United States 

Department of 

Agriculture

Marketing and 

Regulatory 

Programs

Animal and 

Plant Health 

Inspection 

Service

Cattle Fever Tick 

Eradication Program 

Use of Ivermectin Corn

Final Environmental

Assessment, January 2017
i

Cattle Fever Tick Eradication 

Program Use of Ivermectin Corn

Final Environmental

Assessment, January 2017

Agency Contact:

Denise L. Bonilla

Cattle Fever Tick Program Manager/Entomologist

Veterinary Services

Animal and Plant Health Inspection Service

U.S. Department of Agriculture

2150 Centre Avenue

Fort Collins, CO 80526

The U.S. Department of Agriculture (USDA) prohibits discrimination in all its 

programs and activities on the basis of race, color, national origin, sex, 

religion, age, disability, political beliefs, sexual orientation, or marital or family 

status. (Not all prohibited bases apply to all programs.) Persons with 

disabilities who require alternative means for communication of program 

information (Braille, large print, audiotape, etc.) should contact USDA’s 

TARGET Center at (202) 720–2600 (voice and TDD).

To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, 

Room 326–W, Whitten Building, 1400 Independence Avenue, SW, Washington, 

DC 20250–9410 or call (202) 720–5964 (voice and TDD). USDA is an equal 

opportunity provider and employer.

Mention of companies or commercial products in this report does not imply 

recommendation or endorsement by the U.S. Department of Agriculture over 

others not mentioned. USDA neither guarantees nor warrants the standard of 

any product mentioned. Product names are mentioned solely to report 

factually on available data and to provide specific information.
ii

Table of Contents

I. Introduction......................................................................................1

A. Background...................................................................... ..............1

B. Purpose and Need............................................................................7 

II. Alternatives..................................................................... ..............8 

A. No Action ..........................................................................................8 

B. Preferred Alternative.........................................................................8 

III. Potential Environmental Impacts .................................... ..............9 

A. No Action ........................................................................................9 

1. Physical Environment .....................................................................9

2. Livestock and Wildlife ...................................................................14

3. Human Environment.....................................................................16

B. Preferred Alternative.......................................................................18

1. Physical Environment ...................................................................18 

2. Livestock and Wildlife ...................................................................19

3. Human Health and Safety.............................................................20

4. Wildlife Protections .......................................................................23 

a. Endangered Species Act..........................................................23

b. Migratory Bird Treaty Act..........................................................24

c. Bald and Golden Eagle Protection Act .....................................26

5. Environmental Justice...................................................................27

6. Tribal Consultation and Coordination............................................27

7. Historic and Cultural Resources ...................................................28

8. Potential Cumulative Impacts .......................................................29

IV. Agencies Contacted ......................................................................32

V. References ...................................................................................33

VI. Appendix 1.....................................................................................38

VII. Appendix 2.....................................................................................73
1

I. Introduction 

A. Background

The U.S. Department of Agriculture (USDA), Animal and Plant Health Inspection 

Service (APHIS), Veterinary Services (VS) is responsible for (1) protecting and 

improving the health, quality, and marketability of U.S. animals by eliminating animal 

diseases, and (2) monitoring and promoting animal health and productivity. The 

Animal Health Protection Act of 2002, as amended, (7 U.S.C. §§ 8301-8317) provides 

broad authority for APHIS to prevent the introduction into or dissemination within the 

United States of any pest or disease of livestock (§§ 8303-8305). The Act authorizes 

prohibition and restriction of the importation, exportation, and interstate movement of 

animals moving in trade and strays, as well as exportation, inspection, disinfection, 

seizure, quarantine, destruction and disposal of animals and conveyances (§§ 8303-

8308). This includes the ability to “carry out operations and measures to detect, control, 

or eradicate any pest or disease of livestock” and identifies specific cooperative 

programs as one way to achieve these actions (§ 8308).

APHIS defines livestock as all farm-raised animals (9 C.F.R. § 71.1). As it relates to 

tick eradication, Texas defines “animal” as any domestic, free-range, or wild animal 

capable of hosting or transporting ticks capable of carrying Babesia, including (A) 

livestock; (B) zebras, bison, and giraffes; and (C) deer, elk, and other cervid species 

(TEX. AGRIC. CODE § 167.001(1) (see also 4 TEX. ADMIN. CODE § 41.1(2). In Chapter 

167, Texas’ definition of “livestock” means cattle, horses, mules, jacks, or jennets 

(TEX. AGRIC. CODE § 167.001(4)).

APHIS established the Cattle Fever Tick Eradication Program (CFTEP) in 1906 as a 

cooperative State-Federal cattle fever/babesiosis eradication effort, which shared

program costs and cooperation between the Federal government, States, local 

governments, and individual livestock producers. Eradication of two species of cattle 

fever ticks (Rhipicephalus (Boophilus) annulatus Say, 1821 [Acari: Ixodidae] and R. 

microplus Canestrini, 1888) from the United States occurred by 1943, except in the 

Permanent Tick Quarantine Zone in South Texas that extends more than 500 miles 

from Del Rio, Texas to the Gulf of Mexico (figure 1; USDA APHIS, 2015). 

To ensure U.S. animal health continues to be unaffected by cattle fever ticks and

associated diseases, it is essential to prevent their establishment in this country. With 

increasing U.S. trade and animal traffic, the risk of cattle fever tick entry and 

establishment increases. In Central and South America and Africa, cattle diseases and 
2

cattle fever ticks are endemic and therefore highly likely to travel with transported 

animals and associated materials as they enter this country (CFSPH, 2008; Lew-Tabor,

2011; Nakayima et al., 2014). Additionally, there are greater numbers of wildlife hosts 

such as deer and nilgai antelope. For these reasons, APHIS established and continues to 

maintain port-of entry inspections and the original Permanent Tick Quarantine Zone. 

Figure 1. Map of the location of the permanent quarantine line, tick quarantine zone, 

and quarantine zone counties. 

By preventing the establishment of cattle fever ticks in the United States, the CFTEP

simultaneously eliminates the disease bovine babesiosis from the U.S. cattle 

population. Babesiosis is a severe and often fatal disease of livestock caused by 

protozoan pathogens (Babesia bovis V. Babes, 1888 [Piroplasmida: Babesiidae] and B. 

bigemina Smith and Kilbourne, 1893) that is vectored by the two species of cattle ticks. 

Transmission of this disease requires the presence of these vectors. Babesiosis breaks 

down the cellular membrane of red blood cells leading to anemia, jaundice, and death. 

Infected cattle may exhibit neurological disturbances characterized by incoordination, 
3

seizures, muscle tremors, hyperexcitability, aggressiveness, blindness, head pressing, 

and coma. In addition, the two tick species are capable of causing blood loss, 

significant damage to hides, and an overall decrease in the condition of livestock. There 

is additional biological information and history of the CFTEP in the “Cattle Fever Tick 

Eradication Program – Tick Control Barrier, Maverick, Starr, Webb, and Zapata 

Counties, Texas, Draft Environmental Impact Statement – June 2013”, which is 

incorporated by reference (USDA APHIS, 2013) and referred to as the CFTEP DEIS.

Between October 2011 and September 2014, the incidence of infested premises outside 

of the Permanent Quarantine Zone remained at 20 or fewer. By December 2014, more 

than 20 premises in the tick free zone were infested, and that number continues to rise 

(figure 2). Figure 3 shows the locations of the infestations occurring outside of the 

Permanent Tick Quarantine Zone and in the Texas Temporary Preventative Quarantine 

Area. The current increase in infestations combined with a high frequency of 

infestations occurring outside the Permanent Tick Quarantine Zone suggest current 

CFTEP inspection and treatment activities may be insufficient to ensure adequate 

protection of the U.S. livestock industry in the future.

Figure 2. Infested premises in the Tick Eradication Quarantine Area (TEQA; 

permanent quarantine zone) and outside of the permanent tick quarantine zone (tick 

free area) from FY 2009-2016.
4

Figure 3. Map of Infestations in Fiscal Year (FY) 2016 highlighting the Quarantine 

Zone Counties and Temporary Preventative Quarantine Area.

In this environmental assessment (EA), APHIS analyzes the potential impacts on the 

human environment associated with adding an additional treatment method to the 

CFTEP. APHIS is proposing to treat white-tailed deer with ivermectin to control tick 

vectors of cattle fever in Cameron, Hidalgo, Jim Wells, Kinney, Maverick, Starr, Val 

Verde, Webb, Willacy, and Zapata counties in South Texas (figure 4). White-tailed 

deer would be fed ivermectin-treated corn from a closed gravity feeder placed in areas 

where cattle fever is a concern. The information presented in this EA is consistent with 

requirements in the National Environmental Policy Act of 1969 as amended (42 U.S.C. 

§§ 4321 et seq.), NEPA regulations promulgated by the Council on Environmental 

Quality (40 C.F.R. §§ 1500-1508), and APHIS implementing procedures (7 C.F.R. pt. 

372).
5

Figure 4. The 10 South Texas counties included in the proposed action.

APHIS uses Ivomec® or Ivomax® pour-on for cattle formulation mixed with whole 

kernel corn. Ivomec® pour-on for cattle is sold by Merial, Inc., and Ivomax® pour-on 

for cattle is a generic product (FDA ANADA 200-272). The treated corn is placed in 

gravity flow feeding stations from February through July to control cattle fever ticks in 

deer populations (nilgai do not eat corn and thus, are not treated). The gravity flow 

feeder is a commercially made plastic bin device with four feed tubes below the bin, 

and a lid (figure 5). Each feeder has a holding capacity of approximately 300-350 

pounds of corn. Each feed site will include one gravity flow feeder. In areas with non-
6

target animals (such as hogs, javelina, or livestock) in the vicinity, feeders will be 

enclosed with welded wire panels to exclude non-target animals. The feeders will be 

serviced and filled weekly. The program office will maintain records including wildlife 

treatment feeding/bait station data sheets, service record sheets, and a map showing the 

number and location of each feeding station (USDA APHIS, 2016a). 

Figure 5. Closed gravity feeder system used to dispense ivermectin-treated corn to 

white-tailed deer.

For dosing, 200 milliliters (ml) of the formulation containing 5 milligrams (mg) 

ivermectin/ml will be pumped into 100 pounds of clean corn to produce 10 mg of 

ivermectin active ingredient per pound of corn. The daily intake dose of the deer is 

approximately 0.22 mg/kilogram (kg) assuming a 100 pound white-tailed deer eats 1 

pound of corn per day. A previous study concluded that a feeding rate of 0.22 mg/kg 

should produce maximum blood serum levels of approximately 30 parts per billion 

(ppb) (Pound et al., 1996). The target concentration of 30 ppb assures a high degree of 

efficacy even in those deer that may consume as little as one-third of the targeted 

dosage. Serum levels of just 10 ppb (one-third of the dosage) should produce 100 

percent efficacy against ticks attempting to feed on treated animals (Pound et al., 1996; 

Nolan et al., 1985; Miller et al., 1989).

The number of feed sites will be determined based on number and density of deer (1 

feeder per 20–30 deer to minimize excessive competition and social dominance), and 

density of feeders per area (deer do not have to travel more than 1⁄4- to 1⁄2-mile to access 
7

feed = 1 per 125 acres to 1 per 500 acres). In general, ivermectin-treated corn stations 

are placed in areas of high deer use. Remote sensing data is used to identify areas on 

the property that are likely to be used by deer. This data is combined with local 

observation identifying areas with obvious evidence of deer use. More specifically, 

these areas should show evidence of significant deer utilization such as those areas with 

high density of deer tracks, high volume of excrement, and heavy browse-line on 

vegetation. A site for the feeder is selected that is relatively flat and level. There is a 

minimum 30-foot diameter perimeter barrier around feeders in areas where other large 

animals could potentially access them.

B. Purpose and Need

In 1968, cattle fever ticks were discovered on white-tailed deer in ranches in Dimmit 

County, and additional cattle fever ticks were discovered on deer in other areas in later 

years, raising concern about the role of white-tailed deer in cattle fever tick outbreaks 

in the tick-free area. Since then, more evidence has been gathered on the role of whitetailed deer as suitable cattle fever tick hosts and their importance in tick eradication 

efforts.

Numerous studies have shown that white-tailed deer are suitable hosts and reservoirs 

for cattle fever ticks (Graham et al., 1972, in Pound et al., 2010; George, 1990). In the 

1970s, chronic cattle fever tick infestations on ranch properties in Webb County north 

of Laredo, Texas, were not resolved by pasture vacation (Pound et al., 2010). In 1979, 

after sampling white-tailed deer in an area of Webb County, a study demonstrated that 

white-tailed deer sustained the existence of and spread cattle fever ticks within pastures 

vacated of cattle (Gray et al., 1979, in Pound et al., 2010). This study reinforced the 

conclusion from an earlier study (Graham et al., 1972) that white-tailed deer can 

support cattle fever ticks within vacated pastures and from another study that whitetailed deer distribute the ticks from infested to non-infested pastures.

Failure to control ticks on wildlife hosts greatly compromises efforts to eradicate ticks 

on livestock and poses a substantial threat of infestation and disease establishment 

throughout South Texas. In the 1930s, white-tailed deer infestation with cattle fever 

ticks presented a significant obstacle to cattle fever eradication in Florida (Pound et al., 

2010). Dense populations of deer and other wildlife species in South Texas threaten the 

effort to keep Boophilus spp. ticks from becoming reestablished in the southern United 

States (George, 1990). Current efforts to control cattle fever ticks on wildlife include a 

partial tick control barrier fence and population control of certain hosts such as nilgai. 

Studies show that feeding corn treated with ivermectin to white-tailed deer could also 
8

be an effective tool in minimizing the movement and maintenance of cattle fever ticks

(Pound et al., 1996; Miller et al., 1989). 

To control ticks on deer, APHIS proposes the use of ivermectin-treated whole kernel 

corn bait in feeding stations to deliver a systemically active acaricide. The U.S. Food 

and Drug Administration (FDA) does not object to an extra-label drug use as long as all 

applicable provisions of 21 CFR 530 are met, which includes establishing a withdrawal 

period that ensures no illegal drug residues occur in treated deer (email from Vitolis E. 

Vengris, Deputy Director, Division of Surveillance, Center for Veterinary Medicine, 

U.S. Food and Drug Administration, to Thomas R Kasari, Cattle Health Staff 

Veterinarian, USDA-APHIS-VS (March 14, 2016 1:00PM DST) on file with APHIS). 

The CFTEP has a 2015 agreement with the Texas Animal Health Commission (TAHC) 

and the Texas Parks and Wildlife Department (TPWD) for treating wildlife hosts for 

cattle fever ticks by feeding them with an ivermectin-treated corn product 

(Memorandum of Understanding between TAHC and TPWD and USDA APHIS VS 

(Dec. 2, 2015) on file with APHIS).

II. Alternatives

A. No Action

Under the no action alternative, APHIS would not place ivermectin corn in feeders in 

Cameron, Hidalgo, Jim Wells, Kinney, Maverick, Starr, Val Verde, Webb, Willacy, 

and Zapata counties in South Texas to minimize the spread of cattle fever ticks by 

white-tailed deer. APHIS would continue to carry out current activities that help 

prevent the spread of ticks and potential exposure of cattle to babesiosis. These 

activities include inspection of livestock at selected South Texas markets handling 

livestock originating from the Permanent Tick Quarantine Zone, patrols for stray or 

smuggled livestock in the Permanent Tick Quarantine Zone along the Rio Grande, 

inspection and pesticide treatment of tick-host livestock (mostly cattle and horses) on 

quarantined premises, and nilgai removal. 

B. Ivermectin-treated Corn (Preferred Alternative)

Under the preferred alternative, APHIS would strategically place ivermectin-treated

corn in feeders on private and public lands in Cameron, Hidalgo, Jim Wells, Kinney, 

Maverick, Starr, Val Verde, Webb, Willacy, and Zapata counties. Ivermectin-treated 

corn would most likely be used in locations where cattle fever ticks have been detected 

along the permanent quarantine line, or in adjacent areas of those counties that are 

determined to be at risk of incurring an infestation due to movements of tick-infested 
9

deer. Jim Wells and Willacy counties are included as part of the potential action area to 

prepare in advance that ivermectin corn can be used in these counties in the event that 

cattle fever ticks are detected there. The use of ivermectin corn will serve as an 

additional tool in needed areas to help CFTEP personnel prevent cattle fever tick 

infestations beyond the Permanent Tick Quarantine Zone. The CFTEP surveillance, 

population reduction, and eradication activities described under the no action 

alternative would continue.

III. Potential Environmental Impacts

In this chapter, APHIS summarizes the potential impacts to the physical, biological, 

and human environment of Cameron, Hidalgo, Jim Wells, Kinney, Maverick, Starr, Val 

Verde, Webb, Willacy, and Zapata counties in South Texas. The affected environment 

as described in the CFTEP DEIS (USDA APHIS, 2013) for Maverick, Starr, Webb, and 

Zapata counties is incorporated by reference.

A. No Action

1. Physical Environment

According to the Texas State Historical Association, Jim Wells, Kinney, Maverick, 

Webb, and Zapata county lies within an area designated as the Rio Grande Plain. The 

Rio Grande Valley includes Cameron, Hidalgo, Starr, and Willacy Counties, while Val 

Verde is part of the Edwards Plateau (figure 6; TSHA, 2015). As part of the Coastal 

Plain Physiographic Province in Texas, this region’s mean annual soil temperature 

regime is hyperthermic, and the soil moisture regime is aridic. Entisol, Inceptisol, 

Mollisol, and Vertisol soil orders dominate (TAMU, 2014). Soils are fine- to coarsetextured, well drained, with limited soil moisture available for use by vegetation during 

the growing season (McNab and Avers, 1994). Soils range from alkaline to slightly 

acidic clays and clay loams. Table 1 summarizes characteristics of each county (TSHA, 

2015) not already described in the CFTEP DEIS (USDA APHIS, 2013).
10

Figure 6. Physical regions of Texas 

(http://texasalmanac.com/sites/default/files/images/maps/PhysicalReg.pdf) (TSHA, 

2015)

Table 1. Summary of County Characteristics (TSHA, 2015)

County Area 

(square 

miles)

Elevation 

above 

sea level 

(feet) 

Soil types in most of the county Vegetation 

Cameron 905 0-60 Brownish to reddish soils, with 

loamy to clayey surface layers, 

and clayey subsoils

Gulf Prairie 

and Marsh

Hidalgo 1,596 40-200 Sandy and light loamy soils over 

deep reddish or mottled clayey 

subsoils; or moderately deep to 

deep loamy surfaces over clayey 

subsoils, or brown to red clays

South Texas 

Plains
11

County Area 

(square 

miles)

Elevation 

above 

sea level 

(feet) 

Soil types in most of the county Vegetation 

Jim 

Wells

845 200-400 Light to dark, with loamy surfaces 

over reddish, clayey subsoils over 

limestone; or gray to black, 

cracking, clayey soils

South Texas 

Plains

Kinney 1,359 1,000-

2,000

Rocky and hilly with some loamy 

soils; or gray to black, cracking, 

clayey soils over limestone with 

light-colored loamy soils

Edwards 

Plateau or 

South Texas 

Plains

Maverick 1,287 540-960 Gray to black, cracking, clayey 

soils with high shrink-swell 

potential; or light-colored loamy 

soils with limestone bedrock

Mesquite, live 

oak, cat’s 

claw, huajilla, 

cenizo, and 

prickly pear

Starr 1,226 200-400 Sandy or light-colored loamy soils 

over very deep, reddish, or 

mottled clayey subsoils; or lightcolored, deep to moderately deep, 

well-drained soils; or gray to 

black cracking clay 

South Texas 

Plains

Val 

Verde

3,150 2,248-

2,925

Dark, calcareous stony clays and 

clay loams

Desert shrub; 

or juniper, 

oak, and 

mesquite 

savanna

Webb 3,363 400-700 Clayey and loamy Mesquite, 

grasses, 

thorny shrubs, 

and cacti

Willacy 589 0-50 Dark brown to red loam over deep 

clayey subsoils; or sandy and 

saline (cracking) along the Gulf 

Coast 

Mesquite,

grasses, 

thorny shrubs, 

and cacti

Zapata 999 200-700 Light-colored, loamy soils over 

reddish or mottled clayey subsoils

Mesquite,

grasses, 

thorny shrubs, 

and cacti

Throughout the proposed action area, the vegetation is relatively consistent except near 

the coast. The majority of the vegetation in the South Texas Plains area includes live 

oaks, mesquite, brush, weeds, cacti, and grasses. The Edwards Plateau is predominantly 

short grasses, mesquite, and cacti. Val Verde County has desert shrub or juniper, oak, 
12

and mesquite savanna. Along the Gulf Coast in Willacy County, the vegetation shifts to 

cordgrasses, seashore saltgrass, and marsh millet. In Cameron County, Gulf Prairie and 

Marsh vegetation is predominantly marsh grasses, bluestems, and grama grasses

(TSHA, 2015).

The climate in South Texas is considered subtropical. Temperatures in the summer are 

hot, with average high temperatures near 100 degrees Fahrenheit. Winter weather is 

mild, with average minimum temperatures near 45 degrees Fahrenheit. Precipitation 

averages approximately 20 inches annually, with more precipitation in the summer than 

in the winter (table 2). A southerly wind is the predominant wind condition in South 

Texas. In most areas of the State, the average wind speeds are between 7 and 15 miles 

per hour (Bomar, 2008). 

Table 2. Summary of climatic information (TSHA, 2015)

County Temperature 

(average winter low 

to average summer 

high in oF)

Average annual 

rainfall (in 

inches)

Average growing 

season (length in 

days)

Cameron 50 to 94 26 320

Hidalgo 47 to 96 23 320

Jim Wells 44 to 96 28 304

Kinney 36 to 96 22 272

Val Verde 35 to 97 17 300

Willacy 48 to 95 27 318

Drought has historically been a disturbance in the Rio Grande Plain, and 90 percent of 

the area has been converted from natural vegetation to dry-land pasture for cattle 

grazing (McNab and Avers, 1994). According to the U.S. Global Climate Change 

Research Program (Karl et al., 2009), average temperatures in the Southern Great 

Plains (Texas) increased across the region, and are projected to continue this increase. 

Cold days are becoming less frequent and hot days are more frequent. Precipitation is 

projected to decrease, causing the area to become drier. In 2011, Texas experienced the 

most intense one-year drought since statewide weather records were initially 

maintained in 1895 (Nielsen-Gammon, 2011). These climatic trends are expected to 

shift the ranges of native plants as they become less competitive in areas where they are 

not well adapted. Weather events affecting soil erosion also are associated with shifts in 

host plant density, and may alter the ease and frequency of wildlife movements.

Therefore, as the area becomes hotter and drier, and water as well as food preferred by 

wildlife becomes scarcer, wildlife migration has the potential to spread ticks over 

greater areas. Similarly, drought and higher average temperatures would reduce 

vegetation available for livestock grazing, which may lead the ranching industry to 
13

move northward in search of areas where cattle can thrive with fewer heat impacts. 

Further climate change in South Texas is likely to create suitable cattle fever tick

habitat in northern areas that may not have been suitable for ticks in the past (Perez de 

Leon et al., 2012). Under the no action alternative, APHIS would not use ivermectin 

corn to help decrease the potential for ticks to spread into new areas.

Agricultural production in the proposed action area also includes commercial producers 

of hogs, sheep, and goats located in Cameron, Hidalgo, Kinney, Val Verde, and 

Willacy Counties. Major crops in Cameron, Hidalgo, and Willacy Counties include 

sugarcane, grain sorghum, and citrus (Combs, 2008; TSHA, 2015). Onion production in 

Cameron, Hidalgo, Starr, and Willacy Counties is relatively famous in the United 

States (Combs, 2008).

All of the counties in the proposed action area are in the Texas Commission on 

Environmental Quality, Air Quality Control Area Region 14 (Jim Wells), Region 15 

(Cameron, Hidalgo, Kinney, Starr, Val Verde, and Willacy), and Region 16 (Maverick, 

Webb, and Zapata) (https://www.tceq.texas.gov/publications/gi/gi-002.html). South 

Texas counties tend to have better air quality than many of the other major urban areas 

around the State (Combs, 2008), although air quality information is not available for 

Jim Wells, Kinney, Starr, Val Verde, Willacy, and Zapata Counties (U.S. EPA, 2016). 

From January – September 2016, most of the monitoring in Cameron, Hidalgo, 

Maverick and Webb Counties found the Air Quality Index (AQI) values were “Good” 

or “Moderate”. There were 2 days when the AQI values exceeded 100 (rating 

“Unhealthy for Sensitive Groups”) in Cameron County and 1 in Hidalgo County (U.S. 

EPA, 2016). Across these four reporting counties, the main pollutants were Particulate 

Matter 2.5 and Ozone. There was 1 day in Webb County when Particulate Matter 10 

was the main pollutant. There were no days that the air quality (in the Texas cities 

where air quality was measured) in the identified counties exceeded U.S. EPA 

standards (U.S. EPA, 2016). Additional discussion on air quality is not directly 

applicable under the circumstances because CFTEP activities are not known to impact 

air resources.

A watershed is an area of land that contributes water to a river or stream. The overall 

project area includes reservoirs, streams, rivers, ephemeral (short-lived), intermittent, 

and perennial drainage features (TCEQ, 2015) that are considered jurisdictional waters 

of the United States subject to Clean Water Act regulations (40 C.F.R. §§ 136, 230-

233). The principal rivers associated with the area of the proposed action include 

portions of the Rio Grande and Nueces. Minor water bodies include streams and 

arroyos. The proposed action would be located in or along the (a) Rio Grande 

watershed, (b) Neuces River Basin watershed, (c) Nueces-Rio Grande Coastal Basin, 
14

and (d) the Texas Coastline (the Brownsville Ship Channel and South Bay in Cameron 

County) (TCEQ, 2015). Major reservoirs in the South Texas Region include the 

Amistad, Choke Canyon, and Falcon Reservoirs, and the lakes include the Anzalduas 

Channel Dam, Casa Blanca, Corpus Christi, Delta, Loma Alta, and the Upper Nueces 

Lake (Combs, 2008). This region relies less on groundwater than surface water 

supplies, but irrigation accounted for more than 65 percent of the region’s groundwater 

use (Combs, 2008).

Federal activities must seek to avoid or mitigate actions that would adversely affect 

wild and scenic rivers or areas immediately adjacent to the designated rivers (National 

Wild and Scenic Rivers Act of 1968, as amended (16 U.S.C. §§ 1271-1287)). There are 

one hundred ninety-six miles of the Rio Grande extending from Mariscal Canyon to the

Terrell/Val Verde County line designated as a wild and scenic river. The habitat 

associated with this area is not conducive to cattle fever ticks; therefore, APHIS will 

not be placing ivermectin corn feeders nearby.

There are four major and three minor aquifers in the South Texas region (Combs, 

2008). The proposed action will occur several miles away from outcrop parts of the 

Edwards (in Val Verde and Kinney Counties) and Gulf Coast (in Cameron, Hidalgo, 

Jim Wells, Starr, Willacy, and Zapata Counties) aquifers, in addition to the 

southwestern extent of the Carrizo-Wilcox aquifer (in Maverick and Webb Counties) 

where it lies or dips below other formations (George et al., 2011). Outcrop areas are the 

part of an aquifer lying at the land surface. Parts of Kinney County are in the Edwards 

Aquifer, and the remainder is in part of the Edwards-Trinity Plateau aquifer. The only 

minor aquifer reported within the proposed action area is an outcrop area of the YeguaJackson aquifer in Webb, Zapata, and a small portion of Starr County. As of 2010, 

areas with groundwater conservation districts included Kinney, Jim Wells, and Starr 

Counties, in addition to small areas in Hidalgo and Willacy Counties. At that time, 

groundwater management areas placed Val Verde and parts of Kinney in district 7 with

the remainder of Kinney in district 10; Maverick, Webb, and Zapata Counties in district 

13, with the remainder of Webb County in district 16; and Starr, Hidalgo, Cameron, 

Willacy, and Jim Wells Counties in district 16 (George et al., 2011). Additional 

discussion of impairments and sole source aquifers is not directly applicable under the 

no action alternative because current CFTEP activities are not known to impact water 

resources.

2. Livestock and Wildlife

The effectiveness of the CFTEP requires an understanding of the roles many organisms 

play in cattle fever tick infestations and the spread of disease pathogens. All of the tick 

hosts (primary, alternate, and occasional) may be infected, experience disease, and 
15

harbor the protozoan pathogens in addition to suffering blood loss from tick feeding. 

Tick feeding can transmit a variety of pathogens. Table 3 summarizes the organisms 

and their roles. To simplify discussion, throughout this EA we refer to cattle fever ticks

and diseases to collectively include all the pertinent organisms.

Table 3. Summary of pertinent organisms and their roles in the CFTEP

Common name Scientific name1 Role

Cattle Fever Ticks Rhipicephalus (Boophilus) 

annulatus and R. microplus

Pests and vectors of 

diseases 

Protozoan 

Pathogens

Babesia bovis and B. 

bigemina

Causal agents of the disease 

bovine babesiosis 

Rickettsia 

Pathogen

Anaplasma marginale 

Theiler, 1910

Causal agent of the disease 

bovine anaplasmosis

Spirochaete 

Pathogen

Borrelia theileri Laveran 

1903, Bergey et al., 1925

Causal agent of the disease 

relapsing fever (borreliosis)

Cattle

Bos taurus taurus L., 1758 

[Mammalia: Bovidae], B.

taurus indicus L., 1758

Livestock -- Primary Host

Horse Equus caballus L., 1758 

[Mammalia: Equidae]

Used by inspectors, ranch 

hands, and landowners

Livestock -- Primary Host

Axis deer

Axis axis Erxleben, 1777 

(Cervus axis) [Mammalia: 

Cervidae]

Wildlife or

Livestock -- Alternate Hosts

Fallow deer Dama dama L., 1758 

[Mammalia: Cervidae]

Red deer, 

American elk (or 

wapiti)

Cervus elaphus L., 1758 

[Mammalia: Cervidae], 

Cervus elaphus canadensis

Erxleben, 1777

American Bison Bison bison L., 1758 

[Mammalia: Bovidae]

Aoudad sheep Ammotragus lervia Pallas, 

1777 [Mammalia: Bovidae]

White-tailed deer

Odocoileus virginianus

Zimmerman, 1780

[Mammalia: Cervidae] Wildlife -- Alternate Hosts 

Nilgai antelope

Boselaphus tragocamelus

Pallas, 1766 [Mammalia: 

Bovidae]

Pigs Sus scrofa L., 1758 

[Mammalia: Suidae]

Occasional Hosts2
16

Common name Scientific name1 Role

Dogs Canis lupes familiaris L., 

1758 [Mammalia: Canidae]

Occasional Hosts

1

Taxonomic information from ITIS online database, last accessed Oct. 13, 2015 

http://www.itis.gov. 2 Wild pigs and dogs exhibit limited potential to spread ticks (Barros and Fighera, 

2008).

One of the greatest concentrations of exotic animals in Texas occurs within the South 

Texas Plains ecoregion. Free-ranging exotic species in Texas include blackbuck and 

nilgai antelope; axis, fallow, and sika deer; aoudad and mouflon sheep; feral swine 

(Sheffield, 2013), and red deer or wapiti (Pound et al., 2010) (see table 3). The 

identification of exotic species as tick hosts presents a threat to the CFTEP by 

compromising the success of ongoing eradication efforts. Reducing population levels of 

potential hosts limits the likelihood that cattle fever tick host populations will move 

northward and spread cattle fever ticks into new regions.

TPWD controls “Wildlife Management Areas” (WMA) in portions of Cameron, 

Hidalgo, Starr, and Willacy Counties as part of the Las Palomas WMA, Lower Rio 

Grande Valley Units. The 3,311 acres of land in the Las Palomas WMA preserves

native brush nesting habitat, some farmland, and wetlands for white-winged doves. 

There are 18 units with tracts ranging from two to 604 acres in size. Las Palomas 

WMA features hiking, hunting, and wildlife viewing (TDPW, 2016). There are no 

WMA in Jim Wells, Kinney, Maverick, Val Verde, Webb, and Zapata counties 

(TDPW, 2016).

Impacts from a spreading cattle fever tick infestation could occur beyond Cameron, 

Hidalgo, Jim Wells, Kinney, Maverick, Starr, Val Verde, Webb, Willacy, and Zapata 

Counties. States, counties, and areas without an active tick or disease eradication 

program may find herds and breeding stock become infested by ticks or infected with 

diseases during trade. Consequently, indirect impacts associated with the No Action

alternative include the likelihood that, without the use of ivermectin corn, States other 

than Texas may need to create their own tick eradication programs, alter conditions of 

sale for herds and breeding stock, reactivate regulations previously used to deal with 

cattle fever ticks and diseases, or create precautionary measures restricting trade.

3. Human Environment

Human populations in the identified counties include residents, hunters, ranchers, and 

CFTEP employees. Residents include but are not limited to adults and children living in 

Colonias. Exposure to various ticks is unlikely for most residents during the course of 

their normal activities. There is an increased risk of exposure to ticks when children 
17

and adults participate in outdoor activities, and the overall risk of exposure to ticks may 

increase under the no action alternative as a result of untreated deer that could maintain 

many varieties of ticks. However, the risk of exposure to cattle fever ticks, specifically, 

is minimal because humans are not a host for these ticks.

Hunters experience increased risk of tick exposure during their hunting seasons. In 

general, tick exposure by ranchers is highly unlikely during the course of their normal 

activities on uninfested lands. Ranchers and those working with livestock are at 

increased risk of tick exposure when they travel across tick-infested areas or encounter 

infested stray livestock or wildlife. If tick populations establish in their pastures, then 

exposure of people working with livestock in those areas is likely. Exposure to ticks 

occurs for CFTEP and TAHC employees during the course of their work duties; 

however, risk of exposure to cattle fever ticks is minimal because they do not infest 

humans.

The demographics for this area of South Texas indicate the overall population has a 

large proportion of Hispanics who are likely to have graduated high school, but are not 

likely to speak English at home (table 4). In general, county-level poverty estimates are 

not comparable to other geographic levels (state or national) because the poverty 

estimates may come from sample data with associated sampling errors (e.g. see table 4

footnote 2). Nevertheless, roughly 20 to 40 percent of the population in each county in 

the proposed program area appears to be below the poverty level.

Table 4. Select Demographics in the Program Area.

Location Total 

population1

Percent 

white

Percent 

Hispanic 

(all 

races)

Percent 

language 

other than 

English at 

home

Percent 

high 

school 

graduate 

or higher

Percent 

below 

poverty 

level2

State of 

Texas

26,956,958 80.0 38.6 34.7 81.2 17.6

Counties within Texas

Cameron 420,392 97.2 88.7 73.1 63.7 32.4

Hidalgo 831,073 97.0 91.2 84.9 61.8 34.0

Jim Hogg 5,255 97.2 91.6 81.9 66.1 24.0

Kinney 3,526 94.4 56.5 58.1 68.9 19.3
18

Location Total 

population1

Percent 

white

Percent 

Hispanic 

(all 

races)

Percent 

language 

other than 

English at 

home

Percent 

high 

school 

graduate 

or higher

Percent 

below 

poverty 

level2

Maverick 57,023 96.9 95.1 94.2 56.6 24.4

Starr 62,955 98.8 95.8 95.6 45.0 36.3

Val Verde 48,974 95.2 80.4 72.2 67.0 23.9

Webb 266,673 97.6 95.2 91.3 64.2 30.6

Willacy 21,903 95.3 87.7 45.4 62.2 43.1

Zapata 14,319 98.7 93.9 87.2 54.8 29.5

1

Based on U.S. Census Bureau data from 2014 estimates (Total Population) or 2009-

2013 (other categories), last accessed Nov. 6, 2015 

http://quickfacts.census.gov/qfd/states/48000.html.

2

Based on the official poverty definition that uses monetary income before taxes and 

does not include capital gains or noncash benefits (such as public housing, Medicaid, 

and food stamps). If the total income for a family is less than the threshold, then that 

family (and every individual in it) is considered in poverty.

An increase in the number of cattle fever tick infestations over an increasingly large 

area could result in adverse economic and health impacts on affected producers and 

consumers, such as higher consumer prices, loss of local employment, reduced 

nutritional options, loss of market share, compromised mental and physical health, loss 

of property, and so on. These indirect impacts are likely to occur to a lesser extent 

under the preferred alternative.

B. Preferred Alternative

1. Physical Environment

Short-term disturbances to soil, water, vegetation, or air resources could occur, but are 

unlikely, as a result of the use of ivermectin-treated corn. Theoretically, greenhouse gas 

release occurs during the routine use of vehicles and the production of ivermectin used 
19

for the program. Ivermectin has low volatility and is unlikely to partition into the 

atmosphere. 

Ivermectin binds strongly to soil particles making it unlikely to leach into groundwater 

or runoff to surface water in a dissolved state (USDA APHIS, 2016b). Ivermectin 

breaks down to less bioactive compounds via photo and aerobic degradation; 

degradation in soil varies with the soil type and properties, sorption capacity, and 

temperature (USDA APHIS, 2016b). Ivermectin half-lives in soil are 7 to 14 days at 

high temperatures in summer, but can be much longer (91 to 217 days) at low 

temperatures in the winter. Photolysis in water is less than 0.5 day in summer, and 39 

hours in winter. When directly exposed to sunlight, its photolytic half-life was 

approximately 3 hours on a thin, dry film (USDA APHIS, 2016b). In addition, studies 

in cotton and food crops (sorghum, lettuce, carrots, and turnips) show that plants uptake 

little ivermectin from direct applications to plants or from soil (USDA APHIS, 2016b).

2. Livestock and Wildlife

Ivermectin is absorbed into blood, distributed throughout the mammal's body, and 

deposited in the body fat and liver (USDA APHIS, 2016b). The absorption rate varies 

with the route of administration, formulation, and animal species. Ruminant species 

appear to have a slower absorption process than monogastric animals. The safety data 

sheet for formulated ivermectin reports oral LD50 values for ivermectin component B1a 

and component B1b are ~80 mg/kg (dog), 25 mg/kg (mouse), 50 mg/kg (rat), and 

higher than 24 mg/kg (rhesus monkey) (USDA APHIS, 2016b).

Ivermectin blocks the transmission of neural signals of the parasites by binding 

selectively and with high affinity to the glutamate-gated chloride channels in nerve and 

muscle cells of invertebrates, and acting as an agonist of the gamma-aminobutyric acid 

(GABA) neurotransmitter in the peripheral nervous system of invertebrates. However, 

ivermectin has low toxicity in mammals because GABA is found only in the central 

nervous system of mammals and is protected by the blood-brain barrier. There are 

greater toxic effects from ivermectin reported in some dog breeds such as collies and 

sheepdogs, because these breeds carry a mutation of the multiple drug resistance gene 

(MDR1) causing an abnormality in the blood-brain barrier that allows increased 

ivermectin into the central nervous system (USDA APHIS, 2016b).

Ivermectin is considered highly toxic to most aquatic invertebrates; however, the 

proposed method of application, program controls, and environmental fate of 

ivermectin suggest there will not be significant exposure in aquatic habitats. Ivermectin 

is used as a therapeutic treatment for fish and amphibians with 96-hour LC50 values 
20

ranging from 3.3 to 5.3 μg/L for the rainbow trout, Onchorynchus mykiss, and bluegill 

sunfish, (Lepomis microchirus) respectively (USDA APHIS, 2016b). 

APHIS expects ivermectin risks to most fish and wildlife species to be very low. There 

is the possibility of exposure to some non-target terrestrial wildlife who may consume 

corn spilled onto the ground during feeding by white-tailed deer. The use of exclusion 

barriers (welded wire panels precluding swine, javelin, and livestock) combined with 

weekly monitoring of the feeders reduces this potential exposure. The low probability 

of exposure and administration of therapeutic doses of ivermectin suggest that risk to 

non-target terrestrial vertebrates who may consume some spilled treated corn will be 

low. 

There is some risk to terrestrial invertebrates, such as beetles and flies, if they solely 

depend on droppings from white-tailed deer for development. Studies show that 

approximately 90 percent of the ivermectin dose administered parenterally or orally is 

excreted in the feces (USDA APHIS, 2016b). Localized impacts to this group of 

invertebrates depend on site-specific conditions influencing ivermectin degradation and 

availability (USDA APHIS, 2016b).

3. Human Health and Safety

Human health risks associated with the use of ivermectin-treated corn in feeding 

stations to control cattle fever ticks in deer populations are determined based on the 

toxicity of ivermectin, and the potential for exposure. The potential for exposure 

depends on APHIS’ proposed application method and the environmental fate profile for 

ivermectin.

As previously noted in the Livestock and Wildlife section, ivermectin has low toxicity 

in mammals because GABA is found only in the central nervous system of mammals 

and is protected by the blood-brain barrier. Therefore, ivermectin is sometimes used as 

a human drug for the treatment of strongyloidiasis and onchocerciasis in the United 

States (FDA, 2016), and the treatment of scabies, lice, and ascariasis in other countries 

(NIDDK, 2016). The FDA-approved ivermectin human drugs currently include 

Stromectol® (3 milligram (mg) oral tablet), Sklice® (0.5% topical lotion), and 

Soolantra® (1% topical cream) (FDA, 2016).

Ivermectin can be toxic to humans if accidental overdose or significant exposure to 

veterinary formulations occurs. The reported adverse human health effects include rash, 

edema, headache, dizziness, asthenia, nausea, vomiting, and diarrhea (USDA APHIS, 

2016b). Exposure to the Ivomax® pour-on formulation may cause the following 

adverse effects: 1) eye irritation (direct eye contact); 2) irritation and/or drying and 
21

cracking of the skin (prolonged or repeated contact); 3) mild irritation of the nose and 

throat (vapor exposure of isopropyl alcohol in the formulation) and nausea, headache 

and mild narcosis (prolonged exposures to isopropyl alcohol vapor above the 

occupational exposure standard); 4) decreased activity, slow rate of breathing, dilation 

of the pupils, muscle tremors, and in-coordination (overexposure to ivermectin); and 5) 

burning of the gastrointestinal tract, nausea, vomiting and central nervous system

depression (ingesting a large amount of isopropyl alcohol) (Aspen, 2013). The safety 

data sheet for the Ivomec pour-on for cattle formulation reports oral LD50 for 

ivermectin comp. B1a and comp. B1b is greater than 15 mg/kg (adult human) (Merial, 

2010). In humans, ivermectin metabolism occurs in the liver by cytochrome P450 

enzymes. Deposition tends to occur in body fat and the liver, where it is progressively

metabolized until excreted. Approximately 90 percent of excretion occurs in feces, less 

than 2 percent in urine, and less than 2 percent in breast milk (USDA APHIS, 2016b). 

Humans with potential exposure to ivermectin-treated corn include livestock feed mill 

employees formulating the medicated corn, program workers filling feeders, and the 

general public living in the vicinity of feeding stations. Occupational exposure to 

ivermectin may occur through oral, inhalation, and dermal contact. However, direct 

contact to ivermectin during outdoor application in well ventilated areas is not expected 

to occur under normal conditions with proper worker hygiene and properly functioning 

personal protective equipment (PPE). Drift from the application of ivermectin will not 

occur because ivermectin has low volatility and corn is loaded directly into the gravity 

flow feeders. In addition, APHIS program workers are licensed pesticide applicators 

who understand the risks associated with pesticide use and disposal.

For the general public, potential direct exposure to ivermectin-treated corn is unlikely 

based on the method of application and program requirements that restrict access to 

feeders (USDA APHIS, 2016b). Feeders are strategically placed on private properties 

that will have a restricted access point. Comparable security will be provided at refuges 

and other public sites. A sign in both English and Spanish will be posted if feeders are 

placed on public lands. Feeders are checked weekly so that damaged feeders can be 

repaired or removed, reducing the potential for exposure to the public from corn that 

may spill onto the ground if the feeder is damaged. Therefore, the potential exposure 

for the general public to ivermectin via inhalation, dermal exposure, or through 

ingestion of treated corn is not expected. There is potential for a person to ignore the 

signage and breach the exclusion fencing to access treated corn due to the fence’s 

height of 34 inches, however, in general, risk to the general public from direct contact 

exposure to ivermectin-treated corn in feeders in restricted access feeding stations is 

expected to be low.
22

Potential exposure of the general public from dietary consumption of meat from 

ivermectin-treated deer is unlikely because APHIS will discontinue feeding deer with 

ivermectin-treated corn 60 days prior to the deer-hunting season. The proposed 

treatment period is annually from February through July. The withdrawal time of 60 

days allows ivermectin residues to decrease to below the tolerance levels in white-tailed 

deer (USDA APHIS, 2016b). Consequently, risks to the general public from dietary 

consumption of ivermectin in harvested deer meat is expected to be negligible. 

Potential exposure of the general public from dietary consumption of meat from feral 

swine that have ingested ivermectin-treated corn is unlikely because of the installation 

of exclusion fencing, the design of feed ports, and the time of year associated with 

hunting swine for food. Feeders will be enclosed with welded wire panels to exclude 

non-target animals, such as hogs, and serviced weekly. The exclusion fencing 

surrounding each deer feeder has a height of 34 inches, which is optimum to prevent 

feral swine from accessing the corn feeders (Rattan et al., 2010). However, while 

uncommon, breach of fencing by feral swine could occur (Cooper and Ginnett, 2000). 

When a breach occurs, the program will repair the fencing and report the finding to 

landowners for them to implement lethal feral swine population control measures (D. 

Baca, pers. comm., email dated Oct. 19, 2016). The feed ports on the feeders are not 

easily accessible to feral swine because they are above ground level with a small 

opening and ventral lip to minimize spillage. The available corn to feral swine if they 

breach the fencing is minimal—it is estimated that there is less than 5% of the total 

treated corn in the feeder that ends up on the ground (D. Baca, pers. comm., email 

dated Oct. 19, 2016). Although hunting for feral swine is allowed year round, APHIS 

has observed most of the hunting for human consumption occurring during the colder 

months of the year, generally from November through February. Hunting does occur 

during the warm to hot months of the year, but this is mostly for population control and 

trophy hunts for mature boars; meat is not usually salvaged. 

There are no anticipated threats to ground and surface water that may be used as 

drinking water because of the proposed treatment method and environmental fate of 

ivermectin. Feeding stations are placed a minimum of 50 feet from any aquatic areas 

and it is unlikely that any treated corn would be transported during a rain event to any 

surface water source used for drinking water. Small amounts of ivermectin may be 

released to the soil in deer droppings; however, ivermectin concentrations in soil would 

be extremely low due to the small amounts of droppings in a given area. In addition, 

ivermectin is unlikely to leach into groundwater or runoff to surface water because it 

binds to soil and has low solubility, and will further degrade in environment. 
23

Overall, the use of ivermectin-treated corn in feeding stations to control cattle fever 

ticks is expected to pose minimal risks to human health under the APHIS proposed 

applications in the preferred alternative. 

4. Wildlife Protections

a. Endangered Species Act

Section 7 of the Endangered Species Act (ESA) and its implementing regulations 

require Federal agencies to ensure that their actions are not likely to jeopardize the 

continued existence of endangered or threatened species or result in the destruction or 

adverse modification of critical habitat. There are 26 federally listed species and 

species proposed for listing in the program area (Cameron, Hidalgo, Jim Wells, Kinney, 

Maverick, Starr, Val Verde, Webb, Willacy, and Zapata Counties). 

Potential effects of the proposed action to listed species and critical habitat include 

toxicity of ivermectin to non-target species, removal of brush that serves as species 

habitat from areas where feeders are placed, runoff of ivermectin into aquatic areas, 

trampling of listed plants, and species disturbance by feeder set up and weekly 

servicing. 

APHIS has determined that the proposed action may affect, and is likely to adversely 

affect, the Gulf Coast jaguarondi (Puma yagouaroundi cacomitli), ocelot (Leopardus 

pardalis), northern aplomado falcon (Falco femoralis septentrionalis), and whooping 

crane (Grus americana). APHIS requested formal consultation with the U.S. Fish and 

Wildlife Service (FWS) for these species.

APHIS has determined that the proposed action may affect, but is not likely to 

adversely affect the black-capped vireo (Vireo atricapilla), golden-cheeked warbler

(Setophaga chrysoparia), Devils River minnow (Dionda diaboli) and its critical habitat, 

ashy dogweed (Thymophylla tephroleuca), black lace cactus (Echinocereus 

rechenbachii var. albertii), least tern (Sternula antillarum), South Texas ambrosia

(Ambrosia cheiranthifolia), star cactus (Astrophytum asterias), Texas ayenia (Ayenia 

limitaris), Texas snowbells (Styrax platanifolius subsp. texanus), Tobusch fishhook 

cactus (Sclerocactus brevihamatus subsp. tobuschii), Walker’s manioc (Manihot 

walkerae), and Zapata bladderpod (Lesquerella thamnophila). FWS concurred with 

these determinations. 

APHIS has determined that the the proposed action will not jeopardize the continued 

existence of the Texas hornshell (Popenaias popeii), a mussel that is proposed for 

listing as an endangered species. 
24

APHIS has determined that the proposed action will have no effect on the West Indian 

manatee (Trichechus manatus) and its critical habitat, piping plover (Charadrius 

melodus), red knot (Calidris canutus), critical habitat of the whooping crane, hawksbill 

sea turtle (Eretmochelys imbricata), Kemp’s Ridley sea turtle (Lepidochelys kempii), 

leatherback sea turtle (Dermochelys coriacea), loggerhead sea turtle (Caretta caretta), 

and critical habitat of Zapata bladderpod. 

APHIS will avoid adverse effects by surveying potential feeder sites for presence of 

listed plants and nesting birds, placing feeders in areas already dominated by nonnative vegetation, avoiding creation or widening of trails to access feeders, 

implementing buffers for feeder placement from aquatic areas, avoiding removal of 

native vegetation and brush, avoiding placement of feeders within designated critical 

habitat, and using feeders that prevent access to treated corn by non-target species. 

APHIS outlined these mitigations in its biological assessment and submitted it to the 

FWS, Texas Coastal Ecological Services Field Office. 

FWS issued a biological opinion in January 2017 indicating that the proposed use of 

ivermectin-treated corn in feeding stations will not likely jeopardize the continued 

existence of the ocelot, Gulf coast jaguarundi, northern aplomodo falcon, or whooping 

crane. Terms and conditions included in the biological opinion outline reporting and 

monitoring requirements. Terms and conditions include:

1) information regarding reporting take of an ocelot, jaguarundi, northern 

aplomado falcon, or whooping crane to FWS; 

2) a requirement for APHIS to provide ESA training to all CFTEP personnel; 

3) a protocol for use of game cameras to monitor wildlife visiting the ivermectin 

corn feeders; 

4) a requirement for bi-annual ivermectin sensitivity testing of ticks, and; 

5) how to submit annual reports to FWS.

The biological assessment (prepared by T. Willard, USDA-APHIS, October 17, 2016) 

and the biological opinion (including concurrence on “not likely to adversely affect”

species) (prepared by E. Reyes, FWS, Ecological Services, January 2017) are included 

in the administrative record for this EA. 

b. Migratory Bird Treaty Act

Federal law prohibits an individual to pursue, hunt, take, capture, kill, attempt to take, 

capture or kill, possess, offer for sale, sell, offer to purchase, purchase, deliver for 

shipment, ship, cause to be shipped, deliver for transportation, transport, cause to be 

transported, carry, or cause to be carried by any means whatever, receive for shipment, 
25

transportation or carriage, or export, at any time, or in any manner, any migratory bird 

or any part, nest, or egg of any such bird (16 U.S.C. §§ 703-712; 50 CFR § 21).

Texas occurs within the Central Flyway, a bird migration route that is composed of the 

States of Montana, Wyoming, Colorado, New Mexico, Texas, Oklahoma, Kansas, 

Nebraska, South Dakota, and North Dakota, and the Canadian provinces of Alberta, 

Saskatchewan, and the Northwest Territories. Many of the migratory bird species of the 

Central Flyway winter in Central and South America. Some migrate across the Western 

Hemisphere to the Arctic Circle, and others migrate to South America (NAS, 2016a). 

Birds in this flyway include the American oystercatcher (Haematopus palliatus), black 

skimmer (Rynchops niger), brown pelican (Pelecanus occidentalis), greater sagegrouse (Centrocercus urophasianus), least tern, lesser prairie chicken (Tympanuchus 

pallidicinctus), piping plover, reddish egret (Egretta rufescens), redhead (Aythya 

americana), red knot, ruddy turnstone (Arenaria interpres), sanderling (Calidris alba), 

sandhill crane (Grus canadensis), whooping crane, and Wilson’s plover (Charadrius 

wilsonia) (NAS, 2016a). Birds that migrate along this route depend on stopover habitat, 

such as native prairie and wetland areas.

Disturbance of nesting migratory birds is not expected because APHIS will not remove 

brush or native vegetation that migratory birds would use as nesting substrate; as much 

as possible, feeders will be placed in areas dominated by bare ground, buffelgrass, or 

other not native vegetation. APHIS will place feeders a minimum of 50 feet from 

aquatic areas to avoid disturbance of nesting shorebirds from placement and servicing 

of feeders. 

Exposure and toxicity of ivermectin is another potential effect to migratory birds. 

APHIS prepared an environmental risk assessment for ivermectin-treated corn that is 

included in Appendix 1 of this document. The risk assessment concluded that direct 

risk to non-target birds is expected to be low based on the method of application for 

ivermectin-treated corn and low toxicity of ivermectin to birds. The use of the closed 

gravity feeder will reduce risk to most terrestrial non-target birds and other animal 

species. APHIS will place feeders a minimum of 50 feet from aquatic areas to prevent 

ivermectin runoff that could affect fish and invertebrate prey. Insectivorous birds would 

not likely ingest ivermectin-treated corn. Small insects that would serve as prey for 

these birds are also not expected to ingest ivermectin-treated corn. If birds were to 

ingest the treated corn or prey that has ingested ivermectin-treated corn, acute toxicity 

of ivermectin to birds is low to moderate depending on the species. The oral median 

lethal dose (LD50) for ivermectin to bobwhite quail (Colinus virginianus) is 2,000 

milligrams (mg)/kilogram (kg) and 88 mg/kg for the mallard duck (Anas 

platyrhynchos). Similar sensitivities are seen in dietary studies, with median lethal
26

concentration (LC50) values of 3,102 and 383 mg/kg, for the bobwhite quail and 

mallard, respectively (Bloom and Matheson, 1993).

c. Bald and Golden Eagle Protection Act

The Bald and Golden Eagle Protection Act (16 U.S.C. 668-668c) prohibits anyone, 

without a permit issued by the Secretary of the Interior, from "taking" bald eagles

(Haliaeetus leucocephalus), including their parts, nests, or eggs. The Act provides 

criminal penalties for persons who "take, possess, sell, purchase, barter, offer to sell, 

purchase or barter, transport, export or import, at any time or any manner, any bald 

eagle ... [or any golden eagle], alive or dead, or any part, nest, or egg thereof." The Act 

defines "take" as "pursue, shoot, shoot at, poison, wound, kill, capture, trap, collect, 

molest or disturb."

Golden eagles (Aquila chrysaetos) are rare to locally uncommon in the Panhandle and 

western Trans-Pecos area, and very rare to casual throughout the remainder of Texas 

(Lockwood and Freeman, 2004). For bald eagles, breeding populations occur mainly in 

the eastern half of the state and along coastal counties from Rockport to Houston

(TPWD, undated). Nonbreeding populations of bald eagles are found mainly in the 

Panhandle, Central, and East Texas, and in other suitable habitat throughout Texas 

(TPWD, undated). FWS does not indicate that bald eagles occur in any of the 10 

program counties (FWS, 2016). Because bald and golden eagles are unlikely to be in 

the program area, activities associated with ivermectin-treated corn are not expected to 

cause disturbance to eagles. 

Bald eagles in Texas commonly eat coots, catfish, rough fish, and soft-shell turtles 

(TPWD, undated). Carrion is also common in the diet of bald eagles, especially in 

younger birds (TPWD, undated). Golden eagles eat a variety of foods, mainly

mammals ranging in size from ground squirrels up to prairie-dogs, marmots, and 

jackrabbits (NAS, 2016b). They may also prey on smaller rodents, birds, snakes, 

lizards, large insects, and carrion (NAS, 2016b). Should bald or golden eagles occur in 

an area where ivermectin-treated corn feeders are placed, direct risk to them from 

feeding on prey that has fed on ivermectin-treated corn is expected to be low because of 

the method of application for ivermectin-treated corn and low toxicity of ivermectin to 

birds (see the Migratory Bird Treaty Act section for additional toxicity information). 

The use of the closed gravity feeder will reduce exposure to most non-target birds and 

other animal species that could serve as prey to eagles. In addition, eagles would have

other food sources that would not contain ivermectin residues, further reducing risk to 

them. Ivermectin is used therapeutically to treat raptors, including bald and golden 

eagles, for helminth parasites (parasitic worms, such as tapeworms and roundworms) 

(Smith, 1993). 
27

5. Environmental Justice

Federal agencies identify and address disproportionately high and adverse human 

health or environmental effects of its proposed activities as described in Executive 

Order 12898, “Federal Actions to Address Environmental Justice in Minority 

Populations and Low-Income Populations,” and Executive Order 13045, “Protection of 

Children from Environmental Health Risks and Safety Risks.” Adverse direct or 

indirect effects on vulnerable populations are not likely to occur when there is proper 

handling of the treated corn combined with an effective communication with program 

area residents.

Federal agencies must ensure their programs and activities are accessible to persons 

with limited English proficiency as directed by EO 13166. To meet this need, APHIS 

conducts outreach to English-speaking and Spanish-speaking communities through a 

variety of public notices and informational brochures about CFTEP program activities. 

APHIS will invite all stakeholders, including Colonia ombudspersons and residents of 

Colonias, to any public meetings. If this EA leads to a finding of no significant impact

(FONSI), then APHIS will provide a Spanish translation of the FONSI to program and 

TAHC representatives for their use when working with the public. In addition, if a 

FONSI is reached, APHIS will also notify the Director of the Colonia Initiatives 

Program in South Texas about the new CFTEP activities.

6. Tribal Consultation and Coordination

APHIS met with representatives of the Kickapoo Tribe on February 4, 2013 to review 

the Tribe’s needs, interests, and concerns (Roberta Duhaime, pers. comm., 30 January, 

2014). The Texas Kickapoo Indian Reservation in Maverick County includes 125 acres 

of trust land along the Rio Grande, an additional 13,000 acres in Maverick County, and 

has an interest in a 9,000 acre cattle ranch in Spofford, Texas. This is the only 

federally-recognized tribe reported in the 10 county region, using the Native American 

Graves Protection and Repatriation Act Online Databases (NPS, 2013; 25 U.S.C. §§ 16 

3001 et. seq.). While conducting scoping for the CFTEP DEIS (USDA APHIS, 2013), 

APHIS contacts with the Tonkawa Tribe of Oklahoma (which once inhabited this area 

of Texas) indicated their interests extend only to the disposition of artifacts that may be 

inadvertently uncovered. The proposed action will not disturb the ground, so it is 

unlikely to affect Native American sites or artifacts. To ensure that tribal communities 

near the 10 county action area are aware of the proposed action, APHIS mailed the 

Alabama-Coushatta Tribe of Texas, Ysleta Del Sur Pueblo, and Kickapoo Traditional 

Tribe of Texas an informational letter inviting further discussions. The letter (prepared 
28

by M. Gray, USDA APHIS, November 16, 2016) is included in the administrative 

record for this EA. 

The Archaeological Resources Protection Act of 1979 (16 U.S. Code (U.S.C.) §§ 

470aa-mm), secures the protection of archaeological resources and sites on public and 

tribal lands. If APHIS discovers any archaeological resources, APHIS will notify the

appropriate individuals. If the presence of tick-infested animals warrants expansion of 

the proposed activities onto Tribal lands, program officials will initiate consultation 

with the governing Tribal authorities and local Tribal Historic Preservation Officers 

before taking further action.

7. Historic, Cultural, and Visual Resources

The visual resources for the listed counties in Texas are the rangeland and pastures 

serving as habitat for animals. In general, these counties are of minimal recreational or 

scenic interest except for areas directly along the Rio Grande River. Hunting occurs in 

some areas. The visual resources also include any buildings, street patterns and road 

characteristics, view corridors, and vistas.

The National Historic Preservation Act of 1966, as amended (16 U.S. Code § 470 et 

seq.), requires Federal agencies to consider the impact on properties included in, or 

eligible for inclusion in, the National Register of Historic Places (36 Code of Federal 

Regulations §§ 63 and 800). APHIS identified the listed historic places present in each 

County (Texas Atlas of Historic Places, https://atlas.thc.state.tx.us/), and summarized 

the findings in table 5. The museums may be inside historic structures or listed as 

historic places, or both. A full list of historic places considered during the preparation 

of this EA is available in the administrative record (prepared by E. Sutker, USDA

APHIS, October 25, 2016).

Table 5. Summary of the number of listed historic sites present in the 10 Texas action 

area counties.

County Nationally Listed Places Cemeteries Museums 

Cameron 26 (and 4 battlefields) 19 3

Hidalgo 23 (and 1 battlefield) 13 2

Jim Wells 0 4 1

Kinney 2 1 2

Maverick 2 1 0

Starr 9 2 1

Val Verde 3 2 3

Webb 9 3 1

Willacy 1 1 0

Zapata 6 1 0
29

APHIS’ program activities will not alter, change (restore or rehabilitate), modify, 

relocate, abandon, or destroy any historic buildings, edifices, or nearby infrastructure. 

Therefore, APHIS program activities will not directly or indirectly alter characteristics 

of a historic property that qualifies it for inclusion in the National Register of Historic 

Properties. The listed historic cemeteries may provide habitat corridors for deer and 

ticks, nevertheless, treated corn will not be placed on the property of any listed historic 

property. The proposed action will not disturb the ground for construction. This 

proposed action does not include mowing, herbicidal treatments, or removal of plant 

material from any site; private land managers and owners will decide if these activities 

are to occur on the properties they manage. The proposed activities will not use heavy 

equipment that creates noise levels requiring auditory protection. Any visual, 

atmospheric, or auditory effects during delivery and consumption of treated corn will 

be limited in duration, intensity, and area. APHIS initiated consultation with the State 

Historic Preservation Office (SHPO) for Texas.

8. Potential Cumulative Impacts

The Council on Environmental Quality defines cumulative impacts as “the impact on 

the environment which results from the incremental impact of the action when added to 

other past, present, and reasonably foreseeable future actions regardless of what agency 

(Federal or non-Federal) or person undertakes such other actions. Cumulative impacts 

can result from individually minor but collectively significant actions taking place over 

a period of time.”(40 CFR § 1508.7). 

The CFTEP program along with trail use by other Federal agencies has been associated 

with minimal impacts to vegetation and soil compaction. Impacts to vegetation and soil 

occur to a limited degree as a result of CFTEP trail maintenance to survey for cattle 

coming from Mexico, U.S. Department of Homeland Security use of sites associated 

with potential illegal border crossings, and FWS trail use to monitor wildlife. 

Ivermectin corn feeders would be placed in previously disturbed areas resulting in little 

to no additional native vegetation or soil compaction.

Ivermectin is a widely used anti-parasitic drug in humans, livestock, and pets (Crump 

and Omura, 2011). There would be increased environmental loading from the use of 

ivermectin-treated corn for white-tailed deer where there are also ivermectin uses for 

cattle and other domestic animals. The impacts to white-tailed deer are expected to be 

incrementally negligible when put in context with other stressors because the dose of 

ivermectin is considered therapeutic and not intended to result in adverse effects. 

Domestic animals that are receiving ivermectin for other purposes are also not expected 

to have cumulative impacts resulting from the proposed use of ivermectin-treated corn 
30

because domestic animals will not be able to access the feeders. Cumulative impacts to 

aquatic organisms will also be negligible because there is an extremely low probability 

of exposure to aquatic habitats from the proposed use of ivermectin-treated corn 

(USDA APHIS, 2016b).

Cumulative impacts to human health from the proposed use of ivermectin-treated corn 

are not anticipated because of the proposed use pattern. Human exposure and risk is 

very low for the general public. The probability of exposure is greatest for workers who 

mix and fill the feeders. However, the risk to this group of the population will be 

negligible based on the low risk of ivermectin when using the appropriate PPE. There is 

the potential for worker exposure to other chemicals that may be used in the CFTEP.

Coumaphos is an organophosphate insecticide used to treat cattle for ticks that may 

vector cattle fever. The potential for cumulative impacts related to exposure to both 

pesticides by workers will be reduced by the use of PPE. Cumulative risk to the public 

from exposure to mixtures of both chemicals is also not anticipated because of the 

method of application, program controls, and restriction of public access to treatment 

areas. 

Pesticide use in other Federal programs potentially contributes to cumulative impacts 

with chemicals used in cattle fever tick management. Ongoing APHIS programs in 

South Texas using pesticides include the Boll Weevil Cooperative Eradication Program 

and fruit fly control programs. These programs do not treat livestock. Spinosad is a 

commonly used agricultural insecticide. It is highly unlikely there would be any 

pesticide drift of spinosad from fields and orchards or groves into livestock production 

areas where CFTEP activities occur. Subsequently, the probability of exposure to the 

public and workers to both spinosad and ivermectin at the same time would be very low 

because of the use patterns and intended target for each pesticide (USDA APHIS, 

2016b). 

There has been research to show synergistic effects between ivermectin and antibiotics 

(doxycycline, erythromycin, rifampicin, and azithromycin) in controlling body lice. 

These interactions are not expected with the proposed use of ivermectin because 

treatments will be directed at wildlife that would not be receiving antibiotic treatment. 

These types of mixture exposures would not be anticipated for humans either because 

of the method of application and other measures to prevent exposure to the public and 

workers who mix ivermectin with corn and load the feeders (USDA APHIS, 2016b). 

In summary, the cumulative impacts associated with the preferred alternative when 

assessed in relation to the current baseline and past, present, and future activities 

constitutes a small incremental change to the human environment. Some of these 
31

cumulative changes may be positive such as the reduction in cattle fever ticks and the 

associated economic benefits from having tick-free cattle. To preserve environmental 

quality for the human population and ecological resources, the CFTEP would minimize 

potentially negative cumulative impacts by following best management practices. 

APHIS does not find that any reasonably foreseeable effects caused by the program 

activities will occur later in time or be farther removed in time.
32

IV. Agencies Contacted

U.S. Department of Agriculture

Animal and Plant Health Inspection Service

Environmental and Risk Analysis Services

Policy and Program Development

4700 River Road, Unit 149

Riverdale, MD 20737

U.S. Department of Agriculture

Animal and Plant Health Inspection Service

Cattle Fever Tick Eradication Program

Surveillance, Preparedness, and Response Services

Veterinary Services

2150 Centre Avenue

Fort Collins, CO 80526

Texas Animal Health Commission

2105 Kramer Lane

Austin, TX 78758
33

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38

Appendix 1. Ivermectin Risk Assessment

Human Health and Ecological Risk Assessment for the Use of 

Ivermectin-treated Whole Kernel Corn to Control Cattle Fever 

Ticks in White-Tailed Deer Populations
EXECUTIVE SUMMARY 39

EXECUTIVE SUMMARY

United States Department of Agriculture (USDA) Animal and Plant Health Inspection Service 

(APHIS) Veterinary Services (VS) is proposing the use of ivermectin-treated corn in its cattle 

fever tick eradication program. Cattle fever is a serious disease that poses a threat to U.S. cattle 

production, in particular in south Texas. The disease is vectored by the cattle fever tick and 

southern cattle tick that can parasitize livestock and wildlife. A part of the integrated cattle fever 

tick eradication program (CFTEP) is the treatment of white-tailed deer using the parasiticide, 

ivermectin, to control both tick vectors of cattle fever. White-tailed deer are fed ivermectintreated corn from a closed gravity feeder placed in areas where cattle fever is a concern.

USDA APHIS evaluated the potential human health and ecological risks from the proposed use 

of ivermectin and determined that the risk to human health and non-target fish and wildlife is low 

based on the potential for exposure, and the toxicity and environmental fate profile for 

ivermectin. Ivermectin is mixed with whole kernel corn and placed in feeders that are used to 

dispense treated corn. The potential for exposure to people is greatest for workers who mix 

ivermectin with the corn and fill the feeders. However, the toxicity of ivermectin to mammals,

as well as label directions regarding mixing and application, suggests that the risk to this group 

of the population will be low. Risk to the general public will be negligible based on the method 

of application and program requirements that restrict access to feeders. Threats to ground and 

surface water that may be used as drinking water are not anticipated because of the proposed 

treatment method and environmental fate of ivermectin. Risk to the general public from the 

consumption of meat from ivermectin-treated deer will be negligible because feeding deer with 

ivermectin-treated corn will be discontinued 60 days prior to the deer hunting season. 

The risk of ivermectin to most fish and wildlife species is expected to be very low. Ivermectin is 

considered toxic to aquatic organisms; however, the proposed method of application, program 

controls, and the environmental fate of ivermectin suggest that significant exposure to aquatic 

habitats will not occur. There is the possibility of exposure to some non-target terrestrial wildlife 

who may consume corn that is spilled onto the ground during feeding by white-tailed deer. The 

use of a barrier and weekly monitoring of the feeders will reduce this type of exposure to many 

non-target terrestrial vertebrates. The low probability of exposure and therapeutic doses of 

ivermectin being administered suggest that risk to non-target terrestrial vertebrates who may 

consume some spilled treated corn will be low. There is some risk to terrestrial invertebrates that 

depend on deer droppings for development. Some beetles and flies that use deer droppings for 

development may be impacted due to the presence of ivermectin. Impacts to this group of 

invertebrates will be variable based on site specific conditions that will impact ivermectin 

degradation and availability. 

 
INTRODUCTION 40

1.0 INTRODUCTION

This human health risk assessment (HHRA) and ecological risk assessment (ERA) provide a 

qualitative and quantitative evaluation of the potential risks and hazards to human health, nontarget fish, and wildlife as a result of exposure to ivermectin. APHIS proposes the use of 

ivermectin-treated whole kernel corn in feeding stations to control cattle fever ticks in whitetailed deer populations. 

The methods used in this HHRA to assess potential human health effects follow standard 

regulatory guidance and methodologies (NRC, 1983; USEPA, 2015), and generally conform to 

other Federal agencies such as U.S. Environmental Protection Agency, Office of Pesticide 

Programs (USEPA/OPP). The methods used in this ERA to assess potential ecological risk to 

non-target fish and wildlife follow USEPA and other published methodologies regarding ecorisk assessment, with an emphasis on those used by USEPA/OPP in the pesticide registration 

process. 

The risk assessment is divided into four sections beginning with the problem formulation 

(identifying hazard), then a toxicity assessment (the dose-response assessment), and an exposure 

assessment (identifying potentially exposed populations and determining potential exposure 

pathways for these populations). In the fourth section (risk characterization) the information 

from the exposure and toxicity assessments are integrated to characterize risk of ivermectin use

to human health and the environment. 
PROBLEM FORMULATION 41

2.0 PROBLEM FORMULATION

Cattle fever is a severe and often fatal disease of cattle transmitted by the cattle fever tick, 

Rhipicephalus (=Boophilus) annulatus, and southern cattle tick, R. microplus. These parasites 

typically attach themselves to the skin inside an animal’s thigh, flanks, and forelegs or along the 

belly and brisket, and spread cattle fever through infected saliva. Infection can cause anemia, 

seizures, aggressiveness, jaundice, and death. To control cattle fever ticks, APHIS created the 

Cattle Fever Tick Eradication Program (CFTEP) in 1906. With the help of mounted patrol 

inspectors (also known as tick riders) and systematic quarantines, the CFTEP eradicated cattle 

fever and cattle fever tick populations from the continental United States in 1943, with the 

exception of a permanent quarantine “buffer” zone between Texas and Mexico. Today, this 

buffer zone extends over 500 miles from Del Rio, Texas, to the Gulf of Mexico (USDA APHIS, 

2015).

Failure to control ticks on wildlife hosts greatly compromises efforts to eradicate ticks on 

livestock and poses a substantial threat of infestation and disease establishment throughout South 

Texas. Current efforts to control cattle fever ticks include a partial tick control barrier fence, 

livestock movement quarantines, population control of certain hosts such as Nilgai, and tick 

treatments for cattle and deer. While these methods are effective, the free-ranging movement of 

deer and stray livestock across non-fenced properties and an increase in the overall white-tailed 

deer population has recently led to increased fever tick infestations in South Texas. As of 

October 2009, 72 premises outside the quarantined area were found to be infested, compared to 8 

infested premises in October 2006 — a nine-fold increase in 3 years (USDA APHIS, 2015).

To control ticks in deer, APHIS proposes the use of ivermectin-treated whole kernel corn bait in 

feeding stations as a way to deliver a systemically active acaricide. The FDA does not object to 

an extra-label drug use as long as all applicable provisions of 21 CFR 530 are met, which 

includes establishing a withdrawal period that ensures no illegal drug residues occur in treated 

deer (Email from Vitolis E. Vengris, Deputy Director, Division of Surveillance, Center for 

Veterinary Medicine, U.S. Food and Drug Administration, to Thomas R Kasari, Cattle Health 

Staff Veterinarian, USDA-APHIS-VS (March 14, 2016 1:00PM DST) (on file with APHIS). 

APHIS-VS has a 2015 agreement with the Texas Animal Health Commission and the Texas 

Parks and Wildlife Department on treating hosts for cattle fever ticks by feeding them with an 

ivermectin-treated corn product (Memorandum of Understanding between Texas Animal Health 

Commission (TAHC) and the Texas Parks and Wildlife Department (TPWD) and the United 

States Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) 

Veterinary Services (VS) (Dec. 2, 2015) on file with APHIS).

Ivermectin is a macrocyclic lactone and semisynthetic chemical derivative of avermectin that is 

produced by soil microorganisms (Streptomyces avermitilis). Ivermectin is a broad spectrum 

antiparasitic drug. Similar to other macrocyclic lactones, ivermectin blocks the transmission of 

neural signals of the parasites by binding selectively and with high affinity to the glutamategated chloride channels in nerve and muscle cells of invertebrates, and acting as an agonist of the 

gamma-aminobutyric acid (GABA) neurotransmitter in the peripheral nervous system of 

invertebrates and in the central nervous system (CNS) of vertebrates (Liebig, et al., 2010; 

Junquera, 2015; Crump and Omura, 2011). Without the transmission of neural signals, parasites 
PROBLEM FORMULATION 42

are paralyzed and expelled out of the body, or starve (Junquera, 2015). In vertebrates, the 

GABA receptors occur only within the CNS (usually in the brain) where the blood-brain barrier 

prevents larger molecules, such as ivermectin, from entering the brain (Omura, 2008; Crump and 

Omura, 2011). As a result, ivermectin is much less toxic to vertebrates than invertebrates that 

lack a blood-brain barrier. This situation allows for a high safety margin when there is 

ivermectin use on livestock and pets against invertebrate pests. 

Ivermectin is approved as a human drug under the name of Stromectol® in the United States for 

the treatment of strongyloidiasis and onchocerciasis (USFDA, 2016a). It is also used for the 

treatment of scabies, lice, and ascariasis in other countries (NIDDK, 2016). The U.S. Food and 

Drug Administration (FDA)-approved ivermectin human drugs currently include Stromectol® (3 

milligram (mg) oral tablet), Sklice® (0.5% topical lotion), and Soolantra® (1% topical cream) 

(USFDA, 2016a). Ivermectin is also widely used as a veterinary medicine to treat various ectoand endoparasites on a variety of mammals, birds, reptiles, amphibians, and fish used in 

agriculture, or that are held in captivity such as zoos (Panayotova-Pencheva, 2016).

2.1 Chemical Description and Product Use

Ivermectin (CAS No. 70288-86-7, C48H74O14) is a mixture of mostly avermectin H2B1a (RN 

71827-03-7) with smaller quantities of avermectin H2B1b (RN 70209-81-3), that are macrolides 

from the bacteria, Streptomyces avermitilis (NCBI, 2016) (Figure 2-1).

Figure 2-1. The chemical structure of ivermectin

Ivermectin is the active ingredient (a.i) in several animal medicines including the formulation

Ivomec® pour-on for cattle. Ivomec® pour-on for cattle contains 5 mg ivermectin per milliliter 

(ml) (Merial, 2015). The formulation consists of 0.5% ivermectin compound B1a and compound

B1b, 80% isopropanol, and 19.5% non-hazardous inert ingredients (Merial, 2010). Ivomec pouron® for cattle, was originally approved by FDA (New Animal Drug Application No. 140-841) in 

1990 for the treatment and control of gastrointestinal nematodes, lungworms, cattle grubs, 
PROBLEM FORMULATION 43

sucking and biting lice, and sarcoptic mange mites with a supplemental approval in 1997 for 

persistent control of gastrointestinal roundworms. The same formulation was given a second 

supplemental approval in 2003 for persistent activity against Dictyocaulus viviparus (lungworm)

for 28 days after treatment, Cooperia surnabada (gastrointestinal roundworm) for 14 days after 

treatment, and Damalinia bovis (lice) for 56 days after treatment. The same approval also 

allowed for extended persistent activity periods for Oesophagostomum radiatum (gastrointestinal

roundworm) from 14 to 28 days after treatment and C. punctata and Trichostrongylus axei

(gastrointestinal roundworms) from 14 days to 21 days after treatment. The revised labeling is to 

reflect updated environmental information, to speciate Cooperia spp. (gastrointestinal

roundworms) in the treatment and control section, and to add a veal calf warning statement to 

the residue information section (USFDA, 2016b). Ivomec® pour-on for cattle is sold by Merial, 

Inc. but there is a generic product, Ivomax® pour-on for cattle (FDA ANADA 200-272), that is 

also available for use (Aspen, 2016). To control ticks in deer, APHIS proposes the use of 

ivermectin-treated whole kernel corn bait delivered to deer using a closed gravity feeding station

from February through July at a rate of 10 mg of ivermectin per pound of corn. 

2.2 Physical and Chemical Properties

Ivermectin is a white to yellowish-white, non-hygroscopic, crystalline powder with a molecular 

weight of 875 grams (g)/mole (mol) (Merck, Sharp, & Dohme Corp., 2010). It has a low vapor 

pressure of 1.5 x 10-9 millimeters of mercury (mm Hg) or less, suggesting it is not likely to 

volatilize into the atmosphere. Ivermectin has an octanol/water coefficient of 1,651 suggesting it 

may partition into lipids and is hydrophobic. Ivermectin is not considered soluble in water 

(water solubility = 4 mg/liter (L)) (Bloom and Matheson, 1993). The Ivomec® pour-on for cattle 

formulation is a blue liquid with an alcohol odor. It is not an oxidizing agent but is considered

flammable (Merial, 2010). 

2.3 Environmental Fate

The environmental fate describes the processes by which ivermectin moves and is transformed in 

the environment. The environmental fate processes include: 1) mobility, and migration potential 

to groundwater and surface water, 2) persistence and degradation, and 3) plant uptake. 

Ivermectin has low volatility and is unlikely to partition into the atmosphere. Ivermectin binds 

strongly to soil particles based on the high soil organic carbon binding coefficients that range 

from 4,000 to 15,700 L/kilogram (kg) (Halley et al., 1989a; Liebig et al., 2010). Variations in 

soil binding coefficients are due to the various soil types and organic carbon concentrations 

tested in each soil. The strong binding affinity of ivermectin to soil and sediment means it is 

unlikely to leach into groundwater or runoff to surface water in a dissolved state. Ivermectin was 

not detected in leachate in a leaching study with soils containing 2.3 and 6.3% organic carbon 

content (Oppel et al., 2004). Halley et al. (1989a) reported 39% to 45% of the applied 

radioactivity remained in the top 5 centimeters (cm) of the soil column, and 10% to 48% leached 

as metabolites or degradation products. No ivermectin parent material was detected in the 

leachate.
PROBLEM FORMULATION 44

Table 2-1. Environmental fate half-lives for ivermectin.

Compartment Half-life (days) Reference

Water (dissipation) <0.25-5 Prasse et al., 2009; Loffler et 

al., 2005; Sanderson et al. 

2007

Water (degradation) <1-30 Bloom and Matheson, 1993; 

Liebig et al., 2010

Water/sediment (dissipation) 15-130 Prasse et al., 2009; Loffler et 

al., 2005; Liebig et al., 2010

Sediment (degradation) 130 Liebig et al., 2010

Soil (dissipation) 16.1-36.1 Krogh et al., 2009

Soil (degradation) 7-217 Halley et al., 1989a

Ivermectin breaks down to less bioactive compounds via photo and aerobic degradation (Halley 

et al., 1993). Degradation of ivermectin in soil depends on the soil type and properties, sorption 

capacity, as well as temperature (Krogh et al., 2009). Ivermectin half-lives in soil are 7 to 14 

days at high temperatures in summer, but can be much longer (91 to 217 days) at low 

temperatures in the winter (Bloom and Matheson, 1993; Halley et al., 1993). Photolysis of 

ivermectin in water is less than 0.5 day in summer and 39 hours in winter (Halley et al., 1993; 

Bloom and Matheson, 1993). Its photolytic half-life was approximately 3 hours on a thin, dry 

film exposed to direct sunlight (Halley et al., 1989a). 

Studies in cotton and food crops (sorghum, lettuce, carrots, and turnips) show that plants uptake 

little ivermectin from direct applications to plants or from soil (Merck & Co., Inc., 1991). Greenhouse and field trial studies were performed in seventeen terrestrial species by exposing the 

plants to high levels of several ivermectin analogs. The studies did not show phytotoxic effects 

at foliar application rates as high as 10 kg/hectare (ha) of ivermectin-like material. The study 

results also indicated that ivermectin analogs are not readily translated into plants from the soil 

or from directly spraying (Bloom and Matheson, 1993).

2.4 Hazard Identification for Human Health 

FDA has approved ivermectin for human use; however, it can be a hazard to human health in

accidental intoxication or when significant exposures to veterinary formulations occur. The most 

frequently reported adverse effects include rash, edema, headache, dizziness, asthenia, nausea, 

vomiting, and diarrhea (Merck, Sharp, & Dohme Corp., 2010). The Ivomec® pour-on for cattle 

formulation has a narcotizing effect and is irritating to eyes. Vapors of the formulation may 

cause drowsiness and dizziness (Merial, 2010). Symptoms from overexposure to the Ivomax® 

pour-on for cattle formulation may include decreased activity, slow rate of breathing, dilation of 

the pupils, muscle tremors, and in-coordination (Aspen, 2013). The Ivomax® pour-on 

formulation can cause eye irritation from direct eye contact, and may cause irritation and/or 

drying and cracking of the skin from prolonged or repeated contact.
PROBLEM FORMULATION 45

2.4.1 Toxic Effects

Acting as a GABA agonist, ivermectin has reduced toxicity in mammals and other vertebrates 

compared to invertebrates because GABA is found only in the CNS of mammals and is normally 

protected by the blood-brain barrier. In humans, the symptoms of toxic effects from overdose 

exposure to ivermectin include rash, edema, headache, dizziness, asthenia, nausea, vomiting, and 

diarrhea. Other symptoms include seizure, ataxia, dyspnea, abdominal pain, paresthesia, and 

urticarial and contact dermatitis (Merck, Sharp, & Dohme Corp., 2010).

Higher ivermectin toxic effects are reported in some dog breeds such as collies, Shetland 

sheepdogs, old English sheepdogs, Australian collies, and their crosses. These breeds are more 

sensitive to ivermectin due to a mutation of the multiple drug resistance gene (MDR1) causing 

an abnormality in the blood-brain barrier and allowing increased ivermectin into the CNS. The 

symptoms include mydriasis, ataxia, and tremors, as well as collapse, coma, and respiratory 

collapse at higher doses (Merck, Sharp, & Dohme Corp., 2016).

Some dogs are homozygous for a mutation to produce a severely truncated P-glycoprotein 

(<10% of the normal amino acid sequence). Dogs with this mutation will develop ivermectin 

toxicity at any of the dosages used to treat demodicosis. Idiosyncratic reactions may develop 

in any breed (Merck, Sharp, & Dohme Corp., 2016). 

2.4.2 Metabolism

Ivermectin is absorbed into blood, distributed throughout the mammal's body (Canga et al., 

2009). Ivermectin tends to be deposited in the body fat and the liver, where it is progressively 

metabolized (biotransformation) and excreted. Main factors affecting absorption are route of 

administration, formulation, and animal species. Compared to monogastric animals, the 

ruminant species appear to have a slower absorption process after oral administration, and a 

longer plasma elimination half-life after intravenous administration (Canga et al., 2009). Animal 

studies show approximately 90% of the dose administered parenterally or orally is excreted in 

the feces with unchanged parent compound in tissue residues (Canga et al., 2009, Campbell, 

1985). Only 0.5 to 2% of the dose is excreted in urine (Campbell et al., 1983), and less than 1% 

appeared in the urine and less than 2% in breast milk (Temple and Smith, 1992). Approximately

half (39-45%) of the ivermectin in feces of subcutaneously treated cattle are the parent 

compound and 54% are polar metabolites (Halley et al., 1989). In humans, ivermectin is 

primarily metabolized in the liver by cytochrome P450 enzymes (Canga et al., 2008). 

2.4.3 Human Case Study

Guzzo et al. (2002) performed a dose escalation study in humans to evaluate ivermectin CNS 

effects and general toxicity of ivermectin. Sixty-eight healthy adults were administered in higher 

and/or more frequent doses (three times a week of 30 or 60 mg, or single dose of 90 or 120 mg) 

than the approved dose for human use. Mydriasis quantitated by pupillometry was the primary 

safety endpoint. The study showed that ivermectin is generally well tolerated by healthy adults, 

with no indication of associated CNS toxicity at levels up to 10 times the highest FDA-approved 

dose of 200 μg/kg. 
PROBLEM FORMULATION 46

Approximately 40 cases of self-injection with ivermectin solutions intended for animal use 

reported the major effect as pain at the injection site. Other reported effects include nausea, 

paresthesia, variable blood pressure, urticaria, and cellulitis. Dermatitis was the main reaction 

following dermal exposure. Approximately 10 cases of accidental ingestion of ivermectin 

solution or tablets by adults reported adverse effects of mydriasis, vomiting, tachycardia, and 

somnolence (Woodward, 2016).

An acute liver injury was reported from a single dose of ivermectin (15 mg orally) used as a 

subsequent treatment for low levels of microfilariae in the blood in a 20 year old woman from 

Cameroon (NIDDK, 2016; Veit et al., 2006). The patient developed hepatitis and experienced 

abdominal pain and elevated serum aminotransferase levels one month after treatment. A liver 

biopsy showed intralobular inflammatory infiltrates, confluent necrosis, and apoptosis

compatible with drug-induced liver disease. Symptoms improved and serum aminotransferase 

levels fell rapidly within days and the patient fully recovered four months later.

2.4.4 Acute Toxicity

The acute oral LD50s are 25 mg/kg in mice and 50 mg/kg in rat. The acute dermal LD50s are 

higher than 660 mg/kg in rats and 406 mg/kg in rabbits. The acute oral LD50 in dogs without the

MDR1 gene defect is 80 mg/kg. The acute oral LD50 for rhesus monkey is >24 mg/kg (Lankas 

and Gordon, 1989). The acute oral LD50 for dogs with the MDR1 gene defect is 0.2 mg/kg 

(Hopper et al., 2002).

The safety data sheet for the formulation reports oral LD50 values for ivermectin comp. B1a and 

comp. B1b are greater than 15 mg/kg (adult human), ~80 mg/kg (dog), 25 mg/kg (mouse), 50 

mg/kg (rat), and higher than 24 mg/kg (rhesus monkey). The dermal LD50s are 406 mg/kg 

(rabbit) and >660 mg/kg (rat). Inhalation LC50 values are 0.4 mg/kg (rat) and 1.6 mg/kg (adult 

human). The intraperitoneal LD50 is 55 mg/kg (rat), and the subcutaneous LD50 is ~10 mg/kg 

(juvenile dog) (Merial, 2010).

2.4.5 Subchronic and Chronic Toxicity

A 3-month oral toxicity study in rats exposed to ivermectin via in utero at dose levels of 0.4, 0.8, 

and 1.6 mg/kg/day reported a no observable effect level (NOEL) of 0.4 mg/kg/day. At 0.8 

mg/kg/day and above, splenic enlargement and reactive hyperplasia of the bone marrow were 

observed (Lankas and Gordan, 1989).

A 3-month oral toxicity study in beagle dogs exposed to ivermectin via gavage at dose levels of 

0.5, 1.0, and 2.0 mg/kg/day reported a NOEL of 0.5 mg/kg-day. Mydriasis was observed at 1.0 

mg/kg/day. Tremors, ataxia, and anorexia were observed at 2.0 mg/kg/day (Lankas and Gordan, 

1989).

A 16-day oral toxicity study in immature rhesus monkey exposed to ivermectin via nasogastric 

intubation at dose levels of 0.3, 0.6, and 1.2 mg/kg/day reported a NOEL of >1.2 mg/kg/day with 

no treatment-related effects (Lankas and Gordan, 1989).
PROBLEM FORMULATION 47

2.4.6 Nervous System Effects

CNS effects and visual disturbances have been observed at relatively high doses in 

mammals. Some mammals such as rodents are more sensitive to ivermectin CNS effects than 

others such as primates. Rats administered ivermectin intravenously at 4 mg/kg, produced 

moderate incoordination. A dose of 6 mg/kg induced a state resembling anaesthesia (started one 

minute after injection and lasted for four to five hours). Higher doses caused respiratory 

depression and resulted in death (Hayes and Laws, 1991). 

 

A 14-week oral toxicity study in dogs reported no treatment-related effects in dogs given 0.5 

mg/kg/day. Dogs given 1 mg/kg/day developed mydriasis and lost a small amount of 

weight. Dogs given 2 mg/kg/day developed tremors, ataxia, anorexia, and became dehydrated 

(Temple and Smith, 1992).

Campbell and Benz (1984) reported ataxia with tremors in dogs given oral doses of 10 mg/kg 

ivermectin. Death occurred at 40 mg/kg due to respiratory depression. Collie dogs have been 

shown to be more sensitive than other dogs to the toxic effects of ivermectin. Depression, 

tremors, mydriasis, ataxia, coma, and death have been seen in collie dogs at 100 μg/kg orally and 

greater, but not at the recommended dose of the commercial product (6 ug/kg) (Campbell and 

Benz, 1984; Hopper et al., 2002).

2.4.7 Reproductive or Developmental Effects

Lankas and Gordon (1989) reported ivermectin’s toxic effects in a reproductive and neonatal 

toxicity study in several generations of rats. The study results showed ivermectin (oral 

administered dose as low as 0.4 mg/kg body weight per day) caused increased postnatal pup 

mortality and a decrease in pup body weight in survival offspring. Neonatal rats are more 

sensitive to ivermectin because they form the blood-brain barrier during the postnatal 

period. Other mammals such as humans, sheep, and rabbits form the blood-brain barrier during 

the prenatal period.

A 3-year study in Liberia identified 203 children born to women inadvertently treated with 

ivermectin during pregnancy. There was no increased incidence of birth defects in these 

children, when compared with children born to untreated mothers in the same population or in a 

reference population (Pacque, et al., 1990).

2.4.8 Carcinogenicity and Mutagenicity

A one-year carcinogenic study using male Wistar rats (0 and 2 parts per million (ppm) of 

ivermectin in diet) reported negative results (O'Connor et al., 2001). Ivermectin is not listed as 

carcinogenic by the U.S. National Toxicology Program or the International Agency for Research 

on Cancer.
DOSE-RESPONSE ASSESSMENT 48

Mutagenicity studies (the AMES Assay and a mouse lymphoma mutation assay) with ivermectin 

have been negative. Ivermectin did not induce unscheduled DNA synthesis in a human 

fibroblast cell culture or damage DNA (Temple and Smith, 1992).

2.4.9 Endocrine System Effects

The available data does not indicate ivermectin has endocrine system effects. 

2.4.10 Immune System Effects

The available data does not indicate ivermectin has immune system effects. An ivermectin 

immune study in rabbits and rats did not show direct immune response (Uhlir and Volf, 1992). 

2.4.11 Toxicity of Other Ingredients

The Merial safety data sheet lists 80% of the Ivomec® pour-on for cattle formulation as

isopropanol (CAS # 67-63-0), 19.5% non-hazardous inert ingredients and the remaining 0.5% as 

ivermectin (Merial, 2010). More than 60% of the Ivomax® pour-on for cattle formulation is 

isopropanol (Aspen, 2013). Isopropanol has low toxicity (Category III) in oral dosing and very 

low toxicity (Category IV) in dermal and inhalation exposures. The oral LD50 in rats is 4,570 

mg/kg. The dermal LD50 in rabbits is 13,400 mg/kg and the inhalation LC50 in rats is 30 mg/L 

(Merial, 2010). Ingestion of a large amount of isopropyl alcohol will cause burning of the 

gastrointestinal tract, nausea, vomiting, and CNS depression (Aspen, 2013).

3.0 DOSE-RESPONSE ASSESSMENT

3.1 Human Health Dose-Response Assessment

A dose-response assessment evaluates the dose levels (toxicity criteria) for potential human 

health effects including acute and chronic toxicity. 

The FDA has established tolerances for the proposed ivermectin formulation in the CFTEP. For 

22,23-dihydroavermectin B1a (marker residue) in liver (target tissue), the tolerances are 1.6 ppm 

in cattle, 20 parts per billion (ppb) in swine, 30 ppb in sheep, and 15 ppb in reindeer and 

American bison. Muscle residues are not indicative of the safety of other edible tissues. For 

22,23-dihydroavermectin B1a (marker residue) in muscle, the tolerances established include 20 

ppb for swine, and 0.65 ppm for cattle (USFDA, 2016c).

3.2 Ecological Dose-Response Assessment

3.2.1 Wild Mammal, Avian and Reptile Toxicity

Ivermectin has been used as a treatment for internal and external parasites in various mammals, 

birds and reptiles (Panayotova-Pencheva, 2016). The toxicity of ivermectin to mammals is 

characterized in the above human health section that covers acute and chronic effects in 
DOSE-RESPONSE ASSESSMENT 49

laboratory animal studies. Acute oral toxicity values range from 25 to approximately 80 mg/kg,

suggesting ivermectin is moderately to slightly toxic to mammals.

Acute toxicity of ivermectin to birds is considered low to moderate depending on the test species. 

The oral LD50 for ivermectin to bobwhite quail is 2,000 mg/kg and 88 mg/kg for the mallard

duck. Similar sensitivities are seen in dietary studies with LC50 values of 3,102 and 383 mg/kg, 

for the bobwhite quail and mallard, respectively (Bloom and Matheson, 1993). Little toxicity 

data appears to be available for reptiles; however, tortoises have been reported as less tolerant of 

therapeutic doses of ivermectin than other terrestrial vertebrates (Halley et al., 1989b). These 

studies appear to be based on injected doses of ivermectin in reptiles, thus the applicability to 

this risk assessment is limited because the primary route of exposure would be from the 

consumption of ivermectin-treated corn. USEPA/OPP uses avian data as a surrogate to represent 

the sensitivity of reptiles to pesticides. There is uncertainty in that assumption however. With 

the lack of oral dosing toxicity data for reptiles, ivermectin is considered low to moderately toxic

to reptiles.

3.2.2 Terrestrial Invertebrate Toxicity

Terrestrial invertebrate toxicity data for ivermectin is primarily related to soil dwelling and dung 

dependent invertebrates because of the method of application for most ivermectin uses, and 

residues that may occur in droppings or manure from treated animals. Jensen et al. (2003) 

reported EC50 values and no observable effect concentrations (NOEC) of 1.7 and 0.3 mg/kg dry 

soil for collembolans, and 36 and 3 mg/kg for enchytraeids, respectively, in 21-day exposures. 

Forster et al. (2011) evaluated the impacts of ivermectin on a variety of soil borne invertebrates 

from field collected soil that was dosed at various ivermectin levels and found collembolans to 

be the most sensitive taxa. Rombke et al. (2010a) showed similar results when comparing lethal 

and sublethal effect levels in laboratory exposures using the collembolan, Folsomia candida, 

earthworm, Eisenia fetida, and predatory mite, Hypoaspis aculeifer. The EC50 and NOEC for the 

collembolan, earthworm, and predatory mite were 1.7 and 0.3, 5.3 and 2.5, and 17.8 and 3.2 mg/ 

kg dry soil, respectively. The toxicity values estimated for the earthworm are some of the lower 

reported values and may be due to differences in organic carbon content in the test soils between 

studies which can alter the bioavailability of ivermectin. 

Invertebrates in the Diptera and Coleoptera insect orders are the primary taxa with species that 

are dependent on dung for development. Work evaluating invertebrate colonization of cattle 

dung containing ivermectin has shown negative effects to various insect taxa and impacts to 

species diversity (Suarez et al., 2003; Jochman et al., 2015). The range of doses and effects that 

cause adverse effects are variable and depend on the type of formulation being tested, dosing 

method used, species dosed, and environmental variables in fields where the studies were 

conducted (Wall and Beynon, 2012). Blackenhorn et al. (2013) evaluated the effects of 

ivermectin on 21 species of sepsid dung and black scavenger flies and found a wide range of 

sensitivities. Lethality values (LC50) ranged from 0.05 to 18.55 micrograms (μg) ivermectin/kg 

dung. Larval Diptera and Coleoptera are the life stage most affected by ivermectin, and 

cyclorrhaphous dipteran larvae are more sensitive than coleopteran larvae (Rombke et al., 2010b; 

Steel and Wardhaugh, 2002). The impact to invertebrates from ivermectin has shown mixed 
DOSE-RESPONSE ASSESSMENT 50

effects regarding whether there are impacts to dung degradation which could impact nutrient 

cycling (Floate et al., 2005). 

3.2.3 Terrestrial Plant Toxicity

No terrestrial phytoxicity data appears to be available for ivermectin. However, studies using

analogs of ivermectin have shown low phytotoxicity when applied directly to plants at levels up 

to 10 kg/ha (Bloom and Matheson, 1993). Ivermectin that may occur in soil or manure, or 

applied directly to plants will not translocate and result in adverse effects to terrestrial plants.

3.2.4 Aquatic Toxicity 

Ivermectin is considered highly toxic to most aquatic invertebrates based on available laboratory 

and field toxicity studies. Acute toxicity values range from 25 nanograms (ng)/L to 400 μg/L

with cladocerans being the most sensitive aquatic invertebrate species (Table 3-1). The 

sensitivity of cladocerans to ivermectin has also been confirmed in mesocosm studies showing 

acute impacts to cladoceran populations at 30 ng/L. Other invertebrates such as ostracods, 

copepods, and ephemeropterans were less sensitive in acute exposures with predicted NOECs

ranging from 100 to 1,000 ng/L (Sanderson et al., 2007). Chronic toxicity to aquatic 

invertebrates is also high. Lopes et al. (2009) reported NOECs of 0.0003 and 0.001 ng/L for the 

freshwater cladocerans, Daphnia magna and Ceriodaphnia dubia, respectively. Garric et al. 

(2007) also reported a NOEC of 0.0003 ng/L for D. magna in 21-day exposures that assessed

impacts on reproduction and growth. Aquatic invertebrate toxicity from ivermectin can be 

ameliorated due to the partitioning of ivermectin in the presence of organic material. CarbonellMartin et al. (2011) report a 48-hour EC50 of 0.50 μg/L for D. magna in soil leachate which is an 

approximate 10-100 fold reduction in toxicity compared to values reported in water without 

organic matter. Two of the primary hydrolytic metabolites of ivermectin have been shown to be 

between 16 and >680 times less toxic to D. magna than the parent material (Halley et al., 1989b).
DOSE-RESPONSE ASSESSMENT 51

Table 3-1. Acute toxicity of ivermectin to freshwater and marine invertebrates.

Species EC/LC50 Reference

Daphnia magna, cladoceran 5-25 ng/L Garric et al., 2007; Halley et al., 

1989b

D. similis 70 ng/L Dal Basco et al., 2011

Neomysis integer, mysid shrimp 70 ng/L Davies et al., 1997

Gammarus pulex, freshwater 

amphipod

1.8-2.1 μg/L* Alonso et al., 2010

G. fusarium 2.1-3.6 μg/L* Alonso et al., 2010

Mytilus edulis, Eastern oyster 400 μg/L Davies et al., 1997

Tubifex tubifex, oligochaete 2.0 mg/L Gerhardt, 2009

*value approximated from graph for adults and juveniles

Effects to benthic inverterbrates have also been evaluated for various species due to the low 

solubility of ivermectin and its preferential binding to soil and sediment. Egeler et al. (2010) 

reported 28-day EC50 values for the chironomid, Chironomus riparius, ranging from 9.0 to 64 

μg/kg and NOECs ranging from 3.1 to 25 μg/kg, depending on the endpoint. The most sensitive 

NOEC was based on statistical differences in individual dry weight. The oligochaete, 

Lumbriculus variegatus, was less sensitive compared to C. riparius in a similar 28-day exposure 

with an EC50 value of 2,980 μg /kg and NOEC of 160 μg/kg based on the total number of 

worms. Allen et al. (2007) evaluated the impacts of sediment-bound ivermectin to the marine 

amphipod, Corophium volutator and polychaete, Arenicola marina in 10-day exposures. The 

lowest LC50 and NOEC values were reported as 14.8 and 2.0 μg/kg for the amphipod, and an 

LC50 of 21.9 for A. marina. Brinke et al. (2010) reported NOECs of 0.62 and 0.06 μg/kg for 

meiobenthic and nematode communities, respectively, in ivermectin-dosed sediments in 

freshwater mesocosms. 

Ivermectin is used as a therapeutic treatment for fish and amphibians. The toxicity of ivermectin 

to aquatic vertebrates is much less when compared to aquatic invertebrates with 96-hour LC50

values ranging from 3.3 to 5.3 μg/L, for the rainbow trout, Onchorynchus mykiss, and bluegill 

sunfish, (Lepomis microchirus) respectively (Halley et al., 1989b). The acute NOEC in the 

rainbow trout study was reported as 0.9 μg /L. The sea bream, Sparus aurata, is less sensitive 

than the bluegill sunfish and rainbow trout with no mortalities reported in 96-hour exposures 

between 0.056 and 0.32 mg/L (Mladineo et al., 2006). The 96-hr LC50 for the Atlantic salmon, 

Salmo salar, is reported as 0.017 mg/L (Kilmartin et al., 1996). The zebrafish, Danio rerio, has 

comparatively lower sensitivity to ivermectin with a 144-hour LC50 value of 0.44 mg/L and a 

NOEC of 0.22 mg/L (Carlsson et al., 2013). Ivermectin sensitivity for amphibians appears to be 

in the range of sensitivities for fish based on one 96-hour LC50 value of 5.5 μg/L, with a NOEC 

of 1.1 μg/L, for the African clawed frog (Xenopus laevis) (Martini et al., 2012). 
DOSE-RESPONSE ASSESSMENT 52

Ivermectin has low toxicity to aquatic plants with no impacts noted for green algae, Chlorella 

pyrenoidesa, in a 14-day exposure at concentrations up to 9.1 mg/L (Halley et al., 1989b).
EXPOSURE ASSESSMENT 53

4.0 EXPOSURE ASSESSMENT

4.1 Human Health Exposure Assessment

The exposure assessment estimates the potential exposure of humans to ivermectin. The 

exposure assessment begins with the use and application method for ivermectin in the CFTEP.

A complete exposure pathway for ivermectin includes (1) a release from an ivermectin source, 

(2) an exposure point where contact can occur, and (3) an exposure route such as ingestion, 

inhalation, or dermal contact by which contact can occur. In this way, the potentially exposed 

human population and complete exposure pathways are identified. Finally, exposures for the 

identified human populations are qualitatively evaluated for each exposure pathway.

APHIS uses the Ivomec® or Ivomax® pour-on for cattle formulation mixed with whole kernel 

corn that is placed in gravity flow feeding stations from February through July to control cattle 

fever ticks in deer populations. The gravity flow feeder is a commercially made plastic bin 

device with four feed tubes below the bin, and a lid (Figure 4-1). Each feeder has a holding 

capacity of approximately 300-350 pounds of corn. Each feed site will include one gravity flow 

feeder. Feeders will be enclosed with welded wire panels to exclude non-target animals (such as 

hogs, javelina, or livestock). The feeders will be serviced and filled weekly. The program office 

will maintain records including wildlife treatment feeding/bait station data sheets, service record 

sheets, and a map showing the number and location of each feeding station (USDA APHIS, 

2016). 

Figure 4-1. Closed gravity feeder system used to dispense ivermectin-treated corn to whitetailed deer.
EXPOSURE ASSESSMENT 54

For dosing, 200 ml of the formulation containing 5 mg ivermectin/ml will be pumped into one 

hundred pounds of clean corn to produce 10 mg of ivermectin a.i. per pound of corn. The 

feeding rate for deer is approximately 1% of body weight per day or approximately 1 pound per 

100 pound deer per day. The daily intake dose of the deer is approximately 0.22 mg/kg

assuming a 100 pound white-tailed deer eats 1 pound of corn per day. A previous study 

concluded that a feeding rate of 0.22 mg/kg should produce maximum blood serum levels of 

approximately 30 ppb (Pound et al., 1996). The target concentration of 30 ppb assures a high 

degree of efficacy even in those deer that may consume as little as one-third of the targeted 

dosage. The serum levels of 10 ppb should produce 100% efficacy against ticks attempting to 

feed on treated animals (Pound et al., 1996; Nolan et al., 1985; Miller et al., 1989). The 

ivermectin-treated corn will be fed to white-tailed deer throughout the treatment period 

(February through July).

The number of feed sites will be determined based on number and density of deer (1 feeder per 

20-30 deer to minimize excessive competition and social dominance), and density of feeders per 

area (deer do not have to travel more than 1/4 to 1/2 mile to access feed= 1 per 125 acres to 1 per 

500 acres).

Based on the application method, workers in the program are the most likely human population 

segment to be exposed to ivermectin. Occupational exposure to ivermectin may occur through 

incidental ingestion, inhalation, and dermal contact during application (mixing the formulation 

with corn and loading it into the feeding station). However, direct contact to ivermectin during 

outdoor application is not expected to occur under normal conditions with proper worker hygiene 

and properly functioning personal protective equipment (PPE). The safety data sheet states to 

use the product only in well ventilated areas, and use a suitable respiratory protective device 

when aerosol or mist is formed (Merial, 2010). The PPE for program workers applying the 

material to whole kernel corn baits includes vinyl, nitrile, or rubber gloves, a waterproof bibapron, and suitable eye protection (Merial, 2010; Aspen, 2013). Accidental exposure may occur 

during mixing of ivermectin with corn from liquid splashing or otherwise being transferred from 

contaminated gloves or clothing to an unprotected skin area (usually the face). However, these 

types of accidental exposures are unlikely with well-trained applicators. Drift from the 

application will not occur because baits are placed directly into the gravity feeding stations.

A complete exposure pathway associated with direct contact to ivermectin from the proposed 

application method is unlikely for the general public because they will have restricted access to 

the feeding stations. A lock will be installed at points of access to the property to allow 

systematic access to the premises/feeders (USDA APHIS, 2016). Comparable security will be 

provided at refuges and other public sites. Based on the APHIS feeding station use pattern, the 

potential for the general public to be exposed to ivermectin is not expected via inhalation, dermal

exposure, or through ingestion of treated corn.

A complete exposure pathway is not identified for dietary consumption of deer meat from the 

treated deer because the APHIS program requires no less than 60 days of withdrawal time before 

the start of the hunting season (USDA APHIS, 2016). FDA does not have an established 

withdrawal time for white-tailed deer. The FDA established a withdrawal time for reindeer of 56 

days based on a subcutaneous dose of 10 mg per 50 kg body weight and a liver (target tissue) 
EXPOSURE ASSESSMENT 55

tolerance of 15 ppb (USFDA, 2016d; 21 CFR 522.1192; Dieterich and Craigmill, 1990). Based 

on available data, the residual ivermectin serum and tissue concentrations in white-tailed deer 

should decrease to non-detectable levels and less than the tolerance level of 15 ppb, respectively 

before the withdrawal time of 60 days. Mackintosh et al. (1985) reported the elimination halflife of 6 days in the blood plasma of red deer calves after a subcutaneous administration of 

ivermectin at 0.2 mg/kg. The half-lives in tissue after a single subcutaneous treatment of 

ivermectin at 0.2 mg/kg were 7.1 days in back fat, 2.9 days for the injection site, 4.9 days for 

muscle, 5.8 days for liver, and 5.7 days for kidney (EMEA, 2004). A withdrawal time of 60 days 

(10x the elimination half-life of 6 days) was estimated based on the extra label use in food 

animals (Riviere et al., 1998; Baynes et al., 2000). The use of 10x the elimination half-life is a 

conservative approach that will insure eliminate 99.9% of the drug (Riviere et al., 1998). The 

ratio of 22,23-dihydroavermectin B1a in tissues at 28 days following a single percutaneous

administered dose of 1 mg/kg to red deer was 6.8:2.5:1.3:1 for fat, liver, kidney and muscle. The 

mean concentrations of 22,23-dihydroavermectin B1a in fat, liver, muscle, and kidney at 28 

days after treatment were 13.2, 9.3, 1.4, and 3.6 μg/kg, respectively. The ratio of 22,23-

dihydroavermectin B1a in tissues at 17 days after a single subcutaneous dose of 0.2 mg 

ivermectin/kg in reindeer was 9.5:6.6:2.6:1 in fat, liver, kidney and muscle, respectively (EMEA, 

2004). A study using penned female and male white-tailed deer administered by direct 

subcutaneous ivermectin injection or ingesting ivermectin-treated whole corn, showed 

ivermectin serum concentrations decreased below detectable levels (<2 ppb) within 21 days after 

injection and 14 days after ingestion (Pound et al., 2004). The potential exposure to ivermectin 

residues higher than the tolerance level from consumption of deer meat treated with ivermectincorn for the general public is precluded by cessation of feeding of ivermectin-treated corn to 

allow a withdrawal period of 60 days before the start of the hunting season.

A complete exposure pathway is not identified for the groundwater medium. Small amounts of 

ivermectin may be released to the soil in deer droppings; however, ivermectin soil concentrations 

would be extremely low due to the small amounts of droppings and ivermectin degradation and 

low solubility. In addition, any parent material would bind to soil and be unavailable to leach 

into groundwater (see Section 2.3).

A complete exposure pathway is not identified for the surface water medium. Significant surface 

runoff of ivermectin is not expected to occur from the use of treated corn. Feeding stations are 

placed a minimum of 50 feet from any aquatic areas and it is unlikely that any treated corn would 

be transported during a rain event to any surface water source used for drinking water. Any 

ivermectin transported to surface water would degrade and/or partition into sediment.

4.2 Ecological Exposure Assessment

4.2.1 Terrestrial Exposure Assessment

Exposure of ivermectin-treated corn to terrestrial non-target organisms will be minimized by the 

use of a closed gravity system feeder designed to be accessed by white-tailed deer through four 

spouts (Figure 4-1). Each spout has a baffle which can be adjusted to minimize spillage during 

feeding. Panels that are 34 inches high by 16 feet long are placed in a 15-foot radius around the 

feeder to reduce access by non-target wildlife and domestic animals. Long-term exposure will 

also be minimized by placement of the feeders in target areas from February to July. 
EXPOSURE ASSESSMENT 56

Although the amount of spillage from the feeders is expected to be minimal (<5%), the spilled

treated corn would be consumed by white tailed deer, or could be consumed by nontarget 

animals that are able to get within the panels. Expected ivermectin residues in corn are 22 ppm 

based on the mixing instructions to ensure white-tailed deer receive an effective dose. 

In addition to the exposure of non-target animals to ivermectin-treated corn, the metabolism of 

ivermectin in mammals suggests that some of the chemical will be present in feces from the 

target organism, as well as any non-target organisms that may feed on the treated corn that is 

spilled on the ground. This type of exposure would be greatest for soil-borne invertebrates and 

those invertebrates that depend on deer droppings for development. Exposure will decrease over 

time due to environmental degradation of ivermectin in feces and soil.

4.2.2 Aquatic Exposure Assessment

The potential for exposure to aquatic organisms from ivermectin-treated corn is expected to be 

negligible. Ivermectin is mixed with whole kernel corn and dispensed from a closed gravity 

feeder system to deer (Figure 4-1). Drift of ivermectin to aquatic areas is not anticipated based 

on the use pattern. Runoff would also not be anticipated because the corn is contained within a 

feeder that is accessed by deer. There is the possibility of some spillage from deer feeding at the 

feeders although the amount of corn would be minor because that corn would likely be 

consumed by deer or other non-target organisms. Previous studies to evaluate ivermectin 

effectiveness in controlling ticks in white-tail deer populations have shown that little to no 

treated corn is available on the ground (Pound et al., 1996; Rand et al., 2000). Any corn left on 

the ground would not be expected to runoff into aquatic areas due to the size of the kernels with 

a very low probability of movement in a rain event. Ivermectin has low water solubility and 

partitions strongly to soil and organic matter and would not be expected to be in solution in 

detectable levels if there was a rain event that could result in transport of treated corn into 

aquatic habitats. The program does not place feeders within 50 feet of aquatic habitats, further 

reducing the probability of any aquatic exposure. Deer droppings containing ivermectin may be 

transported as runoff or deposited directly into aquatic habitats but this is not expected to be a 

major pathway of exposure for most aquatic organisms. Ivermectin in deer droppings would be 

bound to organic matter and not available to most aquatic organisms. Sediment dwelling 

invertebrates could be exposed due to the preferential binding of ivermectin to organic matter. 

However, the low probability of significant quantities of deer droppings being deposited into 

aquatic habitats, and the degradation of ivermectin would suggest that exposure to benthic 

aquatic invertebrate populations would be negligible. 
RISK CHARACTERIZATION 57

5.0 RISK CHARACTERIZATION

5.1 Human Health

The use of ivermectin-treated corn in feeding stations to control cattle fever ticks is expected to

pose minimal risks to human health under the APHIS proposed applications. 

The greatest risk to human health is to workers who mix the ivermectin with the corn and those 

workers who fill the feeders. The risk to these types of workers from ivermectin via oral, 

inhalation, and dermal exposure is minimized by adherence to the proper use of PPE. Although 

ivermectin is a hazard to humans at high doses, the low potential for exposure to ivermectin from 

the bait application suggests that adverse effects to workers will not occur. Accidental exposure 

may occur from splash to unprotected body areas during mixing of ivermectin with corn. The 

exposure frequency would be low following the label safety precautions; therefore, risk from 

accidental exposure is minimal. 

The risk to the general public from exposure to ivermectin-treated corn from the feeders is 

expected to be negligible. The feeders are placed on properties that will have a point of access 

that will be locked. Feeders are checked weekly so that those that are damaged will be repaired 

or removed, reducing the potential for exposure to the public from corn that may spill out onto 

the ground if the feeder is damaged. Risk to the public from the consumption of harvested 

white-tailed deer will also be negligible because the ivermectin-treated corn is removed from the 

feeder 60 days prior to the hunting season to allow ivermectin residues to decrease to nondetectable levels in white-tailed deer.

5.2 Terrestrial and aquatic risk characterization

Direct risk to non-target mammals, birds, and reptiles is expected to be low based on the method 

of application for ivermectin-treated corn and toxicity of ivermectin to birds, reptiles, and 

mammals. The use of the closed gravity feeder will reduce risk to most non-target birds, reptiles,

and mammals. In addition, the use of panels surrounding the feeders will further prevent access

for many terrestrial non-target organisms and exposure to any corn that may spill from the 

baffles during feeding. Feeders are checked weekly by program staff and adjusted to reduce corn 

from spilling onto the ground during feeding which could then be consumed by non-target 

vertebrates. The available toxicity profile for terrestrial vertebrates and dosing levels for whitetailed deer suggest that non-target domestic and wildlife vertebrate species would have to ingest 

more than their typical daily consumption rates to reach a dose that could result in adverse 

effects. 

The risk to non-target birds and mammals that could enter the enclosures and consume any 

ivermectin-treated corn left on the ground was estimated using the bobwhite quail, mallard, and 

deer mouse because toxicity data is available for all three species. Doses for species were 

estimated using average body weights and daily food consumption rates reported by EPA 

(USEPA, 1993). The concentration of ivermectin in treated corn was assumed to be 22 ppm 

based on the target concentration needed for effective tick control for white-tailed deer. Using 

the body weight, daily food consumption rates, and ivermectin residues, a daily dose of 

ivermectin for each species could be calculated and then compared to the available toxicity data 
RISK CHARACTERIZATION 58

to determine whether consumption of ivermectin-treated corn would exceed effect levels. The 

assumption was that both non-target species would consume ivermectin-treated corn and would 

not consume any other food material.

Table 5.1. Estimates of exposure and risk to non-target bird and mammal species from 

ivermectin-treated corn.

Test Species Body 

Weight (g)

Food 

Ingestion 

Rate (g/gday)

Ivermectin 

consumed (mg 

ivermectin/animal)

Ivermectin 

Dose 

(mg/kg)

LD50

(mg/kg)

Bobwhite 

quail

154 0.067 0.23 1.49 2000

Mallard 1134 0.055 1.38 1.21 88

Deer Mouse 21 0.415 0.19 9.05 25

Based on the estimates of ivermectin doses calculated for both species the bobwhite quail would 

have to consume approximately 1,342 times its daily food consumption rate to exceed the 

median lethality value. The mallard would have to consume 73 times its daily consumption rate 

to exceed its medial lethality value. The deer mouse would have to consume approximately 2.7

times its daily food consumption rate to exceed the median lethality value. These estimates 

suggest that the acute risk to small non-target birds and mammals that consume ivermectintreated corn would be low. No toxicity data is available for reptiles; however, assuming that the 

avian effects data is comparable would suggest that any reptiles that consume ivermectin-treated 

corn would be at low acute risk. The risk to listed birds, such as the whooping crane would also 

be low based on the above estimates of risk for the mallard and bobwhite and food preference. 

Whooping crane diet in southern Texas consists primarily of blue crabs, clams, snails, wolfberry 

fruit, acorns and crayfish (Hunt and Slack, 1989). The risk to aquatic food items from the use of 

ivermectin-treated corn is low based on the use pattern for ivermectin and the use of an 

application buffer from aquatic areas that will reduce the risk to aquatic organisms that may 

serve as a food source for the whooping crane. Risks to the wolfberry fruit and oak species from 

ivermectin-treated corn are negligible due to the method of application and the low toxicity of 

ivermectin analogs to terrestrial plants. Whooping cranes may also consume waste corn from 

fields so there is a possibility that they could enter enclosures and feed on spilled corn from the 

feeders. The low risk to other avian species such as the bobwhite and mallard suggests that the 

whooping crane would have to consume many times its daily food consumption rate of 

ivermectin-treated corn to exceed an effect threshold. 

Risk to vertebrate scavengers and predators that may consume white-tailed deer that have 

ingested ivermectin-treated corn would be expected to be low because the ivermectin dose being 

delivered is therapeutic. In addition, white-tailed deer are only receiving ivermectin-treated corn 

February through July and would have reduced levels immediately after feeding for 60 days with 

no detectable levels for the remainder of the year. The risk to vertebrate scavengers and 

predators that rely on prey items that may consume spilled corn is also expected to be low. 

Estimate of risks to small mammals and birds (e.g., bobwhite quail and deer mouse) suggests that 

both groups would have to eat well above their daily food consumption rates to receive a lethal 

dose. 
RISK CHARACTERIZATION 59

The risk to federally listed predators such as the ocelot, jaguarundi and aplomado falcon from the 

consumption of prey items containing ivermectin residues would also be considered low. The 

ocelot consumes primarily small mammals and birds while the jaguarundi consumes small 

mammals, birds, and reptiles (FWS, 2013; 2016). The aplomado falcon has a more varied diet 

consuming mammals, birds, reptiles, and insects with birds being the predominant food source

(Hector, 1985; FWS, 2014). Most insect groups that the aplomado falcon would consume (e.g.,

moths, dragonflies, grasshoppers, wasps) would not be impacted by the use of ivermectin-treated 

corn because they do not use dung pellets from treated white-tailed deer, or other animals, to 

complete their life cycle (Hector, 1985; TPWD, 2013). The risk to scavengers and predators 

that may consume small mammals and birds that have consumed ivermectin-treated corn is 

expected to be low. Acute ivermectin toxicity data is not available for the ocelot or jaguarundi;

however, these values can be approximated using the below equation (EPA, 2005):

Adjusted LD50 = LD50 (TW/AW).25

TW = body weight of the test animal (g)

AW = body weight of assessed animal (g)

The LD50 value used in these estimates was the lower value that was reported for the mouse (25 

mg/kg). The body weight of the mouse was assumed to be 21 g. The average weight of the 

ocelot was 11.5 kg and for the jaguarundi, 6.4 kg (FWS, 2013; 2016). Using the above equation 

and the average body weights, the ocelot and jaguarundi adjusted LD50 values were 5.17 and 

5.98 mg/kg, respectively. Consumption rate data for the ocelot states they consume between 

0.56 and 0.84 kg of meat per day or an average of 0.7 kg/day. Assuming they only eat deer mice 

with an average weight of 21 grams an ocelot would consume approximately 33 mice per day or 

6.27 mg ivermectin (33 mice * 0.19 mg ivermectin/mouse). The expected dose for the ocelot in 

mg/kg body weight would be 0.54 mg/kg. When comparing the expected dose to the estimated 

LD50, the ocelot would have to consume approximately 9.6 times its daily consumption rate of 

mice to exceed the adjusted LD50 value. This exposure scenario assumes that ocelots only prey 

on mice that have consumed their maximum daily food rate with ivermectin-treated corn and that 

no depuration or metabolism of ivermectin occurred in the mice from the time they ingested the 

corn until they were consumed by the ocelot. Food consumption rates were not reported for the 

jaguarundi; however similar results would be expected based on the comparable adjusted LD50

values. Although there is some uncertainty in the estimated LD50, values the conservative 

assumptions regarding exposure suggest that predatory mammals such as the ocelot and 

jaguarundi would be at low risk from the consumption of prey items that fed on ivermectintreated corn. Risk to the aplomado falcon would also be low based on the conservative exposure 

assumptions and available avian toxicity data discussed above. In addition, the aplomado falcon 

has other food sources that would not be expected to contain ivermectin residues, further 

reducing the risk. Acute median lethality values do not account for potential sublethal risks to 

these species so there is some uncertainty regarding these types of impacts. However, the 

conservative estimate of exposure for scavengers and greater than 10 fold safety margins for 

direct acute risk suggest that sublethal risk to this group of non-target organisms would also be 

low. Chronic exposures are not anticipated because the feeders are checked weekly and any 
RISK CHARACTERIZATION 60

residual corn would be removed. In addition the feeders are used between February and July 

reducing exposure to any non-target mammals and birds that may serve as a food source for the 

ocelot, jaguarandi and aplomado falcon. 

Indirect risk to insectivorous birds and mammals that feed on terrestrial invertebrates that may 

use deer droppings containing ivermectin is not anticipated. Most insectivorous birds, mammals,

and reptiles are generalist feeders and would have other prey items available for consumption. 

In addition, the impact to dung-dependent invertebrates is variable and site specific, and may not 

result in any impacts to this group of invertebrates.

 

There is some risk to dung-dependent invertebrates, such as dung beetles and certain dipterans

that would use droppings from deer that have consumed ivermectin-treated corn. These impacts 

would be localized to areas where feeders are placed and in cases where an invertebrate species 

relies solely on droppings from white-tailed deer. As stated above in the summary of ivermectin 

effects, the impacts to this group of invertebrates are variable based on several factors. The 

range of doses and effects that cause adverse effects are variable and depend on the type of 

formulation being tested, dosing method used, species dosed, and environmental variables in 

fields where the studies were conducted (Kruger and Scholtz, 1998a,b; Wall and Beynon, 2012). 

Any impacts would be primarily to dung beetles and flies that utilize deer droppings as part of 

their development and are sensitive to the effects of ivermectin. The spatial and temporal effects 

from the proposed use of ivermectin-treated corn are difficult to quantify because of the 

variability of effects, or lack of, in various studies exposing ivermectin to dung-dependent 

invertebrates. Available data would suggest there is risk to some invertebrates that would feed 

on deer droppings containing ivermectin; however, the range of sensitivities to various dungdependent species suggests that not all species would be impacted (Rombke et al., 2010b; Steel 

and Wardhaugh, 2002; Floate et al., 2005; Blackhorn et al., 2013). This is supported by field 

level evaluations where ivermectin has been shown to have impacts to certain invertebrates 

resulting in impacts to diversity but with no changes in overall abundance (Wall and Beynon, 

2012; Jochman et al., 2015). The potential for area-wide impacts to invertebrates that use deer 

droppings that contain ivermectin is also difficult to quantify; however, these types of impacts 

are expected to be low. Invertebrates sensitive to ivermectin would also utilize dung from other 

mammals that have not fed on ivermectin-treated corn. The greatest risk to dung-dependent 

beetles and flies will occur between February and July when the feeders are present. After that 

time the risk would decrease since white-tailed deer and other mammals are not feeding on 

ivermectin-treated corn and any droppings in the field containing ivermectin will degrade. The 

risks to sensitive invertebrates would be expected to last for approximately one to two weeks 

after the treated corn is removed based on a summary of studies that evaluated oral dosing of 

ivermectin and effects to certain dung dependent species (Floate et al, 2005). The risks to 

sensitive invertebrates would be localized to areas in proximity to the feeders. Deer may range 

over larger areas; however, they would be expected to congregate near feeders resulting in more 

deer droppings that could contain ivermectin in a smaller area. Deer that forage further away 

from the feeders after receiving a dose of ivermectin could also impact sensitive invertebrates but 

the risk would be lower because there would be dung from other animals that do not contain 

ivermectin. 
RISK CHARACTERIZATION 61

The risk to aquatic organisms is expected to be negligible due to the lack of significant exposure. 

Treated corn is not expected to be transported to aquatic habitats in sufficient amounts that could 

result in risk to aquatic fauna. The feeder is a closed system that can only be accessed through 

spouts that contain baffles that can be adjusted to minimize spillage on the ground. A 

conservative estimate of spillage is 5% based on field observations. Assuming that a feeder is 

loaded with 300 pounds of whole kernel corn at a rate of 10 mg ivermectin per pound of corn,

the total amount of ivermectin in the feeder would be 3,000 mg. Assuming 5% of the treated 

corn falls onto the ground during feeding, this would result in 150 mg of ivermectin available for 

transport to aquatic habitats from runoff. Conservative estimates from broadcast liquid pesticide 

applications suggest that up to 10% of the applied material may move off-site. The amount 

would be much lower in this example because the program is not using broadcast liquid 

treatments. However, 10% was used in this example as a very conservative estimate of off-site 

transport. The quantity of ivermectin (15.0 mg) entering an aquatic habitat one acre in size and 

one foot deep would result in a concentration of approximately 12 ng/L, assuming no 

degradation or dissipation. When compared to the available aquatic toxicity data for 

invertebrates, fish, and amphibians, the residue level is below the range of acute toxicity values

for aquatic invertebrates, with the exception of the lowest reported Daphnia magna toxicity 

value, and is three orders of magnitude below the lowest effect concentration for aquatic 

vertebrates, suggesting a lack of risk to aquatic species (Figure 5-1). This value is also two 

orders of magnitude below the range of acute NOECs reported for fish (0.9-220 μg/L). The 

estimated residue is very conservative and would not occur in the field because it assumes there 

is no buffer between the site of the feeder and the aquatic habitat. In addition, it assumes that all 

of the ivermectin in the corn that enters the aquatic habitat would be present in the water and not 

degrade, which would not be the case because ivermectin preferentially binds to organic matter

and degrades based on available environmental fate data (see Section 2.3). Degradation of 

ivermectin in water in the proposed area of treatment and in a shallow body of water would be 

expected to be rapid due to the rapid breakdown of ivermectin at higher temperatures and in the 

presence of light. The overly conservative assumptions were used in the estimate of an aquatic

residue as a way to account for uncertainty in the available toxicity data while demonstrating 

safety to aquatic organisms.
RISK CHARACTERIZATION 62

Figure 5-1. Acute aquatic risk characterization for ivermectin-treated corn.

The method of application and setting feeders a minimum of 50 feet from aquatic habitats

suggests that any ivermectin-treated corn would be unlikely to enter surface water. In addition 

the environmental fate of ivermectin suggests that any material that would enter surface water 

would quickly dissipate and bind to organic matter or sediment, reducing bioavailability and risk 

to aquatic organisms. There is the potential for risk to aquatic organisms from droppings from 

deer that have consumed ivermectin-treated corn that enter surface water directly, or through 

runoff; however, the risk would be low because ivermectin would be bound to organic matter in 

the droppings. The risk to sediment-dwelling organisms would be greater than those 

invertebrates in the water column; however, the risk is also expected to be low for benthic 

invertebrates because the pathway for this exposure is not expected to be significant. Any 

impacts to sediment-dwelling invertebrates would be localized to those that would primarily 

consume deer droppings because other organic material would be available. 

1 10 100 1000 10000 100000

Log Ivermectin Concentrations (ng/L)

Acute Invertebrate Effects

Estimated Acute Aquatic Concentration

Acute Fish/Amphibian Effects
UNCERTAINTIES AND CUMULATIVE IMPACTS

63

6.0 UNCERTAINTIES AND CUMULATIVE IMPACTS

The uncertainties associated with this risk evaluation arise primarily from lack of information 

about the effects of ivermectin, its formulations, metabolites, and potential mixtures to non-target 

organisms that can occur in the environment. These uncertainties are not unique to this 

assessment but are consistent with uncertainties in human health and ecological risk assessments 

with any environmental stressor. In addition, there is uncertainty in the number and location of 

feeding stations, which are based on number and density of deer in the program area.

Another area of uncertainty is the potential for cumulative impacts to human health and the 

environment from the proposed use of ivermectin in the CFTEP. Areas where cumulative 

impacts could occur are: 1) repeated worker and environmental exposures to ivermectin from 

program activities; 2) co-exposure to other chemicals with a similar mode of action; and 3) 

exposures to other chemicals in mixtures and how that may affect the toxicity of ivermectin.

Ivermectin is a widely used anti-parasitic drug in humans, livestock, and pets (Crump and 

Omura, 2011). There would be increased environmental loading from the use of ivermectintreated corn for white-tailed deer where there are also ivermectin uses for cattle and other 

domestic animals. The impacts to white-tailed deer are expected to be incrementally negligible 

when put in context with other stressors because the dose of ivermectin is considered therapeutic 

and not intended to result in adverse effects. Domestic animals that are receiving ivermectin for 

other purposes are also not expected to have cumulative impacts resulting from the proposed use 

of ivermectin-treated corn because domestic animals will not be able to access the feeders. 

Cumulative impacts to aquatic organisms will also be negligible because there is an extremely 

low probability of exposure to aquatic habitats from the proposed use of ivermectin-treated corn.

Cumulative impacts to human health from the proposed use of ivermectin-treated corn are not 

anticipated because of the proposed use pattern. Human exposure and risk is very low for the 

general public. The probability of exposure is greatest for workers who mix and fill the feeders. 

However, the risk to this group of the population will be negligible based on the low risk of 

ivermectin when using the appropriate PPE. There is the potential for worker exposure to other 

chemicals that may be used in the CFTEP. Coumaphos is an organophosphate insecticide used 

to treat cattle for ticks that may vector cattle fever. Coumaphos affects the nervous system by 

inhibiting chlolinesterase (USEPA, 2006). Ivermectin and coumaphos have different modes of 

action and the literature does not indicate whether there could be synergism, potentiation, 

additive, or antagonistic effects from these two compounds. The potential for cumulative 

impacts related to exposure to both pesticides, in particular, to workers will be reduced by the 

use of PPE. Cumulative risk to the public from exposure to mixtures of both chemicals is also 

not anticipated because of the method of application, program controls, and restriction of public 

access to treatment areas. A study on pharmacokinetic interaction of ivermectin and spinosad in 

dogs indicates spinosad can increase the risk of ivermectin neurotoxicity by acting as a Pglycoprotein inhibitor (Dunn et al., 2011). Spinosad is a commonly used insecticide in 

agriculture and is used in other APHIS programs. The probability of exposure to the public and 

workers to both spinosad and ivermectin at the same time would be very low because of the use 

patterns and intended target for each pesticide. There has also been research to show synergistic 

effects between ivermectin and antibiotics (doxycycline, erythromycin, rifampicin, and 
UNCERTAINTIES AND CUMULATIVE IMPACTS

64

azithromycin) in controlling body lice (Sangare et al., 2016). These interactions are not expected 

with the proposed use of ivermectin because treatments will be directed at wildlife that would 

not be receiving antibiotic treatment. These types of mixture exposures would not be anticipated 

for humans either because of the method of application and other measures to prevent exposure 

to the public and workers who mix ivermectin with corn and load the feeders. 
REFERENCES 65

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APPENDIX 2

Summary of and Responses to Comments Received on the 

Environmental Assessment Prepared for the Cattle Fever Tick 

Eradication Program Use of Ivermectin Corn

Issue: Two commenters support the use of ivermectin-treated corn on all private properties and 

public lands that are at an increased risk of hosting cattle fever ticks. 

Response: Program feeders are strategically placed where there have been cattle fever 

tick infestations in the vicinity of white-tailed deer populations. Feeders are stocked with 

ivermectin-treated corn annually from February through July so that deer have a large 

window of opportunity to feed on the corn. After consuming ivermectin-treated corn, it is 

beneficial for the deer to move around because they can pick up ticks that may be located 

elsewhere, and then those ticks will also die. Additionally, while it seems logical to treat 

all deer, rather than a few, the program does not have the resources to treat all deer in the 

affected counties. This treatment will be used, along with others, to create an integrated 

strategy for tick control.

Issue: One commenter stated that he received a plan from APHIS that described how APHIS 

would supply non-medicated corn during the months when medicated corn was withdrawn from 

his ranch; however, non-medicated corn was never put in the feeders. The commenter believes 

supplying non-medicated corn would retain deer on ranches and minimize risk of reinfestation of 

treated deer with cattle fever ticks from neighboring ranches. Similarly, another commenter 

supported the use of non-medicated corn August through January to eliminate the need to move 

feeder locations based on deer population changes from herd movements.

Response: APHIS appreciates the comments from, and the landowners participating in,

the CFTEP. Through established policies and procedures, we will attempt to treat all deer 

within a 2-mile range of where cattle fever ticks are discovered. There are occasions 

where this cannot be accomplished because of compliance challenges, variable deer 

population densities, resource limitations, and mission priorities at other locations. 

Concerning use of non-treated corn, when deemed strategic and as resources permit, 

APHIS does attempt to employ that tool as part of the management of potential cattle 

fever tick hosts.

Issue: One commenter mentioned a rule stating that equine must receive deworming 30 days 

prior to slaughter. Based on this rule, the commenter wants to know how this would affect cattle 

and deer if they were slaughtered within 30 days of consuming ivermectin, and what impact 

would occur on animals and humans consuming these carcasses.
74

Response: APHIS is uncertain of the commenter’s reference to withdrawal period for 

deworming equine. There are no labeled ivermectin products for use in horses slated for 

food production. Horse slaughter for the production of food has not occurred in the 

United States since 2007, and APHIS is not familiar with the laws governing horse 

slaughter for human consumption in other countries. Regardless, APHIS does not 

anticipate that animals or humans will be negatively impacted by the use of ivermectintreated corn in South Texas. APHIS has corresponded with the FDA, and the FDA does 

not object to the proposed extra-label use of ivermectin in deer. It should be noted that 

the FDA has approved ivermectin for human use to treat parasitic infections. As 

discussed in Appendix 1 of the EA, in a study where healthy adults received more than 

the intended dose of ivermectin, the results showed that ivermectin is generally welltolerated by healthy adults with no indication of associated central nervous system 

toxicity at levels up to 10 times the highest FDA-approved dose (Guzzo et al., 2002).

To minimize the potential exposure of the general public to ivermectin, APHIS 

established a 60-day withdrawal period prior to the deer hunting season. A withdrawal 

period of 60 days was based on the extra-label use of ivermectin in food animals, and is 

10 times longer than the elimination half-life of 6 days. Therefore, the potential exposure 

to ivermectin residues higher than the tolerance level from consumption of deer meat 

treated with ivermectin-corn for the general public is precluded by withdrawal of 

ivermectin-treated corn 60 days prior to the start of the hunting season. In addition, 

although cattle will not be fed ivermectin corn as part of this proposed action, for 

comparison purposes, various ivermectin products used in cattle have 35-48 day 

withdrawal periods (depending upon formulations). 

Issue: One commenter asked if nilgai would have access to the feeders.

Response: While nilgai would have access to the feeders, APHIS noted in the EA that 

nilgai do not eat corn. Therefore, nilgai would not be treated with ivermectin in this 

manner. APHIS will, however, use periodic game cameras to monitor non-target species 

accessing the ivermectin corn feeders.

Issue: One commenter is concerned that feeding ivermectin corn to deer will unintentionally kill 

beneficial insects that feed on deer feces. The commenter is also concerned that feeding 

ivermectin corn to deer could lead to resistant gastrointestinal nematodes that survive ivermectin 

treatment causing internal parasite issues that could lead to the death of individual animals.

Response: As stated in the risk assessment in Appendix 1 of the EA, APHIS recognizes 

that there is some risk to dung-dependent invertebrates that would use droppings from 

deer that have consumed ivermectin-treated corn. The range of doses and effects that 

cause adverse effects are variable and depend on the formulation being tested, dosing 

method used, species dosed, and environmental variables; subsequently, not all species 
75

would be impacted. Any impacts would be primarily to dung beetles and flies that use 

deer feces as part of their development and are sensitive to the effects of ivermectin. 

These impacts would be localized to areas where feeders are placed.

Ivermectin is a widely used broad-spectrum, anti-parasitic drug in humans, livestock, and 

pets. Evidence of ivermectin-resistant gastrointestinal nematodes is increasing worldwide 

in goat and sheep flocks and bovine herds (Canul-Ku et al., 2012). Application of subtherapeutic doses of ivermectin can reduce its efficacy and favors gastrointestinal 

nematode resistance (Canul-Ku et al., 2012). As discussed in Appendix 1, the feeding 

rate for deer is approximately 1% of body weight per day, or 1 pound per 100 pound deer 

per day. Based on this information, the daily intake dose of deer feeding on ivermectintreated corn is approximately 0.22 mg/kg, which is an effective dose against ticks and 

gastrointestinal nematodes (Garris et al., 1991). 

Frequency of ivermectin use can also contribute to gastrointestinal nematode resistance in 

farm animals. Selection pressure for resistance in gastrointestinal nematodes is increased 

if ivermectin is used for tick control every 30-35 days throughout the year (Canul-Ku et 

al., 2012). Use of ivermectin corn to treat deer is less likely to cause gastrointestinal 

nematode resistance because it will only be used a few months each year in limited areas. 

This use frequency will allow for maintenance of a nematode population that maintains 

the genes for susceptibility within its population, thereby reducing the development of 

gastrointestinal nematode resistance (Kenyon et al., 2009).

Issue: One commenter urged APHIS to coordinate with private landowners when administering 

ivermectin-treated corn so that the agency does not infringe upon a landowner’s ability to 

harvest wildlife. The commenter opposes extending the use of ivermectin-treated corn into 

months that would cause the withdrawal period of ivermectin-treated corn to overlap with the 

hunting season.

Response: APHIS communicates with landowners prior to placing feeders on private 

property and is sensitive to a landowner’s ability to harvest wildlife from their property. 

As such, APHIS will not extend the use of ivermectin-treated corn beyond the February 

through July window.

Issue: One commenter suggested there should be a committee formed that consists of cattle 

ranchers, wildlife landowners, farmers, and veterinarians to identify solutions to the cattle fever 

tick problem. Another commenter mentioned the need to meet with landowners to address their 

concerns.

Response: APHIS recognizes the need to communicate more broadly with individuals in 

South Texas regarding the CFTEP. A partner agency in the cattle fever tick response, the 
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Texas Animal Health Commission, has been spearheading communications, educational 

forums, and working group meetings with respective stakeholders.

Issue: One commenter expressed concern about nilgai, suggesting that they should be protected 

on both private properties and refuges. Two commenters stated that nilgai provide a valuable 

resource to the community and removal of nilgai by helicopter can economically impact sporting 

good sales, hunting leases, meat processors, taxidermists, the sale of hunting licenses, and family 

memories. Another commenter further stated that he is strongly opposed to the removal of nilgai 

on state or federal lands adjacent to private lands.

Response: This topic is outside of the scope of the “Cattle Fever Tick Eradication 

Program Use of Ivermectin Corn” Draft Environmental Assessment. This topic is, 

however, addressed more thoroughly in the “Population Reduction of Nilgai in the Boca 

Chica Beach, Bahia Grande, and Brownsville Navigation District Areas, Cameron 

County, Texas” Final Environmental Assessment published in September 2014. 

Issue: Two commenters suggested darting nilgai with ivermectin via helicopter as an alternative 

to lethally removing nilgai.

Response: APHIS appreciates the suggestion to dart nilgai as an extension of the 

proposed program use of ivermectin corn to treat white-tailed deer. Cost, efficacy, and 

remote dart technologies would need to be studied prior to determining if this is a viable 

option and is outside the scope of this EA.

Issue: One commenter asked how ivermectin will affect quail since quail are drawn to feeders 

with corn.

Response: The risk assessment in Appendix 1 of the EA describes that by using body 

weight, daily food consumption rates, and ivermectin residues, a daily dose of ivermectin 

corn for bobwhite quail could be calculated. This data was then compared to the available 

toxicity data to determine whether consumption of ivermectin-treated corn would exceed 

effects levels. The assumption was that bobwhite quail would consume only ivermectintreated corn and no other food material. Based on the calculation, bobwhite quail would 

have to consume approximately 1,342 times its daily food consumption rate to exceed the 

median lethality value. This information suggests that the acute risk to small non-target 

birds that consume ivermectin-treated corn would be low.

Issue: One commenter stated that every USDA employee they had met stated that deer do not 

host cattle fever ticks. The commenter is supportive of treating them with ivermectin if their 

understanding is incorrect.
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Response: APHIS apologizes if any of its employees failed to properly communicate the 

role deer play in the life cycle of cattle fever ticks. Numerous studies have shown that 

deer are suitable hosts and reservoirs for cattle fever ticks (Graham et al., 1972, in Pound 

et al., 2010; George, 1990). In 1968, cattle fever ticks were discovered on white-tailed 

deer in ranches in Dimmit County, and southern cattle ticks were discovered on deer in 

other areas in later years, raising concern about the role of white-tailed deer and cattle 

fever tick outbreaks in the tick-free area (Giles, 2014). Since then, more evidence has 

accrued on the role of white-tailed deer as suitable cattle fever tick hosts and their 

importance in tick eradication efforts (Busch 2014). More information regarding the role 

of deer in distribution and maintenance of cattle fever ticks can be found in the “Cattle 

Fever Tick Eradication Program—Tick Control Barrier, Maverick, Starr, Webb, and 

Zapata Counties, Texas” Draft Environmental Impact Statement published in June 2013.

Issue: One commenter asked APHIS to consider the following as part of their management 

program: 1) Identify the parameters used to determine length of the program; 2) Survey for 

resistance in parasites in deer; 3) Identify non-target species accessing ivermectin-treated corn 

directly or as part of the food chain; 4) Survey for development of resistance due to 

subtherapeutic dosing in susceptible parasites; 5) Publish the methods APHIS plans to use to 

determine the effectiveness of treatment; 6) Include Texas Parks and Wildlife Department during 

Endangered Species Act consultation activities; 7) Discuss alternatives to the use of ivermectin 

to deliver therapeutic doses to deer while reducing antiparasitic resistance pressures; and, 8) 

Provide surveillance activity findings (e.g., effectiveness, resistance development, non-target 

species impact) to the public periodically.

Response: APHIS intends to implement the feeding of ivermectin-treated corn to whitetailed deer annually and has made this clarification in the Final EA. Briefly, the 

parameters used are locations where cattle fever ticks have been detected along the 

permanent quarantine line, or in adjacent areas of those counties that are determined to be 

at risk of incurring an infestation due to movements of tick-infested deer, or locations 

with identified infestations beyond the Permanent Tick Quarantine Zone. Another key 

parameter is a determination by veterinary epidemiologists that there is a risk of incurring 

infestations due to movements of tick-infested deer.

APHIS is aware of the potential for resistance to develop in gastrointestinal nematodes 

following the use of therapeutic (and potentially subtherapeutic) levels of an acaricide 

such as ivermectin. Since many nematodes are generalist parasites and have the potential 

to infect a broad range of hosts (Walker and Morgan, 2014), and ivermectin is frequently 

used to treat cattle in South Texas, conducting surveys for resistant parasites is unlikely 

to yield actionable data identifying the degree to which ivermectin-resistant 

gastrointestinal nematodes are occurring, if at all, as a result of APHIS’ proposed action.
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The primary measurement of effectiveness of this program will be cattle fever tick 

infestation rates or spread within, adjacent to, or beyond the Permanent Tick Quarantine 

Zone. The use of ivermectin-treated corn for deer is one component of APHIS’ integrated 

pest management (IPM) programs and practices.

APHIS is legally required to consult with FWS during its ESA consultation activities, 

which it has done. FWS can involve additional parties in the consultation process if it 

chooses to do so. APHIS and TAHC periodically consult with the Texas Parks and 

Wildlife Department regarding native wildlife unrelated to federally-listed threatened and 

endangered species.

APHIS will use game cameras to occasionally monitor non-target species accessing the 

ivermectin-corn feeders. In addition, APHIS will continue to partner with stakeholders to 

assess science-based alternatives and potential improvements to controlling cattle fever 

ticks on deer while having minimal impact on the environment. APHIS will periodically 

provide surveillance activity findings to these stakeholder groups and the public.

Issue: One commenter noted the confirmed presence of cattle fever ticks in Live Oak County on 

November 30, 2016, as well as the two infestations in Kleberg County in 2016. The commenter 

advocates for a process to be in place to efficiently add additional affected counties as the needs 

arise.

Response: APHIS considered including all counties where cattle fever ticks could 

potentially be found in its documentation prepared for the ESA consultation and NEPA 

process but ultimately decided to move forward with the 10 counties discussed in the EA. 

After this decision, the confirmed presence of cattle fever ticks in Live Oak County 

occurred. Subsequently, APHIS worked with FWS to add Live Oak County, in addition 

to Kleburg and Kenedy counties, to the ESA consultation process. This will enable 

APHIS to move forward in a more expeditious manner if it chooses to place ivermectintreated corn in these three additional counties.