increased blood coagulation time; nr = not reported b registered baits are 0.005% a.i.

c the 2 owls that survived the initial 1-day exposure were subsequently re-exposed for 3 days and again for 6 days; the owls were allowed to recover for 75 to 79 

days between exposure periods

d the amount of food offered to the raptors was “limited” to prevent overindulgence on any given day

e Cited in US EPA (2004) Potential Risks of Nine Rodenticides to Birds and Nontarget Mammals: a Comparative Approach
Page 106 of 183

Table 5-18. Observed Effects of Warfarin on Birds Consuming Contaminated Carcasses

Secondary 

species Prey offered to birds No. prey offered daily per bird

No. days birds 

exposed

No. birds 

exposed

No. birds 

dead

No. survivors 

with signs of 

warfarin 

toxicitya

Reference

(MRID)

Tawny owl Mice fed bait for 3 

days 1 every other day 9028

4

2c

0

0

0

0

Townsend et al. 

1981

Black-billed 

magpie

Rats fed 0.05% baitc

for 4-7 days Ad libitum 5 14 0 0 Mach 1997

Barn owl Rats fed 0.005% baitc 4 total 5-7 4 2 nr Lee 1994b

Eurasian 

buzzard Rat/mouse Ad libitum 18 1 0 nr Telle 1955b

a eb = external bleeding; ih = internal hematoma; bl = bleeding (unspecified); ct = increased blood coagulation time; nr = not reported b Cited in US EPA (2004) Potential Risks of Nine Rodenticides to Birds and Nontarget Mammals: a Comparative Approach

c rat and mouse registered baits in the U.S. are 0.025% a.i. and 0.054% a.i

Table 5-19. Observed Effects of Chlorophacinone on Birds Consuming Contaminated Carcasses

Secondary 

species Prey offered to birds No. prey offered daily per bird

No. days birds 

exposed

No. birds 

exposed

No. birds 

dead

No. survivors 

with signs of 

chlorophacinone 

toxicitya

Reference

(MRID)

Barn owl

Rats fed choice of 

0.005% bait or 

untreated bait for 5 

days

1-2 10 2 0 0

Mendenhall and 

Pank 1980

MRID # 

46750931

Black-billed 

magpie

Rats fed 0.005% bait 

for 5 days Ad libitum 5 20 0 0

Baroch 1997

MRID#

44631402

American 

kestrel

Voles fed 0.01% bait 

until dead

1

1 every 3 days

21

61

10

10

0

0

10 (eb/ih)

10 (eb/hi)

Radvanyi et al. 

1988

E39765
Page 107 of 183

Red-tailed 

hawk

Voles fed 10 g 

0.0005% bait daily for 

up to 9 days

2 6 5 0 0 Askham 1988

E73126

Askham and 

Poché 1992 Great horned 

owl

Voles fed 10 g 0.005% 

bait daily for up to 9 

days

2 6 1 0 0

Tawny owl Mice fed 0.0075% 

baitb Ad libitum 10 4 0 (ct)

Riedel et al. 

1991c

Eurasian 

buzzard

Mice fed 0.0075% 

baitb Ad libitum

7

10

5-5-5d

40

4

6

3

3

0

0

0

0

(ct)

(ct)

(ct)

(ct)

Carrion crow Mice fed 0.0075% 

baitb Ad libitum 10 4 0 (ct)

Eurasian 

buzzard

Mice fed 0.0075% 

baitb 4 7 4 0 0 Anonymous 1978c

Carrion crow Mice fed 0.0075% 

baitb 3-4 35

12

12

0

0

0

0 Sterner 1978c

White stork Mice fed 0.0075% 

baitb

Ad libitum 

(treated/untreated)

3

14

3

3

0

0

1 or 2 (ct)

1 or 2 (ct) a eb = external bleeding; ih = internal hematoma; bl = bleeding (unspecified); ct = increased blood coagulation time; nr = not reported b baits registered in the U.S. are either 0.005% or 0.01% a.i.

c Cited in US EPA (2004) Potential Risks of Nine Rodenticides to Birds and Nontarget Mammals: a Comparative Approach and Joermann 1998 d the 3 5-day treatment periods are separated by 3 days when the birds were fed untreated mice
Page 108 of 183

Table 5-20. Observed Effects of Diphacinone on Birds Consuming Contaminated Carcasses

Secondary 

species Prey offered to birds No. prey offered daily per bird

No. days birds 

exposed

No. birds 

exposed

No. birds 

dead

No. survivors 

with signs of 

diphacinone 

toxicitya

Reference

(MRID)

Great horned 

owl

Mice fed choice of 

0.01% bait or 

untreated food for 10 

days

2 5 3 2 1 (ct)

Mendenhall and 

Pank 1980

MRID # 

46750931

Saw-whet owl

Mice fed choice of 

0.01% bait or 

untreated food for 10 

days

2 5 1 1 No survivors

Mendenhall and 

Pank 1980

MRID # 

46750931

Barn owl

Mice fed choice of 

0.01% bait or 

untreated food for 10 

days

Ad libitum 10 2 0 0

Mendenhall and 

Pank 1980

MRID # 

46750931

American 

crow

Rats red 0.005% bait 

until death

1

1-2b

1

6

10

11

0

0

0

5 (eb/ct)

Massey et al. 

1997

Golden eagle Meat laced at 2.7 mg 

a.i./kg a.i. 454 g 5

10

4

3

0

0

4 (eb/ct)

3 (eb/ct)c

Savarie et al. 

1979

MRID# 

00079312 a eb = external bleeding; ih = internal hematoma; bl = bleeding (unspecified); ct = increased blood coagulation time; nr = not reported b offered 1 rat per crow for 5 days and 2 rats per crow on day 6

c general weakness of all eagles was observed after 5 days
Page 109 of 183

Mammals

For the rodenticides considered in this assessment, 12 studies were found in which mammalian 

predators or scavengers were exposed to a rodenticide in whole or ground carcasses, usually rats 

or mice, or in spiked meat. 

Mortality of 8 (42%) of 19 individuals (foxes, mustelids, domestic dogs) occurred in 4 

brodifacoum studies. Test subjects included 5 red foxes (Vulpes vulpes) and gray foxes (Urocyon 

cinereoargenteus), 4 mongooses (Herpestes auropunctatus), 4 weasels (Mustela sp.) and 6 

domestic dogs. Signs of toxicity were reported for most survivors (Table 5-21).

There are no secondary studies on the toxicity of difethialone to mammals. 

In 7 studies involving warfarin, only 9 (9%) of 100 individuals died after eating warfarin-treated 

rodents (Table 5-22). Dead animals included 3 mink, 3 least weasels (Mustela nivalis), and 3 

dogs. 

Laboratory studies with chlorophacinone and diphacinone indicate that these chemicals present a 

hazard to mammal predators and scavengers. In 8 chlorophacinone studies, 32 (58%) of 55 

individuals died, including 7 of 8 mongooses, 3 of 7 coyotes, 1 of 4 red foxes, 18 of 35 ferrets, 

and 3 of 4 weasels (Table 5-23). In 3 diphacinone studies, 19 (58%) of 33 test animals died after 

feeding on rodents fed diphacinone, liver tissue from owls fed diphacinone, or fortified meat. 

Species affected included mink (Mustela vison), mongooses, ermine, deer mice, rats, and dogs 

(Table 5-24). 

In one study with bromethalin, 4 dogs survived with no observed adverse effects after feeding for 

14 days on rats that were poisoned for 1 day (Table 5-25).
Page 110 of 183

Table 5-21. Observed Effects of Brodifacoum on Mammals Consuming Contaminated Carcasses

Secondary 

species

Prey offered to 

mammals

No. prey offered 

daily per 

mammal

No. days 

mammals 

exposed

No.

mammals 

exposed

No. 

mammals 

dead

No. survivors 

with signs of 

brodifacoum 

toxicitya

Reference

(MRID)

Red fox and 

Gray fox

Rats dosed at 15 mg 

a.i./kgc 400 g

1

3

4

2

1

2

0

1

1

2 (eb/ih)

No survivors

1 (eh/ih)

ICI Americas, 

Inc 1978a

Mongoose Rats fed 0.002% baitc

for 5 days 1

1

3

6

10

1

1

1

1

0

1

0

0

nr

No survivors

nr

nr

Pank and Hirata 

1976

Weasel Mice fed 0.002% baitc Ad libitum 16-25 4 4 No survivors Anonymous 

1981b

Dog 

(domestic)

Rats dosed at 15 mg 

a.i./kgc 650 g 1-4 6 1 4 (eb/ih) ICI America, Inc 

1978a a eb = external bleeding; ih = internal hematoma; bl = bleeding (unspecified); ct = increased blood coagulation time; nr = not reported b Cited in US EPA (2004) Potential Risks of Nine Rodenticides to Birds and Nontarget Mammals: a Comparative Approach

c rat and mouse registered baits in the U.S. are 0.005% a.i.

Table 5-22. Observed Effects of Warfarin on Mammals Consuming Contaminated Carcasses

Secondary 

species

Prey offered to 

mammals

No. prey offered 

daily per 

mammal

No. days 

mammals 

exposed

No. 

mammals 

exposed

No. 

mammals 

dead

No. survivors 

with signs of 

warfarin 

toxicitya

Reference

(MRID)

Mink Nutria fed 0.025% bait 

for at least 7 days Ad libitum 8-15 3 3 No survivors Evans and Ward 1967b

Mink

Rabbits fed 25 or 67 

mg a.i./kg baitc for 5 

weeks

Ad libitum 28 50 0 0 Aulerich et al 

1987
Page 111 of 183

Least weasel

Mice fed 0.001% bait,

0.005% bait,

Or 0.02% bait for 3 

daysc

Ad libitum

90

29-90

12-57

2

2

2

0

1

2

2 (ct)

1 (ct)

No survivors

Townsend et al. 

1984

European 

ferret

Prairie dogs fed 0.05% 

baitc for 15 days 1 7 10 0 0

Carlet and Mach 

1997

MRID# 

44995003

European 

ferret

Prairie dogs fed 0.05% 

baitc for 15 days Ad libitum 5 10 0 0

Mach 1998

MRID# 

44820001

Raccoon Rats fed 0.025% bait 

for 5 days

1

3

5

5

8

10d

0

0

0

0 EPA 1982

Dog 

(domestic)

Nutria fed 0.025% bait 

for at least 7 days Ad libitum 8-16 3 1 2 (eb/ct) Evans and Ward 1967b

Dog 

(domestic)

Mice fed 0.025% bait,

0.05% bait,

Mice dosed with 2.5 

mg a.i.,

10 mg a.i.,

40 mg a.i.

4-10

10

1

1

1

56

56

56

25

17

4

1

1

1

1

0

0

0

1

1

0

0

0

No survivors

No survivors

Prier and Derse 

1962b

a eb = external bleeding; ih = internal hematoma; bl = bleeding (unspecified); ct = increased blood coagulation time; nr = not reported b Cited in US EPA (2004) Potential Risks of Nine Rodenticides to Birds and Nontarget Mammals: a Comparative Approach

c rat and mouse registered baits in the U.S. are 0.025% a.i. d the 10 test animals included the 8 individuals from the first trial plus 2 additional untested individuals

Table 5-23. Observed Effects of Chlorophacinone on Mammals Consuming Contaminated Carcasses

Secondary 

species

Prey offered to 

mammals

No. prey offered 

daily per 

mammal

No. days 

mammals 

exposed

No. 

mammals 

exposed

No. 

mammals 

dead

No. survivors 

with signs of 

chlorophacinone 

toxicitya

Reference

(MRID)
Page 112 of 183

Mongoose Rats fed 0.005% bait 

for 5 days 1

1

3

5

6

7

9

10

1

1

2

1

1

1

1

0

1

2

1

1

1

1

nr

no survivors

no survivors

no survivors

no survivors

no survivors

no survivors

Pank and Hirata 

1976

MRID# 2467

Coyote

Ground squirrels fed 

15 g of 0.01% bait for 

6 days

1 5 7 3 0

Marsh and 

Howard 1986

MRID# 

42760902

European 

ferret

Rats fed 0.005% bait 

for 5 days Ad libitum 5 20 11 nr

Ahmed et al. 

1996

MRID# 

44631401

European 

ferret

Prairie dogs fed 25 g 

of 0.0025% bait daily 

for 6 daysc

4 (1 every other 

day) 8 6 5 nr

Fisher and Timm 

1987

European 

ferret

Voles/mice fed 

0.0075% baitc 5 total 4 2 1d (ct) Bachhuber and 

Beck 1988f

Red fox Mice fed 0.0075% 

baitc 20 total 4 1 1e no survivors

European 

ferret

Muskrats fed 0.0075% 

baitc Ad libitum 4

8

2

1

0

1

1 (bl)

no survivors Jobsen 1798f

European 

ferret

Voles fed 0.0075% 

baitc Ad libitum 3 4 0 (ct) Anonymous 1983f

Weasel Mice fed 0.005% bait At libitum 90 4 3 0 Anonymous 

1981f
Page 113 of 183

a eb = external bleeding; ih = internal hematoma; bl = bleeding (unspecified); ct = increased blood coagulation time; nr = not reported b ground squirrels were fed no-choice for 3 days followed by 3 days in which they had a choice of bait or untreated laboratory chow c baits registered in the U.S. are either 0.005% or 0.01% a.i.

d individual recovered from moribund state after administration of antidote, but assumed ‘dead’ without antidote treatment

e individual was sacrificed but considered ‘dead’ based on coagulation index

f Cited in US EPA (2004) Potential Risks of Nine Rodenticides to Birds and Nontarget Mammals: a Comparative Approach and Joermann 1998

Table 5-24. Observed Effects of Diphacinone on Mammals Consuming Contaminated Carcasses

Secondary 

species

Prey offered to 

mammals

No. prey offered 

daily per 

mammal

No. days 

mammals 

exposed

No. 

mammals 

exposed

No. 

mammals 

dead

No. survivors 

with signs of 

diphacinone 

toxicitya

Reference

(MRID)

Mink

Nutria fed 0.01% 

carrot bait for up to 10 

days

Ad libitum 5-18 3 3 no survivors Evans and ward 

1967 b

Mongoose Rats fed 0.005% bait 

for 5 days 1

1

3

5

6

7

8

10

1

1

2

1

1

1

1

0

1

2

1

1

1

1

nr

no survivors

no survivors

no survivors

no survivors

no survivors

no survivors Pank and Hirata 

1976

Ermine MRID# 2467 Deer mice fed 0.01% 

bait for 10 days 2 5 2 1 nr

Striped skunk Deer mice fed 0.01% 

bait for 10 days 2 5 5 0 nr

Deer mouse

Liver from 

diphacinone poisoned 

owls

1 g daily 7 4 1 3 (ct)

Rat Meat containing 0.5 

mg a.i./kg Ad libitum 6 8 4 nr

Savarie et al. 

1979
Page 114 of 183

Dog 

(domestic)

Nutria fed 0.01% 

carrot bait for up to 10 

days

Ad libitum 6-10 3 3 no survivors Evans and Ward 

1967b

a eb = external bleeding; ih = internal hematoma; bl = bleeding (unspecified); ct = increased blood coagulation time; nr = not reported b Cited in US EPA (2004) Potential Risks of Nine Rodenticides to Birds and Nontarget Mammals: a Comparative Approach

Table 5-25. Observed Effects of Bromethalin on Mammals Consuming Contaminated Carcasses

Secondary 

species

Prey offered to 

mammals

No. prey offered 

daily per 

mammal

No. days 

mammals 

exposed

No. 

mammals 

exposed

No. 

mammals 

dead

No. survivors 

with signs of 

bromethalin 

toxicitya

Reference

(MRID)

Dog 

(domestic)

Ground meat from rats 

fed 0.005% a.i. bait for 

1 day

600 g 14 4 0 0 Van Lier 1981
Page 115 of 183

5.4 Uncertainties

In addition to the uncertainties discussed in the primary risk characterization section (Section 4 

of this document), the following uncertainties apply to the analysis of secondary risk:

Accumulation of dose: Calculations used to estimate accumulation of rodenticides in target 

animals are dependent on available toxicity data and the elimination potential for each chemical. 

Though this analysis assessed accumulation in both the house mouse and Norway rat, the LD50 

values used are from laboratory rat studies. These species may have different sensitivities to the 

various rodenticides. However, EFED does not anticipate the sensitivity ranges for these species 

are greatly different given that the rodenticides are formulated to kill both mice and rats and 

available information suggests that target animals accumulate internal doses of rodenticides 

beyond the median lethal dose. This analysis also made the conservative assumptions that prey 

animals consumed 100% of their diet as bait for the duration of the accumulation period (1 to 6 

days) and that100% of residues in the prey is available to the consumer. To determine how 

conservative calculated body burdens in target animals are and to provide a lower end secondary 

exposure estimate, an empirical analysis on whole body residues in prey was conducted. Risks 

were similar when considering both the estimated and empirical residues, indicating that the 

conservative assumptions in the estimated residue levels did not impact overall risk 

characterization. 

In addition, elimination half-lives used to assess the accumulation of rodenticides in target 

animals are based on liver half-lives, where available, which is a conservative surrogate value for 

what is likely a multi compartment process. However, the conservative nature of these liver halflife values are potentially counterbalanced by EFED’s use of first order elimination equations, 

which likely underestimate elimination half-lives that are generally biphasic in nature. In 

addition, LD50 values used to calculate RQs are based on external doses and as such do not 

account for absorption either. In order to allow for comparisons of half-lives among chemicals, 

liver half-lives for brodifacoum, difethialone, warfarin, and chlorophacinone from the same 

study by Vandenbroucke et al. (2011) were selected. Although this approach reduces variability 

due to differences attributed to laboratories, it is not necessarily the most conservative approach 

that could be undertaken. For instance, the liver half-life value for difethialone that was reported 

by Vandenbroucke (28 days) is lower than provided in other studies (74 days from Belleville, 

1991). This difference does not impact the ultimate risk characterization for difethialone, but it 

may mean that the potential for risk has been underestimated for this chemical, especially in the 

number of target animals that is required to reach an LD50 dose in secondary consumers. Though 

a liver half-life value is available for diphacinone, it is based on pig data, which differs from the 

organisms used in the other studies. There is some indication from the available half-life studies 

that species react to anticoagulant rodenticides differently. Therefore, this pig value may 
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overestimate or underestimate the liver half-life of rats for this chemical. For bromethalin, there 

is no available data on the liver half-life for bromethalin, so a blood plasma value is used instead. 

It is uncertain how a blood plasma half-live would differ from the liver half-life for this 

chemical. 

Secondary toxicity data: For this assessment, EFED used standard bird and mammal test species 

to estimate weight-adjusted LD50 values for secondary animals. This approach was employed 

because acceptable secondary toxicity studies were not available for all of the assessed 

rodenticides and use of the standard test species, which were common among all of the 

rodenticides, allowed for comparisons of results among chemicals. However, this method may 

not be conservative. As discussed in the toxicity section above, for diphacinone, an LD50 of 96.8 

mg a.i/kg-bwt is available for one raptor species (American kestrel, Falco sparverius) (Rattner et 

al. 2011). This study suggests that American kestrels may be more sensitive than the standard 

test species (i.e., based on the available studies, 17 to 21 times more sensitive than bobwhite 

quail and 33 times more sensitive than mallard ducks). None of the other evaluated rodenticides 

have data for which an LD50 for raptors, or, more generally, predatory birds is available. If the 

sensitivity of the American kestrel to diphacinone is representative of other species of raptors, 

this information suggests that raptors may be more sensitive to diphacinone than either bobwhite 

quail or mallard duck. The potential for increased sensitivity of raptors to warfarin was observed 

by Watanabe et al (2010), who demonstrated that hydroxylation rates of warfarin in the great 

horned owl (Bubo virginianus) and snowy owl (Bubo scandiacus) were much less than that 

observed in mallards, chickens (Gallus gallus), and laboratory rats. This study suggests that 

raptors may metabolize and detoxify warfarin at slower rates compared to mallard ducks. Taken 

together, these data may indicate that raptors, which represent secondary consumers of 

rodenticides, are more sensitive to rodenticides when compared to the standard test species used 

to derive the RQs calculated in this assessment.

Secondary exposure data: Though secondary exposure studies provide information on the 

potential effects from rodenticide exposure that may occur under field conditions, it is often not 

possible to quantify the level of exposure or the sensitivity of the exposed animals because 

rodenticide residues are not quantitatively sampled in either the prey organisms or secondary 

consumers. Most studies exposed prey animals (i.e., rats, mice, or voles) to the rodenticide for a 

variable number of days before providing the moribund animals to the predators. Even in those 

instances where predators were fed ground meat spiked with specific doses of rodenticide, the 

study designs vary too widely and the sample sizes are far too small to draw any quantitative 

conclusions. In addition, secondary exposure studies are not available for all of the assessed 

rodenticides.

Residue data: Available residue studies used in empirical risk characterization vary widely in the 

study organisms used, sample design, and analytical methods. Many of the studies are field 
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studies which makes it difficult to quantify the level of exposure. In addition, residue data was 

not available for all of the assessed rodenticides. 

5.5 Discussion of Risks of Rodenticides to Secondary Birds and Mammals

The assessment of secondary exposure risks for wildlife involved a series of assessment 

methodologies that encompassed both theoretical accumulation of rodenticides in a prey base as 

well as empirical measurements of rodenticide residues in prey. Based on the analysis of number 

of prey organisms necessary to reach an LD50 dose in secondary birds and mammals, the 

following general conclusions can be made: 1) brodifacoum, difethialone, and bromethalin pose 

greater secondary risk than warfarin, chlorophacinone, and diphacinone, 2) mammals are more at 

risk from consumption of contaminated prey than birds, 3) smaller secondary consumers are at a 

greater risk than larger secondary consumers, and 4) prey that have accumulated more 

rodenticide through longer exposure periods pose greater secondary risks to secondary birds and 

mammals. 

For brodifacoum and difethialone, a combination of toxicity and biological persistence results in 

a trigger for secondary exposure risk concerns for all mammals and birds regardless of the 

assessment method (theoretical and empirical). The delayed time to mortality for brodifacoum 

coupled with its slow clearance time results in small mammal target accumulation of this 

rodenticide to levels that are two orders of magnitude above the lethal toxicity endpoints (i.e., 

LD50 values) suggesting an enhanced opportunity for secondary exposure. Furthering the concern 

for secondary exposure, an analysis of the number of target individuals necessary for 

consumption to achieve a lethal dose in the secondary consumers reveals that brodifacoum 

residue in a single prey item is sufficient to trigger lethal risk concerns for birds and mammals. 

These conclusions are supported by available data from secondary toxicity studies where 63% of 

secondary birds and 42% of secondary mammals fed brodifacoum contaminated prey organisms 

died. For difethialone, an analysis of the number of target individuals necessary for consumption 

of a lethal dose in the secondary consumers reveals that, on average, birds would need to 

consume <1 to 27 mice or <1 to 5 rats, depending on weight class and accumulation scenario 

assessed, to reach a lethal dose. For mammals, <1 to 2 rat would be sufficient to reach a lethal 

dose, while up to 11 mice would be necessary to reach the median lethal dose. The difference in 

risk can be attributed primarily to the faster clearance times in mammalian prey. However, as 

noted in the assessment, there is an uncertainty associated with the clearance rate for 

difethialone, and adoption of a biological half-life more in-line with brodifacoum would yield 

results similar to that chemical. In addition, secondary toxicity studies may provide further 

information on the secondary exposure risk concerns for difethialone; however, none are 

currently available.
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For warfarin, there are no LOC exceedances for avian wildlife except for secondary consumer 

birds in the smallest weight class assessed consuming mice exposed to the highest concentration 

of warfarin for six days. An analysis of the number of target individuals necessary to reach an 

LD50 dose in secondary birds supports this conclusion since birds would need to consume 3 to 

1,587 mice or <1 to 284 rats exposed to the high concentration warfarin (0.054%) bait, 

depending on weight class and accumulation scenario assessed. Birds exposed to the low 

concentration warfarin (0.025%) bait would have to consume 7 to 3,429 mice or 1 to 614 rats to 

reach the median lethal dose. Based on this calculation it is not likely that secondary birds of the 

weight classes assessed would be able to consume enough mice or rats necessary to accumulate a 

lethal dose. However, this analysis suggests that small birds consuming small and large prey 

exposed to warfarin bait for six consecutive days may be at risk. In this case, birds would only 

need to consume up to approximately 6 mice or 1 rat, depending on concentration of warfarin 

bait, to reach a lethal dose. This analysis is somewhat conservative because it assumes that prey 

animals are consuming 100% of their diet as bait for 6 consecutive days. Secondary toxicity 

studies had no mortalities and few signs of warfarin toxicity were observed. Conclusions for 

mammalian secondary risk are somewhat mixed and depend on the concentration of bait and the 

analysis method. However, secondary toxicity studies conducted with secondary mammals 

indicate that there is the potential for lethal exposure to occur. These conclusions are further 

supported by an analysis of the number of target species necessary for consumption of a lethal 

dose in the secondary consumers, which reveals that residue levels in less than one prey item is 

sufficient to trigger lethal risk concerns for mammals, except for larger mammals consuming 

mice. 

There are no LOC exceedances identified for avian wildlife exposed to chlorophacinone based 

on risk calculations. An analysis of the number of target species necessary to reach an LD50 dose 

in secondary birds supports this conclusion since birds would need to consume 23 to > 11,000 

mice or 4 to 2,094 rats to reach the median lethal dose. Based on this calculation it is not likely 

that secondary birds of the weight classes assessed would be able to consume enough mice or 

rats necessary to accumulate a lethal dose. Available secondary toxicity data, where no 

mortalities and few signs of chlorophacinone toxicity were observed, support this conclusion. 

For mammals, there were LOC exceedances for all mammals on a dietary basis considering both 

theoretical and empirical estimates of exposure. On an acute basis, empirical data would suggest 

that there are no LOC exceedances for mammals exposed to chlorophacinone residues in prey. 

However, theoretical exposure calculations yield concerns for all mammals eating mice exposed 

to a single dose and an accumulation dose. These conclusions are supported by available data 

from secondary toxicity studies where 58% of secondary mammals fed chlorophacinone 

contaminated prey organisms died and signs of chlorophacinone toxicity were observed in 

survivors. These conclusions are further supported by an analysis of the number of target 

individuals necessary for consumption of a lethal dose in the secondary consumers reveals that, 

mammals would need to consume <1 to 60 mice or < to 11 rats to reach a lethal dose. It is 
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important to note that mammals in the smallest weight class assessed would be at greater risk 

than larger mammals. Mammals in this weight class would need to consume <1 to 3 mice, 

depending on the accumulation scenario, or < 1 rat. 

For diphacinone, there were no LOC exceedances as a result of secondary exposure for avian 

wildlife considering both theoretical and empirical estimates of exposure. An analysis of the 

number of target individuals necessary to reach an LD50 dose in secondary birds supports this 

conclusion since birds would need to consume 183 to >17000 mice or 33 to >13,000 rats to reach 

the median lethal dose. Based on this calculation it is not likely that secondary birds of the 

weight classes assessed would be able to consume enough mice necessary to accumulate this 

dose. However, secondary toxicity studies conducted with selected raptors indicate that there 

may be differences in sensitivity for these birds as compared to the species that effects endpoints 

used the risk assessment, potentially leading to underestimation of risks of diphacinone to 

secondary birds. A study by Rattner et al. (2011) found that American Kestrels (Falco 

sparverius) may be nearly 20 times more sensitive to diphacinone than the species (bobwhite 

quail and mallard duck) providing endpoints for this assessment. For further understanding of the 

implications of this observation on the findings of the risk assessment as a whole, the reader is 

directed to the cumulative evaluation of all lines of evidence elsewhere in the document (Section 

7). For mammals, risk concerns were identified for all mammals on a dietary basis considering 

both theoretical and empirical estimates of exposure. On an acute basis, empirical data would 

suggest that there were no LOC exceedances for mammals exposed to diphacinone residues in 

prey. However, RQs based on theoretical exposure calculations exceeded LOCs for all mammals 

eating mice exposed to a single dose and an accumulation dose. These conclusions are further 

supported by an analysis of the number of target species necessary for consumption of an LD50

dose in the secondary consumers reveals that mammals would need to consume <1 to 18 mice or 

<1 to 3 rats to reach this dose. It is important to note that mammals in the smallest weight class 

assessed would be at greater risk than larger mammals. Mammals in this weight class would 

need to consume less than one mouse or rat exposed to diphacinone bait for one to six days. 

For the non-anticoagulant rodenticide, bromethalin, there are LOC exceedances for birds based 

on the single oral dose and the six-day accumulation for all weight classes assessed except for 

the medium and large weight classes consuming rats. An analysis of the number of target 

individuals necessary for consumption of a lethal dose in the secondary consumers reveals that 

birds would need to consume <1 to 104 mice and <1 to 17 rats to reach an LD50 dose. The small 

and medium weight classes assessed would be at greater risk than larger birds. Birds in the 

smallest weight class would only need to consume 1 mouse or rat exposed to bromethalin bait, 

while birds in the medium weight class would need to consume up to 3 rats exposed to 

bromethalin bait. There are no secondary feeding studies available to support or refute this risk 

conclusion for birds. For mammals, RQs based on theoretical exposure calculations exceeded 

LOCs for all mammals on a single oral dose, six-day accumulation, and dietary basis. However, 
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in the absence of bromethalin residue data this could not be assessed empirically. Secondary 

toxicity studies indicate that there is limited toxicity to mammals exposed to bromethalin 

residues in prey organisms. An analysis of the number of target species necessary for 

consumption of a lethal dose in the secondary consumers reveals that residue in <1 to 11 mice 

and <1 to 2 rats is sufficient to trigger lethal risk concerns for mammals. However, a recent 

study by Pitt et al. (2011) found that time to death for house mice, Polynesian rats (Rattus 

exulans), and black rats (Rattus rattus) offered bromethalin may be more rapid than suggested in 

registrant submitted data for the Norway rat, which may reduce the potential for bromethalin to 

accumulate in prey organisms. Data from this study also suggest that house mice and Polynesian

rats exposed to bromethalin bait may stop feeding on the bait after reaching a lethal dose, further 

reducing the potential for accumulation. In addition, an analysis of the grams of bait necessary to 

consume an LD50 dose in primary consumers previously discussed (see Section 4) suggests that 

large mammals would only need to consume 13 grams of bait to reach this dose, which is 

significantly less than would be required to reach an LD50 dose in a secondary consumer. Risk 

assessment based on dietary concentration and dietary toxicity supports this contention for birds. 

However, the absence of bromethalin residue data limits the confidence in the assumption of low 

prey accumulation.

6 Wildlife Incidents

6.1 Mortality Incidents

6.1.1 Introduction

This section reviews reported incidents of wildlife mortality that have been linked to rodenticide 

exposure. As the ecological risks from exposure to rodenticides are only to terrestrial vertebrates 

(as opposed to aquatic organisms, plants, or invertebrates), the incident review was carried out 

only for incidents of adverse effects to wild terrestrial vertebrates (hereafter referred to as 

‘wildlife’). The incidents generally involved mortality, but occasionally included incapacitated 

animals which recovered after treatment by a veterinarian. Incidents to pets and domesticated 

animals were not included in this analysis, but are reviewed in the HED document 

“Rodenticides: Tier 2 Pet Incident Report in Support of NOIC” (October, 2011). In this report, 

the Agency evaluates wildlife mortality incidents for the four rodenticides involved in the NOIC 

(brodifacoum, difethialone, warfarin, and bromethalin) as well as chlorophacinone and 

diphacinone, which are rodenticides with current conforming labels that may serve as

alternatives for residential commensal rodent control. In addition, the Agency compiles and 

evaluates incidents of documented rodenticide exposures in wildlife which were not necessarily 

associated with any observed adverse effects. These exposure incidents were generally reports 

of anticoagulant rodenticides being measured in the liver tissues of wild animals which were 

determined to die from causes unrelated to rodenticide poisoning.
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Reported incidents for each of the active ingredients are summarized as counts of reported 

incidents and compiled based on three variables: exposure type (primary or secondary), certainty 

level, and location (rural/field versus urban/suburban). The certainty levels indicate the 

likelihood that a particular rodenticide ingredient caused the effects observed in the animal. 

These levels were assigned to the incidents by the EPA when the incident records were entered 

into the Ecological Incident Information System (EIIS). For this analysis, incidents were further 

categorized by exposure type and location using the methods described in the next section. The 

purpose of the exposure type categorization was to evaluate how many of the incidents were 

likely related to primary exposure, i.e. animals directly consuming the rodenticide bait, versus 

secondary exposure, i.e. animals consuming other animals that ingested the bait and had 

detectable levels of rodenticide residues in their bodies. The purpose of the location 

categorization was to evaluate the occurrence of incidents in urban/suburban areas, which are 

thought to be predominantly related to residential and commercial uses, versus incidents that 

occurred in rural/field areas which may be related to agricultural uses of rodenticides. The 

analyses performed in this section focus on incidents that have been reported in the United 

States. Incidents from other countries are dealt with separately in Section 6.1.2

Interpretation of incident data is difficult and may be confounded by a number of factors (see 

Section 6.1.2.1 for a discussion of limitations and uncertainties). The report of an incident can 

demonstrate that a complete exposure pathway resulting in an effect (i.e., mortality) has 

occurred. The lack of incidents or the presence of fewer incidents in and of itself for various 

chemicals does not necessarily suggest that fewer incidents are occurring or that one chemical is 

safer than another. However, incident data can corroborate results from the deterministic risk 

assessment presented in this assessment. In addition, the incident data were used in this 

assessment to describe general land use patterns where effects have occurred and to describe 

whether mortality has been reported for different exposure types (e.g., primary exposure vs.

secondary exposures) as described in more detail in subsequent sections. 

The number of incidents that have been reported are thought to represent a very small fraction of 

actual incidents that have occurred. Also, a number of factors may influence reporting frequency 

that can differ across the chemicals including location of the labeled uses (agricultural vs.

commensal rodent control), timing of high-end use relative to efforts to document incidents (for 

example, warfarin use has been low relative to its peak for a number of years), and chemical 

properties such as persistence in a carcass. For these reasons, it is difficult to make meaningful 

comparisons across the different rodenticides with respect to the number of incidents for each 

one. Nonetheless, some comparisons of primary and secondary risks can be made when 

incorporating all lines of evidence used to describe potential risks in this document, including the 

presence of incidents. Overall conclusions related to all lines of evidence, including risk 

characterization and incident reports, are presented in Section 7 of this assessment. 
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6.1.2 Methods

Databases

Incidents were primarily obtained from the Ecological Incident Information System (EIIS), a 

database of incidents of adverse effects to nontarget wild animals and plants attributed to 

pesticide exposure which is maintained by the Environmental Fate and Effects Division (EFED) 

of the Office of Pesticide Programs (OPP). 1 A summary of each of the incidents for the 

rodenticides considered in this assessment is provided in Appendix D. 

Ecological incident reports entered into EIIS are generally from two main sources: pesticide 

registrants and state government offices. The majority of the reported incidents used for this 

analysis were provided by the states (predominantly New York and California); registrant 

submitted incidents make up a smaller number of reported incidents. 

Incidents recorded in EIIS were cross-referenced with those found in the Incident Data System 

(IDS). The IDS database, unlike EIIS, is maintained by the Information Technology and 

Resources Management Division (ITRMD) of OPP. IDS tracks incident reports submitted by 

registrants in compliance to the Federal Insecticide Fungicide and Rodenticide Act (FIFRA) 

section 6(a)(2) regulations (which requires registrants to report incidents of adverse effects) as 

well as incident reports that are obtained from other sources, such as offices of state 

governments. In most cases, incidents within the EIIS corresponded with an incident report 

recorded in IDS 

The Contaminant Exposure and Effects – Terrestrial Vertebrate (CEE-TV)2 Database maintained 

by the U.S. Geological Survey was also queried for rodenticide-related wildlife incidents. Only 

one incident was identified that was linked to the ingredients evaluated here and was not already 

recorded in EIIS. 

The Avian Incident Monitoring System3 (AIMS), an on-line database of pesticide-related avian 

incidents that is maintained by the American Bird Conservancy, was also consulted, but no 

unique rodenticide incidents were identified in this database that were not in EIIS.

The ITRMD maintained Aggregate Incidents Database was not consulted primarily due to the 

fact that EIIS and IDS provide more robust reports. The Aggregate Incidents Database provides 

only counts for mammalian and fish incidents (grouped), terrestrial plants, and other non-target 

 1 EFED is a division part of OPP. OPP is one of the offices that is part of the Office of Chemical Safety and 

Pollution Prevention (OCSPP).

2 http://www.pwrc.usgs.gov/contaminants-online/pages/CEETV/CEETVintro.htm 3 http://www.abcbirds.org/abcprograms/policy/toxins/aims/aims
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wildlife. Other than providing the relative counts, these reports are not accompanied by any 

certainty, legality, location information, pathology report, or residue analysis. Therefore, this 

database was not queried for wildlife morality incident reports.

An incident report within EIIS will have information about the legality of use (e.g., registered 

use or a misuse), certainty (how certain a particular pesticide caused the incident), and use 

location associated with that incident (see below). The legality index refers to the determination 

of the incident investigation as to whether the rodenticide used linked to the incident was in 

compliance with the label. In all but 25 of the incidents evaluated for this report, the legality was 

“undetermined,” typically because information was not available to linking the incident to a 

specific product and use. A few incidents were excluded from the analysis because they were 

associated with intentional misuse of rodenticides, primarily because products labeled for 

commensal rodent controls were intentionally used to control pest species not on the label, such 

as squirrels.

An important consideration when examining the incident data is that one reported wildlife 

mortality incident does not necessarily correspond with a single dead animal. Several of the 

rodenticide incidents involved multiple animals of the same species and sometimes multiple 

animals of different species. Since brodifacoum had the majority of incidents associated with it,

this separate analysis is of the totals of the individual organisms affected was performed along 

with the number of incidents only for this chemical.

A certainty index was assigned for each active ingredient linked to each incident. The certainty 

index indicates the likelihood that a particular ingredient was the cause of the adverse effects 

observed in the incident. Certainty index values range from “unlikely” to “highly probable” 

“Unrelated” is also a certainty category used in EIIS but incidents with this certainty value were 

excluded from the analysis. 27 incidents in which rodenticide residues were detected in tissue 

but were assigned a certainty of “unlikely” are discussed in Section 6.1.2 because they provide 

information on rodenticide exposure in wildlife. In many cases, the certainty index value was 

determined by the wildlife pathologist who performed the necropsy and compiled the chemical 

residue analysis results (where applicable). In incidents where the certainty determination was 

not provided in the submitted report, staff of EFED assigned the certainty index based on tissue 

residue data and other diagnostic and circumstantial evidence provided in the incident report. 

However, whenever the submitted report included a certainty conclusion made by the wildlife 

pathologist who investigated the incident, this conclusion was never altered.

Characterizing Incidents as Primary and Secondary Exposure

An attempt was also made to characterize the exposure type of each incident as either “primary” 

or “secondary.” Primary exposure refers to incidents where the animal was exposed to the 
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rodenticide through direct consumption of bait, whereas secondary exposure refers to incidents 

where animals prey or scavenge on other animals which consumed rodenticide bait. This 

designation was based primarily on the diet of the species affected. For example, raptors such as 

owls, hawks, and eagles were assigned an exposure code of “S” due to their carnivorous feeding 

habits and the likelihood that they preyed or scavenged upon a rodenticide-exposed animal. In 

contrast, strict herbivores such as squirrels and deer were assigned “P” because they likely fed on 

the bait directly. For omnivorous species such as crows, raccoons, and opossums, for which the 

exposure could have been either primary or secondary, the findings of the necropsy analysis and 

reported the gastrointestinal (GI) were consulted when available. When the GI contents to 

contain granules or material resembling bait, probable primary exposure was assigned. When 

the necropsy analysis noted bone fragments or tissues of another organism in the GI tract, 

probable secondary exposure was assigned. Finally, when the incident involved an omnivorous 

species and the necropsy report provided no useful information on the contents of the GI tract, 

unknown exposure was assigned. 

Characterizing the Location of Incidents

One question that was investigated through analysis of incident data is whether use of 

rodenticide baits by homeowners and businesses to control commensal rodents in urban and 

suburban areas pose a hazard to nontarget wildlife. This question was investigated by 

considering the locations of the reported incidents relative to urban, suburban, and rural areas. 

When possible, the locations of incidents were categorized as being “urban/suburban” or “rural” 

based on the location where the affected animals were found. This categorization was often 

based on information provided in the incident report on the land use type of the site of the 

incident and is recorded in the Habitat field of EIIS. In some instances, this field was entered as 

“not reported” (N/R) due to lack of information. In other cases, the location category was often 

determined from information given in the wildlife pathologist report on the location where the 

carcass was picked up. In some reports, the actual address of the site of the incident was 

provided. In those instances, the address was located using mapping software to ascertain its 

proximity to developed areas. A few incidents, including ones that occurred from use in zoos or 

from use in island rat eradication programs, were not considered representative of either typical 

urban or rural areas and thus were pooled with the “not reported” incidents. This spatial type of 

analysis was only performed for brodifacoum as the incident reports were numerous and detailed 

enough to conduct that type of analysis. 

6.1.3 Primary Exposure Incidents

6.1.3.1 Brodifacoum 

Counts of reported wildlife mortality incidents occurring in the United States, which are 

attributed to exposure to brodifacoum, are provided below (Table 6-1). Counts are shown 
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accumulated by three factors, (1) whether the exposure was believed to be primary or secondary 

(or unknown), (2) whether the incident occurred in a rural or urban/suburban location (or 

unknown), and (3) the certainty level assigned indicating the likelihood that the given active 

ingredient was the cause of the incident. Because some incidents involve the death of multiple 

animals, Table 6-2 provides a summary of the number of mortalities observed in incidents 

attributed to brodifacoum. A summary of each of these incidents included in these counts are 

provided in Appendix D.

Table 6-1. Summary of Wildlife Incidents Linked to Brodifacoum Exposure.

Exposure Location Possible Probable Highly Probable Total

Primary Rural 1 0 3 4

Urban/Suburban 1 8 25 34

Unknown 0 5 1 6

Total 2 13 29 44

Secondary Rural 5 9 36 50

Urban/Suburban 7 29 61 97

Unknown 5 12 22 39

Total 17 50 119 186

Unknown Rural 4 2 2 8

Urban/Suburban 1 5 13 19

Unknown 3 1 1 5

Island (rat eradication) 0 0 2 2

Other (Zoo) 0 2 1 3

Total 8 10 19 37

Grand Total 27 73 167 267
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Table 6-2. Summary of Number of Mortalities (Total Number of Individuals Affected) in 

Wildlife Incidents Linked to Brodifacoum Exposure

Exposure Location Possible Probable Highly Probable Total

Primary Rural 0 0 10 10

Urban/Suburban 0 10 40 50

Unknown 1 12 1 14

Total 1 22 51 74

Secondary Rural 4 14 49 67

Urban/Suburban 6 24 55 85

Unknown 4 12 32 48

Other (Golf Course, 

Military base) 0 1 1 2

Total 14 51 137 202

Unknown Rural 4 29 6 39

Urban/Suburban 1 5 10 16

Unknown 4 1 3 8

Other (Zoo) 0 28 5 33

Total 9 63 24 96

Grand Total 24 136 212 372

Many wildlife mortality incidents have been attributed to primary poisoning by brodifacoum, 

and the majority of the primary incidents have been linked to rodenticide poisoning with high 

certainty. Of the 35 incidents which were attributed to primary poisoning, all but one has a 

certainty index rating of “probable” or “highly probable.” Most of these incidents have involved 

deer, squirrels, chipmunks, and passerine birds, all of which are believed to have been poisoned 

by directly consuming brodifacoum bait. Of the 35 primary-exposure incidents, 25 (71%) were 

identified as occurring in urban or suburban/residential areas. This indicates that use of 

brodifacoum as a rodenticide represents a complete exposure pathway for non-target wildlife via 

primary exposure that can result in mortality for birds and mammals. 

Stone et al. (1999) reported on six primary poisoning incidents of white-tailed deer which 

occurred on Fire Island, a barrier island on the south shore of Long Island, NY. Four of the 

incidents were associated with brodifacoum and two were associated with diphacinone. The 

incidents were believed to have resulted from outdoor residential use of rodenticides by 

homeowners on the island. They concluded that the risk of deer ingesting rodenticide bait is 

high in suburban residential areas like Fire Island, where favorable habitat and restriction of 

hunting creates high deer density, and thus the population food limited. A similar situation 

occurred in the winter of 2008 on Big Pine Key in southern Florida, where the death of three key 
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deer (Odocoileus virginianus clavium) was linked to exposure to anticoagulant bait placed by 

homeowners in the area (EIIS Incident I020713-002). Surveys of wildlife mortality following 

the use of brodifacoum by the US Fish and Wildlife Service for rat eradication on islands also 

has documented mortality of birds of species which would likely have been exposed by primary 

exposure (e.g. emperor goose, gray-crowned rosey finch, and rock ptarmigan), although the 

majority of mortality was to species which could have been exposed by secondary exposure 

(Ornithological Council, 2010).

An additional 37 incidents could not be identified as being caused by primary or secondary 

poisoning. Most of these incidents were of omnivorous species which potentially could have 

consumed either the bait directly or a living or dead animal which had fed on the bait. The most 

common species in this group are the American crow (Corvus brachyrhynchos), the red fox

(Vulpes vulpes), the gray fox (Urocyon cinereoargenteus), and the raccoon (Procyon lotor). 

These incidents were also associated with residential and commercial uses of bait, with 19 of the 

37 incidents (51%) occurring in areas identified as urban or suburban, whereas only 8 out of 

(22%) occurred in areas identified as rural. The location could not be characterized in the 

remaining 10 incidents.

Spatial analysis was performed on wildlife incidents which occurred in the state of New York 

(Figure 6-1). Symbols are used to distinguish incidents associated with primary exposure 

(triangles) and secondary exposure (stars). Only incident reports which had exact location 

information where the carcasses were found were included in this analysis. Locations of 

incidents were overlaid on a graphical representation of human population density by county. 

Figure 1shows that reported primary incidents (as well as secondary incidents) have been 

reported in areas of high population density. The majority of primary incidents were clustered in 

the highly populated urban and or suburban/residential areas of New York City, Albany, Buffalo, 

Syracuse, and Utica. The association of reported primary wildlife mortality incidents with urban 

could in part be the result of more people being present in urban areas to observe and report dead 

carcasses, thereby making the reporting rates higher in those areas compared to rural areas. 

Thus, it is not known if there is a difference in the frequency of occurrence of primary incidents 

between urban and rural areas; however, the incident data do show that primary incidents 

involving brodifacoum occur in urban/suburban areas.
Page 128 of 183

Figure 6-1. Spatial analysis of wildlife mortality incidents attributed to primary and 

secondary exposure to brodifacoum in New York State.

6.1.3.2 Difethialone

Counts of reported wildlife mortality incidents occurring in the United States, which are 

attributed to exposure to difethialone are provided below (Table 6-3). Counts are shown 

accumulated by three factors, (1) whether the exposure was believed to be primary or secondary 

(or unknown), (2) whether the incident occurred in a rural or urban/suburban location (or 

unknown), and (3) the certainty level assigned indicating the likelihood that the given active 

ingredient was the cause of the incident. There was one reported incident attributed to primary 

exposure and 4 reported incidents attributed to secondary exposure. All reported incident for 

difethialone (primary and secondary exposures) occurred in urban/suburban areas. A summary 

of each of these incidents included in these counts are provided in Appendix D.
Page 129 of 183

Table 6-3. Summary of Wildlife Incidents Linked to Difethialone Exposure

Exposure Location Possible Probable Highly Probable Total

Primary Rural 0 0 0 0

Urban/Suburban 1 0 0 1

Unknown 0 0 0 0

Total 1 0 0 1

Secondary Rural 0 0 0 0

Urban/Suburban 0 4 0 4

Unknown 0 0 0 0

Total 0 4 0 4

Grand Total 1 4 0 5

6.1.3.3 Warfarin

Counts of wildlife mortality incidents reported to the Agency which are attributed to primary and 

secondary poisoning warfarin are shown in Table 6-4. Three incidents have been identified as 

being caused by primary exposure to warfarin. This indicates that use of warfarin as a 

rodenticide represents a complete exposure pathway for non-target wildlife via primary exposure 

that can result in mortality for birds and mammals. Two of the incidents were assigned a 

certainty level of “probable” and the other one was assigned a level of “possible.” Species 

affected include squirrels and northern bobwhite quail. Two incidents occurred in 1981 and one 

occurred in 1971. The lack of recent reported incidents involving warfarin may be attributed to 

decreased use; however, primary incidents were reported during the years when warfarin use was 

widespread. The number of incidents are too few to draw any conclusions concerning the 

frequency of incidents with use in rural versus urban/suburban areas. A summary of each of 

these incidents included in these counts are provided in Appendix D.

Table 6-4. Summary of Wildlife Mortality Incidents Linked to Warfarin Exposure

Exposure Location Possible Probable

Highly 

Probable Total

Primary Rural 1 0 0 1

Urban/Suburban 0 0 0 0

Unknown 0 2 0 2

Total 1 2 0 3

Secondary Rural 0 1 0 1

Urban/Suburban 0 1 0 1

Unknown 0 1 2 3

Total 0 3 2 5

Grand Total 1 5 2 8
Page 130 of 183

6.1.3.4 Bromethalin

One primary incident has been reported for bromethalin (Table 6-5). That incident involved the 

death of a chipmunk following consumption of a rodenticide bait product which contained 

bromethalin. This indicates that use of bromethalin as a rodenticide represents a complete 

exposure pathway for non-target wildlife via primary exposure that can result in mortality for 

mammals. 

The low number of primary incidents for bromethalin could be explained, at least in part, by the 

failure of investigations of wildlife incidents to identify this ingredient as the cause of primary 

poisoning incidents. This may be attributed to two factors. First, bromethalin has a different 

chemical structure compared to anticoagulant rodenticides and requires a different analytical 

method for detection and quantification. Bromethalin is not normally included in the analytical 

screen for rodenticides conducted on tissue from wildlife carcasses, which normally include only 

a screen of anticoagulant rodenticides (Personal communication, Joseph Okoniewski, New York 

Department of Environmental Conservation and Stella McMillin, California Department of Fish 

& Game). Second, bromethalin poisoning does not result in clinical signs that are obvious in 

gross necropsy examinations, and thus there may not be cues that would trigger the investigator 

to seek analysis to detect bromethalin. A summary of each of these incidents included in these 

counts are provided in Appendix D.

Table 6-5. Summary of Wildlife Mortality Incidents Linked to Bromethalin Exposure

Exposure Possible Probable Highly Probable Total

Primary 0 1 0 1

Total 0 1 0 1

6.1.3.5 Chlorophacinone 

Chlorophacinone is linked to 11 total incidents, six of which can be attributed to primary 

poisoning (Table 6-6). The primary exposure incidents involved quail, turkeys, and squirrels. 

Primary incidents have occurred in both rural and urban/suburban locations. This indicates that 

use of chlorophacinone as a rodenticide represents a complete exposure pathway for non-target 

wildlife via primary exposure that can result in mortality for birds and mammals. A summary of 

each of these incidents included in these counts are provided in Appendix D.

Table 6-6. Summary of Wildlife Mortality Incidents Linked to Chlorophacinone Exposure

Exposure Location Possible Probable Highly Probable Total

Primary Rural 1 1 1 3

Urban/Suburban 1 1 1 3

Total 2 2 2 6
Page 131 of 183

Table 6-6. Summary of Wildlife Mortality Incidents Linked to Chlorophacinone Exposure

Exposure Location Possible Probable Highly Probable Total

Secondary Rural 0 0 1 1

Urban/Suburban 0 1 0 1

Unknown 0 1 1 2

Total 0 2 2 4

Unknown Rural 0 0 1 1

Grand Total 2 4 5 11

6.1.3.6 Diphacinone

Diphacinone is linked to 19 total incidents, 10 of which are for primary consumers (Table 6-7). 

The primary exposure incidents involved deer, squirrels, pigeons and kangaroo rats Primary 

incidents have occurred in both rural and urban/suburban locations. This indicates that use of 

diphacinone use as a rodenticide represents a complete exposure pathway for non-target wildlife 

via primary exposure that can result in mortality to birds and mammals. A summary of each of 

these incidents included in these counts are provided in Appendix D.

Table 6-7. Summary of Wildlife Mortality Incidents Linked to Diphacinone Exposure

Exposure Location Possible Probable Highly Probable Total

Primary Rural 0 1 0 1

Urban/Suburban 1 1 3 5

Unknown 1 2 1 4

Total 2 4 4 10

Secondary Rural 0 0 2 2

Urban/Suburban 0 2 0 2

Unknown 1 2 2 5

Total 1 4 4 9

Grand Total 3 8 8 19

6.1.4 Secondary Exposure Incidents

6.1.4.1 Brodifacoum 

Analysis of incident reports attributed to brodifacoum exposure show 186 incidents associated 

with secondary exposure. The majority of these incidents (164 out of 186, 88%) had high 

certainty for being caused by brodifacoum, with a certainty index value of “probable” or “highly 

probable.” This is because most of the incidents were identified by the presence of brodifacoum 
Page 132 of 183

residues in the liver of the poisoned species, often along with supporting evidence of 

anticoagulation poisoning from the necropsy examination. Indecent reports pertained to 

mortalities of a wide variety of raptor species, including hawks, eagles, and owls. A variety of 

predatory and scavenger mammals were also reported in association with brodifacoum 

poisoning, including foxes, coyotes, bobcats, and mountain lions. Brodifacoum has no labeled 

field uses (other than island eradication programs) and is only currently labeled for use to control 

commensal rodents; therefore, these incidents show that use of brodifacoum for control of 

commensal rodents can result in secondary poisoning of raptors and mammalian predators which 

commonly feed on rodents. 

Incidents of wildlife mortality from secondary exposure to brodifacoum have been reported

throughout the United States. The largest numbers of reported incidents have been from New 

York and California, which is thought to be due to the presence of a more comprehensive or 

systematic programs for investigating and reporting pesticide-related wildlife incidents in those 

states compared to other states. States other than New York and California that have reported 

secondary exposure incidents for rodenticides included in this NOIC include Florida, Georgia, 

Virginia, Massachusetts, Wisconsin, and Kansas. Additionally, incidents of widespread wildlife 

mortality attributed to secondary exposure to rodenticides have been documented in numerous 

other countries, including Canada (Alber et al., 2009), France (Berny et al., 1997), Britain (Shore 

et al, 1999), and New Zealand (Eason et al., 2001, Dowding et al., 2006). 

As with primary incidents, the location of reported secondary poisoning incidents was associated 

with use in urban and suburban areas. Ninety-three out of 183 reported incidents (51%) are 

associated with secondary exposure to brodifacoum (excluding the one with an unlikely

certainty) were located in areas known to be urban or suburban areas, whereas 50 out of 183

(27%) were located in rural areas. All 5 of the incidents of secondary poisoning by difethialone 

also occurred in an urban area. The map of reported secondary incidents in New York State 

shows that many secondary incidents have been reported in urban areas, with clusters of 

secondary incidents (as well as primary incidents) occurring near the urban centers of New York 

City, Albany, Buffalo, Syracuse, and Utica (Figure 6-1). 

A spatial analysis of brodifacoum incidents reported in California (all of which were attributed to 

secondary exposure) indicated that numerous incidents have been reported in or near the 

metropolitan areas of Los Angeles, San Francisco, and Sacramento (Figure 6-2). Most of the 

reported secondary poisoning incidents were located in counties with dense human population, 

and the majority occurred near major metropolitan areas such as Los Angeles, Santa Barbara, 

San Francisco, and Sacramento. Other incidents with exact location information are scattered 

throughout the state but generally occur in areas that have moderate to high population densities 

as indicated by the legend on the California map.
Page 133 of 183

In a study conducted by McMillin et al (2008), the California Department of Fish and Game’s 

Pesticide Investigations Unit (in conjunction with the Endangered Species Recovery Program’s 

Urban Kit Fox Project), San Joaquin Kit Foxes were monitored from the Bakersfield, CA 

population. This study corresponds with the EIIS incident number I016100-001. Necropsies and 

liver tissue samples were collected from kit fox carcasses. A non-urban population of kit foxes 

from Lokern Natural Area (a 40,000 acre habitat 30 miles west of Bakersfield) was used as a 

control. Between 1999 and 2007, tissue samples from various animals were analyzed for 

residues of anticoagulant rodenticides. The fox carcasses were part of an ongoing monitoring 

effort dating back to 1977 from which 10-20 carcasses are collected from both the Bakersfield 

and Lokern area per year. The compounds identified included brodifacoum and 

chlorophacinone. Out of the 30 San Joaquin Kit Foxes analyzed from the Bakersfield 

population, 27 contained at least one anticoagulant and the most commonly detected from that 

set was brodifacoum (26 out of 30 or 87%). Chlorophacinone residues were detected in one fox 

(3%). All control foxes from the Lokern population had no anticoagulant residues detected. 

The authors report that results from this study confirm that San Joaquin Kit Foxes are exposed to 

anticoagulants in urban environments. 
Page 134 of 183

 

Figure 6-2. Distribution of wildlife mortality incidents attributed to brodifacoum and 

difethialone reported in California. 

In conclusion, available incident data show that brodifacoum has been associated with a number 

of incidents of secondary poisoning of wildlife throughout the United States. Since the only 

registered uses of brodifacoum are for commensal rodent use, this suggests that for the uses 

considered for this NOIC, brodifacoum represents a complete exposure pathway for secondary 

mammals and birds. Incidents have been reported in urban and suburban areas. The occurrence 

of secondary poisoning incidents in urban and suburban areas is not surprising because products 

for commensal rodent control are heavily used urban and residential areas, and rodent predators 

such as hawks, owls, foxes, and coyotes are often abundant in urban parks and residential areas.

6.1.4.2 Difethialone

A series of mortalities was reported in Golden Gate Park in San Francisco in 2007 following 

widespread use of difethialone in bait stations which were used at the park for rodent control 
Page 135 of 183

(incidents I018414-001 through I018414-005). The four secondary exposure incidents included 

three red tailed hawks (Buteo jamiacensis) and one red fox (Vulpes vulpes).

6.1.4.3 Warfarin

Counts of wildlife mortality incidents reported to the Agency that are attributed to secondary 

poisoning by warfarin are shown in Table 6-4. Five of the 8 reported incidents were attributed 

to secondary exposure to warfarin. These incidents were all linked to warfarin with high 

certainty, with two incidents assigned a certainty level of “highly probable” and the other three 

with the certainty level of “probable.” Species affected were all raptors, including two bald 

eagles (Haliaeetus leucocephalus), a peregrine falcon (Falco mexicanus), a great horned owl 

(bubo virginianus), and a Cooper’s hawk (Accipiter cooperii). These incidents occurred when 

use of warfarin was more widespread than it is today. One incident occurred in the mid 1980’s, 

three occurred in the 1990’s, and one occurred in the year 2000. 

6.1.4.4 Bromethalin

No incident of wildlife mortality has been reported that has provided convincing evidence of 

secondary exposure to bromethalin. One incident was reported involving the death and 

incapacitation of several raptors that were being held in captivity on Amicalola Falls State Park, 

Georgia, as part of an educational program (I014717-001). Rodenticides that were reportedly in 

use in the park were products that contained bromadiolone and bromethalin. However, evidence 

in this case strongly suggests that the raptors died from exposure to anticoagulant rodenticides 

rather than from bromethalin. Both gross pathological and microscopic examinations observed 

moderate to severe hemorrhaging in the carcasses of the dead birds. In addition, the 

incapacitated birds were treated with vitamin K (an antidote to anticoagulant poisoning) and 

recovered. Finally, while no use of a product containing brodifacoum was reported, analysis of a 

liver samples from the raptors found brodifacoum at 77 mg/kg-bw in an owl and at 7 mg/kg-bw 

in a hawk. 

As previously mentioned, bromethalin is not normally included in the analytical screen for 

rodenticides conducted on tissue from wildlife carcasses, which normally include only a screen 

of anticoagulant rodenticides. Furthermore, bromethalin poisoning does not result in clinical 

signs that are obvious in gross necropsy examinations, and thus there may not be cues that would 

trigger the investigator to seek analysis to detect bromethalin. Therefore, the lack of reported 

secondary incidents for bromethalin does not necessarily indicate that poisoning of nontarget 

wildlife by secondary exposure to bromethalin is not occurring.

6.1.4.5 Chlorophacinone 

Counts of wildlife mortality incidents for chlorophacinone are provided in Table 6-6. 

Chlorophacinone has four incidents associated with secondary poisoning. One of the incidents 
Page 136 of 183

was in an urban/suburban location, one was in a rural location, and two others were in

uncharacterized locations. Incidents involved mortality of both avian and mammalian predators. 

The avian predator was the red-tailed hawk (Buteo jamiacensis) and the mammalian predator 

was the bobcat (Lynx rufus). 

After the above analysis was completed, the Agency received two additional incidents of 

secondary poisoning of wildlife linked to chlorophacinone. One was the poisoning of a bald 

eagle in Nebraska in 2007, and one was a poisoning of an American badger (Taxidea taxus) in 

California in 2009. A certainty level of “highly probable” was assigned to each incident for 

chlorophacinone being the cause of death. Both incidents occurred in rural locations. 

It should be noted chlorophacinone is widely used in agricultural sites for control of rodent pests 

other than commensal rodents. As most if not all of the chlorophacinone incidents may be 

associated with these uses, they do not necessarily provide evidence of secondary risk concern 

for use in bait products for commensal rodent control.

6.1.4.6 Diphacinone

Diphacinone has nine incidents associated with secondary exposure. Two incidents were located 

in rural locations, three were located in urban/suburban locations, and the other five incidents 

were in locations that could not be characterized. Incidents involved mortality of both avian and 

mammalian predators. Avian predators include the turkey vulture (Cathartes aura), the redtailed hawk (Buteo jamiacensis), the barred owl (Strix varia), and the Snowy Owl (Bubo 

scandiacus). Mammalian predators include the San Joaquin Kit Fox (Vulpes macrotis mutica), 

raccoon (Procyon sp.), coyote (Canis latrens), and mountain lion (Puma concolor). 

It should be noted diphacinone is widely used in agricultural sites for control of rodent pests 

other than commensal rodents. As most if not all of the chlorophacinone incidents may be 

associated with these uses, they do not necessarily provide evidence of secondary risk concern 

for use in bait products for commensal rodent control; however, these incidents do suggest that 

there is risk of secondary poisoning in both birds and mammals from diphacinone use. 

6.2 Open Literature Evaluations

Some researchers who have studied wildlife mortality incidents caused by anticoagulant 

rodenticides have reported a large proportion of incidents occurring in highly populated areas 

when compared to rural or agricultural areas. Stone et al. (2003) found that 39% of positive 

cases of rodenticide mortality of raptors in the state of New York between 1998 and 2001 

occurred in the New York City/Long Island region. This is consistent with the current analysis 

involving brodifacoum incidents reported in New York State (Figure 6-1). The authors of this 
Page 137 of 183

paper stated that “urban and suburban origins were more common than rural origins,” although 

noted that the difference could be at least in part due to greater probability of discovery and 

reporting of incidents in heavily populated areas. 

Murray (2010) reports on cases of anticoagulant poisoning of wildlife investigated by a wildlife 

clinic in central Massachusetts between 2006 and 2010. Of the nine raptors they positively 

identified as being killed by brodifacoum intoxication, six were found in urban areas, two from 

suburban towns, and one from a farm. 

A study of anticoagulant rodenticide exposure in owls from Western Canada from 1998 through 

2003 found that the majority of owls (42%) with positive detections of rodenticides came from 

the Upper Fraser Valley, and area characterized with relatively high population as well as 

intensive agriculture. The highest brodifacoum liver residue was observed in a barred owl (Strix 

varia) from an urban area in West Vancouver, and the highest bromadiolone liver residue was 

observed in a barred owl from Surrey, and suburb of Vancouver. 

Lima and Salmon (2010) studied the distribution of detections of anticoagulant rodenticides in 

raptor carcasses collected in San Diego County and the largely agricultural counties of Fresno, 

Kern, and Madera. A spatial analysis of the location of raptor incidents found an association of 

carcasses with positive detections of rodenticides with locations in urban areas with higher 

population density, a pattern similar to the one observed with wildlife mortality incidents in this 

current paper (Figure 6-2).

As noted by Stone et al. (2003), the tendency for more reported incidents to be located in 

urban/suburban areas may in part be the result of greater rate of reporting of wildlife mortality 

incidents in areas with higher human population. In urban/suburban areas, more people are 

present to observe and report wildlife carcasses, and thus more mortality incidents are likely to 

be reported to authorities for investigation. Since no data exists on carcass reporting rates in 

urban and suburban areas, it is not possible to make conclusions on the relative frequency of 

secondary poisoning incidents between urban/suburban areas and rural areas. The authors 

reported that it is clear that a sizable number of secondary poisoning incidents do occur in 

urban/suburban areas, indicating that use of residential and commercial uses of rodenticides for 

commensal rodent control can result in secondary poisoning for wildlife predators and 

scavengers in these areas.

An available radiotelemetry study with bobcats (Lynx rufus) in southern California (Riley et al., 

2007) provides evidence that secondary exposure to rodenticides is associated with their use of 

urban areas. When the livers of dead radiocollared bobcats were analyzed, the total 

concentration of anticoagulant rodenticides was significantly related to the percentage of the 
Page 138 of 183

animal’s home range that was made up of developed areas and to the percentage of 

radiolocations which occurred in developed areas. 

In another study in California, Lima and Salmon (2010) found that many more raptor carcasses 

contained brodifacoum and difethialone, which are generally used for commensal rodent control 

around structures, whereas relatively few contained chlorophacinone, diphacinone, and warfarin, 

which are registered for agricultural uses in California. The authors’ findings support the 

conclusion that residential and commercial uses of rodenticides in urban and suburban areas pose 

a concern for secondary poisoning of wildlife.

The most likely cause of secondary poisoning of wildlife is from predatory and scavenger 

animals consuming rats and mice that have been contaminated by consumption of bait containing 

anticoagulant rodenticides; however, this is not always the case. Poisoning of two mountain 

lions (Felis concolor) in Simi Hills area near Los Angeles was suspected of being caused by 

tertiary poisoning when the lions consumed a coyote which had in turn consumed rodents 

poisoned by brodifacoum and bromadiolone (Riley et al., 2007). In investigating the mortality of 

New Zealand dotterels (Charadrius obscures) following an application of brodifacoum for in a 

rat eradication program in New Zealand, Dowding et al. (2006) found that sandhoppers 

(Talorchestia spp.), a terrestrial crustacean commonly found on beaches, accumulated 

brodifacoum and served as a potential route of secondary exposure to the dotterels and other 

birds which feed on them. Also, some of the mortality of captive birds at the Philadelphia Zoo 

which followed use of a rodenticide bait stations containing brodifacoum was suspected of being 

caused by the birds consuming insects which had consumed bait from the stations (EIIS incidents 

I011274-001 and I011274-001).

After analyzing mortality incidents to wildlife (Stone et al. 1999), and more recent specifically to 

raptors (Stone et al., 2003), wildlife managers of the New York Department of Environmental 

Conservation concluded that “brodifacoum appears to have the greatest potential for nontarget 

mortality of all anticoagulants currently in use” (Stone et al., 2003). While they did not do a 

comparative analysis, Thomas et al. (2010) conducted a probabilistic analysis which related 

observed liver residues of brodifacoum, difethialone to mortality in predatory birds. They 

concluded that the use of brodifacoum and difethialone to control rodents in Canada pose a risk 

to raptors, with an estimate that 11% of the sampled great horned owl population was at risk of 

mortality from exposure to these rodenticides.

6.2.1 Summary and Analysis of Numbers of Incidents

In this section, wildlife incidents reported in the United States were summarized, and the relative 

amount of incidents in various types of environments (e.g., urban, suburban, and rural) and types 

of exposures (i.e., primary and secondary) were documented for the rodenticides included in this 

assessment. Only recent (2003 to 2008) incidents were included in this analysis to attempt to 
Page 139 of 183

capture recent use patterns. The purpose of this assessment was to document the types of 

exposures and incident locations associated with recent incidents. In addition, the number of 

incidents reported may be a function of many factors, one of which is the chemical’s use. 

As previously discussed, warfarin, chlorophacinone, and diphacinone are used in agricultural 

settings to control several other mammal pests (e.g., voles, gophers, prairie dogs, and rabbits), 

and thus the incident counts and usage data included in this analysis for those rodenticides 

include those non-commensal rodent uses. Two wildlife incidents have been reported for the 

neurotoxic rodenticide bromethalin. One of these incidents was not included because it had a 

certainty index of “unlikely,” and the other one was not included because it occurred prior to 

2002. Therefore, for this analysis, the incident count for bromethalin is zero. 

The counts of incidents associated with rodenticide active ingredients are shown in Figure 2. 

The number of incidents attributed to brodifacoum (133) greatly exceeded the number for all 

other rodenticides. The large number of secondary poisoning incidents accounts for a large 

proportion (74%) of the reported brodifacoum incidents.

Difethialone had 6 reported incidents during this time. It was similar to brodifacoum in having 

more secondary exposure incidents (5) than primary exposure incidents (1). Diphacinone had 

five reported incidents and chlorophacinone had four. Both primary and secondary incidents 

were reported for these indandione rodenticides during these years. Only one wildlife incident 

was attributed to warfarin during this time, and that one was a primary poisoning incident. 

Figure 6-3. Number of reported wildlife incidents during the years 2000-2011, excluding 

incidents associated with misuse and those with a certainty value less of unrelated and 

unlikely.

0

20

40

60

80

100

120

140

Unknown

Primary

Secondary
Page 140 of 183

Figure 6-3 compares the proportion of all reported wildlife mortality incidents that were 

attributed to primary and secondary exposures. For brodifacoum, the majority of incidents 

(71%) were attributed to secondary poisoning, whereas only 14% of the incidents were attributed 

to primary exposure. Difethialone had a similar pattern with 80% of incidents being attributed to 

secondary poisoning and 20% being attributed to primary poisoning. In contrast, secondary 

exposure incidents comprised smaller proportions of total incidents for chlorophacinone and 

diphacinone. The percentage of incidents attributed to secondary poisoning were 33% for 

chlorophacinone and 50% for diphacinone. Percentages are not displayed for warfarin because 

only one wildlife incident was reported during this time, which was attributed to primary 

poisoning. When incidents from earlier years are included, 62.5% (5 out of 8) were attributed to 

secondary exposure and 37.5% (3 out of 8) were attributed to primary exposure. Again, a 

number of factors may influence the relative number of primary and secondary incidents 

including species affected, and location and behavior of affected animals. 

6.3 Exposure Incidents

Over the last two decades, widespread accumulation of residues of anticoagulant rodenticides, in 

the liver tissue of various predatory and scavenging wildlife species has been documented. This 

section reviews reported incidents of anticoagulant rodenticides being detected and measured in 

tissue of wildlife without necessarily evidence of adverse effects. Anticoagulant rodenticide 

residues have been found to be prevalent in populations of avian and mammal species which 

feed on rodents, being reported in every recent publication involving monitoring of rodenticide 

residues in wildlife which was reviewed for this effort. Although some of these studies rely on 

the carcasses of dead animals others studies also analyzed incapacitated animals or were part of 

surveillance efforts for West Nile Virus. The studies show that when wildlife pathologists 

(whether it was a pointed effort or secondary to other purposes in the study) looked for 

rodenticide residues in wildlife, they usually found them. Table 6-8 summarizes the percent of 

detections of rodenticides in liver tissues as reported in published wildlife monitoring studies. 

These studies generally investigated the presence of rodenticides in carcasses of wild animals 

that died from causes unrelated to rodenticide poisoning (e.g. killed in automobile accidents or 

caught in traps) or that were found dead from unknown causes. The percentages given represent 

the prevalence of rodenticides observed in the sampled population of carcasses. The prevalence 

of detections ranging from 18% in tawny owls (Stix aluco) in Britain to 100% in great horned 

owls in central Massachusetts and mountain lions in southern California. Species in which 

widespread accumulation of anticoagulant rodenticides in liver tissue has been observed include 

owls, hawks, vultures, mustelids, bobcats, and mountain lions. While this information does not 

provide an indication of risk, it does provide evidence that a complete route of exposure exists 

for predatory and scavenging birds and mammals for rodenticides and that exposure is relatively 

widespread.
Page 141 of 183

Table 6-8. Prevalence of Anticoagulant Rodenticides in Liver Samples of Wildlife

Location, Years, and Description of 

Collection (Reference)

Land 

Use

Species 

Examined

Percentage with Positive Detections

1,2

All3 Brodifa Difeth Warf Chloroph Diphac

Central Massachusetts, 

2006-2010. 

Dead or incapacitated birds brought to 

wildlife clinic. (Murray 2011)

Mixed

Red-tailed 

hawks 

(Buteo 

jamaicensis)

89%

(71/80)

89%

(71/80)

1.3%

(1/80) ND 0%

(0/80)

0%

(0/80)

Barred owls

(Strix varia)

75%

(30/40)

73%

(29/40)

2.5%

(1/40) ND 0%(0/40)

0%

(0/40)

Eastern 

screech owl

(Megascops 

asio)

87%

(20/23)

87%

(20/23)

0%

(0/23) ND 0%

(0/23)

0%

(0/23)

Great horned 

owl

(Bubo 

virginianus)

100%

(18/18)

100%

(18/18)

0%

(0/18) ND 0%

(0/18)

0%

(0/18)

San Diego County, California.

Dead birds delivered to Public Health 

Services for West Nile Virus 

surveillance.

(Lima and Salmon 2010)

Urban Various 

raptors

92%

(49/53)

83%

(44/53)

9.4%

(5/53)

0%

0/53

0%

0/53

0%

0/53

Central Valley, California.

Dead birds collected by public and 

delivered to the CA Dept. of Fish &

Game

(Lima and Salmon 2010)

Agr. Various 

raptors

79%

(34/43)

12%

(5/43)

0%

(0/43)

0%

(0/43)

4.7%

(2/43)

0%

(0/43)
Page 142 of 183

New York State, 1998-2001

Dead birds submitted by the public to 

the NY Depart. of Environ. Conserv., 

augmented by dead birds collected as 

part of a West Nile Virus surveillance 

program

(Stone et al., 2003)

Mixed

Various 

raptors and 

vultures

49%

(128/265)

41%

(108/265) ND 0.75%

(2/265)

0.38%

(1/265)

1.1%

(3/265)

British Columbia and Yukon 

Territory, Canada, 1988-2003.

Dead birds collected by individuals 

and wildlife rehabilitation centers.

(Albert et al, 2009) Mixed

Barn owl

(Tyto alba)

62%

(48/78)

45%

(17/78)

13%

(10/78)

1%

(1/78)

0%

(0/78)

4%

(3/78)

Barred owls

(Strix varia)

92%

(23/25)

68%

(17/25)

4%

(1/25)

4%

(1/25)

16%

(4/25)

4%

(1/25)

Great horned 

owl

(Bubo 

virginianus)

70%

(43/61)

46%

(28/61)

5%

(3/61)

5%

(3/61)

5%

(3/60)

7%

(/61)

Southern California near Los 

Angeles, 

1997-2003

Radiotracked animals found dead in 

suburban area north and west of Los 

Angeles,

(Riley et al. 2007)

Urban

Bobcat

(Lynx rufus)

90%

(35/39)

79%

(31/39)

26%

(10/39)

0%

(0/39)

0%

(0/39)

31%

(12/39)

Mountain lion 

(Puma 

concolor)

100%

(4/4)

100%

(4/4)

25%

(1/4)

0%

(0/4)

0%

(0/4)

0%

(0/4)

Britain,

1990-1993

Dead birds submitted by the public to 

the Predatory Bird Monitoring Scheme

(Walker et al. 2008)

Mixed Tawny owl

(Stix aluco)

18%

(16/88)

8.0%

(7/88) ND ND ND ND

Britain,

2003-2005

Dead birds submitted by the public to 

the Predatory Bird Monitoring Scheme 

(Walker et al. 2008)

Mixed Tawny owl

(Stix aluco)

20.2%

(17/84)

1.2%

(1/84) ND ND ND ND
Page 143 of 183

Britain,

1993-2000

Dead birds submitted by the public to 

the Predatory Bird Monitoring Scheme

(Shore et al. 2005)

Mixed Barn owl

(Tyto alba)

40%

(21/53)

3.8%

(2/53) ND ND ND ND

Britain,

2007-2008

Dead birds submitted by the public to 

the Predatory Bird Monitoring Scheme 

(Walker et al. 2010) Mixed

Barn owl

(Tyto alba)

81%

(79/98)

29%

(28/97)? ND 0%(0/98)

4.1%

(4/98)

0%

(0/98)

Kestrel

(Falco 

tinnunculuc)

68%

(15/22)

27%

(6/22) ND 0%

(0/22)

0%

(0/22)

0%

(0/22)

Red Kite

(Milvus 

milvus)

91%

(29/32)

53%

(17/32) ND 3.1%

(1/32)

3.1%

(1/32)

3.1%

(1/32)

Britain

1992-1994

Animals accidentally killed on roads 

and in traps.

(Shore et al. 1996)

Unkn.

Polecat

(Mustela 

putorius)

Liver:

29%

(7/24)

Stomach 

wall:

40%

(2/5)

Liver:

4.2%

(1/24)

Stomach 

wall:

0%

(0/5)

ND ND ND ND

Britain

1992-1997

Animals killed accidentally on roads 

and in traps. Includes 24 liver samples 

reported in Shore et al 1996. 

(Shore et al. 1999)

Unkn.

Polecat

(Mustela 

putorius)

26%

(13/50)

2.0%

(1/50) ND ND ND ND

Britain

1996-1997

Animals trapped or shot by 

gamekeepers.

(McDonald et al. 1998)

Rural

Stout 

(Mustela 

erminea)

22.5%

9/40

2.5%

(1/40) ND 0%

(0/40) ND ND

Weasel

(Mustela 

nivalis)

30%

3/10

0%

(0/3) ND 0%

(0/10) ND ND
Page 144 of 183

Mapara Wildlife Management 

Reserve, New Zealand, 1993-1995.

Mustelids were trapped and birds were 

shot following rat eradication program 

using brodifacoum. (Murphy et al. 

1998) 

Rural

Stout 

(Mustela 

erminea)

78%

31/40

78%

31/40 ND ND ND ND

Weasel

(Mustela 

nivalis)

71.4%

(10/14)

71.4%

(10/14) ND ND ND ND

Ferrets

(Mustela furo)

56%

(9/16)

56%

(9/16) ND ND ND ND

1 Includes unquantifiable (trace) detections. ND=not determined (i.e., compound was not included in the rodenticide analytical screen). 2 Brodif=Brodifacoum, Difeth=Difethialone, Warf=Warfarin, Chloroph=Chlorophacinone, Diphac=Diphacinone

3 Detection of one or more of all of the anticoagulant rodenticides analyzed in the study.
Page 145 of 183

Brodifacoum residues are prevalent in the tissue of owls, hawks, eagles and vultures. Numerous 

monitoring studies have found brodifacoum present in over half of individuals of one or more 

species studied (Murray 2011; Lima and Salmon, 2010; Albert et al., 2009). Murray (2011) 

found that the prevalence of brodifacoum in great horned owls (Bubo virginianus), red-tailed 

hawks (Buteo jamaicensis), and eastern screech owls (Megascops asio) in Massachusetts to be 

100%, 89%, and 87%, respectively. Alberts et al., (2009) report a prevalence of brodifacoum of 

68% in barred owls in western Canada. Lima and Salmon (2010) report a prevalence of 

brodifacoum of 83% in various raptors monitored in San Diego County, CA. Brodifacoum has 

also been found to be prevalence in wildlife monitored in Europe, with Walker et al (2010) 

finding brodifacoum in 53% of red kites and 29% of barn owls (Tyto alba) in Britain. 

Brodifacoum was by far the most frequently detected rodenticide in studies monitoring raptors 

and vultures in the United States, including in Massachusetts (Murray 2011), San Diego County, 

CA (Lima and Salmon, 2010), and New York State (Stone et al., 2003). In 2000, the California 

Dept. Fish and Game detected brodifacoum in four golden eagles (Aquila chrysaetos), that were 

found dead in Ventura County, CA (CEE-TV database, records 232266, 232273, 232274, 

232218). Difethialone was also detected in raptors in Central Massachusetts (Murray 2011) and 

in San Diego County, CA, although with fairly low prevalence (1.3-9.4%). 

Difethialone, warfarin, chlorophacinone, and diphacinone have been detected in raptors 

monitored in New York State (Stone et al., 2003), Western Canada (Albert et al., 2009), and 

Britain (Walker et al., 2010); however, the prevalence was generally considerably lower than for 

brodifacoum. Difethialone has been found to be moderately prevalent in some studies, with 

percent detections of 9.4% of raptors in San Diego County (Lima and Salmon, 2010) and 5-13% 

of raptors in Western Canada (Albert et al, 2009). In general, warfarin, chlorophacinone, and 

diphacinone were detected in no more than 7% of individuals. The single exception was barred 

owls in Western Canada, in which chlorophacinone was detected in 16% of individuals (Albert 

et al, 2009). 

The Predator Monitoring Scheme (PBMS) in Britain provides information on the prevalence of 

rodenticides in the liver tissue of raptors in Britain over the last two decades. PBMS 

(http://pbms.ceh.ac.uk) is a long-term national monitoring program run by the Centre for 

Ecology and Hydrology that measures concentrations of contaminants in predatory birds in 

Britain. This program analyzes contaminants in carcasses of dead birds that were submitted by 

the public. Results of analysis for brodifacoum and difenacoum for birds submitted during the 

periods of 1990-1993 and 2003-2205 were published by Walker et al. (2008), and results for 

birds submitted during 2007 and 2008 were published by Walker et al. (2010). Table 6-9 

provides a summary of results compiled from both papers for brodifacoum, and difenacoum. 

The most recent results from 2007-2008 show that these two compounds are present in the tissue 

of a high percentage of barn owls, kestrels (Falco tinnunculuc), and red, and a somewhat lower 

percentages of tawny owls. Unlike the United States, the frequency of detection was generally 
Page 146 of 183

higher for difethialone than for brodifacoum. The prevalence of brodifacoum appears to have 

increased from the early 1990’s to the early 2000’s, and then increased even more in the period 

of 2007-2008. Better analytical methodology could be partially responsible for this observed 

increase. 

Table 6-9. Percent of Positive Detections of Difenacoum and Brodifacoum in Four Raptor 

Species Analyzed by the Predator Monitoring Scheme in Britain.

Species Years Percentage (Ratio) with Positive Detection

Difenacoum Brodifacoum

Barn Owl 1990-1993 13.9%

(33/237)

1.3%

(3/237)

2003-2005 20.1%

(39/194)

3.1%

(6/194)

2007-2008 62.2%

(61/98)

28.6%

(28/98)

Kestrel 1990-1993 -- --

2003-2005 50.7%

(37/73)

11.0%

(8/73)

2007-2008 54.5%

(12/22)

27.3%

(6/22)

Red Kites 1990-1993 -- --

2003-2005 -- --

2007-2008 87.5%

(28/32)

53.1%

(17/32)

Tawny Owl 1990-1993 5.7%

(5/88)

8.0%

(7/88)

2003-2005 6.0%

(5/84)

1.2%

(1/84)

Brodifacoum and difethialone also have been found to be widespread in the liver tissues of 

predatory mammals. During a long-term radiotelemetry study conducted in a mountainous 

suburban area north and west of Los Angeles, CA, from 1997 through 2003, brodifacoum was

found in 79% of 39 bobcats analyzed and 100% of four mountain lions. Difethialone was 

somewhat prevalent, being found in 26% of bobcats and 25% of mountain lions. Diphacinone 

was found in 31% of the bobcats examined, but only 4 (10%) had measurable levels above the 

minimum detectable limit of 0.25 ppm. Warfarin was found in 1 of the 39 bobcats, and 

chlorophacinone was found in none. No diphacinone, chlorophacinone, or warfarin was found in 

any of the four mountain lions analyzed (Riley et al., 2007).

Monitoring conducted in New Zealand has also found brodifacoum to be widespread in 

mammals in the weasel family (Mustilidae). Murphy et al. (1998) studied the prevalence of 

brodifacoum in mustelids in a wildlife management reserve in New Zealand after this area was 

baited with this rodenticide in an attempt to eradicate rats between 1993 and 1995. They found 
Page 147 of 183

brodifacoum present in the liver tissue of 78% of stouts (n=30), 71% of weasels (n=14), and 56% 

of ferrets (n=16).

The Ecological Incident Information System (EIIS) also contains some information on tissue 

residues of rodenticides in cases where the cause of death was determined to be unrelated to 

rodenticide poisoning. Concentrations for the 27 rodenticide incidents in EIIS with a certainty 

index assignment of “unrelated” are shown in Table 6-10. These data all came from wildlife in 

the state of California that was analyzed for rodenticides by the California Department of Fish & 

Game. Results of many of the cases of exposure incidents in California wildlife are also 

recorded in the Contaminant Exposure and Effects-Terrestrial Vertebrates (CEE-TV) database 

managed by the US Geological Survey (http://www.pwrc.usgs.gov/contaminantsonline/pages/CEETV/CEETVintro.htm). Brodifacoum was identified in all of these cases. 

Brodifacoum liver concentrations ranged from 0.004 ppm to 0.66 ppm, with a mean of 0.17 ppm. 

Brodifacoum concentrations ranged from 0.07 ppm to 1.1 ppm, with a mean of 0.39 ppm. 

Diphacinone was detected in two cases, two coyotes which had liver concentrations of 0.081 and 

0.13 ppm. Chlorophacinone was detected in one case, a coyote which had a liver concentration 

of 0.43 ppm. 

Table 6-10. Liver Tissue Concentrations of Rodenticides from Records of the 

EIIS Database for Cases in which the Cause of Death was Unrelated to 

Rodenticide Poisoning

Incident ID Year Species Rodenticide Concentration

(ppm)

R000-02-051 2000 Bobcat

(Lynx rufus)

Brodifacoum 0.024

R000-02-007 1998 Coyote

(Canis latrans)

Brodifacoum 0.50

Diphacinone 0.081

Chlorophacinone 0.43

R000-02-008 1998 Coyote

(Canis latrans)

Brodifacoum 0.50

R000-02-009 1998 Coyote

(Canis latrans)

Brodifacoum 0.66

Bromadiolone 0.22

R000-02-004 1995 Great horned owl

(Bubo virginianus)

Brodifacoum 0.015

R000-02-012 1998 Coyote

(Canis latrans)

Brodifacoum 0.04

R000-02-013 1998 Coyote

(Canis latrans)

Brodifacoum 0.08

R000-02-018 1999 Coyote

(Canis latrans)

Brodifacoum 0.33

R000-02-020 1999 Coyote

(Canis latrans)

Brodifacoum 0.03

R000-02-021 1999 Coyote Brodifacoum 0.03
Page 148 of 183

Table 6-10. Liver Tissue Concentrations of Rodenticides from Records of the 

EIIS Database for Cases in which the Cause of Death was Unrelated to 

Rodenticide Poisoning

Incident ID Year Species Rodenticide Concentration

(ppm)

(Canis latrans)

R000-02-022 1999 Coyote

(Canis latrans)

Brodifacoum 0.28

R000-02-023 1999 Coyote

(Canis latrans)

Brodifacoum 0.06

R000-02-015 1999 Coyote

(Canis latrans)

Brodifacoum 0.36

R000-02-016 1999 Coyote

(Canis latrans)

Bromadiolone 0.07

Brodifacoum 0.3

R000-02-017 1999 Coyote

(Canis latrans)

Bromadiolone 0.09

Brodifacoum 0.23

R000-02-038 1999 Golden eagle

(Aquila chrysaetos)

Brodifacoum 0.02

R000-02-042 2000 Golden eagle

(Aquila chrysaetos)

Brodifacoum 0.026

R000-02-043 2000 Golden eagle

(Aquila chrysaetos)

Brodifacoum 0.004

R000-02-044 2000 Golden eagle

(Aquila chrysaetos)

Brodifacoum 0.01

R000-02-031 1999 Kit fox

(Vulpes velox)

Bromadiolone 0.72

Brodifacoum 0.47

R000-02-039 1999 Kit fox

(Vulpes velox)

Brodifacoum 0.22

R000-02-050 2000 Kit fox

(Vulpes velox)

Brodifacoum 0.18

R000-02-010 1998 Raccoon

(Procyon lotor)

Brodifacoum 0.082

Bromadiolone 1.1

Diphacinone 0.13

R000-02-011 1998 Raccoon

(Procyon lotor)

Bromadiolone 0.41

Brodifacoum 0.011

R000-02-027 1999 Red-shouldered 

hawk

(Buteo Laneatus)

Brodifacoum 0.015

R000-02-028 1999 Red-tailed hawk

(Buteo jamaicensis)

Brodifacoum 0.01

R000-02-029 1999 Red-shouldered 

hawk

(Buteo Laneatus)

Brodifacoum 0.01

Bromadiolone 0.28
Page 149 of 183

6.3.1 Uncertainties and Limitations

Many steps must successfully occur for a rodenticide-related wildlife kill to be reported to the 

EPA. First, the animal killed must be observed. Since rodenticide incidents usually involve only 

one or a few animals per incident, they likely often go unnoticed. Carcasses of poisoned wildlife 

are often difficult to observe because the animals actively seek cover before dying, or may 

happen to die in an area where they are obscured by vegetation (Vyas, 1999). Furthermore, 

studies have shown that the majority of bird carcasses are removed by scavengers within one day 

(Balcomb, 1986; Wobeser and Wobeser, 1992), thus there often may be only a short window of 

opportunity for wildlife carcasses to be observed. Wildlife kills also must be reported to the 

appropriate wildlife agency or organization which is capable of diagnosing the cause. That 

agency or organization must have a program in place for investigating wildlife kills through 

conducting necropsy examinations and conducting residue analysis of tissue samples, and must 

have adequate funding available to pay for the analysis. The residue analysis must include an 

analysis for the rodenticide active ingredient that caused the poisoning and the methodology 

must be adequate to detect the ingredient at the levels present in the tissue. Finally, the agency 

or organization must document the incident into a report and that report must be made available 

to the EPA. If any of these steps fail to take place, the incident will not be known to the EPA. 

Furthermore, the nature of rodenticide incidents is that only one or a few dead animals are 

typically discovered per incident. Even if several non-target animals are poisoned by a 

rodenticide, the carcasses of the killed animals would likely be widely scattered. Mortality 

incidents involving one or a few animals are less likely to be observed, reported, and investigated 

than ones involving large numbers of animals. For these reasons, the Agency believes that the 

rodenticide-related wildlife incidents reported here represent only a fraction of the incidents 

which are occurring.

Wildlife mortality incidents attributed to rodenticide exposure differ from those that are related 

to most other pesticides. Because of the delayed toxicity of rodenticides, the discovery of 

animals killed by rodenticide poisoning is typically removed, both spatially and temporally, from 

the exact site of rodenticide use. Therefore, when a dead animal that has been poisoned by a 

rodenticide is discovered, there is often no suspicion at that time that an animal died as the result 

exposure to rodenticides. Typically, incidents are only linked to rodenticide exposure only when 

they are reported to some authority, usually a state fish and wildlife office, and that office 

conducts an investigation to diagnose the cause of the incident. Investigations of wildlife 

mortality incidents usually begin with a necropsy examination. Observation of hemorrhaging 

and bleeding during the necropsy may cause suspicion that the mortality was caused by an 

anticoagulant rodenticide, which may trigger a toxicological analysis for rodenticides. Animals 

poisoned by bromethalin, which is not an anticoagulant, usually do not have any diagnostic 

symptoms that are obvious in the necropsy exam. The investigating agency may then take tissue 

samples from the animal, normally from the liver, and have laboratory analyses performed to 
Page 150 of 183

screen for the presence of various anticoagulant rodenticides, as well as other pesticides. Only 

when the residue analysis finds one or more rodenticides in the tissue of the dead animal does the 

incident become diagnosed as a suspected or confirmed rodenticide incident. Because this 

analysis only identifies the presence of the active ingredient, the causal agent of a rodenticide 

incident is generally identified only on an active ingredient basis, not a product. 

Because of the reliance on detection of residues in the tissue of the carcasses, wildlife mortality 

incidents may go undetected if the rodenticide ingredient that caused the death is not included in 

the screen of rodenticides performed on the tissue sample, or if the analytical method is not 

sensitive enough to detect the rodenticide at the levels present. Incidents caused by bromethalin 

in particular may be underreported because the rodenticide screens conducted in investigations of 

wildlife mortality incidents often only include anticoagulant compound, but not bromethalin, and 

thus the number of bromethalin incidents may be underreported. Furthermore, since bromethalin 

poisoning does not produce toxicity symptoms that are easily observed in gross morphology 

examinations, the necropsy examination generally do not provide cues that would cause the 

investigator to suspect bromethalin poisoning and include it in the toxicological analysis. 

Therefore, a lack of reported bromethalin incidents may not necessarily reflect a lack effects in 

the field.

Only states which have personnel and resources devoted to investigating and reporting on 

wildlife mortality incidents report such incidents to the EPA. Only two states, New York and 

California, have had programs that have systematically reported wildlife mortality incidents, and 

thus are responsible for the majority of the Agency’s known rodenticide incidents. Most of the 

incidents have been reported from only two states, New York and California, which have had 

effective and well-funded incident investigation and reporting programs. Reporting of wildlife 

incidents in many states have been inconsistent because of sporadic funding of wildlife 

programs. Many states have never reported any rodenticide-related wildlife mortality incidents 

to the EPA. 

Changes in the level of effort that states devote to monitoring and reporting wildlife incidents 

change overtime due to a changes of resources. During the late 1990’s, many dead birds were 

collected and analyzed by some state and local health departments as part of surveillance for 

West Nile virus, resulting in a temporary increase in incident reporting during those years. 

Reporting of rodenticide-related incidents decreased after these West Nile virus surveillance 

programs were discontinued. In recent years, reductions of state budgets has impacted funding 

for state programs that are responsible for investigating wildlife mortality incidents, further 

reducing the incident reporting rates. 

Reporting of rodenticide-related wildlife incidents by pesticide registrants has also decreased. 

Changes made in 1998 to the FIFRA 6(a)(2) relaxed reporting requirements and resulted in 
Page 151 of 183

reduced numbers of incident reports submitted by registrants. Registrants are now only required 

to submit individual reports on “major” wildlife incidents, whereas “minor” wildlife incidents 

may be reported only as aggregated counts without information on the individual events. Since 

no information is generally provided on individual minor incidents, these incidents cannot be 

entered in the EIIS database and are not included in analyses based on EIIS data. The majority 

of rodenticide-related incidents fall into the “minor” category of the 6(a)(2) rule because they 

involve mortality of only one or a few individual animals. 

The National Wildlife Health Center (NWHC) of the US Geological Survey investigates and 

reports on wildlife mortality incidents. However, the NWHC generally only becomes involved 

with investigations of only larger-scale mortality events, such as those caused by wildlife disease 

outbreaks. They normally are not involved with mortality events involving only one or a few 

animals, especially when the cause is easily diagnosed, as is true for most rodenticide-related 

incidents. For this reason, wildlife mortality incidents caused by rodenticides are generally 

lacking from the USGS/NWHC Quarterly Mortality Reports. 

The Agency recognizes that there are limitations with available information on rodenticiderelated wildlife incidents and use of rodenticides, that there are limited information linking 

incidents and rodenticide use. Reporting of rodenticide-related incidents to the EPA has been 

limited and inconsistent. Incidents from those states may not be completely representative of 

adverse effects occurring in other states. Also, the frequency of reported rodenticide-related 

wildlife incidents are influenced by factors other than those directly related to the frequency of 

effects in the field. For example, more incidents may be reported in areas with high human 

populations in part because more people are there to observe and report the carcasses of the 

poisoned animals. This difference in reporting rates likely affected the analysis of the prevalence 

of incidents in rural/field versus urban/suburban locations as the reporting rate is likely greater in 

urban/suburban areas. The rate of occurrence of incidents related to a particular rodenticide 

ingredient is possibly related to the amount of use of that ingredient. 

Since several rodenticides accumulate in animal tissue over time, more than one rodenticide 

active ingredient is sometimes found in tissue of animal carcasses, and thus the same incident 

may be associated with exposure to more than one active ingredient. This is of particular interest 

for the anticoagulant rodenticides, which have the same mode of action. Sometimes, the level of 

detection of one rodenticide ingredient predominated and indicated that it is likely the primary 

cause of death, but in other cases the primary cause is unclear. In such cases, the uncertainty is 

reflected in the certainty value assigned to each ingredient. In the summary counts of incidents 

for various ingredients shown in Tables 6-1 through 6-7, incidents are counted for each active 

ingredient and thus incidents linked to multiple active ingredients were included in the counts for 

more than one active ingredient.
Page 152 of 183

Some of the rodenticides considered in this document have uses to control pests other than 

commensal rodents. Chlorophacinone, diphacinone, and warfarin have numerous products 

registered for use to control agricultural mammalian pests such as prairie dogs, voles, gophers 

and rabbits. Bromethalin is registered for use to control moles as well as rats and mice. 

Unfortunately, the intended target species of the use of rodenticides associated with wildlife 

mortality incidents is usually unknown. Incidents considered in this analysis thus may include 

ones for uses other than commensal rodent control. Therefore, inferences made concerning the 

relative frequency and proportions of incidents for these rodenticides may be influenced by these 

other uses and may not be completely representative of the commensal rodent uses. For 

example, since chlorophacinone and diphacinone have many uses for controlling agricultural 

pests, they may be used more in rural areas where reporting rates of incidents are lower. In 

addition, chlorophacinone and diphacinone have registered uses that allow broadcast application 

of bait over large areas, which may not be representative of uses for commensal rodent control in 

and around buildings. Rodenticide bait containing chlorophacinone and diphacinone for use to 

control commensal rodents have only been registered in the United States in the past three years. 

Since most reported incidents for these ingredients took place during years before these new 

products were registered, they likely are largely associated with uses to control agricultural pests 

other than commensal rodents. 

Finally, uncertainty is introduced by changes in use patterns over time, particularly relative to 

incident collection or survey efforts. The most dramatic shifts in usage occurred mostly in the 

late 1980’s, when there was a large shift in the rodenticide market from warfarin to secondgeneration anticoagulant rodenticides. 

6.3.2 Incident Conclusions

Despite the uncertainties described above, incident data provide compelling evidence that use of 

rodenticides for commensal rodent control can cause mortality of nontarget wildlife as the result 

of primary and secondary exposure. Investigation of many wildlife mortality incidents 

diagnosed rodenticide exposure as the cause of death with high certainty. This confirms that a 

complete exposure route exists for non-target species of nontarget wildlife to be exposed to 

rodenticides at lethal doses. 

From this evaluation of wildlife mortality and exposure incident associated with rodenticide 

active ingredients, the following conclusions are evident:

• Incident data support the conclusion that all rodenticide ingredients pose a risk of primary 

poisoning of nontarget mammals, both in urban and rural areas. Some of these mortality 

incidents result from large mammals such as deer consuming rodenticide bait. 
Page 153 of 183

• Incidents of mortalities to birds that represent primary consumers of rodenticides have 

been reported for brodifacoum, warfarin, chlorophacinone and diphacinone.

• Uses of brodifacoum, difethialone, warfarin, chlorophacinone and diphacinone as 

rodenticides represent complete exposure pathways for non-target wildlife via secondary 

exposure that can result in mortality for birds and mammals. This conclusion is 

supported by reported incidents in species of mammals and birds that represent predators 

and scavengers of target pests of rodenticides.

• Although few incidents have been reported for bromethalin, this may be because this 

ingredient is not often included in analysis of tissue residues in investigations of wildlife 

mortality incidents.

• Considerable numbers of incidents of primary and secondary poisoning occur in urban 

and suburban areas, suggesting that residential and commercial uses of rodenticides for 

commensal rodent control can result in primary and secondary poisoning of nontarget 

wildlife.

• Studies in the United States and elsewhere have reported frequent detections of residues 

of brodifacoum in numerous species of avian and mammalian predators and scavengers. 

Difethialone has also been detected in several studies. Accumulated residues of other 

rodenticides considered in this document (i.e., warfarin, chlorophacinone, and 

diphacinone) have been reported less frequently in survey data in the open literature.

7 Synthesis of Conclusions

All of the anticoagulants identified in the NOIC present risk to terrestrial wildlife in one form or 

another. However, there are differences in the risk picture relative to taxonomic group, body size 

and dietary parameters. This assessment was not conducted with the primary purpose of 

comparing risks across the rodenticides. However, over the course of the conduct of the 

assessment a few comparative conclusions are evident. Potential primary and secondary risks 

were identified for all rodenticides assessed, but brodifacoum stands out for risks to terrestrial 

wildlife, particularly for secondary exposure potential based on the lines of evidence considered 

in this assessment. The results of this analysis confirm those from previous analyses and are 

summarized in Tables 7-1 through 7-4. 
Page 154 of 183

Table 2-1. Avian RQ analysis: primary exposure of passerine birds to bait and secondary exposure of 

carnivore/scavenger birds to residues in consumed rodents. 

Body 

weights (g)

BrodifacoumDifethialoneWarfarin 

(0.025%)

Warfarin 

(0.054%)

ChlorophacinoneDiphacinoneBromethalin

Primary exposure

Single day dose RQ

20 19.61* 29.80* 0.23 0.50* 0.07 0.01 7.76*

100 22.67* 18.39* 0.14 0.31 0.04 0.01 4.76*

1000 27.89* 9.22* 0.07 0.16 0.02 <0.01 2.40*

Six day dose RQ

20 117* 168* 1.36* 2.94* 0.40 0.05 34*

100 135* 104* 0.84* 1.81* 0.25 0.03 21*

1000 166* 52* 0.42 0.91* 0.12 0.01 11*

Dietary RQ NA 38* 45* 0.40 0.86* 0.89* 0.06 0.48

Theoretical secondary exposure

House mouse consumption: 1 

day accumulation in prey RQ

100 6.60* 5.84* 0.05 0.10 0.01 <0.01 1.52*

1000 5.15* 1.85* 0.01 0.03 <0.01 <0.01 0.48

5000 4.33* 0.83* 0.01 0.01 <0.01 <0.01 0.22

Norway rat consumption: 1 

day accumulation in prey RQ

100 1.75* 1.54* 0.01 0.03 <0.01 <0.01 0.40

1000 1.36* 0.49 <0.01 0.01 <0.01 <0.01 0.13

5000 1.15* 0.22 <0.01 <0.01 <0.01 <0.01 0.06

House mouse consumption: 3 

day accumulation in prey RQ

100 19.77* 17.09* 0.14 0.30 0.04 0.01 4.06*

1000 15.42* 5.42* 0.04 0.09 0.01 <0.01 1.29*

5000 12.96* 2.43* 0.02 0.04 0.01 <0.01 0.58*

Norway rat consumption: 3 

day accumulation in prey RQ

100 5.25* 4.52* 0.04 0.08 0.01 <0.01 1.07*

1000 4.08* 1.43* 0.01 0.02 <0.01 <0.01 0.34

5000 3.43* 0.64* 0.01 0.01 <0.01 <0.01 0.15

House mouse consumption: 6 

day accumulation in prey RQ

100 39.40* 32.98* 0.27 0.58* 0.08 0.01 NA

1000 30.74* 10.45* 0.09 0.19 0.02 <0.01 NA

5000 25.84* 4.68* 0.04 0.08 0.01 <0.01 NA

Norway rat consumption: 6 100 10.43* 8.73* 0.07 0.15 0.02 <0.01 NA
Page 155 of 183

day accumulation in prey RQ 1000 8.14* 2.77* 0.02 0.05 0.01 <0.01 NA

5000 6.84* 1.24* 0.01 0.02 <0.01 <0.01 NA

House mouse consumption: 

dietary RQ NA 17.81* 2069* 0.19 0.41 0.32 0.02 0.20

Norway rat consumption: 

dietary RQ NA 4.71* 5.48* 0.05 0.11 0.09 <0.01 0.05

Empirical

Secondary 

Exposure

Single day dose RQ

100 4.42* 4.64* <0.01 <0.01 <0.01 ND

1000 5.01* 1.47* <0.01 <0.01 <0.01 ND

5000 4.21* 0.66* <0.01 <0.01 <0.01 ND

Dietary RQ NA 5.25* 5.54* <0.01 0.02 <0.01 ND

NA = not applicable; ND = no data; * = RQ exceeds the LOC of 0.5
Page 156 of 183

Table 2-2. Avian Risk Characterization: Additional lines of evidence

Body 

weights (g)

BrodifacoumDifethialoneWarfarin 

(0.025%)

Warfarin 

(0.054%)

ChlorophacinoneDiphacinoneBromethalin

Primary exposure

Days to consume sufficient bait 

to reach the LD50

20 <1 <1 4-5 1-2 16-17 >365 <1

100 <1 <1 7-8 3-4 30-31 >365 <1

1000 <1 <1 14-15 6-7 117-118 >365 <1

Grams bait consumed to reach 

the LD50

20 0.3 0.2 25 10 86 >1800 0.7

100 0.9 1.1 160 80 620 >7300 4.2

1000 5 15 2100 990 17000 >52000 59

Secondary exposure

Number of house mice 

consumed to reach the LD50: 1 

day accum. in prey

100 <1 <1 38 18 130 821 1

1000 2 4 539 249 1836 11602 16

5000 5 27 3429 1587 11690 73852 104

Number of Norway rats to 

consume to reach the LD50: 1 

day accum. in prey

100 <1 <1 7 3 23 147 <1

1000 <1 <1 97 45 329 2079 3

5000 <1 5 614 284 2094 13232 17

Secondary feeding study 

summary counts

(#mortalities/ #tested)

NA 63/149 ND 2/23 0/112 3/34 ND

Incident data 

summary 

counts

primary exposure NA 3 0 1 3 1 0

secondary exposure NA 164 3 5 3 5 0

unknown exposure NA 21 0 0 1 0 0

NA = not applicable; ND = no data; * = RQ exceeds the LOC of 0.5
Page 157 of 183

Table 2-3. Mammalian RQ analysis: primary exposure of rodents to bait and secondary exposure of 

carnivore/scavenger mammals to residues in consumed rodents.

Body weights 

(g)

BrodifacoumDifethialoneWarfarin 

(0.025%)

Warfarin 

(0.054%)

ChlorophacinoneDiphacinoneBromethalin

Primary exposure

Single day dose RQ

15 9.93* 4.30* 8.36* 18.05* 0.82* 1.97* 5.1*

35 8.48* 3.67* 7.14* 15.42* 0.70* 1.68* 4.4*

1000 4.55* 1.97* 3.83* 8.27* 0.38 0.90* 2.4*

Six day dose RQ

15 59* 24* 49* 106* 4.7* 11.6* 23*

35 51* 21* 42* 90* 4.0* 9.9* 19*

1000 27* 11* 22* 48* 2.1* 5.3* 10*

Dietary RQ NA 94* ND 57* 123* 44* 22* ND

Theoretical secondary exposure

House mouse consumption: 1 day 

accumulation in prey RQ

50 3.71* 1.62* 2.97* 6.41* 0.28 0.94* 1.62*

1000 4.60* 2.01* 3.68* 7.95* 0.35 1.16* 2.01*

3000 4.98* 2.17* 3.99* 8.61* 0.38 1.26* 2.17*

Norway rat consumption: 1 day 

accumulation in prey RQ

50 0.98* 0.43 0.79* 1.70* 0.08 0.25 0.43

1000 1.22* 0.53* 0.97* 2.11* 0.09 0.31 0.53*

3000 1.32* 0.58* 1.05* 2.28* 0.10 0.33 0.58*

House mouse consumption: 3 day 

accumulation in prey RQ

50 11.10* 4.74* 8.81* 19.03* 0.84* 2.49* 4.31*

1000 13.78* 5.88* 10.93* 23.61* 1.04* 3.09* 3.53*

3000 14.91* 6.37* 11.83* 25.56* 1.12* 3.35* 5.79*

Norway rat consumption: 3 day 

accumulation in prey RQ

50 2.94* 1.25* 2.33* 5.04* 0.22 0.66* 1.14*

1000 3.65* 1.56* 2.89* 6.25* 0.27 0.82* 1.42*

3000 3.95* 1.68* 3.13* 6.76* 0.30 0.89* 1.53*

House mouse consumption: 6 day 

accumulation in prey RQ

50 22.13* 9.15* 17.35* 37.48* 1.63* 4.20* NA

1000 27.46* 11.35* 21.53* 46.51* 2.02* 5.21* NA

3000 29.72* 12.28* 23.30* 50.33* 2.18* 5.64* NA

Norway rat consumption: 6 day 

accumulation in prey RQ

50 5.86* 2.42* 4.59* 9.92* 0.43 1.11* NA

1000 7.27* 3.00* 5.70* 12.31* 0.53* 1.38* NA

3000 7.87* 3.25* 6.17* 13.32* 0.58* 1.49* NA
Page 158 of 183

House mouse consumption: dietary 

RQ NA 44.69* ND 26.64* 57.54* 20.42* 10.11* ND

Norway rat consumption: dietary 

RQ NA 11.23* ND 7.05* 15.23* 5.41* 2.68* ND

Empirical 

secondary 

exposure

Single Day RQ

50 3.59* 1.41* 0.26 0.07 0.19 ND

1000 4.46* 1.75* 0.32 0.08 0.23 ND

3000 4.83* 1.89* 0.34 0.08 0.25 ND

Dietary RQ NA 13.17* NA 0.67* 1.39* 0.61* ND

NA = not applicable; ND = no data; * = RQ exceeds the LOC of 0.5
Page 159 of 183

Table 2-4. Mammalian Risk Characterization: Additional lines of evidence from the mammal risk assessment.

Body 

weights (g)

BrodifacoumDifethialoneWarfarin 

(0.025%)

Warfarin 

(0.054%)

ChlorophacinoneDiphacinoneBromethalin

Primary exposure

Days to consume sufficient bait

to reach the LD50

15 <1 <1 <1 <1 1-2 <1 <1

35 <1 <1 <1 <1 1-2 <1 <1

1000 <1 <1 <1 <1 1-2 <1 <1

Grams bait consumed to reach 

the LD50

15 0.3 0.7 0.3 0.2 6 1.1 0.6

35 0.5 1.3 0.7 0.3 9 2.1 1.1

1000 6.7 16 8.0 3.7 61 25 13

Secondary exposure

Number of house mice 

consumed to reach the LD50: 1 

day accum. in prey

50 <1 <1 <1 <1 3 <1 <1

1000 2 5 3 1 27 8 5

3000 5 11 6 3 60 18 11

Number of Norway rats to 

consume to reach the LD50: 1

day accum. in prey

50 <1 <1 <1 <1 <1 <1 <1

1000 <1 <1 <1 <1 5 1 <1

3000 <1 2 1 <1 11 3 2

Secondary feeding study summary counts

(#mortalities/ #tested) NA 8/19 ND 9/100 32/55 19/33 0/4

Incident data 

summary 

counts

primary exposure NA 41 1 2 3 9 1

secondary exposure NA 22 1 0 1 4 0

unknown exposure NA 16 0 0 0 0 0

NA = not applicable; ND = no data; * = RQ exceeds the LOC of 0.5
Page 160 of 183

Table 7-4. Additional lines of evidence from the mammal risk assessment.

Body 

weights (g)

BrodifacoumDifethialoneWarfarin 

(0.025%)

Warfarin 

(0.054%)

ChlorophacinoneDiphacinoneBromethalin

Primary exposure

Days to consume sufficient bait 

to reach the LD50

15 <1 <1 <1 <1 1-2 <1 <1

35 <1 <1 <1 <1 1-2 <1 <1

1000 <1 <1 <1 <1 1-2 <1 <1

Grams bait consumed to reach 

the LD50

15 0.3 0.7 0.3 0.2 6 1.1 0.6

35 0.5 1.3 0.7 0.3 9 2.1 1.1

1000 6.7 16 8.0 3.7 61 25 13

Secondary exposure

Number of house mice 

consumed to reach the LD50: 1 

day accum. in prey

50 <1 <1 <1 <1 3 <1 <1

1000 2 5 3 1 27 8 5

3000 5 11 6 3 60 18 11

Number of Norway rats to 

consume to reach the LD50: 1 

day accum. in prey

50 <1 <1 <1 <1 <1 <1 <1

1000 <1 <1 <1 <1 5 1 <1

3000 <1 2 1 <1 11 3 2

Secondary feeding study summary counts

(#mortalities/ #tested) NA 8/19 ND 9/100 32/55 19/33 0/4

Incident data 

summary 

counts

primary exposure NA 41 1 2 3 9 1

secondary exposure NA 22 1 0 1 4 0

unknown exposure NA 16 0 0 0 0 0

NA = not applicable; ND = no data; * = RQ exceeds the LOC of 0.5
Page 161 of 183

7.1 Primary Exposure Risk Conclusions

7.1.1 Quantitative Risk Assessment Results across the Rodenticides 

As with all rodenticides, brodifacoum and difethialone represent a lethal primary risk to nontarget small mammals that may eat either loose treated bait or utilize a bait station. In addition, 

the quantitative assessment also demonstrates that bait containing these two rodenticides 

constitutes a primary exposure risk concern for larger mammals as well as all avian size classes. 

This conclusion is consistent across all primary exposure risk analyses performed (single oral 

dose, six-day accumulated dose, and dietary concentration). EPA’s analysis indicates birds and 

mammals could attain a median lethal dose (LD50) of either brodifacoum or difethialone upon 

feeding for less than day on treated bait. The brevity of feeding required to reach a potentially 

lethal exposure represents an increased opportunity (relative to all other rodenticides assessed) 

for non-target birds and mammals to chance upon loose treated bait and be lethally exposed.

Warfarin also demonstrates a primary exposure risk concern to small mammals. The assessment 

of warfarin indicates a risk to large mammals for all assessment methods employed as well. 

EPA’s analysis indicates that feeding on treated bait for less than a day is sufficient to attain a 

median lethal dose in all evaluated size classes of mammals, suggesting a similar opportunity, 

relative to brodifacoum and difethialone, for non-target mammals to chance upon treated bait and 

consume a lethal dose. In contrast, warfarin’s primary exposure risk for birds is limited to small 

passeriforms and other medium sized birds. These results are dependent on the assessment 

method EPA considered. While a single day dose risk assessment suggests only a risk to small 

passeriforms from the highest concentration formulation, accounting for the potential for 

multiple day accumulation on a dose basis expands the concerns to include passeriforms and 

other small and medium sized birds at all formulation concentrations assessed. The dietary 

assessment does not consider body weight as a determinant factor. However, this assessment 

method still predicts a risk concern at the highest formulation concentration. EPA’s analysis 

indicates that attainment of a lethal warfarin dose can occur after 1 to 7 days of feeding (0.054% 

warfarin bait) or after 4 to 15 days of feeding (0.025% warfarin bait), depending on body weight 

of the passerine in question. This finding suggests that, for birds there is less of an opportunity 

for chance lethal encounters with warfarin when compared with brodifacoum and difethialone, 

since it would require multiple days of feeding on warfarin treated bait.

For bromethalin, as can be expected for a relatively fast acting compound (target mortality 1 to 7 

days, Pitt et al. 2011, and registrant submitted studies), acute risk concern levels are exceeded for 

all mammalian size classes feeding on treated bait based on a single day of exposure. Primary 

risk to birds from bromethalin exceeds risk concern levels in all assessment methodologies with 

the exception of the dietary exposure methodology, which returns a risk quotient that closely 

approaches but does not exceed the acute risk LOC. EPA’s analysis indicates that less than a 

single day’s feeding of bromethalin bait is sufficient to attain a median lethal dose in birds and 
Page 162 of 183

mammals, suggesting that like brodifacoum and difethialone, adverse effects to non-target 

wildlife are likely from primary exposure events with treated bait. 

Two anticoagulant alternatives, chlorophacinone and diphacinone, also present primary exposure 

risk concerns for nearly all rodent and mammal sizes assessed using all primary exposure 

assessment methods (large rodents consuming chlorophacinone bait for one day are not expected 

to experience risk concerns). EPA’s analysis indicates that <1 day (diphacinone) and <1 to 3 

days (chlorophacinone) of feeding on bait are sufficient to attain a median lethal dose in 

mammals. This is not materially different from the other rodenticides with respect to the 

opportunity for non-target mammals to chance upon and feed lethally on treated bait. In contrast 

to mammals, avian risk is below concern levels for these two compounds. This may be a 

limitation in the analysis of primary exposure models terminating at six days (i.e., longer 

exposure periods might provide sufficient exposure to bait for adverse accumulation to occur); 

however, the analysis indicates that, unlike brodifacoum and difethialone, a very consistent and 

protracted feeding period for either diphacinone or chlorophacinone is required to attain a lethal 

dose in birds (i.e., daily feedings for the former versus weeks to months for the latter). 

7.1.2 Incident Analysis Findings as they Relate to Primary Exposure Risks 

The quantitative risk assessment results cannot provide direct evidence relating risks to wildlife 

for various land uses; however, the incident data can be used to characterize the spatial locations 

of mortalities that have been attributed to specific rodenticides. The incident data show a sizable 

number of primary incidents occurring in urban/suburban areas. One can conclude that 

residential and commercial use of rodenticides in these areas pose complete exposure pathways 

to a variety of wildlife and that primary exposure lethal incidents involving non-target wildlife

can occur.

Incident data support the quantitative risk assessment’s contention that primary exposure to 

brodifacoum poses lethal risk to a variety of sizes of mammals and birds. Available incidents 

representing primary brodifacoum exposure include mortalities of non-target mammalian 

wildlife for species ranging in size from chipmunks to white-tailed deer. These incidents are 

consistent with results of the deterministic assessment. For birds, incidents concluded to 

represent likely primary exposure involved sizes ranging from robins through geese. Again, these 

incidents are consistent with the risk model predictions. Incident data indicate that likely lethal 

events from primary exposure to brodifacoum for non-target wildlife can occur in rural, suburban 

and urban environments.

The incident data set for difethialone is much more limited than brodifacoum. Analysis of the 

toxicity and retention time of difethialone indicates that it is toxicologically similar to 

brodifacoum, suggesting that the lack of observed incidents for this ingredient is likely due to the 
Page 163 of 183

low use or to other factors not related to risk. Nevertheless, there is a lethal incident involving a 

species likely directly exposed to difethialone treated bait: the Key deer, in a suburban 

environment and this incident is consistent with the primary risk assessment. There are no 

incidents involving difethialone and bird species expected to involve primary exposure to treated 

bait materials; however, the lack of primary avian incidents involving this chemical neither 

supports or refutes the primary risk assessment findings. 

Warfarin incidents with non-target wildlife generally parallel the findings of the quantitative risk 

assessment for primary exposure to this chemical, i.e., risk was identified in the assessment for 

primary birds and mammals and there are incidents involving these taxa. Little can be said 

regarding the land use characteristics in areas where incidents were reported. This may be more 

a factor of the age of the incidents reports and the relative low recent use of warfarin than any 

real differences in the pattern of occurrence of wildlife exposures.

The incident data set for primary animals exposed to bromethalin is limited to a reported 

mortality of a chipmunk and this incident involved a commensal rodent control product. This 

limited information is consistent with a quantitative risk assessment that indicates a risk to small 

mammals exposed to treated bait, yet provides little support risk assessment predictions for 

larger mammal. There are no incidents involving bromethalin and bird species expected to 

involve primary exposure to treated bait materials. There may be an extenuating issue that 

explains the absence of reported incidents for this chemical. As discussed in the incident 

sections, bromethalin is not a commonly assayed compound when investigating wildlife 

mortality incidents.

Chlorophacinone incidents of high certainty involving mammals of likely primary exposure to 

bait are scarce but include reports or mortalities in squirrels (Central Park, New York) which 

would be in agreement with the primary exposure risk assessment predictions for small 

mammals. Larger mammal incidents attributable to primary exposure to this chemical are not 

reported. An incident report with a likely primarily exposed bird (California quail in an orchard) 

suggests a possible lack of appropriate conservatism in the chlorophacinone primary risk 

assessment. The report of this incident identified crop content in the mortal individual as 

containing physical residue of parafinized rodenticide pellets. 

The diphacinone incident data set for mammals is similar to that for chlorophacinone, in that the 

only incident involves a squirrel in an urban setting (Central Park, New York). Again, as for 

chlorophacinone, there is a diphacinone incident involving a likely primarily exposed bird, i.e., a

turkey, again suggesting a possible lack of quantitative risk assessment conservatism. This 

incident involved birds feeding in the vicinity of a prairie dog town, not a commensal rodent 

control operation. 
Page 164 of 183

7.1.3 Implications for Mitigation

For primary exposure lethalities to non-target wildlife to occur, the organisms must encounter the 

treated bait formulation and ingest it. 

The RMD mitigations for all rodenticides required the placement of commensal rodent control 

chemicals in bait boxes and in forms not readily removable from the box. It is qualitatively 

anticipated that bait boxes represent a reduction in the geographic extent of bait dispersal across 

the landscape relative to other application methods that may involve scattering of bait. Exposed 

bait (e.g., scattered pellets, uncovered place packs, and exposed bait blocks) may offer an 

increased opportunity for casual encounter with rodenticides when compared with a more focally 

placed bait box, and indeed, for such chemicals as brodifacoum, difethialone, bromethalin and 

warfarin, only a few feeding episodes are sufficient to cause mortality. Moving to bait boxes is 

likely to result in a reduction of encounters of wildlife with rodenticides.

In addition to the issue of geographical distribution of rodenticide, the requirement for bait boxes 

may further limit access of non-target animals to the bait itself. Because bait boxes are 

optimized for the commensal rodent body size (mice and rats), larger mammals (e.g., deer) are 

physically limited by the box portal size, which prevents them from reaching the bait within. 

Bird behavior and feeding patterns suggest that they would be unlikely to enter bait boxes to feed 

even if the bird was small enough to enter. In addition, the requirement for bait to be not readily 

removable from the boxes indicates that even boxes disturbed by larger organisms will present a 

reduced opportunity for contact with the enclosed bait. 

The RMD also required that commensal rodent control bait box placement occur within and not 

more than 50 feet from the exterior of structures. This likely represents a reduced opportunity 

for wildlife to encounter rodenticides. By geographically limiting the areas that may be treated 

with the chemicals the wildlife, co-occurrence will be likely limited to individual animals and 

species that are more tolerant of human activities and only in areas immediate to human 

structures and sites of habitation.

The RMD also required that second generation rodenticides (including brodifacoum and 

difethialone) be applied in a domestic setting by a licensed applicator. There are opportunities 

with this requirement to further reduce the potential for wildlife to encounter these rodenticides. 

Effective rodent control methods employed by licensed applicators probably incorporate an 

integrated pest management approach. In addition to the rodenticide application, licensed 

applicators will likely take additional steps to limit the shelter, water, and food resources 

surrounding infested domestic areas. This integrated approach may have the ancillary effect of 

reducing the attractiveness of such residential areas to wildlife, thereby potentially reducing 

encounters of non-target wildlife with rodenticides.
Page 165 of 183

7.2 Secondary Exposure Risk Conclusions 

7.2.1 Quantitative Risk Assessment Results Across the Rodenticides 

Rodenticide intake based on consumption of bait by the primary consumers (prey) was 

calculated for the house mouse and Norway rat. For this analysis, rodenticide accumulation in 

prey over a one or six day period was calculated in order to bracket the available data from 

residue studies submitted to the agency and occurring in open literature. Accumulation was 

determined based on daily food intake rates for the prey, assuming 100% of their diet consisted 

of dry bait. It was assumed that the form of the bait would not influence intake. Based on the 

assumed weight of the primary consumers, the percent active ingredient in the bait, and daily 

food intake, the concentration within the prey animal at the end of a day’s feeding was 

determined. Accumulation of rodenticide within the prey animal over time was calculated using 

an elimination rate constant based on the liver half-lives for each chemical to approximate 

elimination. 

Exposure to secondary consumers was determined in a similar manner. Secondary consumers 

were assumed to be mammals weighing 50g, 1000g, or 3000g, or birds weighing 100g, 1000g, or 

5000g. These weights were assumed based on the range of secondary consumers identified in 

rodenticide incidents in the Environmental Incident Information System (EIIS) database. The 

general equation for food intake was used to calculate the daily rodenticide intake for 

predator/scavenger birds and mammals consuming house mice and Norway rats exposed to 

rodenticide bait for three or six days.

RQs were also calculated based on subacute dietary LC50 values and expected concentration of 

the rodenticides in prey items. Daily rodenticide intake values (mg a.i./kg-bw/day) for primary 

consumers based on their consumption of bait were used as an estimate of the a.i. concentration 

in prey items after one day of feeding. This value can be considered as the dietary concentration 

(mg a.i./kg diet) for secondary consumers. RQs were calculated by dividing this value by the 

LC50 values for birds and mammals.

A combination of toxicity and biological persistence with brodifacoum and difethialone results in 

a trigger for secondary exposure risk concerns for all assessed mammals and birds, regardless of 

the method used to estimate exposure (theoretical and empirical). The delayed time to mortality

for brodifacoum, coupled with its slow clearance time, results in accumulation of this rodenticide 

in target mammals to levels that are two orders of magnitude above the median lethal dose. This 

suggests an enhanced opportunity for secondary exposure in that the rodenticide body burden 

reaches levels far above that which is necessary to kill the target organism. Subsequent ingestion 

of these targets (live or moribund) by wildlife represents dietary doses may occur in excess of 

concentrations in the rodenticide bait itself. These factors combine to yield risk quotients for 
Page 166 of 183

brodifacoum and difethialone that indicate that there are concerns for lethality in all assessed 

sizes of mammalian and avian scavengers/predators consuming rats and mice. 

The warfarin quantitative analysis for secondary risks to mammalian wildlife is similar to 

brodifacoum and difethialone. However, the avian scavenger/predator results depart from the 

observations for the other two rodenticides. Only the smallest scavenger/predator birds, 

consuming the smallest target pest and only the pests consuming bait over protracted periods of 

time is sufficient to raise risk concerns.

The bromethalin secondary risk assessment results are similar to brodifacoum and difethialone

with respect to mammals. As with warfarin, the bird risk assessment results for bromethalin 

depart somewhat from brodifacoum and difethialone. For birds that consume target animals 

exposed to bromethalin, no rat consumption triggered risk concerns yet mouse consumption 

triggered concerns in all size avian scavenger/predators regardless of exposure duration. 

Mammalian secondary risk concerns for diphacinone and chlorophacinone are triggered for 

consumption of target pests (mice only as these compounds are not labeled for rats). Both of 

these compounds do not present secondary risk concerns for birds. Available toxicity data for 

diphacinone suggest that raptors are more sensitive than the test species used to derive the RQs 

for secondary avian species. Although incorporation of the raptor data would not materially 

affect conclusions of the deterministic risk assessment, it may at least in part explain the 

presence of secondary poisoning incidents for diphacinone in the absence of LOC exceedances 

(See Section 7.2.4).

7.2.2 Secondary Feeding Studies

Concerns for secondary risks with brodifacoum are supported by available data from secondary 

toxicity studies showing mortality in 63% of predator/scavenger birds and 42% of 

predator/scavenger mammals fed brodifacoum contaminated target organisms. Secondary 

feeding studies include data on mammalian species including foxes, mongoose. Weasels, and 

dogs and avian species including hawks, kestrels, owls, and gulls. No secondary feeding data are 

available for difethialone. 

In the case of warfarin, secondary feeding studies involving mammal predators/scavengers show 

mixed results and for some species, the differences extended to a dose-dependent response. No 

mortalities were observed in raccoons and European ferrets (no dose dependency and bait 

concentrations equal to or higher levels found in US registered products). Other studies showed 

lethalities in mink, least weasels and dogs, though the results often involved few test organisms. 

These findings with warfarin do little to support or refute globally the results of the quantitative 

analysis and may reflect a wide range in sensitivity of predator and scavenger mammals to 
Page 167 of 183

warfarin. Another possible explanation for high variability, given the span of dates when the 

studies were conducted, is that responses may also reflect differences in active ingredient 

potency as discussed in the primary toxicity section’s description of available acute toxicity 

results. Avian secondary feeding studies involved testing of four species. In three of the four 

species tested there was no observed mortality. Interestingly, the results from feeding tests, 

when considering the weights of the birds, do not completely parallel the results of the 

quantitative secondary risk assessment. For example no mortality was observed in the feeding 

studies with black-billed magpies (~170 g bodyweight) while the risk assessment suggests a 

concern for a 100 gram bird. Conversely, mortality results were mixed for larger birds of similar 

weights (death in barn owls but not in tawny owls both ~400-500 g body weight). These 

discrepancies may indicate that allometric relationships for predicting effects of warfarin toxicity 

do not completely explain the factors contributing to species sensitivity variability.

Bromethalin secondary feeding data are limited to a single case. In that study, domestic dogs (n 

= 4) were fed intoxicated rats. No mortalities were observed. For this chemical, the number of 

species tested is limited and so there is insufficient evidence from these data alone to make 

definitive conclusions regarding the accuracy of the secondary quantitative risk assessment 

results. No bird feeding studies were available for bromethalin.

Chlorophacinone secondary toxicity results include studies with five mammal species. Mortality 

responses were variable and ranged from approximately 50 percent to 100 percent of individuals 

tested. In one single species, the European ferret, different studies yielded highly variable 

results, ranging from zero to 100 percent mortality of tested individuals. It should be noted again 

that all these studies involve a low number of test individuals and differences in perceived 

sensitivity may be as much a product of low number of individuals as from real differences in 

toxic response across species. Avian feeding studies with chlorophacinone span 10 species with 

a wide range of bodyweights (170 to 4400 g). In no cases were mortalities observed, which is 

consistent with the secondary risk assessment.

Diphacinone secondary feeding data span studies from seven mammal species. The effects range 

from 0 to 100 percent mortality in tested individuals. Of particular note is the study involving 

actual secondary feeding with rats. This is of note because (1) the secondary exposure risk 

assessment used a rat acute toxicity endpoint as a surrogate for predators and scavengers and (2) 

the assumed prey item concentrations in the risk assessment were 10 to 20 times higher than 

tested in the feeding study. The feeding study with rats yielded a 50 percent mortality response 

under conditions of similar toxicity and lower exposure than modeled conditions showing risk. 

Looking back on the quantitative risk results, substituting the modeled diphacinone dietary 

concentrations with the actual concentrations used in the feeding study would still trigger 

secondary acute risk concerns. These results are considered supportive of the secondary risk 

assessment with mammals. Avian feeding studies with diphacinone span 5 species of varying 

bodyweights. In three species there were no mortalities, which is expected based on the 
Page 168 of 183

secondary risk assessment results. In the cases of two owl species, mortalities were observed, 

which is contrary to the risk assessment results. However, as discussed earlier, this may be 

explained by the toxicity data used to derive RQs for secondary birds in that these data were less 

conservative than available data for a raptorial species exposed to this chemical.

7.2.3 Analysis of the Opportunity for Secondary Exposure Lethality 

As evaluated in the secondary risk assessment, residues reach levels in target organisms that, 

when consumed by predators and scavengers, can result in risk of mortality. When considering 

those risks among the assessed rodenticides, it is important to consider the likelihood of 

predators and scavengers encountering enough intoxicated target prey to reach potential lethal 

doses. Furthering the evaluation of secondary exposure of rodenticides are the results of 

analyses of the number of individual target organisms necessary for dietary consumption to 

achieve a lethal dose in predators and scavengers. The fewer the number of intoxicated target 

organisms necessary to reach a lethal dose, the less efficiently and intently a predator or 

scavenger must locate and consume these organisms to reach lethal body burdens. The analysis 

considered different bodyweights of predator/scavengers consuming either rats or mice which 

have ingested bait for a single day or up to six days. Again, compounds that can present a lethal 

dose to predators and scavengers after consumption of target organisms with only a single day of 

exposure are likely to represent greater opportunities for lethal secondary exposure events than 

those which require multiple days of target organism exposure in target organisms

With brodifacoum, and rats as a diet, every size predator/scavenger mammal need only consume 

a fraction of single target organism to reach lethal endpoint exposures. Comparing these results 

to other NOIC rodenticides does not reveal a marked difference among the chemicals 

(diphacinone and chlorophacinone are not labeled for rat control). Obviously, owing to the 

smaller size of targeted mice, consumption of these organisms intoxicated with brodifacoum 

must be slightly more intensive for the predator/scavengers to reach lethal endpoint exposure (<1 

to 5 organisms). These results are similar to the results for the other NOIC rodenticides. 

However, the brodifacoum results are substantially lower than diphacinone and chlorophacinone, 

suggesting an increased opportunity for chance encounter with brodifacoum intoxicated target 

organisms to trigger a lethal incident with predator/scavenger wildlife.

Similar analyses were conducted to determine the target organism numbers necessary to trigger 

lethal incidents in scavenger/predator birds. Brodifacoum results indicate that consumption of 

only a fraction of an intoxicated rat, regardless of rat exposure duration, is needed to reach lethal 

endpoint exposures in all bird sizes modeled. The results for difethialone are similar to those of 

brodifacoum; however, for warfarin and bromethalin, the results indicate that a secondary bird 

would need to consume many more target mammals to reach the median lethal dose. For 

brodifacoum, mouse consumption would have to be higher than for rats, though not substantially 
Page 169 of 183

so (<1 to 5 organisms consumed) and again results are comparable to difethialone and slightly 

lower than bromethalin. However, similar analyses conducted with warfarin, chlorophacinone, 

and diphacinone reveal that consumption of intoxicated target organisms must reach drastically 

larger and more intent levels to reach lethal exposure endpoints. This suggests that brodifacoum

has a substantially greater opportunity to result in mortality following exposure of a 

predator/scavenger bird to intoxicated target organisms. 

7.2.4 Incident Analysis Findings as they Relate to Secondary Exposure Risks

As in the case of primary exposure risk assessment, the quantitative secondary exposure risk 

assessment cannot, by itself, provide evidence that land use is or is not a factor in where 

secondary exposure wildlife mortalities may occur. Again incident data are important in this 

regard and demonstrate for some chemicals that such incidents of wildlife mortality involving 

predators and scavengers do occur in urban and suburban land use areas.

Incident data support the quantitative risk assessment’s conclusion that secondary exposure to 

brodifacoum poses lethal risk to a variety of sizes of mammals and birds. The presence of a 

large number of secondary mortality incidents together with the results of the deterministic risk 

assessment increase confidence in the conclusion that brodifacoum exposure poses a secondary 

risk concern to birds and mammals. From available incidents concluded to represent likely 

secondary exposure to brodifacoum, non-target mammalian wildlife include lethalities in species 

ranging foxes to mountain lions, consistent with the modeled size classes determined in this 

assessment to be at risk. For birds, incidents involving likely secondary exposures included a 

variety of raptors, which was again consistent with the risk predictions for larger birds. There 

are incidents involving corvid species that are closer to the lowest bodyweight assessed, but it is 

not completely known if all these are primary or secondary exposure results. Incident data 

indicate that likely lethal events from secondary exposure to brodifacoum for non-target wildlife 

can occur in rural, suburban and urban environments.

The incident data set for difethialone is much more limited than brodifacoum. Nevertheless, 

there are lethal incidents involving predatory birds and mammals. These results are consistent 

with the secondary risk assessment findings. All the available incidents are associated with 

suburban and urban areas.

Warfarin incidents with non-target secondary mammals are not available. The lack of secondary 

mammal incidents cannot be considered to be supportive or contrary to the finding of the 

secondary risk assessment. Secondary incidents with warfarin include a variety of raptorial

species and these results are not inconsistent with the conclusions of the secondary risk 

assessment that did not identify risks for larger secondary birds. However the incidents are in 

partial agreement with the mixed available secondary feeding toxicity studies. On balance, the 
Page 170 of 183

quantitative risk assessment may be lacking appropriate levels of conservatism with respect to 

raptorial birds and there exists a real concern for secondary exposure raptor mortality with 

warfarin. Incident data indicate that likely lethal events from secondary exposure to warfarin for 

non-target wildlife can occur in rural, suburban and urban environments.

The incident data set for secondary animals exposed to bromethalin is limited to birds. This 

limited information is can neither support nor refute the mammalian secondary risk assessment 

but is consistent with the quantitative risk assessment for secondary exposure in birds.

Chlorophacinone incidents of high certainty involving mammals and birds of likely secondary

exposure include reports of mortalities in wild felines (bobcats) and raptors (red-tailed hawk). 

The mammal incident is consistent with the secondary risk assessment findings; however, the 

bird incident is inconsistent with the secondary risk assessment. It may be possible that 

chlorophacinone mirrors diphacinone in terms of the sensitivity of raptors being possibly greater 

than the test species employed in the risk assessment (i.e., a 20 fold increase in sensitivity). If 

the chlorophacinone secondary risk assessment was adjusted for this possible increased 

sensitivity in raptors the secondary risk conclusions would then be consistent with reported 

incidents for dietary subacute portions of the analysis but not for the single oral dose analysis. 

On balance, secondary lethalities to birds may occur but the strength of this prediction for this 

chemical is limited.

Diphacinone incidents of high certainty involving mammals and birds of likely secondary

exposure but include reports of mortalities in wild canids, mustilids and felids (fox, coyote, 

raccoon, and mountain lion) and raptors (snowy and barred owls, red-tailed hawk). The mammal 

incidents are consistent with the secondary risk assessment findings. The bird incidents are 

inconsistent with the modeling portions of secondary risk assessment but consistent with 

available feeding studies, especially for owls. If the diphacinone secondary risk assessment was 

adjusted for the possible increased sensitivity in raptors the secondary risk conclusions would 

still yield risk quotients below concern levels. On balance, secondary lethalities to birds may 

occur but the strength of this prediction for this chemical is limited, though the uncertainty in 

concluding the possibility of a lethal risk is less than for chlorophacinone. 

7.2.5 Implications for Mitigation

For secondary exposure lethalities in non-target wildlife to occur, the organisms must encounter 

prey organisms exposed to the treated bait formulation and ingest them. 

The RMD mitigations for all rodenticides required the placement of commensal rodent control 

chemicals in bait boxes and in forms not readily removable from the box. It is qualitatively 

anticipated that bait boxes represent a reduction in the geographic extent of bait dispersal across 

the landscape relative to other application methods that may involve scattering of bait. A 
Page 171 of 183

scattered bait may offer an increased opportunity for casual encounter of prey species with 

rodenticides and therefore, for wide ranging predators and scavengers, an increased opportunity 

to have significant proportions of the diet composed of intoxicated prey. For bait boxes, the 

placement of rodenticides is more geographically focused and can be expected to involve much 

less of the prey base for the wide ranging predators and scavengers. Moving to bait boxes is 

likely to result in a reduction of encounters of predatory and scavenger wildlife with rodenticide.

Bait boxes are optimized for the commensal rodent body size (mice and rats). Larger mammals 

are physically limited by the box portal size preventing them from reaching the bait within. For 

predators that may utilize a variety of body size prey items, narrowly focusing the sizes of 

rodenticide exposed prey could reduce the proportion of exposed prey in the total diet and so 

reduce the overall availability of a rodenticide to the predator. 

The RMD also required that commensal rodent control bait box placement occur within and not 

more than 50 feet from the exterior of structures. This likely represents a reduced opportunity 

for prey wildlife to encounter rodenticides. By geographically limiting the areas that may be 

treated with the chemicals the prey wildlife co-occurrence will be likely limited to individual 

animals and species that are more tolerant of human activities and only in areas immediate to 

human structures and sites of habitation. Again this reduces the variety of prey items likely to 

encounter bait, as compared to widespread applications of rodenticides and so reduces the variety 

and possibly the proportion of the diet exposed organisms contribute to predators and 

scavengers.

The RMD required that second generation rodenticides (including brodifacoum and difethialone) 

be applied in a domestic setting by a certified applicator. There are opportunities with this 

requirement to further reduce the potential for wildlife to encounter these rodenticides. Effective 

rodent control methods employed by certified applicators likely take an integrated approach. In 

addition to the rodenticide application applicators will likely take additional steps to limit the 

shelter, water and food resources surrounding infested domestic areas. This integrated approach 

may have the ancillary effect of reducing the attractiveness of such residential areas to wildlife, 

thereby potentially reducing encounters of non-target wildlife with rodenticides.

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