United States

Department of

Agriculture

Animal and

Plant Health

Inspection Service

Veterinary

Services

National

Animal Health

Monitoring

System

March 2011

Dairy 2007

Salmonella, Listeria, and Campylobacter on

U.S. Dairy Operations, 1996–2007
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USDA:APHIS:VS:CEAH

NRRC Building B, M.S. 2E7

2150 Centre Avenue

Fort Collins, CO 80526-8117

(970) 494-7000

Email: NAHMS@aphis.usda.gov

http://nahms.aphis.usda.gov

#578.0311

Cover photo courtesy of Agricultural Research Service
USDA APHIS VS / i

This report provides an in-depth look at the

prevalence of food safety pathogens on U.S.

dairy operations from 1996 to 2007, as

identified from three National Animal Health

Monitoring System studies: Dairy 1996, Dairy

2002, and Dairy 2007. Estimates in this report

from bulk-tank milk testing are reported as

population estimates. Estimates based on fecal

culture represent a convenience sample and are

not population estimates.

Here are a few highlights from the report:

In 2007, the percentage of operations on which

a milk filter tested positive for Salmonella

(24.7 percent) was more than double the

percentage of operations on which a bulk-tank

milk sample tested positive (10.8 percent).

Likewise, the percentage of operations on which

a milk filter tested positive for any Listeria

(28.3 percent) was more than three times the

percentage of operations on which a bulk-tank

milk sample tested positive for any Listeria

(9.0 percent). Milk filters were not tested in

2002 or 1996.

The percentage of operations on which bulktank milk tested positive for Salmonella by

RT-PCR was similar in 2002 and 2007 (11.9 and

10.8 percent, respectively). In addition, the

percentage of operations on which bulk-tank

milk tested positive for Listeria monocytogenes

was similar in 2002 and 2007 (3.8 and

3.7 percent, respectively). Bulk-tank milk was

not tested in 1996.

The percentage of operations positive for

Salmonella via fecal culture increased from

1996 to 2007. In 1996, 20.0 percent of

operations had any Salmonella-positive cows

compared with 30.9 percent of operations in

2002 and 39.7 percent in 2007. In 1996 and

2007, the percentage of cows positive for

Salmonella was 5.4 and 13.8 percent,

respectively.

During the Dairy 1996, 2002, and 2007 studies,

a higher percentage of operations with 500 or

more cows were Salmonella positive than

operations with fewer than 100 cows.

The percentage of Salmonella isolates resistant

to at least one antimicrobial decreased from

2002 to 2007 (17.7 and 3.4 percent,

respectively). Similarly, for any specific

antimicrobial to which resistance was observed,

a lower percentage of isolates were resistant in

2007 than in 2002.

In the Dairy 1996, 2002, and 2007 studies,

nearly all operations had at least one cow that

was shedding Campylobacter (100, 97.9, and

92.6 percent of operations, respectively).

In 2002 and 2007, less than 5 percent of

C. jejuni isolates were resistant to any single

antimicrobial tested, with the exception of

tetracycline. In 2007, 62.4 percent of C. jejuni

isolates were resistant to tetracycline compared

with 47.5 percent in 2002.

Selected Highlights ed Highlights
ii / Dairy 2007

Acknowledgments ledgments

This report has been prepared from material

received and analyzed by the U.S. Department

of Agriculture (USDA), Animal and Plant

Health Inspection Service (APHIS), Veterinary

Services (VS) via three national studies of

health management and animal health on U.S.

dairy operations conducted in 1996, 2002, and

2007.

The Dairy 1996, Dairy 2002, and Dairy 2007

studies were cooperative efforts between State

and Federal animal health officials, statisticians,

university researchers, and extension personnel.

We want to thank the National Agricultural

Statistics Service (NASS) enumerators, State

and Federal veterinary medical officers

(VMOs), and animal health technicians (AHTs)

who visited the farms and collected the data.

Their hard work and dedication to the National

Animal Health Monitoring System (NAHMS)

are invaluable. The roles of the producers, Area

Veterinarians in Charge (AVIC), NAHMS

Coordinators, VMOs, AHTs, and NASS

enumerators were critical in providing quality

data for all dairy reports. Recognition also goes

to the personnel at the Centers for Epidemiology

and Animal Health (CEAH) for their efforts in

generating and distributing valuable reports

from NAHMS dairy studies.

Additional biological sampling and testing were

afforded by the generous contributions of

collaborators for the dairy studies, including

• USDA–APHIS, National Veterinary Services

Laboratories;

• USDA–ARS, National Animal Disease

Center;

• USDA–ARS, Beltsville Agricultural Research

Center;

• USDA–ARS, Russell Research Center;

• Antel BioSystems, Inc.;

• BIOCOR Animal Health;

• Colorado State University, College of

Veterinary Medicine and Biomedical

Sciences;

• Cornell University Animal Health Diagnostic

Laboratory;

• IDEXX Laboratories;

• Quality Milk Production Services;

• Tetracore, Inc.;

• TREK Diagnostic Systems;

• University of British Columbia, Canada,

Animal Welfare Program;

• University of California - Davis;

• University of Pennsylvania, New Bolton

Center;

• University of Wisconsin, Madison; and

• Wisconsin Veterinary Diagnostic Laboratory.

All participants are to be commended,

particularly the producers whose voluntary

efforts made the dairy studies possible.

Larry M. Granger

Director

Centers for Epidemiology and Animal Health
USDA APHIS VS / iii

Suggested bibliographic citation for this report:

USDA. 2011. Salmonella, Listeria, and Campylobacter on U.S. Dairy Operations, 1996–2007.

USDA–APHIS–VS, CEAH. Fort Collins, CO

#578.0311

Contacts for further information:

Questions or comments on data analysis: Dr. Jason Lombard (970) 494–7000

Information on reprints or other reports: Ms. Abby Fienhold (970) 494–7000

Email: NAHMS@aphis.usda.gov

Feedback

Feedback, comments, and suggestions regarding the Dairy 2007 study reports are welcome. Please

forward correspondence via email to: NAHMS@aphis.usda.gov, or you may submit feedback via

online survey at: http://nahms.aphis.usda.gov (Click on “FEEDBACK on NAHMS reports.”)
iv / Dairy 2007

Table of Cont able of Contents

Introduction 1

Study Objectives and Related Outputs 5

Terms Used in This Report 7

Section I: Population Estimates 8

A. Salmonella Detection in Bulk-tank Milk and Milk Filters 8

1. Background 8

2. Sampling and testing overview 9

3. Prevalence 10

4. Serotypes 12

B. Listeria Detection in Bulk-tank Milk 13

1. Background 13

2. Sampling and testing overview 14

3. Prevalence 15

Section II: Pathogen Detection in Feces 17

A. Salmonella 17

1. Sampling and testing overview 17

2. Prevalence 19

3. Serotypes 28

4. Comparison of serotypes isolated from cattle and humans 30

5. Antimicrobial susceptibility 34

6. Multidrug resistance patterns 36

B. Campylobacter 37

1. Background 37

2. Sampling and testing overview 38

3. Prevalence 41

4. Antimicrobial susceptibility 45

5. Multidrug resistance patterns 47

Section III: Sampling and Diagnostic Testing 48

A. Farm Selection 48

B. Sampling Methods 50

1. Bulk-tank milk and milk filter sampling 50

2. Fecal sampling 50
USDA APHIS VS / v

C. Laboratory Methods 52

1. Salmonella testing of bulk-tank milk and milk filters 52

2. Listeria testing of bulk-tank milk and milk filters 54

3. Salmonella testing of fecal samples 55

4. Campylobacter testing of fecal samples 58

D. Testing Methods Overview 61

Appendix I: NAHMS Study Methodology—Phase II 62

Appendix II: References 63

Appendix III. Previously Published Material 69

USDA APHIS VS / 1

Introduction

Introduction oduction

There are more than 250 known diseases caused

by bacteria, fungi, viruses, and parasites

transmitted through food to humans. Foodborne

pathogens or toxins enter the body through the

gastrointestinal tract where the first symptoms of

illness often appear. As a result, nausea,

vomiting, abdominal cramps, and diarrhea are

common symptoms in many foodborne diseases.

The majority of foodborne illnesses are mild

and cause symptoms for only 1 to 2 days;

however, some cases are more serious, resulting

in severe illness or death (CDC, 2005).

While the food supply in the United States is

one of the safest in the world, the Centers for

Disease Control and Prevention (CDC)

estimates that each year 76 million people in the

United States get sick from foodborne

pathogens, of which 325,000 are hospitalized

and 5,000 die (Mead et al., 1999). The most

commonly recognized foodborne infections

caused by bacteria are due to Campylobacter,

Salmonella, and Escherichia coli (E. coli)

O157:H7 (CDC, 2005). The Foodborne

Diseases Active Surveillance Network

(FoodNet) of CDC’s Emerging Infections

Program collects data in 10 U.S. States on

diseases caused by enteric pathogens transmitted

commonly through food. In 2008, FoodNet

reported that the incidence per 100,000 people

for Salmonella, Campylobacter, E. coli

O157:H7, and Listeria remained unchanged for

the preceding 3 years (CDC, 2009). Preventing

illness and death associated with foodborne

pathogens remains a major public health

challenge.

In addition to the effect on human health,

foodborne illnesses have an economic impact.

The health-related cost of foodborne illness in

the United States is estimated to be

approximately $152 billion annually (Scharff,

2010).

Many organisms capable of causing foodborne

illness are present in the intestines of healthy

animals raised for food. As a result, food can be

contaminated as it is produced. For example,

meat and poultry carcasses can be contaminated

if they come in contact with small amounts of

intestinal contents during slaughter. Similarly,

fresh fruits and vegetables can be contaminated

if they are washed or irrigated with water

contaminated with animal manure or human

sewage (Doyle and Erickson, 2008; Hanning et

al., 2009). Other foods of animal origin, such as

raw eggs, unpasteurized milk, and raw shellfish

might also be contaminated. In general,

commingling products from many individual

animals—such as bulk raw milk, pooled raw

eggs, or ground beef—presents an increased risk

of contamination; a pathogen present in any one

animal can contaminate products from multiple

animals.

There are several reasons that food safety is of

concern to the dairy industry. Raw milk can

contain Salmonella, Campylobacter, or Listeria,

all of which can cause human disease; however,

outbreaks of disease in humans caused by milk

products have primarily been due to the

consumption of unpasteurized milk or cheeses

made from unpasteurized milk. In addition, cull

dairy cows account for about 17 percent of the
2 / Dairy 2007

Introduction

ground beef available for national consumption

(Troutt and Osburn, 1997) and may be a

potential source of human exposure to

foodborne pathogens if the meat from these

animals is contaminated with fecal material

during slaughter or processing.

This report compares the prevalence and

antimicrobial resistance of Salmonella,

Campylobacter, and Listeria on U.S. dairy

operations as reported in the NAHMS Dairy

1996, Dairy 2002, and Dairy 2007 studies.

These pathogens were selected because data

relating to them could be compared across study

years; only results that could be compared with

Dairy 2007 results were included. For example,

results from the composite fecal sample testing

for Salmonella conducted during Dairy 2007 are

not reported here because composite fecal

Salmonella samples were not collected and

tested during the Dairy 1996 and Dairy 2002

studies.

Further information on NAHMS studies and

reports is available online at:

http://nahms.aphis.usda.gov

For questions about this report or additional

copies, please contact:

USDA–APHIS–VS–CEAH

NRRC Building B, M.S. 2E7

2150 Centre Avenue

Fort Collins, CO 80526-8117

970.494.7000
USDA APHIS VS / 3

Introduction

NAHMS Dairy Studies

The National Animal Health Monitoring System

(NAHMS) is a nonregulatory division of the

U.S. Department of Agriculture’s (USDA)

Animal and Plant Health Inspection Service

(APHIS). NAHMS is designed to help meet the

Nation’s animal-health information needs and

has collected data on dairy health and

management practices through four previous

studies.

The NAHMS 1991-92 National Dairy Heifer

Evaluation Project (NDHEP) provided the dairy

industry’s first national information on the

health and management of dairy heifers in the

United States. Just months after the study’s first

results were released in 1993, cases of acute

bovine viral diarrhea (BVD) surfaced in the

United States following a 1993 outbreak in

Canada. NDHEP information on producer

vaccination and biosecurity practices helped

officials address the risk of disease spread and

target educational efforts on vaccination

protocols. An outbreak of human illness was

reported in 1993 in the Pacific Northwest, this

time related to Escherichia coli 0157:H7.

NDHEP data on the bacteria’s prevalence in

dairy cattle helped officials define public risks

as well as research needs. This baseline picture

of the industry also helped identify additional

research and educational efforts in various

production areas, such as feed management and

weaning age.

Information from the NAHMS Dairy 1996 study

helped the U.S. dairy industry identify

educational needs and prioritize research efforts

on such timely topics as antibiotic usage and

Johne’s disease, as well as digital dermatitis,

bovine leukosis virus, and potential foodborne

pathogens, including E. coli, Salmonella, and

Campylobacter.

A major focus of the Dairy 2002 study was to

describe management strategies that prevent and

reduce Johne’s disease and to determine

management factors associated with

Mycoplasma and Listeria in bulk-tank milk.

Additionally, levels of participation in quality

assurance programs, the incidence of digital

dermatitis, a profile of animal waste handling

systems used on U.S. dairy operations, and

industry changes since the NDHEP in 1991 and

Dairy 1996 were examined.

One of the objectives of the Dairy 2007 study

was calf health, including colostrum

management and passive transfer of immunity.

Additional study topics included an evaluation

of cow comfort and the analysis of hygiene and

hock scores. Additionally, diseases of concern

such as BVD, Johne’s disease, and contagious

mastitis were evaluated. The Dairy 2007 study

also took and in-depth look at reproductive

practices.

An objective for all three studies, Dairy 1996,

Dairy 2002, and Dairy 2007, was to determine

the prevalence of specific food safety pathogens

and to describe antimicrobial resistance patterns

on U.S. dairy operations.
4 / Dairy 2007

Introduction
USDA APHIS VS / 5

Introduction

Study Objectiv tudy Objectives and

Related Outputs ed Outputs

1. Describe trends in dairy cattle health and

management practices

• Part II: Changes in the U.S. Dairy Cattle

Industry, 1991–2007, March 2008

• Part V: Changes in Dairy Cattle Health and

Management Practices in the United States,

1996–2007, July 2009

2. Evaluate management factors related to cow

comfort and removal rates

• Facility Characteristics and Cow Comfort on

U.S. Dairy Operations, 2007, Interpretive

Report, October 2010

3. Describe dairy calf health and nutrition from

birth to weaning and evaluate heifer disease

prevention practices

• Part I: Reference of Dairy Cattle Health and

Management Practices in the United States,

2007, October 2007

• Off-Site Heifer Raising on U.S. Dairy

Operations, 2007, info sheet, November 2007

• Colostrum Feeding and Management on U.S.

Dairy Operations, 1991–2007, info sheet,

March 2008

• Part IV: Reference of Dairy Cattle Health and

Management Practices in the United States,

2007, February 2009

• Calving Intervention on U.S. Dairy

Operations, 2007, info sheet, February 2009

• Heifer Calf Health and Management Practices

on U.S. Dairy Operations, 2007, Interpretive

Report, February 2010

• Passive Transfer in Dairy Heifer Calves,

1991–2007, info sheet, March 2010

4. Estimate the prevalence of herds infected with

bovine viral diarrhea virus (BVDV)

• Bovine Viral Diarrhea (BVD) Management

Practices and Detection in Bulk Tank Milk in

the United States, 2007, info sheet, October

2008

5. Describe current milking procedures and

estimate the prevalence of contagious mastitis

pathogens

• Part III: Reference of Dairy Cattle Health and

Management Practices in the United States,

2007, September 2008

• Milking Procedures on U.S. Dairy Operations,

2007, info sheet, October 2008

• Prevalence of Contagious Mastitis Pathogens

on U.S. Dairy Operations, 2007, info sheet,

October 2008

6. Estimate the herd-level prevalence and

associated costs of Mycobacterium avium

subspecies paratuberculosis

• Johne’s Disease on U.S. Dairies, 1991–2007,

info sheet, April 2008

7. Describe current biosecurity practices and

determine producer motivation for

implementing or not implementing biosecurity

practices

• Part I: Reference of Dairy Cattle Health and

Management Practices in the United States,

2007, October 2007

• Part III: Reference of Dairy Cattle Health and

Management Practices in the United States,

2007, September 2008

• Biosecurity Practices on U.S. Dairy

operations, 1991–2007, Interpretive Report,

May 2010
6 / Dairy 2007

Introduction

8. Determine the prevalence of specific foodsafety pathogens and describe antimicrobial

resistance patterns

• Antibiotic Use on U.S. Dairy Operations, 2002

and 2007, info sheet, October 2008

• Prevalence of Salmonella and Listeria in Bulk

Tank Milk and In-line Filters on U.S. Dairies,

2007, info sheet, July 2009

• Salmonella and Campylobacter on U.S. Dairy

Operations, 2002–07, info sheet, July 2009

• Salmonella, Listeria, and Campylobacter on

U.S. Dairy Operations, 2007, Interpretive

Report, March 2011

• Prevalence of Coxiella burnetii on U.S. Dairy

Operations, 2007, technical brief, March 2011

• Prevalence of Clostridium difficile on U.S.

Dairy Operations, 2007, technical brief, April

2011

Additional information sheets

• Dairy Cattle Identification Practices in the

United States, 2007, info sheet, November

2007

• Bovine Leukosis Virus (BLV) on U.S. Dairy

Operations, 2007, info sheet, October 2008

• Reproduction Practices on U.S. Dairy

Operations, 2007, info sheet, February 2009

• Injection Practices on U.S. Dairy Operations,

2007, info sheet, February 2009

• Methicillin-Resistant Staphylococcus aureus

(MRSA) Isolation from Bulk Tank Milk in the

United States, 2007,technical brief, March

2011
USDA APHIS VS / 7

Introduction

Terms Used in This Report

Herd size: Herd size is based on January 1

dairy cow inventory for each study year. Small

herds are those with fewer than 100 head;

medium herds are those with 100 to 499 head;

and large herds are those with 500 or more head.

Population estimates: The estimates in this

report for bulk-tank milk and milk filter

sampling make inference to all of the operations

with dairy cows in the target population. Data

from the operations responding to the survey are

weighted to reflect their probability of selection

during sampling and to account for survey

nonresponse.

Precision of population estimates: Population

estimates in this report are provided with a

measure of precision called the standard error. A

95-percent confidence interval can be created

with bounds equal to the estimate plus or minus

two standard errors. If the only error is sampling

error, the confidence intervals created in this

manner will contain the true population mean 95

out of 100 times. In the example to the right, an

estimate of 7.5 with a standard error of 1.0

results in limits of 5.5 to 9.5 (two times the

standard error above and below the estimate).

The second estimate of 3.4 shows a standard

error of 0.3 and results in limits of 2.8 and 4.0.

Alternatively, the 90-percent confidence interval

would be created by multiplying the standard

error by 1.65 instead of 2. Most estimates in this

report are rounded to the nearest tenth. If

rounded to 0, the standard error was reported

(0.0). References to population estimates being

higher or lower than other estimates are based

Regions:

West: California, Colorado, Idaho, New

Mexico, Oregon, Texas, Washington

East: Florida, Illinois, Indiana, Iowa, Kentucky,

Michigan, Minnesota, Missouri, Ohio,

Pennsylvania, New York, Tennessee, Vermont,

Virginia, Wisconsin

Standard Errors

(1.0)

10

9

8

7

6

5

4

3

2

1

0

(0.3)

Examples of a 95% Confidence Interval

95% Confidence 

Intervals

on the 95-percent confidence intervals not

overlapping. The estimates in this report without

standard errors are not considered populatin

estimates.
8 / Dairy 2007

Section I: Population Estimates—A. Salmonella Detection in Bulk-tank Milk and Milk Filters

Section I: P Section I: Population opulation

Estimates

A. Salmonella Detection in Bulk-tank Milk and Milk Filters

1. Background Salmonellae are gram-negative bacteria that can

cause gastrointestinal infection in animals and

humans. Salmonella causes an estimated

1.4 million human illnesses and over 500 deaths

annually in the United States (Mead et al.,

1999). Clinical signs of salmonellosis in humans

include diarrhea, fever, and abdominal cramps

12 to 72 hours after infection. Clinical signs

usually last 4 to 7 days, and most people recover

without treatment (CDC, 2008a). In the elderly,

infants, and immunocompromised individuals,

Salmonella infection may spread from the

intestines to the bloodstream and cause more

severe, sometimes life-threatening, infections.

Economic losses associated with human

Salmonella infections have attracted increasing

attention in a number of countries.

Salmonellosis is estimated to cost the United

States $14.6 billion annually (Scharff, 2010).

In dairy cows, Salmonella infection can result in

mortality of adult cows, higher treatment costs,

increased cull rates, higher labor costs, and

lower milk production. Calf mortality and

morbidity also add to the total cost of disease.

Clinical signs of salmonellosis in adult cattle

include depression, dehydration, diarrhea, fever

(106–108°F), anorexia, vaginal discharge,

abortion, and decreased milk production. The

effects of infection can range from no clinical

signs to endotoxemia and death. Calves with

clinical Salmonella infections can present with

diarrhea, fever, lethargy, and an inability to rise.

Infected calves can also become septic and die

(Smith, 2002). Evidence indicates that calves

are more likely to experience mortality than

cows (Cummings et al., 2009b), and preweaned

calves are more likely to be affected by clinical

salmonellosis compared with other cattle

(Cummings et al., 2009a). Cattle can shed

Salmonella in their feces without showing

clinical signs.

Dairy operations represent a potential source of

Salmonella infection for humans. Salmonella

species can colonize the gastrointestinal tracts of

cattle and other animals. Humans can become

infected with Salmonella through fecal

contamination of food products or water.

Several outbreaks of salmonellosis have been

linked to beef and dairy products (CDC, 2003,

2006a, 2006b; Van Duynhoven et al., 2009).

Another source of human infection, primarily

affecting farm families, employees, and visitors,

is direct contact with ill animals (Holmberg et

al., 1984; Troutt and Osburn, 1997). Cull dairy

cows contribute about 17 percent of the ground

beef available for national consumption (Troutt

and Osburn, 1997) and can be a potential source

of human exposure to Salmonella when the meat

is contaminated with fecal material during

slaughter. Pasteurization is very effective against

Salmonella organisms, and foodborne outbreaks

associated with this pathogen in pasteurized

milk or dairy products are very rare.

Testing for Salmonella in milk is not a routine

practice by milk producers. Bacteriological

analysis of raw milk is typically limited to tests

for bacteria (i.e., standard plate count and

coliform count) or for specific mastitis-causing

bacteria (Jayarao et al., 2001). Salmonella

serotyping allows for monitoring changes in the
USDA APHIS VS / 9

Section I: Population Estimates—A. Salmonella Detection in Bulk-tank Milk and Milk Filters

causative organisms. A change in a herd’s

serotype profile could indicate a new source of

infection. Antimicrobial susceptibility testing is

important for determining effective therapy and

for guiding prudent antibiotic use.

Salmonella contamination in bulk-tank milk is

believed to result from fecal contamination

attributable to poor hygiene during the milking

process rather than from intramammary

infection with Salmonella, which is rare (Van

Kessel et al., 2004; Jayarao et al., 2006).

Standard hygiene practices during milking

reduce but do not eliminate the risk of milk

contamination. Pasteurization decreases the

number of pathogenic organisms, decreases

transmission of pathogens, and improves the

safety of milk more than other measures,

including certification of raw milk (Potter et al.,

1984). Interstate sale of raw milk is banned in

the United States by the Food and Drug

Administration, but intrastate sales are allowable

on a State-by-State basis, depending upon each

State’s regulation. Consumption of raw bulktank milk is a common practice among farm

families (Jayarao et al., 2006). Among the

nonfarming population, a growing number of

consumers claim that raw milk is healthier, and

they choose raw milk over pasteurized milk

(Bren, 2004; Jayarao et al., 2006). Pasteurizing

raw milk is an important public health tool for

preventing foodborne disease. Because of

pasteurization, contamination of dairy products

currently accounts for a small percentage of

foodborne illness in the United States. However,

it is clear that consuming raw milk and products

made with raw milk present a risk of foodborne

illness to humans.

2. Sampling and

testing overview

Bulk-tank milk samples were collected and

tested for the presence of Salmonella during the

Dairy 2002 and Dairy 2007 studies. Bulk-tank

milk was not tested for Salmonella in Dairy

1996. In 2002 and 2007, one bulk-tank milk

sample was collected per operation using aseptic

techniques. In addition, a milk filter was

collected from each operation in 2007.

For Dairy 2002, both culture and real-time

polymerase chain reaction (RT-PCR) were used

to detect Salmonella in bulk-tank milk samples,

while only PCR was used in Dairy 2007. Culture

was performed on PCR-positive samples from

Dairy 2007 so that serotyping could be done. In

2002, culture results for bulk-tank milk were

available from 852 dairy operations, and

RT-PCR results were available from 838

operations. In the Dairy 2007 study, test results

from bulk-tank milk or milk filters were

available from 538 dairy operations: 517 from

bulk-tank milk and milk filters, 19 from bulktank milk only, and 2 from milk filters only.

For more information on sampling and

diagnostic testing methods, see Section III, p 48.
10 / Dairy 2007

Section I: Population Estimates—A. Salmonella Detection in Bulk-tank Milk and Milk Filters

3. Prevalence In 2007, the percentage of operations on which

a milk filter tested positive for Salmonella

(24.7 percent) was more than double the

percentage of operations on which a bulk-tank

milk sample tested positive (10.8 percent).

The percentage of operations on which bulktank milk tested positive for Salmonella by

RT-PCR was similar in 2002 and 2007 (11.9 and

10.8 percent of operations, respectively). The

percentage of operations on which a bulk-tank

milk sample tested positive for Salmonella by

RT-PCR was similar across herd sizes in 2002

and 2007.

b. Percentage of operations on which a bulk-tank milk sample tested positive for 

Salmonella by RT-PCR, by herd size 

Herd Size (Number of Cows)

Small 

(Fewer 

than 100)

Medium 

(100–499)

Large 

(500 or More)

All 

Operations 

Study Pct. 

Std. 

Error Pct. 

Std. 

Error Pct. 

Std. 

Error Pct. 

Std. 

Error 

Dairy 2002 12.4 (2.2) 10.2 (2.1) 13.9 (3.1) 11.9 (1.7) 

Dairy 2007 8.1 (2.3) 16.2 (3.2) 19.6 (4.6) 10.8 (1.8) 

a. Percentage of operations on which a bulk-tank milk and/or a milk-filter sample 

tested positive for Salmonella in Dairy 2007, by testing method 

Testing Method 

Bulk-tank Milk RT-PCR Milk Filter RT-PCR 

Bulk-tank Milk or Milk 

Filter RT-PCR 

Percent Std. Error Percent Std. Error Percent Std. Error 

10.8 (1.8) 24.7 (2.4) 28.1 (2.6) 
USDA APHIS VS / 11

Section I: Population Estimates—A. Salmonella Detection in Bulk-tank Milk and Milk Filters

In 2002 and 2007, there was no regional

difference in the percentage of operations on

which a bulk-tank milk sample tested positive

for Salmonella by RT-PCR.

c. Percentage of operations on which a bulk-tank milk sample tested positive for 

Salmonella by RT-PCR, by region 

Region 

East West 

Study Percent Std. Error Percent Std. Error 

Dairy 2002 11.9 (1.8) 11.5 (3.8) 

Dairy 2007 10.7 (2.0) 12.7 (3.1) 
12 / Dairy 2007

Section I: Population Estimates—A. Salmonella Detection in Bulk-tank Milk and Milk Filters

4. Serotypes Eight Salmonella serotypes were found in bulktank milk in 2002. S. Montevideo was found in

bulk-tank milk on seven operations in 2002. In

2007, 14 and 22 Salmonella serotypes were

found in bulk-tank milk and milk filters,

respectively. S. Cerro was identified in the

highest number of both sample types.

a. Number of operations on which the following Salmonella serotypes were 

identified, by sample type used for identification 

Sample Type 

Dairy 2002 Dairy 2007 

Bulk-tank Milk Bulk-tank Milk Milk Filters 

Serotype

Number 

Operations 

(852 Sampled)

Number 

Operations 

(536 Sampled)

Number 

Operations 

(519 Sampled)

Cerro 3 8 27 

Kentucky 0 5 16 

Muenster 2 5 10 

Newport 4 1 9 

Anatum* 1 4 8 

Montevideo 7 2 7 

Meleagridis 2 1 6 

Mbandaka 0 3 5 

Typhimurium* 0 1 4 

Dublin 1 2 3 

Senftenberg 0 1 2 

Give* 0 0 2 

Untypable 3 0 2 

Agona 0 1 1 

Infantis 0 1 1 

Schwarzengrund 0 1 1 

Derby 0 0 1 

Muenchen 0 0 1 

Reading 0 0 1 

Saintpaul 0 0 1 

Soerenga 0 0 1 

Thompson 0 0 1 

*Includes variant species (e.g., Typhimurium var. 5-, formerly Typhimurium var. Copenhagen). 
USDA APHIS VS / 13

Section I: Population Estimates—B. Listeria Detection in Bulk-tank Milk

B. Listeria Detection in Bulk-tank Milk

1. Background Listeria species are gram-positive bacteria that

can cause serious infections in humans. Listeria

monocytogenes is the most important Listeria

species in terms of public health risk and

frequency of appearance in foodstuffs.

L. monocytogenes is widespread in the

environment. The main source of infection for

ruminants is spoiled silage, but cattle may also

ingest the organism by fecal-oral transmission.

Adult cattle observed with clinical disease

(listeriosis) most often have encephalitis, a

nervous system disorder. Signs of disease in

cattle include facial paralysis, depression,

circling, and abortion.

Although the occurrence of human listeriosis is

generally infrequent, it often leads to serious

illness. Listeriosis in humans can be

accompanied by fever, muscle aches, nausea,

and diarrhea. If infection spreads to the nervous

system, symptoms such as headache, stiff neck,

loss of balance, or convulsions can occur.

Infections during pregnancy can lead to

miscarriage or stillbirth. Pregnant women, the

elderly, and those with immunosuppression are

most susceptible to listeriosis. In the United

States, the annual cost of illness in humans due

to L. monocytogenes is estimated at $8.8 billion

(Scharff, 2010). Estimates indicate that

approximately 2,500 listeriosis cases in humans

occur each year in the United States, with nearly

all cases attributed to a food source (Mead et al.,

1999). Approximately 92 percent of individuals

with illness caused by L. monocytogenes

listeriosis are hospitalized (20 percent of these

cases are fatal), making L. monocytogenes

responsible for the highest hospitalization rate

among foodborne pathogens (Mead et al.,

1999).

It is not possible to remove all Listeria

organisms from the environment.

L. monocytogenes is found in soil and water,

which can lead to contamination of fruits,

vegetables, and other foods typically eaten raw.

Listeria is killed by pasteurization and cooking

but is relatively cold tolerant. L. monocytogenes

survives refrigeration temperatures and can

grow under these conditions, an unusual

characteristic among foodborne pathogens

(Walker et al., 1990). With regard to milk and

dairy products, listeriosis is most often

associated with products made from

unpasteurized milk. Because of its ability to

grow under refrigeration, contamination of cold

cuts or other ready-to-eat foods after processing

is a concern and has been associated with human

illness.

Pasteurizing raw milk is an important public

health tool for foodborne disease prevention.

Because of pasteurization, contamination of

dairy products currently accounts for a small

percentage of foodborne illness in the United

States. However, it is clear that consuming raw

milk and products made with raw milk present a

risk of foodborne illness to humans.
14 / Dairy 2007

Section I: Population Estimates—B. Listeria Detection in Bulk-tank Milk

2. Sampling and

testing overview

Bulk-tank milk samples were collected and

tested for the presence of Listeria as part of the

Dairy 2002 and Dairy 2007 studies. Bulk-tank

milk was not tested for Listeria in Dairy 1996.

In 2002 and 2007, one bulk-tank milk sample

was collected per participating operation using

aseptic techniques. In addition, in 2007 a milk

filter was collected from each operation.

Culture methods were used to identify Listeria

in bulk-tank milk samples in 2002 and 2007. In

2002, PCR was used as a component of the

process to confirm isolates as Listeria. Results

for bulk-tank milk testing for Listeria were

available from 851 operations for Dairy 2002

and from 538 operations for Dairy 2007. In

2007, bulk-tank milk or milk-filter results were

available from 538 dairy operations: 517 from

bulk-tank milk and milk filters, 19 from bulktank milk only, and 2 from milk filters only.

For more information on sampling and

diagnostic testing methods, see Section III, p 48.

Photo of Listeria courtesy of CDC.
USDA APHIS VS / 15

Section I: Population Estimates—B. Listeria Detection in Bulk-tank Milk

3. Prevalence The percentage of operations on which a bulktank milk sample tested positive for

L. monocytogenes was similar in 2002 and 2007

(3.8 and 3.7 percent, respectively). In Dairy

2007, the percentage of operations on which a

milk filter tested positive for any Listeria

species (28.3 percent) was more than three times

the percentage of operations on which a bulktank milk sample tested positive for any Listeria

species (9.0 percent).

The percentage of operations on which a bulktank milk sample tested positive forL. monocytogenes was similar across herd sizes

in 2002 and 2007.

a. Percentage of operations on which a bulk-tank milk and/or a milk filter sample 

tested positive for Listeria, by sample type 

Dairy 2002 Dairy 2007 

L. monocytogenes Any Listeria L. monocytogenes 

Sample Type Pct. 

Std. 

Error Pct. 

Std. 

Error Pct. 

Std. 

Error 

Bulk-tank milk 3.8 (0.7) 9.0 (1.9) 3.7 (1.2) 

Milk filter NA 28.3 (2.9) 5.1 (1.2) 

Bulk-tank milk or 

milk filter NA 32.1 (2.9) 7.1 (1.5) 

b. Percentage of operations on which a bulk-tank milk sample tested positive for 

L. monocytogenes, by herd size 

Herd Size (Number of Cows)

Small 

(Fewer 

than 100)

Medium 

(100–499)

Large 

(500 or More)

All 

Operations 

Study Pct. 

Std. 

Error Pct. 

Std. 

Error Pct. 

Std. 

Error Pct. 

Std. 

Error 

Dairy 2002 3.0 (0.9) 5.4 (1.2) 7.8 (2.4) 3.8 (0.7) 

Dairy 2007 2.3 (1.4) 7.5 (2.5) 4.0 (1.7) 3.7 (1.2) 
16 / Dairy 2007

Section I: Population Estimates—B. Listeria Detection in Bulk-tank Milk

There were no regional differences in 2002 and

2007 in the percentage of operations on which a

bulk-tank milk sample tested positive for

L. monocytogenes.

c. Percentage of operations on which a bulk-tank milk sample tested positive for 

L. monocytogenes, by region 

Region 

East West 

Study Percent Std. Error Percent Std. Error 

Dairy 2002 3.9 (0.8) 2.9 (1.4) 

Dairy 2007 3.3 (1.2) 8.3 (4.2) 
USDA APHIS VS / 17

Section II: Pathogen Detection in Feces—A. Salmonella

Section II: P Section II: Pathogen

Detection in F ection in Feces

A. Salmonella

1. Sampling and

testing overview

NAHMS has examined Salmonella occurrence

using individual fecal samples from dairy cows

in three separate studies: Dairy 1996, Dairy

2002, and Dairy 2007. Typically, NAHMS

studies generate population-based estimates, and

appropriate sample sizes are used to arrive at

such estimates. Field resources, laboratory

capacity, and the expense of culturing samples

make it difficult to provide a national estimate

of Salmonella prevalence based on fecal

culturing of individual animals. Therefore, for

the Salmonella estimates in this section, all three

NAHMS dairy studies used a sample of

approximately 100 operations, which is not an

optimal sample size for providing national

estimates of prevalence. Despite this limitation,

the NAHMS studies provide valuable

information on Salmonella occurrence and

antimicrobial susceptibility patterns on

U.S. dairies and represent the only national

examination of Salmonella on dairy operations

in the United States. Other research studies have

examined Salmonella occurrence in dairy cattle

but have been limited to specific regions of the

United States.

At the time of sampling, records were kept as to

whether each cow was sick, healthy (from the

milking string), scheduled for culling (within

7 days of leaving the operation), or dry (dry

cows were sampled only in Dairy 1996 and

Dairy 2002). Dry cows were grouped with

healthy cows in the following estimates. Dairy

1996 compared the prevalence of Salmonella in

milk cows on-farm to that of cows on-farm that

were scheduled for culling within the next

7 days and to cull cows at markets.

Dairy 2007 evaluated strategies for detecting

Salmonella using fecal samples from individual

cows, fecal samples pooled from five cows, and

composite fecal samples from the dairy

environment. To allow for comparison across

the three studies, results presented in this report

are primarily limited to healthy cows. An

operation was classified as infected if one or

more fecal samples were culture positive for

Salmonella. The following table presents an

overview of the sampling procedures used for

the three dairy studies.

For more information on sampling and

diagnostic testing methods, see Section III, p 48.
18 / Dairy 2007

Section II: Pathogen Detection in Feces—A. Salmonella

Salmonella fecal sampling methods, by study 

Study 

Number of 

Operations 

Sampled* 

Sampling 

Period 

Number of 

Samples per 

Operation Notes 

Dairy 

1996 

91 dairy 

operations/ 

19 States 

Feb. 26 to July 

10, 1996 

40 or 50, 

depending on 

herd size, plus 

all cows 

scheduled for 

culling 

All samples were taken rectally from individual cows. 

There were no specific targets as to the number of 

sick, dry, or milking string cows, other than the sample 

was to be representative of the cows on hand on the 

day of the visit. Cow type was noted at the time of 

collection. 

Dairies with 30–99 cows: Operations were visited 

once. Up to 40 fecal samples were collected, which 

included samples from all cows scheduled for culling 

present on the day of the visit. 

Dairies with 100 or more cows: Operations were 

visited three times. During one visit, 50 cows (from 

milking string, dry, or sick) were sampled along with 

up to 20 cows scheduled for culling in the next 7 days. 

On 2 other visits, up to 20 samples were taken from 

cows scheduled for culling in the next 7 days. 

97 cull cow 

markets/ 

20 States 

Feb. 26 to July 

10, 1996 25 

Twenty-five fresh fecal samples per market—either by 

rectal retrieval from individual cows or from pen floors 

if restraining facilities were not available. 

Dairy 

2002 

97 dairy 

operations/ 

21 States 

Mar. 27 to 

Sept. 25, 2002 40 

The goal was to collect 40 individual fecal samples 

during a single visit, all via rectal retrieval. If the herd 

had fewer than 40 cows, all cows were sampled. 

There were no specific targets for number of sick, dry, 

or milking string cows to be sampled, but cow type 

was noted at the time of collection. 

Dairy 

2007 

121 dairy 

operations/ 

17 States 

Feb. 28 to 

Aug. 30, 2007 35 

The goal was to collect 35 individual fecal samples 

during a single visit, all via rectal retrieval. Up to five 

sick cows and up to five cows scheduled for culling 

(within 7 days of leaving the operation) were sampled, 

with the remainder (up to 35) being from cows with 

saleable milk. 

260 dairy 

operations/ 

17 States 

Feb. 28 to 

Aug. 30, 2007 6 

Manure/slurry (composite fecal) samples from six 

different adult cow areas where manure accumulates 

were taken (area samples). Each area sample was 

composed of about 4 oz of manure/slurry from each of 

six sites within the area. Areas recommended for 

sampling included common alleyways, common pens, 

exits from parlors, floors of holding pens, flush water, 

gutter cleaner, lagoons or manure pits, and manure 

spreaders. 

*Operations with 30 or more dairy cows. 
USDA APHIS VS / 19

Section II: Pathogen Detection in Feces—A. Salmonella

2. Prevalence The table below presents both herd- and animallevel Salmonella prevalence estimates from the

three NAHMS dairy studies. For purposes of

comparison, estimates are limited to healthy

cows because these sample numbers remained

relatively consistent across the three studies.

Culture methods were similar for the three

NAHMS dairy studies. In 2007, the percentage

of Salmonella-positive operations was almost

double that in 1996, and the percentage of

Salmonella-positive cows more than doubled

over the same time period. Differences in types

of operations sampled by region and herd size

might account for some of the differences

among the three studies; however, it is possible

that Salmonella is becoming more common on

U.S. dairies.

a. Percentage of operations and percentage of cows fecal-culture positive for

Salmonella1

Study Operations2 Cows 

Dairy 1996 20.0 (18/90) 5.4 (194/3,585) 

Dairy 2002 30.9 (30/97) 7.1 (259/3,645) 

Dairy 2007 39.7 (48/121) 13.7 (523/3,804) 

1

Only cows healthy at the time of collection are included. 

2

Operations with at least one culture-positive cow were considered positive. 

Photo courtesy of Judy Rodriguez
20 / Dairy 2007

Section II: Pathogen Detection in Feces—A. Salmonella
USDA APHIS VS / 21

Section II: Pathogen Detection in Feces—A. Salmonella

In several previous studies, operations with 100

or more dairy cows have been more likely than

operations with fewer than 100 cows to be

Salmonella positive (Warnick et al., 2001; Wells

et al., 2001; Huston et al., 2002; Fossler et al.,

2004; Blau et al., 2005; Davison et al., 2006;

Cummings et al., 2009a). This finding was true

in all three NAHMS dairy studies: the

percentage of large operations culture positive

for Salmonella was at least double that of small

operations.

b. Percentage of operations1 fecal-culture positive for Salmonella, by herd size2

Herd Size (Number of Cows)

Study

Small 

(Fewer than 100)

Medium 

(100–499)

Large 

(500 or More)

Dairy 1996 4.8 (2/42) 29.0 (9/31) 41.2 (7/17) 

Dairy 2002 18.2 (6/33) 28.2 (11/39) 52.0 (13/25) 

Dairy 2007 24.3 (9/37) 44.7 (21/47) 48.7 (18/37) 

1

Operations with at least one culture-positive cow were considered positive. 

2

Only cows healthy at the time of collection are included. 
22 / Dairy 2007

Section II: Pathogen Detection in Feces—A. Salmonella

Salmonella has been found more commonly

during summer months than winter months

(Evans and Davies, 1996; Wells et al., 2001;

Fossler et al., 2005b), although this finding has

not been as consistently observed as the herd

size differences described previously. In all

three NAHMS studies, a higher percentage of

operations sampled during summer (June–

September) were positive compared with

operations sampled during spring (February–

May).

c. Percentage of operations1 fecal-culture positive for Salmonella, by season2

Season 

Study Spring (February–May) Summer (June–September)

Dairy 1996 16.2 (12/74) 37.5 (6/16) 

Dairy 2002 23.5 (12/51) 39.1 (18/46) 

Dairy 2007 29.6 (16/54) 47.8 (32/67) 

1

Operations with at least one culture-positive cow were considered positive. 

2

Only cows healthy at the time of collection are included. 
USDA APHIS VS / 23

Section II: Pathogen Detection in Feces—A. Salmonella

There were no consistent trends in the

percentage of Salmonella-positive operations by

region. In 1996 and 2002, a higher percentage

of operations in the West region than in the East

region were Salmonella positive. In contrast, in

2007 a higher percentage of operations in the

East region than in the West region were

Salmonella positive (43.6 and 20.0 percent,

respectively). It is difficult to draw any

conclusions with regard to regional differences

while ignoring herd size differences. As shown

in table on the next page, during Dairy 1996 and

Dairy 2002 there were fewer participating

operations with 500 or more cows in the East

region than there were in Dairy 2007. Thus, any

apparent trends with regard to regional

differences are likely due to herd sizes within

each region.

d. Percentage of operations1 fecal-culture positive for Salmonella, by region2

Region 

Study East West 

Dairy 1996 13.8 (9/65) 36.0 (9/25) 

Dairy 2002 26.1 (18/69) 42.9 (12/28) 

Dairy 2007 43.6 (44/101) 20.0 (4/20) 

1

Operations with at least one culture-positive cow were considered positive. 

2

Only cows healthy at the time of collection are included. 
24 / Dairy 2007

Section II: Pathogen Detection in Feces—A. Salmonella

For the West region in 1996 and 2002, a higher

percentage of operations with 500 or more cows

were Salmonella positive compared with

operations with fewer than 500 cows. For the

East region in 2007, a higher percentage of

operations with 500 or more cows were

Salmonella positive compared with operations

with fewer than 500 cows (65.2 and

37.2 percent, respectively). In the West region in

2007, there was little difference by herd size in

the percentage of Salmonella-positive

operations, which was also true in the East

region in 1996 and 2002, although there were

relatively few participating operations with 500

or more cows in the East region in 1996 and

2002.

e. Percentage of operations1 fecal-culture positive for Salmonella, by herd size and by 

region2

Dairy 1996 Dairy 2002 Dairy 2007 

Region 

Herd size 

(Number of 

Cows) East West East West East West 

Fewer 

than 500 13.3 (8/60) 23.1 (3/13) 25.8 (16/62) 10.0 (1/10) 37.2 (29/78) 16.7 (1/6) 

500 or more 20.0 (1/5) 50.0 (6/12) 28.6 (2/7) 61.1 (11/18) 65.2 (15/23) 21.4 (3/14) 

1

Operations with at least one culture-positive cow were considered positive. 

2

Only cows healthy at the time of collection are included. 
USDA APHIS VS / 25

Section II: Pathogen Detection in Feces—A. Salmonella

In all three dairy studies, a lower percentage of

healthy cows were Salmonella positive on small

operations than on medium and large operations.

Large operations had the highest percentage of

Salmonella-positive cows in 1996 and 2002.

In 1996 and 2002, a higher percentage of

healthy cows were positive for Salmonella in the

West region than in the East region. In contrast,

in 2007 a higher percentage of cows were

positive for Salmonella in the East region than

in the West region; however, a much smaller

number of cows were sampled in the West

region than in the East region in 2007 (580 and

3,224, respectively).

f. Percentage of healthy cows fecal-culture positive for Salmonella, by herd size 

Herd Size (Number of Cows)

Study

Small 

(Fewer than 100)

Medium 

(100–499)

Large 

(500 or More)

Dairy 1996 0.6 (9/1,494) 6.3 (81/1,292) 13.0 (104/799) 

Dairy 2002 1.8 (21/1,152) 7.7 (118/1,535) 12.5 (120/958) 

Dairy 2007 5.5 (66/1,209) 17.9 (270/1,508) 17.2 (187/1,087) 

g. Percentage of healthy cows fecal-culture positive for Salmonella, by region 

Region 

Study East West 

Dairy 1996 1.6 (39/2,429) 13.4 (155/1,156) 

Dairy 2002 5.3 (136/2,569) 11.4 (123/1,076) 

Dairy 2007 15.5 (499/3,224) 4.1 (24/580) 
26 / Dairy 2007

Section II: Pathogen Detection in Feces—A. Salmonella

One of the goals of the Dairy 1996 study was to

evaluate whether cows scheduled for culling

were more likely to be Salmonella positive than

other cows on the operation. Aside from cows

scheduled for culling, other cows sampled were

to be representative of all cows on the operation

on the day of sampling, including sick cows, dry

cows, and cows in the milking string. It was

noted at the time of sampling whether a cow was

sick, dry, from the milking string, or scheduled

for culling, but there were no requirements for

sampling a specified number of sick cows.

Likewise, for Dairy 2002 there were no

requirements for sampling different types of

cows, but it was noted at the time of sampling

whether a cow was sick, scheduled for culling

within 7 days, dry, or from the milking string. In

contrast, in the Dairy 2007 study there were

specific instructions to collect samples from up

to 5 sick cows and up to 5 cows scheduled for

culling, with the remainder of samples—up to

35—taken from cows with saleable milk.

The following results should be interpreted with

these sampling differences in mind. For all three

NAHMS studies, a higher percentage of cows

designated as sick on the day of the visit were

culture positive for Salmonella compared with

cows designated as healthy. These results are

supported by a previous study which collected

samples from preweaned calves, sick cows,

cows scheduled to be culled, periparturient cows

(within 14 days of calving), and healthy cows

and found that sick cows had the highest odds of

being Salmonella positive (Fossler, 2005a). It is

possible that the primary cause of illness in the

sick cattle was salmonellosis, or that battling

another illness or condition may make the

animals more susceptible to secondary

infections with Salmonella.

In 1996, a higher percentage of cows scheduled

for culling were culture positive for Salmonella

compared with healthy cows. In Dairy 2007,

there was no difference in Salmonella

prevalence between cows scheduled for culling

and healthy cows.

h. Percentage of cows fecal-culture positive for Salmonella, by cow status 

Cow Status Dairy 1996 Dairy 2002 Dairy 2007 

Healthy 5.4 (194/3,585) 7.1 (259/3,645) 13.7 (523/3,804) 

Sick 7.3 (4/55) 30.8 (8/26) 18.2 (40/220) 

Scheduled 

for culling 18.1 (121/668) 0.0 (0/17) 12.6 (17/135) 

All 7.4 (319/4,308) 7.2 (267/3,688) 13.9 (580/4,159) 
USDA APHIS VS / 27

Section II: Pathogen Detection in Feces—A. Salmonella

Longitudinal studies with repeated sampling

suggest that Salmonella can be found on almost

all dairy operations. A study in which 110

dairies were sampled 5 times over the course of

1 year found 92.7 percent of operations to be

culture positive for Salmonella (Fossler et al.,

2004). In that study, between 31 and 55 percent

of farms were positive on a per-visit basis and

25.0 percent of dairies accounted for 75 percent

of the Salmonella-positive samples, implying

that a relatively small percentage of dairy

operations account for a majority of Salmonellapositive cattle.

Each of the three NAHMS dairy studies

sampled operations at a single point in time, and

the majority of operations were negative when

tested for Salmonella. On many of the

operations that tested positive for Salmonella,

less than 10 percent of the cows sampled tested

positive. Among culture-positive operations in

1996, 2002, and 2007, the median within-herd

prevalence was 6.4, 10.3, and 21.9 percent,

respectively. Among culture-positive operations

in 1996, 2002, and 2007, the 75th percentile for

within-herd prevalence was 40.0, 34.6, and

60.0 percent, respectively.

In each of the NAHMS dairy studies,

approximately 10 percent of the sampled

operations accounted for 75 percent or more of

the positive samples from healthy cows. In

1996, 4 of the 90 operations accounted for

77.8 percent of the positive samples from

healthy cows; in 2002, 9 of the 97 operations

accounted for 74.5 percent of the positive

samples from healthy cows; and in 2007, 16 of

the 121 operations accounted for 74.6 percent of

the positive samples from healthy cows.

i. Number of operations by within-herd prevalence of Salmonella* 

Within-herd 

Prevalence Dairy 1996 Dairy 2002 Dairy 2007 

0.0 72 67 73 

0.1 to 10.0 11 16 20 

10.1 to 20.0 1 2 4 

20.1 to 30.0 1 3 4 

30.1 to 40.0 1 3 3 

40.1 to 50.0 0 1 2 

50.1 to 60.0 0 3 4 

60.1 to 70.0 1 0 2 

70.1 to 80.0 0 0 1 

80.1 to 90.0 3 1 4 

90.1 to 100.0 0 1 4 

Total 90 97 121 

*Only cows healthy at the time of collection were included. 
28 / Dairy 2007

Section II: Pathogen Detection in Feces—A. Salmonella

3. Serotypes The following table shows the number of

operations on which each serotype was

identified from at least one cow (i.e., herd-level

results). Cerro and Kentucky were the most

common serotypes isolated from operations in

Dairy 2007. S. Montevideo, one of the most

common serotypes identified in the 1996, 2002,

and 2007 NAHMS studies, has been among the

top 10 identified from humans every year from

1996 through 2006 (CDC, 2008b).

a. Number of operations on which the following Salmonella serotypes were identified1

Dairy 1996 Dairy 2002 Dairy 2007 

Serotype2 Number of Operations 

(90 Sampled)

Number of Operations

(97 Sampled)

Number of Operations 

(121 Sampled)

Cerro 2 2 14 

Kentucky 3 8 14 

Muenster 2 3 8 

Meleagridis 4 5 6 

Montevideo 5 8 6 

Untypable 1 5 6 

Typhimurium3 2 3 4 

Mbandaka 4 5 3 

Anatum3 3 2 2 

Agona 1 3 2 

Bovismorbificans 1 0 2 

Newport 0 5 2 

Senftenberg 1 4 2 

Derby 0 0 1 

Fresno 0 0 1 

Infantis 0 0 1 

Muenchen 3 0 1 

Saintpaul 0 0 1 

Thompson 0 1 1 

Give 2 2 0 

Barranquilla 0 1 0 

Cubana 0 1 0 

Hartford 0 1 0 

Livingstone 0 1 0 

Newington 1 1 0 

Ohio 0 1 0 

Oranienburg 0 1 0 

Reading 0 1 0 

San Diego 0 1 0 

Tennessee 0 1 0 

Uganda 0 1 0 

Worthington 2 0 0 

Enteritidis 1 0 0 

Menhaden 1 0 0 

New Brunswick 1 0 0 

Albany 1 0 0 

Havana 1 0 0 

Niakhar 1 0 0 

Dublin 1 0 0 

1

Only cows healthy at the time of collection are included. 

2

Listed in order by rank for Dairy 2007 study. 

3

Includes variant species (e.g., Typhimurium var. 5-, formerly Typhimurium var. Copenhagen). 
USDA APHIS VS / 29

Section II: Pathogen Detection in Feces—A. Salmonella

The top 10 serotypes identified from Salmonella

isolates for each of the three NAHMS dairy

studies are listed in the following table.

Serotypes not among the top 10 were grouped

into the “other” category. Three serotypes—

Meleagridis, Montevideo, and Kentucky—

ranked in the top five serotypes indentified in

1996, 2002, and 2007. The top 10 account for

81.1, 82.7, and 94.6 percent of total isolates in

1996, 2002, and 2007, respectively.

b. Number of Salmonella isolates from healthy cows, by serotype 

Dairy 1996 Dairy 2002 Dairy 2007 

Serotype 

No. Isolates 

(n=228) Serotype 

No. Isolates 

(n=283) Serotype 

No. Isolates 

(n=556)

Montevideo 49 Meleagridis 71 Cerro 157 

Kentucky 29 Montevideo 34 Kentucky 130 

Menhaden 27 Typhimurium* 29 Montevideo 66 

Cerro 17 Kentucky 28 Mbandaka 47 

Meleagridis 16 Agona 21 Meleagridis 40 

Mbandaka 12 Mbandaka 12 Derby 27 

Anatum 11 Ohio 12 Muenster 18 

New Brunswick 9 Senftenberg 11 Anatum 17 

Muenster 8 Cerro 8 Senftenberg 13 

Albany 7 Newport 8 Newport 11 

Other 43 Other 49 Other 30 

*Includes variant species (e.g., Typhimurium var. 5-, formerly Typhimurium var. Copenhagen). 
30 / Dairy 2007

Section II: Pathogen Detection in Feces—A. Salmonella

4. Comparison of

serotypes isolated

from cattle and

humans

The following tables (a. through c.) compare

Salmonella serotypes identified from cattle and

humans in 1996, 2002, and 2007. Serotype data

on healthy cows were taken from the respective

NAHMS studies. Data on clinically ill cows

originated from diagnostic samples submitted to

the National Veterinary Services Laboratories

(NVSL). Serotype data on humans were

provided by the CDC through the Public Health

Laboratory Information System. Salmonellosis

is on the CDC’s list of Nationally Notifiable

Infectious Diseases.

The two most common serotypes identified in

humans were S. Typhimurium and S. Enteritidis

in 1996, 2002, and 2007. Sources of individual

cases of salmonellosis in humans are often not

identified, and the role of livestock in human

cases of salmonellosis is unknown. There are

many avenues other than food of animal origin,

such as produce, by which people can get sick.

Poultry is generally considered the most

common source of salmonellosis in humans

from S. Enteritidis. Hogs were the most common

source of S. Typhimurium isolates among

clinical animal submissions to NVSL in the most

recent report in 2006. S. Montevideo was the

only serotype that ranked among the 10 most

common serotypes found in healthy cows,

clinically ill cows, and humans in 1996, 2002,

and 2007. However, S. Montevideo was a

relatively uncommon serotype in humans,

making up only 2 to 3 percent of isolates

identified from humans in 1996, 2002, and

2007. S. Typhimurium was among the two most

common serotypes identified in clinically ill

cattle and humans in all 3 years but was

uncommon among healthy cows. A recent study

in which dairy herds were monitored for

approximately 1 year for clinical signs of

salmonellosis found S. Newport and

S. Typhimurium to be the most common

serotypes identified (Cummings, 2009b), which

coincides with the NVSL results on clinically ill

cattle. S. Typhimurium and S. Newport were

among the four most common serotypes

identified from humans in all 3 years. Clinically

affected cattle may pose a greater threat to

public health than healthy cattle (Cummings et

al., 2009b) However, these serotype data alone

do not provide sufficient evidence of

transmission of Salmonella from cattle to

humans.
USDA APHIS VS / 31

Section II: Pathogen Detection in Feces—A. Salmonella

a. Number of Salmonella isolates from healthy dairy cows, clinically affected cattle, and 

humans in 1996, by serotype 

Healthy Cows (NAHMS) Clinical Cattle (NVSL)1 2 Humans (CDC)2 

Serotype 

No. Isolates 

(n=228) Serotype 

No. Isolates 

(n=4,183) Serotype 

No. Isolates 

(n=39,035)

Montevideo 49 Typhimurium3 1,081 Enteritidis 9,570 

Kentucky 29 Montevideo 589 Typhimurium3 9,501 

Menhaden 27 Cerro 239 Heidelberg 1,998 

Cerro 17 Kentucky 230 Newport 1,985 

Meleagridis 16 Anatum 228 Montevideo 1,227 

Mbandaka 12 Dublin 213 Javiana 749 

Anatum 11 Muenster 201 Oranienburg 690 

New Brunswick 9 Meleagridis 172 Hadar 658 

Muenster 8 Menhaden 118 Agona 606 

Albany 7 Give 118 Muenchen 595 

Other 43 Other 994 Other 11,456 

1

Serotypes are from beef and dairy sources. NVSL typically receives diagnostic samples from clinically ill cattle, but they 

may not be exclusively from ill cattle.

2

From the Salmonella Annual Summaries published by the CDC at 

http://www.cdc.gov/ncidod/dbmd/phlisdata/Salmonella.htm 3

Includes variant species (e.g., Typhimurium var. 5-, formerly Typhimurium var. Copenhagen). 
32 / Dairy 2007

Section II: Pathogen Detection in Feces—A. Salmonella

b. Number of Salmonella isolates from healthy dairy cows, clinically affected cattle, and 

humans in 2002, by serotype 

Healthy Cows (NAHMS) Clinical Cattle (NVSL)1 2 Humans (CDC)2

Serotype 

No. Isolates 

(n=283) Serotype 

No. Isolates 

(n=2,674) Serotype 

No. Isolates 

(n=32,308)

Meleagridis 71 Newport 769 Typhimurium3 7,062 

Montevideo 34 Typhimurium3 583 Enteritidis 5,116 

Typhimurium3 29 Dublin 136 Newport 4,204 

Kentucky 28 Agona 124 Heidelberg 1,957 

Agona 21 Montevideo 115 Javiana 1,188 

Mbandaka 12 Uganda 91 Montevideo 717 

Ohio 12 Anatum 89 Muenchen 591 

Senftenberg 11 Muenster 87 Oranienburg 585 

Cerro 8 Kentucky 70 Saintpaul 535 

Newport 8 Mbandaka 54 Infantis 463 

Other 49 Other 556 Other 9,890 

1

Serotypes are from beef and dairy sources. NVSL typically receives diagnostic samples from clinically ill cattle, but they 

may not be exclusively from ill cattle.

2

From the Salmonella Annual Summaries published by the CDC at 

http://www.cdc.gov/ncidod/dbmd/phlisdata/Salmonella.htm 3

Includes variant species (e.g., Typhimurium var. 5-, formerly Typhimurium var. Copenhagen).
USDA APHIS VS / 33

Section II: Pathogen Detection in Feces—A. Salmonella

c. Number of Salmonella isolates from healthy dairy cows, clinically affected cattle, and 

humans in 2007, by serotype 

Healthy Cows (NAHMS) Clinical Cattle (NVSL)1 2 Humans (CDC)2

Serotype 

No. Isolates 

(n=556) Serotype 

No. Isolates 

(n=3,770) Serotype 

No. Isolates 

(n=40,666)

Cerro 157 Newport 436 Typhimurium3 6,872 

Kentucky 130 Typhimurium3 425 Enteritidis 6,740 

Montevideo 66 Orion var. 

15+,34+ 365 Newport 3,373 

Mbandaka 47 Dublin 335 Heidelberg 1,495 

Meleagridis 40 Montevideo 293 Javiana 1,433 

Derby 27 Agona 239 I 4,[5],12:i- 1,200 

Muenster 18 Anatum 210 Montevideo 1,061 

Anatum 17 Kentucky 164 Muenchen 753 

Senftenberg 13 Muenster 163 Oranienburg 719 

Newport 11 Cerro 155 Mississippi 604 

Other 30 Other 985 Other 16,416 

1

Serotypes are from beef and dairy sources. NVSL typically receives diagnostic samples from clinically ill cattle, but they 

may not be exclusively from ill cattle.

2

From the Salmonella Annual Summaries published by the CDC at 

http://www.cdc.gov/ncidod/dbmd/phlisdata/Salmonella.htm 3

Includes variant species (e.g., Typhimurium var. 5-, formerly Typhimurium var. Copenhagen). 
34 / Dairy 2007

Section II: Pathogen Detection in Feces—A. Salmonella

5. Antimicrobial

susceptibility

Salmonella isolates from healthy cows showed

relatively little resistance to antimicrobial agents

in 1996, 2002, and 2007. Of all Salmonella

isolates found in healthy cows and tested for

antimicrobial susceptibility in 1996, 2002, and

2007, 92.3, 82.3, and 96.6 percent, respectively,

were susceptible to all antimicrobials tested. In

each of the studies, no more than 5 percent of

Salmonella isolates from healthy cows were

resistant to two or more antimicrobials.

Photo of S. Typhimurium courtesy of Agriculture Research Service

a. Percentage of Salmonella isolates by number of antimicrobials in which antimicrobial 

resistance1 was observed2

Study 

(n=Number of Isolates)

Susceptible 

to All 

Antimicrobials 

Resistant to 

a Single 

Antimicrobial 

Resistant to Two 

or More 

Antimicrobials Total 

Dairy 1996 (n=220) 92.3 3.6 4.1 100.0 

Dairy 2002 (n=283) 82.3 12.7 5.0 100.0 

Dairy 2007 (n=556) 96.6 0.7 2.7 100.0 

1

Intermediate isolates were classified as susceptible. 

2

Only cows healthy at the time of sample collection are included. 
USDA APHIS VS / 35

Section II: Pathogen Detection in Feces—A. Salmonella

Resistance to amikacin, ciprofloxacin, nalidixic

acid, and trimethoprim-sulfamethoxazole was

not observed in any of the three dairy studies.

Resistance to ceftriaxone was observed in Dairy

2002 and Dairy 2007, but it was observed in

only one isolate in Dairy 2007. Salmonella

resistance to ceftriaxone is of interest because it

is commonly used to treat severe Salmonella

infections in children (Zhao et al., 2003).

b. Percentage of resistant1 isolates from all Salmonella isolates tested for antimicrobial 

susceptibility, by antimicrobial2

Antimicrobial 

Dairy 1996 

(n=220)

Dairy 2002 

(n=283)

Dairy 2007 

(n=556)

Amikacin (AMI) 0.0 0.0 0.0 

Amoxicillin-clavulanic acid 

(AMO) 0.9 4.9 1.8 

Ampicillin (AMP) 3.6 4.6 2.2 

Apramycin (APR) 0.0 NA NA 

Cefoxitin (FOX) NA 3.9 1.6 

Ceftiofur (TIO) 0.0 4.6 2.0 

Ceftriaxone (AXO) 0.0 2.5 0.2 

Cephalothin (CEP) 2.3 4.9 NA 

Chloramphenicol (CHL) 1.4 4.6 2.5 

Ciprofloxacin (CIP) 0.0 0.0 0.0 

Gentamicin (GEN) 0.0 0.7 0.2 

Kanamycin (KAN) 1.4 0.7 0.0 

Nalidixic acid (NAL) 0.0 0.0 0.0 

Streptomycin (STR) 4.1 9.9 2.7 

Sulfamethoxazole3 (SUL) 1.8 3.9 2.3 

Tetracycline (TET) 2.3 12.4 3.1 

Ticarcillin (TIC) 3.2 NA NA 

Trimethoprimsulfamethoxazole (TRI) 0.0 0.0 0.0 

1

Intermediate isolates were classified as susceptible. Resistance break points were those current at the time of 

sample collection. Break points for extended spectrum cephalosporins changed in 2010 and testing was done prior to 

this change. 

2

Only cows healthy at the time of collection are included. 

3

Sulfisoxazole replaced sulfamethoxazole in 2007. 
36 / Dairy 2007

Section II: Pathogen Detection in Feces—A. Salmonella

6. Multidrug

resistance patterns

There were 53 multidrug-resistant isolates

identified over the 3 study years. In 1996 and

2002, more S. Typhimurium isolates were

resistant to multiple drugs compared with other

serotypes. In 2007, however, no multidrugresistant S. Typhimurium was observed. Dairy

2007 was the first NAHMS dairy study in which

multidrug resistance was observed in

S. Montevideo, which was one of the top three

serotypes identified in each of the previous

NAHMS studies.

Number of multidrug-resistant isolates by serotype and by resistance pattern1 

Serotype Resistance Pattern2

Dairy 

1996 

Isolates 

(n=356)

Dairy 

2002 

Isolates 

(n=291)

Dairy 

2007 

Isolates 

(n=620)

Agona AMO, AMP, FOX, TIO, CHL, 

KAN, STR, SUL, TET, TRI 0 0 1 

CHL, STR, SUL, TET 0 0 2 

Albany AMO, AMP, CEP, TIC 1 0 0 

Anatum AMO, CEP 1 0 0 

Cerro CHL, TET 0 0 1 

Dublin AMP, CHL, KAN, STR, SUL, TET, TIC 2 0 0 

Kentucky AMP, CEP, TIC 1 0 0 

Mbandaka AMO, CEP, TET 0 1 0 

 AMP, CEP, TIC 1 0 0 

Menhaden AMO, CEP 1 0 0 

Montevideo AMO, AMP, FOX, TIO, 

CHL, STR, SUL, TET 0 0 8 

AMO, AMP, TIO, CHL, STR, SUL, TET 0 0 1 

 AMP, TIO, CHL, STR, SUL, TET 0 0 1 

Muenster AMP, CEP, TIC 1 0 0 

Newport AMO, AMP, FOX, TIO, AXO, 

CHL, STR, SUL, TET 0 0 1 

AMO, AMP, FOX, TIO, CEP, CHL, 

GEN, KAN, STR, SUL, TET 0 1 0 

AMO, AMP, FOX, TIO, CEP, 

CHL, STR, SUL, TET 0 5 0 

AMO, AMP, FOX, TIO, CEP, 

CHL, STR, TET 0 1 0 

AMO, AMP, FOX, TIO, CHL, 

STR, SUL, TET 0 0 8 

Reading AMO, AMP, FOX, TIO, CEP, CHL, 

GEN, KAN, STR, SUL, TET 0 1 0 

AMO, AMP, FOX, TIO, CHL, 

STR, SUL, TET 0 0 4 

Saintpaul AMP, GEN, TET 0 0 1 

Typhimurium AMO, AMP, CEP, CHL, GEN, KAN, 

STR, SUL, TET, TIC, TRI 1 0 0 

AMO, AMP, FOX, TIO, CEP, 

CHL, STR, SUL, TET 0 2 0 

AMO, AMP, FOX, TIO, CEP, 

CHL, STR, TET 0 1 0 

AMO, AMP, TIO, CEP, CHL, 

STR, SUL, TET 0 2 0 

 AMP, CHL, SUL, TET, TIC 1 0 0 

AMP, KAN, STR, SUL, TET, TIC 1 0 0 

Total multidrugresistant isolates 11 14 28 

1

Healthy, sick, and to-be-culled cows are included. 

2

See previous table for the full name of the antimicrobial corresponding to the abbreviations listed here. 
USDA APHIS VS / 37

Section II: Pathogen Detection in Feces—B. Campylobacter

B. Campylobacter

1. Background Campylobacter is recognized as a major cause

of acute bacterial gastroenteritis in humans

worldwide, comparable with or even surpassing

Salmonella (Friedman et al., 2000). Mead et al.

(1999) estimated that in the United States there

are approximately 2.5 million cases of

Campylobacter jejuni (C. jejuni) infections each

year, 80 percent of which are food related.

Campylobacter coli (C. coli) was estimated to

cause approximately 26,000 cases in 2000 (Tam

et al., 2003). Human cases of campylobacteriosis in the United States are estimated to

cost $18.8 billion annually (Scharff, 2010).

Typical signs of Campylobacter infection in

humans include abdominal cramping, vomiting,

fever, and diarrhea (with or without blood),

lasting from several days to more than a week

(Skirrow and Blaser, 2000). Of individuals that

recover from the disease, 20 percent may

relapse or experience prolonged or severe

illness requiring antimicrobial treatment. The

disease is rarely fatal, and only about 10 percent

of infected individuals are hospitalized.

The recently recognized association between

development of Guillain-Barré syndrome in

humans and prior C. jejuni infection, along with

other sequelae, has increased the level of public

health concern for this pathogen. Guillain-Barré

syndrome is an autoimmune disease of the

nervous system that can result in paralysis, pain,

and muscle wasting; it has an annual incidence

of about 2 in 100,000 persons in the United

States (Allos, 2001). An estimated 0.1 percent of

reported Campylobacter illnesses result in

Guillain-Barré syndrome (CDC, 2010).

C. jejuni and C. coli, commonly found in the

intestinal tracts of food animals, are the most

frequently isolated Campylobacter species

found in cases of human infection (Engberg et

al., 2000). Poultry and poultry products have

been documented as a major source of

Campylobacter infection in humans (Corry and

Atabay, 2001). Beef and dairy cattle are also

common carriers of Campylobacter (Atabay and

Corry, 1998; Wesley et al., 2000; Stanley and

Jones, 2003; Bae et al., 2005). Young animals

are more often colonized than older animals

(Sato et al., 2004). Feedlot cattle are more likely

than grazing cattle to carry Campylobacter

(Giacoboni et al., 1993; Beach et al., 2002).

Although Campylobacter species can be

considered commensal organisms or normal

flora in livestock, they can produce clinical

disease with diarrhea in neonatal calves and may

cause abortion, infertility, and early embryonic

death (Wesley et al., 2000; Smith, 2002).

Campylobacter spp. has been identified from

many livestock species. Although cattle can be

colonized by C. coli, C. jejuni is the most

common Campylobacter species isolated in

cattle (Wesley et al., 2000; Harvey et al., 2004;

Bae et al., 2005).

Foodborne transmission of Campylobacter can

occur through fecal contamination of carcasses

at slaughter, although Campylobacter is not

frequently isolated from cattle carcasses or fresh

beef (Minihan et al., 2004; Whyte et al., 2004;

Hakkinen et al., 2007). Fecal contamination of

milk or water is another potential route of

human exposure (CDC, 2002; Clark et al.,

2003). Unpasteurized milk has emerged as a risk
38 / Dairy 2007

Section II: Pathogen Detection in Feces—B. Campylobacter

factor for human campylobacteriosis in

epidemiological studies (Jacobs-Reitsma, 2000;

Studahl and Andersson, 2000; Neimann et al.,

2003), and numerous outbreaks of human

Campylobacter infection have occurred through

consumption of raw dairy products (Evans et al.,

1996; Altekruse et al., 1999; Schildt et al.,

2006).

2. Sampling and

testing overview

NAHMS has examined Campylobacter

occurrence using individual fecal samples from

dairy cows in three separate studies: Dairy 1996,

Dairy 2002, and Dairy 2007. Typically,

NAHMS studies generate population-based

estimates, and appropriate sample sizes are used

to arrive at such estimates. Field resources,

laboratory capacity, and the expense of culturing

samples make it difficult to provide a national

estimate of Campylobacter prevalence based on

fecal culturing of individual animals. Therefore,

for the Campylobacter estimates in this section,

all three NAHMS dairy studies used a sample of

approximately 100 operations, which is not an

optimal sample size for providing national

estimates of prevalence. Despite this limitation,

the NAHMS studies provide valuable

information on Campylobacter occurrence and

antimicrobial susceptibility patterns on

U.S. dairies and represent the only national

examination of Campylobacter on dairy

operations in the United States. Other research

studies have examined Campylobacter

occurrence in dairy cattle but have been limited

to specific regions of the United States.

Campylobacter monitoring in the NAHMS

studies focused on C. jejuni and C. coli because

these species are most commonly associated

with human disease. Each of the three studies

investigated the prevalence of Campylobacter

on U.S. dairy operations. Antimicrobial

susceptibility patterns of the Campylobacter

isolates were evaluated in 2002 and 2007. At the

time of sampling, records were kept as to

whether each cow was sick, healthy (from the

milking string), scheduled for culling (within

7 days of leaving the operation), or dry (dry

cows were sampled only in Dairy 1996 and

Dairy 2002; dry cows were grouped with

healthy cows in the following estimates). Dairy

1996 compared the prevalence of

Campylobacter in milk cows on-farm to that of

milk cows on-farm scheduled for culling within

7 days, and to cull cows at markets. To allow for

comparisons across the three studies, results

presented in this report focused primarily on

healthy cows.

The methods used to identify samples as

Campylobacter positive varied across the three

NAHMS studies. Dairy 1996 used PCR

methods, and Dairy 2002 and Dairy 2007 used

culture and PCR methods. A different PCR was

used in Dairy 1996 than was used in Dairy 2002

and Dairy 2007. These differences in

identification methods should be noted when

interpreting Campylobacter results for the

NAHMS dairy studies. Because Dairy 1996

identification methods were limited to PCR

only, no antimicrobial susceptibility testing was

performed. In addition, the PCR test from Dairy

1996 identified isolates with a 460-bp fragment

as C. coli and isolates with both 160- and
USDA APHIS VS / 39

Section II: Pathogen Detection in Feces—B. Campylobacter

460-bp fragments as C. jejuni. Because of the

overlap between species at 460 bp, it could not

be determined whether any samples were

positive for both C. jejuni and C. coli. There

were only 14 C. coli isolates in Dairy 1996, and

for the purposes of this report it was assumed

that no samples were positive for both C. jejuni

and C. coli. For Dairy 2002, isolates were

characterized as presumptive positive based on

culture and microscopy, with PCR being used to

confirm isolates as Campylobacter and to

determine species. However, PCR was

performed on only a subset of the presumptivepositive isolates. For Dairy 2007, species

identification was performed on all positive

isolates, and antimicrobial susceptibility testing

was performed on all viable C. jejuni and C. coli

isolates.

Photo of C. jejuni courtesy of CDC.
40 / Dairy 2007

Section II: Pathogen Detection in Feces—B. Campylobacter

Campylobacter fecal sampling methods, by study 

Study 

Number of 

Operations 

Sampled 

Sampling 

Period 

Number of 

Samples per 

Operation Notes 

Dairy 

1996 

31 dairy 

operations/ 

17 States 

Feb. 26 to 

July 10, 

1996 

40 or 50, 

depending on 

herd size, plus 

all to-be-culled 

cows 

All samples were taken rectally from 

individual cows. There were no specific 

targets as to the number of sick, dry, or 

milking string cows, other than the sample 

was to be representative of the cows on hand 

on the day of the visit. Cow type was noted at 

the time of collection. 

Dairies with 30–99 cows: Operations were 

visited once. Up to 40 fecal samples were 

collected, which included samples from all 

cows scheduled for culling present on the day 

of the visit. 

Dairies with 100 or more cows: Operations 

were visited three times. During one visit, 50 

cows (from milking string, dry, or sick) were 

sampled along with up to 20 cows scheduled 

for culling in the next 7 days. On 2 other 

visits, up to 20 samples were taken from cows 

scheduled for culling in the next 7 days. 

36 dairy cull 

cow markets/ 

14 States 

Feb. 26 to 

July 10, 

1996 

25 

Twenty-five fresh fecal samples per market—

either by rectal retrieval from individual cows 

or from pen floors if restraining facilities were 

not available. 

Dairy 

2002 

97 dairy 

operations/ 

21 States 

Mar. 27 to 

Sept. 25, 

2002 

15 

The goal was to collect 15 individual fecal 

samples during a single visit, all via rectal 

retrieval. There were no specific targets for 

number of sick, dry, or milking string cows to 

be sampled, but cow type was noted at the 

time of collection. 

Dairy 

2007 

121 dairy 

operations/ 

17 States 

Feb. 28 to 

Aug. 30, 

2007 

17 

All samples were taken via rectal retrieval 

from individual cows at a single visit. The goal 

was to collect 17 to 18 samples per operation. 

Sampling for Salmonella was performed at 

the same time, with samples numbered from 

1 to 35. While all 35 samples were tested for 

Salmonella, either the odd or even sample 

numbers were tested for Campylobacter. 

There were no specific targets as to the 

number of sick cows or cow scheduled for 

culling to be tested per operation for 

Campylobacter. However, because there 

were specific goals for testing these cow 

groups for Salmonella, in general 2 to 3 sick 

cows and 2 to 3 cows scheduled for culling 

(within 7 days of leaving the operation) were 

tested for Campylobacter, with the remainder 

up to 18 being from cows with saleable milk 
USDA APHIS VS / 41

Section II: Pathogen Detection in Feces—B. Campylobacter

3. Prevalence In 1996, Campylobacter was detected in at least

one cow on all 31 sampled operations. In 2002,

97.9 percent of operations sampled had at least

one cow shedding Campylobacter in feces. In

2007, 92.6 percent of operations had at least one

cow shedding Campylobacter in feces.

a. Percentage of operations and percentage of healthy cows fecal-culture positive 

for Campylobacter 

Study Operations1 Cows 

Dairy 19962 100.0 (31/31) 44.1 (505/1,144) 

Dairy 20023 97.9 (95/97) 51.4 (732/1,424) 

Dairy 2007 92.6 (112/121) 33.7 (635/1,885) 

1

Operations with at least one positive cow were considered positive. 

2

Only milk cow or cull cow was recorded for Campylobacter results for Dairy 1996, so a few operations might 

have had a small number of sick cows sampled. 

3

Data for 2002 were presumptive positives based on culture and microscopy. Confirmatory testing was 

performed only on a subset of these presumptive-positive isolates. 
42 / Dairy 2007

Section II: Pathogen Detection in Feces—B. Campylobacter

C. jejuni was found on all of the

Campylobacter-positive operations from the

1996, 2002, and 2007 studies for which species

identification was performed. In contrast, C. coli

was found on 19.4 to 39.8 percent of

Campylobacter-positive operations during the

three NAHMS studies.

For Dairy 2002 and Dairy 2007, a slightly

higher percentage of sick cows were fecalculture positive for Campylobacter compared

with other cow types.

b. Of the Campylobacter isolates tested for species identification, percentage of 

operations1 and percentage of healthy cows fecal-culture positive for C. jejuni or 

C. coli 

C. jejuni C. coli

Study Percent Cows Percent Cows 

Dairy 19962 100.0 (31/31) 97.2 (491/505) 19.4 (6/31) 2.8 (14/505) 

Dairy 20023 100.0 (93/93) 89.1 (465/522) 39.8 (37/93) 10.9 (57/522) 

Dairy 2007 100.0 (112/112) 90.1 (554/6154) 25.0 (28/112) 9.3 (57/6154)

 

1

Operations with at least one positive cow were considered positive. 

2

Only milk cow or cull cow was recorded for Campylobacter results for Dairy 1996, so a few operations may have had a 

small number of sick cows sampled. 

3

Species identification was performed on a subset of presumptive-positive isolates from Dairy 2002. 

4

Four of the 615 isolates in 2007 were C. lari. Twenty isolates were nonviable at the time of species identification, and 

these are not included in the isolates listed here. 

c. Percentage of cows fecal-culture positive for Campylobacter, by cow type 

Cow Type 

Study Healthy Sick 

Scheduled 

for Culling All Cows 

Dairy 2002 51.4 (732/1,424) 56.3 (9/16) 40.0 (2/5) 51.4 (743/1,445) 

Dairy 2007 33.7 (635/1,885) 46.4 (51/110) 35.4 (28/79) 34.4 (714/2,074) 
USDA APHIS VS / 43

Section II: Pathogen Detection in Feces—B. Campylobacter

During all three study years, a lower percentage

of cows on small operations than on large

operations were fecal-culture positive for

Campylobacter.

In 1996, the percentage of cows PCR positive

for Campylobacter was similar in the East and

West regions. In 2002 and 2007, a slightly

higher percentage of cows in the West region

were culture positive for Campylobacter

compared with cows in the East region.

d. Percentage of healthy cows fecal-culture positive for Campylobacter, by herd 

size 

Herd Size (Number of Cows)

Study

Small 

(Fewer than 100)

Medium 

(100–499)

Large 

(500 or More)

Dairy 1996* 38.5 (150/390) 45.2 (208/460) 50.0 (147/294) 

Dairy 2002 43.7 (211/483) 49.2 (287/583) 65.4 (234/358) 

Dairy 2007 22.1 (133/603) 36.3 (269/742) 43.1 (233/540) 

*Only milk cow or cull cow was recorded for Campylobacter results for Dairy 1996, so a few operations may 

have had a small number of sick cows sampled. 

e. Percentage of healthy cows fecal-culture positive for Campylobacter, by region 

Region 

Study East West 

Dairy 1996* 44.6 (323/724) 43.3 (182/420) 

Dairy 2002 46.9 (477/1,018) 62.8 (255/406) 

Dairy 2007 31.7 (507/1,597) 44.4 (128/288) 

*Only milk cow or cull cow was recorded for Campylobacter results for Dairy 1996, so a few operations may 

have had a small number of sick cows sampled. 
44 / Dairy 2007

Section II: Pathogen Detection in Feces—B. Campylobacter

In 1996, 2002, and 2007 over 90 percent of

operations were positive for Campylobacter.

Among all operations tested, the median withinherd prevalence in 1996, 2002, and 2007 was

42.9, 58.3, and 30.8 percent, respectively.

Among all operations, the top quartile withinherd prevalence was 64.0, 73.3, and

52.9 percent in 1996, 2002, and 2007,

respectively.

The within-herd prevalence for Salmonella and

Campylobacter in healthy cows differed greatly

on dairy operations. The majority of operations

were Salmonella negative, and the highest

percentage of positive herds had a within-herd

prevalence of 10 percent or less. In contrast, for

Campylobacter most operations were

Campylobacter positive, and most positive

herds had a within-herd prevalence of over

10 percent.

f. Number of operations by within-herd prevalence of Campylobacter1

Within-herd Prevalence Dairy 1996 Dairy 2002 Dairy 2007 

0.0 0 2 9 

0.1–10.0 1 4 7 

10.1–20.0 3 7 24 

20.1–30.0 6 7 20 

30.1–40.0 5 17 18 

40.1–50.0 5 6 14 

50.1–60.0 4 22 11 

60.1–70.0 6 7 8 

70.1–80.0 0 15 7 

80.1–90.0 0 4 2 

90.1–100.0 1 6 1 

Total 31 97 121 
USDA APHIS VS / 45

Section II: Pathogen Detection in Feces—B. Campylobacter

4. Antimicrobial

susceptibility

Antimicrobial susceptibility testing was

conducted on a subset of Campylobacter

isolates from Dairy 2002 and on all isolates

from Dairy 2007. Antimicrobial susceptibility

testing was not performed in Dairy 1996. In

Dairy 2002, 49.2 percent of the C. jejuni

isolates from healthy cows were susceptible to

all antimicrobials against which they were

tested. In Dairy 2007, 37.1 percent of the

C. jejuni isolates from healthy cows were

susceptible to all antimicrobials. A relatively

low percentage of isolates were resistant to two

or more antimicrobials.

a. Percentage of C. jejuni isolates by number of antimicrobials in which antimicrobial 

resistance1 was observed2

Study 

(n=Number of Isolates)

Susceptible to All 

Antimicrobials 

Resistant to a 

Single 

Antimicrobial 

Resistant to 

Two or More 

Antimicrobials Total 

Dairy 2002 (n=465) 49.2 47.4 3.4 100.0 

Dairy 2007 (n=553) 37.1 60.9 2.0 100.0 

1

Intermediate isolates were classified as susceptible. 

2

Only cows healthy at the time of collection are included. 
46 / Dairy 2007

Section II: Pathogen Detection in Feces—B. Campylobacter

Of the antimicrobials in the table below,

ciprofloxacin and erythromycin are especially

important because they are often used when

treatment is indicated for Campylobacter

infection in humans (Gupta et al., 2004). Very

few of the C. jejuni isolates were resistant to

ciprofloxacin or erythromycin in 2002 and

2007. The highest percentages of C. jejuni

isolates were resistant to tetracycline in 2002

and 2007 (47.5 and 62.4 percent, respectively).

b. Percentage of resistant1 isolates from all C. jejuni isolates tested for 

antimicrobial susceptibility, by antimicrobial2

Antimicrobial Dairy 2002 (n=465) Dairy 2007 (n=553)

Azithromycin (AZI) 0.9 0.4 

Ciprofloxacin (CIP) 2.6 1.3 

Chloramphenicol (CHL) 0.0 NA 

Clindamycin (CLI) 0.6 0.2 

Erythromycin (ERY) 0.4 0.4 

Florfenicol (FLO) NA 0.0 

Gentamicin (GEN) 0.2 0.0 

Nalidixic acid (NAL) 4.1 1.6 

Telithromycin (TEL) NA 0.0 

Tetracycline (TET) 47.5 62.4 

1

Intermediate isolates were classified as susceptible. 

2

Only cows healthy at the time of collection are included. 
USDA APHIS VS / 47

Section II: Pathogen Detection in Feces—B. Campylobacter

5. Multidrug

resistance patterns

The table below shows resistance patterns for

C. jejuni and C. coli isolates from all cow types

for 2002 and 2007. No isolates were resistant to

more than four antimicrobials. No isolates were

resistant to both ciprofloxacin and erythromycin.

Number of Campylobacter isolates, by resistance pattern1

Species Resistance Pattern2 

Dairy 2002 

Isolates 

Dairy 2007 

Isolates 

jejuni 

AZI, CLI, ERY, TET 0 1 

AZI, CLI, ERY, NAL 1 0 

AZI, CLI, TET 1 0 

AZI, ERY, TET 1 1 

CIP, NAL, TET 6 7 

CIP, NAL 6 3 

NAL, TET 0 2 

AZI, CLI 1 0 

NAL 6 0 

GEN 1 0 

TET 214 381 

Pansusceptible 234 228 

Total 471 623 

coli 

AZI, CLI, ERY, NAL 1 0 

AZI, CLI, ERY, TET 2 1 

AZI, ERY, TET 2 1 

AZI, CLI, NAL 1 0 

CIP, NAL, TET 0 6 

CLI, TET 1 0 

GEN, TET 2 0 

NAL, TET 3 0 

TET 29 32 

Pansusceptible 18 24 

Total 59 64 

1

Healthy, sick, and to-be-culled cows are included. 

2

See preceding table for the full name of the antimicrobials. 
48 / Dairy 2007

Section III: Sampling and Diagnostic Testing—A. Farm Selection

Section III: Sampling and Section III: Sampling and

Diagnos Diagnostic Testing

A. Farm Selection

Dairy 1996

A stratified random sample of dairy operations

from the USDA National Agricultural Statistics

Service (NASS) list frame in each of 20 selected

States,1 representing 80.2 percent of U.S. dairy

operations and 83.1 percent of U.S. dairy cows,

was the basis for selecting participating

operations in the Dairy 1996 study. More than

2,500 and 1,200 dairy producers participated in

Phase I and Phase II of the study, respectively. A

convenience sample of 100 of the 1,200 dairies

was selected for participation in Salmonella

sampling. This sample included 50 dairies with

30 to 99 dairy cows, and 50 dairies with 100 or

more dairy cows. The number of small and large

operations allocated to each State was

proportional to the number of small and large

operations in the State. Cull-cow markets were

also selected for fecal sampling in these

20 States, allocated based on the number of cull

dairy-cow markets within the State. Previous

history of salmonellosis was not a selection

factor. Samples were collected from February 26

to July 10, 1996.

Dairy 2002

A stratified random sample of dairies was

chosen based on herd size from the NASS

listing for each of 21 selected States.2 This

sample represented 85.5 percent of the

U.S. dairy cows and 82.8 percent of U.S. dairy

operations. Dairy operations reporting one or

more milk cows in inventory on January 1,

2002, were eligible for Phase I of the study, and

operations with at least 30 dairy cows were

eligible for Phase II. Participation in the study

included over 2,400 dairy producers in Phase I

and 1,000 producers in Phase II. Of the Phase II

operations, bulk-tank milk samples from

861 operations were collected by Federal and

State veterinary medical officers or animal

health technicians. Samples were collected from

February 27 to July 1, 2002. A convenience

sample of 100 of these operations with at least

30 milk cows was selected for fecal sampling.

Approximately five operations per State were

selected. Previous history of salmonellosis was

not a selection factor. Samples were collected

from March 27 to September 25, 2002.

1

California, Florida, Idaho, Illinois, Indiana, Iowa,

Kentucky, Michigan, Missouri, New Mexico, New York,

Minnesota, Ohio, Oregon, Pennsylvania, Tennessee, Texas,

Vermont, Washington, and Wisconsin.

2

California, Colorado, Florida, Idaho, Illinois, Indiana,

Iowa, Kentucky, Michigan, Minnesota, Missouri, New

Mexico, New York, Pennsylvania, Ohio, Tennessee, Texas,

Vermont, Virginia, Washington, and Wisconsin.
USDA APHIS VS / 49

Section III: Sampling and Diagnostic Testing—A. Farm Selection

Dairy 2007

Data were collected during the NAHMS Dairy

2007 study from dairy operations in 17 major

dairy States3 representing 79.5 percent of

U.S. dairy operations and 82.5 percent of U.S.

dairy cows. The survey design was a stratified

random sample with unequal selection

probabilities within each stratum to ensure that

large dairy operations were well represented in

the sample. Dairy operations reporting one or

more milk cows in inventory on January 1,

2007, were eligible for Phase I of the study, and

3

California, Idaho, Indiana, Iowa, Kentucky, Michigan,

Minnesota, Missouri, New Mexico, New York, Ohio,

Pennsylvania, Texas, Vermont, Virginia, Washington, and

Wisconsin.

operations with at least 30 dairy cows were

eligible for Phase II. Participation in the study

included over 2,194 dairy producers in Phase I

and 582 producers in Phase II. Of the Phase II

operations, bulk-tank milk and in-line milk filter

samples from 538 operations were collected by

Federal and State veterinary medical officers or

animal health technicians. A convenience sample

of 121 of these operations with at least 30 milk

cows was selected for individual cow fecal

sampling. Previous history of salmonellosis was

not a selection factor. Samples were collected

from February 28 to August 29, 2007.

Photo courtesy of Agriculture Research Service
50 / Dairy 2007

Section III: Sampling and Diagnostic Testing—B. Sampling Methods

B. Sampling Methods

1. Bulk-tank milk

and milk filter

sampling

A single bulk-tank milk sample from each

participating operation was collected during the

Dairy 2002 and Dairy 2007 studies and tested

for Listeria and Salmonella. Bulk-tank milk was

not tested for Salmonella or Listeria in Dairy

1996. Milk filters were collected only in 2007.

Dairy 2002 used both culture and PCR methods

for Salmonella detection, but only PCR was

used for Dairy 2007. Bulk-tank milk samples

were aseptically collected only when milk from

at least 70 percent of the operation’s lactating

cows was represented in the sample.

Additionally, for Dairy 2007, milk filters were

collected at the time of sampling. If the milk

filter was not available for removal from the

milk line during the sample visit, farm operators

were requested ahead of time to place the filter

from the most recent milking in a clean plastic

bag and store in the refrigerator. For Dairy

2007, sample collectors were instructed not to

freeze samples. In some cases for Dairy 2002,

the samples were frozen prior to shipping. Bulktank milk and milk filters were shipped

overnight with ice packs to the USDA–

Agricultural Research Service (ARS)

Environmental Microbial Safety Laboratory

(EMSL) in Beltsville, MD.

2. Fecal sampling Each of the three dairy studies had slightly

different objectives with regard to Salmonella

sampling, which led to differences in the types

of cattle sampled. Although the numbers of

samples taken on each operation were similar

across studies, they were not identical. A subset

of samples taken for Salmonella testing during

the three dairy studies was tested for

Campylobacter. Thus, the type of cattle

sampled, the number of cattle sampled, and the

sampling collection methods were the same for

Salmonella and for Campylobacter, but fewer

samples were tested for Campylobacter than for

Salmonella.

Dairy 1996

The Dairy 1996 study set out to determine if

Salmonella prevalence differed among milk

cows on the farm, cows scheduled for culling

within 7 days, and cull cows at livestock

markets. Small dairies (30 to 99 cows) were

visited once for fecal sampling, and up to

40 samples were collected at this single visit. All

cows scheduled for culling within 7 days were

sampled, and the remainder of samples—up to

40—were taken from other cows, including

healthy milking cows, dry cows, and sick cows.

Dairies with 100 or more cows were visited

3 times. At one visit, 50 cows (milking string,

dry, or sick) were sampled along with up to

20 cows scheduled for culling. During the other

two visits, up to 20 samples were taken from

cows scheduled for culling. There were no

specific target numbers for sick, dry, or milking
USDA APHIS VS / 51

Section III: Sampling and Diagnostic Testing—B. Sampling Methods

string cows, other than samples were to be

representative of the cows on hand on the day of

the visit. Cow type was recorded at the time of

collection. At each livestock market, 25 fresh

fecal samples were taken, either by rectal

retrieval or from pen floors if restraining

facilities were not available. Samples were taken

by rectal retrieval, and a separate glove was

used to collect each fecal sample to avoid crosscontamination during sampling. Samples were

placed in sterile screw-top vials. Fecal samples

were approximately golf-ball sized and were

kept on ice and shipped to NVSL. Salmonellapositive samples were sent to the USDA–

ARS Bacterial Epidemiology and Antimicrobial

Resistance Unit (BEAR) [formerly the

Antimicrobial Resistance Research Unit] in

Athens, GA, for antimicrobial susceptibility

testing. At 50 samples per herd (40 per herd for

operations with fewer than 100 cows), this

sample provided 95-percent confidence of

detecting at least 1 positive animal if the withinherd prevalence was greater than or equal to

5 percent, assuming an equal risk of fecal

shedding for each cow sampled. Campylobacter

testing was done on all samples from 31 of the

operations.

Dairy 2002

The Dairy 2002 study set out to estimate

Salmonella prevalence and describe

antimicrobial resistance on U.S. dairies.

Operations were visited once. The goal was to

collect 40 samples per operation regardless of

herd size, or from all cows if the operation had

fewer than 40 cows. There were no specific

targets for numbers of sick, dry, or milking

string cows to be sampled, but cow type was

recorded at the time of collection. Samples were

taken by rectal retrieval, and a separate glove

was used to collect each fecal sample to avoid

cross-contamination during sampling. Samples

were placed in sterile Whirl-pak® bags. Fecal

samples were approximately golf-ball sized and

were shipped on ice to BEAR for culturing and

antimicrobial susceptibility testing.

Campylobacter testing was done on 15 of the

samples per operation.

Dairy 2007

The Dairy 2007 study set out to estimate

Salmonella prevalence and describe

antimicrobial resistance on U.S. dairies. An

additional goal was to evaluate testing strategies

for detecting Salmonella using fecal samples

from individual cows, pooled fecal samples, and

composite fecal samples. Operations were

visited once. Up to

35 fecal samples per operation were taken via

rectal retrieval. The goal was to collect 35 fecal

samples from every operation, regardless of

operation size. Up to 5 sick cows and up to

5 cows scheduled for culling were sampled, with

the remainder (up to 35) taken from cows with

saleable milk. A separate glove was used to

collect each fecal sample to avoid crosscontamination during sampling. Fecal samples

were approximately golf-ball sized. Samples

were placed in sterile Whirl-pak bags. Samples

were kept on ice and shipped to BEAR for

culturing and antimicrobial susceptibility

testing. Samples from individual cows were

pooled at the laboratory, with each pool

representing five cows.
52 / Dairy 2007

Section III: Sampling and Diagnostic Testing—C. Laboratory Methods

C. Laboratory Methods

1. Salmonella

testing of bulktank milk and milk

filters

Dairy 2002

Culture was one of the testing methods used to

detect Salmonella in milk samples (Van Kessel

et al., 2004). Briefly, milk (250 μL) was plated

in triplicate directly onto XLT4 agar (XLT4 agar

base with XLT4 supplement; BD Diagnostics)

using an Autoplate 4000. Plates were incubated

at 37°C and scored for presumptive Salmonella

colonies (black colonies) at 24 and 48 h. For

enrichment of Salmonella, 5 to 10 mL of milk

was added to 90 mL of tetrathionate broth. The

variation in volume was due to variation in

available sample volume. Enrichment bottles

were incubated at 37°C for 24 h and then the

broth was streaked (10 μL) onto XLT4 agar.

Plates were incubated at 37°C and examined at

24 and 48 h for the presence of black colonies.

Isolated, presumptive Salmonella colonies were

transferred from XLT4 plates onto XLT4,

brilliant green, and L-agar. Colonies that

exhibited the Salmonella phenotype (black on

XLT4 and pink on brilliant green) were

preserved for future analysis. Colony biomass

was transferred from the L-agar plates to a vial

containing 0.5 mL of a 1:1 mixture of Lennox

broth and the 2x freezing medium for cells

(Schleif and Wensink, 1981). The isolates were

stored at -80°C. L-agar slants were inoculated

and, after incubation at 37°C for 24 h, sent to

NVSL for serotyping.

PCR was also used to detect Salmonella in milk

samples using RT-PCR, as described by Van

Kessel, et al. (2003). Briefly, 5 to 10 mL of milk

was added to 95 mL of tetrathionate broth. The

variation in volume was due to variation in

available sample volume. Enrichment bottles

were incubated at 37°C for 24 h. After

incubation, enriched samples (1.5 mL) were

centrifuged (13,000 x g) in microcentrifuge

tubes, the supernatants were discarded, and the

pellets were stored at -20°C. DNA was extracted

from bacterial pellets using 200 μL of InstaGene

Matrix (Bio-Rad Laboratories, Hercules, CA)

following the manufacturer’s directions. The

DNA preparations were stored at -20°C and

later analyzed for the presence or absence of

Salmonella via RT-PCR . RT-PCR was carried

out using the Ruggedized Advanced Pathogen

Identification Device (RAPID) [Idaho

Technology Inc., Salt Lake City, Utah].

Premixed, freeze-dried PCR reagents that target

the spaQ gene on the chromosome of

Salmonella were used according to the

manufacturer’s directions using 2 μL of sample.

Preincubation was at 94°C for 60 s. Forty-five

PCR cycles were run under the following

conditions: 95°C for 0 s (the cuvettes are heated

to 95°C but not held there), followed by 60°C

for 20 s with a temperature transition rate of

20°C/s. Other variable parameters included:

channel 2, gain 8, and mode 1. The RAPID

system, in conjunction with the Salmonella

detection kit, has the capability of running

melting point curves on the PCR reaction

products. Melting point curves were run on all

samples that were identified as Salmonellapositive by the RAPID software. The initial

temperature was 94°C for 1 min; the

temperature was reduced to 50°C, and then

increased from 50 to 94°C at a rate of 0.2°C/s.

The fluorescence in the sample was read at each

stage of the temperature gradient and a first

derivative plot of fluorescence vs. temperature

was used to determine the melting point of any
USDA APHIS VS / 53

Section III: Sampling and Diagnostic Testing—C. Laboratory Methods

PCR products present. The software supplied by

the manufacturer provided a score for each

reaction based upon the degree that the

maximum level of fluorescence recorded during

the PCR run differed from the baseline

calculated in the early stages of the run. Thus,

the score depended upon the magnitude of

fluorescent signal generated and the quality of

the baseline. The higher the score, the more the

maximum fluorescent signal varied from the

baseline. For samples with a very low PCR

score, a subjective analysis of the melting point

curve and the RT-PCR amplification curve was

used to decide if a sample was finally

considered Salmonella-positive or Salmonellanegative (Van Kessel et al., 2003). Logistic

regression analysis of the relationship between

RT-PCR signal and the likelihood of obtaining a

positive culture was done using the PROC

PROBIT procedure in SAS 9.1 (SAS Inst. Inc.,

Cary, NC). Samples that gave a positive result in

the real-time assay were subjected to two rounds

of conventional PCR using primer set 139-141

targeting the invA gene as described by Rahn et

al. (1992) and shown by Malorny et al. (2003)

to detect a wide range of Salmonella. The

conditions for the first round of PCR were those

described by Malorny et al. (2003) except that 1

U of Amplitaq Gold (Applied Biosystems,

Foster City, CA) was used per 25 μL reaction, a

10-min incubation at 95°C was added to activate

the enzyme at the beginning of the reaction, and

the PCR was run for 40 cycles. A portion (1 to 3

μL) of the InstaGene preparation from the

tetrathionate broth enrichments of raw milk

samples was added to each reaction. For the

second round of PCR, 5 μL of first-round

product was added to 20 μL of fresh PCR mix to

give the same final composition as the firstround reactions. Amplification was done on a

Biometra Personal Cycler (Biometra, Göttingen,

Germany). The PCR products were separated by

electrophoresis on a 2-percent horizontal

agarose gel in Tris-borate buffer as described by

Maniatis et al. (1982). The gel contained 0.5 μg/

mL ethidium bromide; bands were visualized on

a UV transilluminator, and documented with a

video camera. Detection of a band in the region

of 284 bp indicated the presence of Salmonella.

Dairy 2007

Bulk-tank milk and/or in-line milk filter samples

were analyzed by RT-PCR. Ten mL of milk were

added to 10 mL of 2X tetrathionate broth and

incubated overnight at 37°C. Milk filters were

cut into pieces and mixed with buffered peptone

water in a stomacher bag and pummeled for 2

min. Five mL of the liquid of the stomacher bag

were added to 5 mL of 2X tetrathionate broth

and incubated at 37°C overnight. After

incubation, 1.5 mL of the broth was centrifuged

(16,000 x g) for 2 min in microcentrifuge tubes.

The supernatant was discarded, and the DNA

was extracted from the pellet biomass using 200

μL of InstageneGene Matrix (Bio-Rad Laboratories, Hercules, CA) following manufacturer’s

instructions. The DNA preparations were stored

at -20°C and analyzed for presence of

Salmonella via PCR for the invA gene using the

primers described by Rahn et al. (1992) and

shown by Malorny et al. (2003) to be effective

for the detection of multiple serotypes of

Salmonella. The PCR reactions were run at

EMSL and monitored in real time through the

addition of EVAGreen dye (Biotium, Inc.,

Hayward, CA). PCR-positive samples were then

cultured to allow for Salmonella serotyping.
54 / Dairy 2007

Section III: Sampling and Diagnostic Testing—C. Laboratory Methods

2. Listeria testing

of bulk-tank milk

and milk filters

Dairy 2002

Milk (250 μL) was plated in triplicate directly

onto Modified Oxford Medium (MOX) agar

(BD Diagnostics) using an Autoplate 4000.

Plates were incubated at 37°C and scored for

presumptive Listeria colonies (esculin

hydrolysis, black colonies) at 24 and 48 h. For

enrichment of Listeria, 5 to 10 mL of milk were

added to 90 mL of Modified Listeria

Enrichment Broth (BD Diagnostics).

Enrichment bottles were incubated at 37°C for

48 h, and then the broth was streaked

(10μL) onto MOX agar. Plates were incubated

and scored as described previously. Isolated,

presumptive Listeria colonies were transferred

from MOX plates onto MOX, PALCAM (BD

Diagnostics), and trypticase soy agar with 0.6

percent yeast extract (TSA-YE). Colonies that

exhibited the Listeria phenotype (black on

MOX and gray-green with esculin hydrolysis on

PALCAM) were preserved for future analysis.

Colony biomass was transferred from the

TSA-YE plates to 1.5 mL of tryptic soy broth

and incubated at 37°C for 48 h. The enriched

broth was centrifuged (16,000 x g), and the

supernatants were discarded. The bacterial pellet

was resuspended in 0.5 mL of 1x freezing

medium for cells of Schleif and Wensink (1981),

and the isolates were stored at -80°C.

Presumptive Listeria isolates were grown on

TSA-YE for further testing. Isolates were tested

for oxidase with 1-percent tetramethyl-pphenylenediamine dihydrochloride

(BD Diagnostics), catalase with 3 percent

hydrogen peroxide, and gram-stained using a

3-step staining kit (BD Diagnostics). Hemolytic

activity was determined by stabbing blood agar

(Columbia with 5 percent sheep blood; Remel,

Lenexa, KS) and incubating at 37°C for

48 hours. The Christie-Atkins-Munch-Peterson

test was performed on each isolate using

Staphylococcus aureus Beta Lysin Disks

(Remel) and Rhodococcus equi (ATCC 6939;

American Type Culture Collection, Manassas,

VA) on sheep blood agar. Additionally, RT-PCR

was run on DNA extracts of the presumptive

Listeria isolates. Isolates were grown in 1.0 mL

of tryptic soy broth at 37°C for 48 h. The

enriched broth was centrifuged (16,000 x g),

and the supernatants were discarded. The DNA

was extracted from the bacterial pellets using a

commercially prepared extraction preparation

(InstaGene Matrix; Bio- Rad Laboratories,

Hercules, CA) following the manufacturer’s

directions. The DNA preparations (200 μL)

were stored at -20°C prior to analysis. RT-PCR

was run according to the method described by

Nogva et al. (2000) using a Mx4000 Multiplex

Quantitative PCR System (Stratagene, La Jolla,

CA). Amplification reactions (50 μL) contained

300 nM of each primer, 250 nM probe, 12.6 μg

of BSA, 25 μL of TaqMan Master Mix (Applied

Biosystems, Foster City, CA), and 5 μL of

extracted DNA product. The thermal profile

used for PCR was 50°C for 2 min followed by

95°C for 10 min followed by 40 cycles of 95°C

for 15 s and 60°C for 60 s. Serotyping of the L.

monocytyogenes isolates was conducted using a

previously described ELISA (Palumbo et al.,

2003).
USDA APHIS VS / 55

Section III: Sampling and Diagnostic Testing—C. Laboratory Methods

Dairy 2007

Ten mL of milk was mixed with 90 mL 1X

modified Listeria Enrichment Broth (mLEB) for

enrichment of Listeria and incubated at 37°C for

40 to 48 h. Milk filters were cut into pieces and

mixed with 2 parts (w/w) buffered peptone

water in a stomacher bag and pummeled for

2 min. Then 5 mL of liquid from stomacher bag

was mixed with 5 mL 2X mLEB and incubated

at 37°C for 40 to 48 h. After 48 h, 2 mL of each

enrichment was harvested by centrifuging at

16K x g for 2 min in a 2-mL cryovial. The

supernatant was removed and the pelleted

biomass was suspended in 0.5 mL of

preservation medium and frozen at -80°C to

archive live cells (Preservations). Biomass was

harvested from 1.5 mL of each enrichment in a

1.7-mL microcentrifuge tube. The supernatant

was removed and the pellet was saved for DNA

extraction by freezing at -20°C. A 10-μL loop

was used to streak 10 μL of each enrichment

onto Modified Oxford Agar plates (MOX).

Plates were incubated at 37°C for 48 h and

examined for colonies with morphology

resembling Listeria. Identity of colonies was

confirmed as Listeria and determined to be

L. monocytyogenes vs. non-L. monocytyogenes

by patching suspect colonies onto PALCAM and

BCM media. Any phospholipase-positive

isolates were further characterized with a CAMP

test to distinguish L. ivanovii from

L. monocytyogenes.

3. Salmonella

testing of fecal

samples

Various diagnostic testing methods are available

for detecting Salmonella, including culture,

PCR, and ELISA. Culture methods must be used

if antimicrobial susceptibility testing is to be

performed. Culture was the diagnostic method

used in the Dairy 1996, Dairy 2002, and Dairy

2007 studies, and culture methods were similar

across studies. The following culture methods

apply to Dairy 1996, Dairy 2002, and Dairy

2007, unless noted otherwise.

Approximately 1 g of feces from each sample

was placed into each of two culture media—

gram-negative Hajna broth and tetrathionite

broth—which were incubated at 37°C for 24

and 48 h, respectively. Following primary

enrichments, 100 μL culture aliquots from each

broth enrichment were transferred into

Rappaport R-10 medium for secondary

enrichment, giving two Rappaport secondary

enrichments per sample. In each case,

Rappaport R-10 medium was incubated

overnight at 37°C and then streaked onto

brilliant green agar with sulfadiazine and

xylosine-lysine-tergitol-4 (XLT-4) plates,

resulting in four plates per sample. All plates

were incubated overnight at 37°C. At least three

(Dairy 1996) or four (Dairy 2002 and 2007)

colonies having the typical appearance of

Salmonella were inoculated into triple sugar

iron and lysine iron agar slants. All slants were

incubated overnight at 37°C. All isolates

presumed to be Salmonella were serogrouped

using serogroup-specific sera and sent to NVSL

for serotyping. Isolates with different serogroups

from each sample were kept. If all four colonies

from a sample had the same serogroup, only one

isolate was kept.
56 / Dairy 2007

Section III: Sampling and Diagnostic Testing—C. Laboratory Methods

Salmonella isolates were tested for

antimicrobial drug susceptibility at BEAR. For

Dairy 1996, Dairy 2002, and Dairy 2007,

susceptibility testing was conducted with a

custom-designed panel of antimicrobial drugs

using a Sensititre semi-automated testing system

(TREK Diagnostic Systems, Inc.). Antimicrobial

agents included in the custom designed panel

differed slightly for each of the three NAHMS

studies. The minimum inhibitory concentration

(MIC) for each isolate was determined, and each

isolate was classified as susceptible,

intermediate, or resistant, according to

guidelines published by the National Committee

on Clinical Laboratory Standards for brothmicrodilution susceptibility testing, when

available. When guidelines were not available,

breakpoint interpretations were determined

using National Antimicrobial Resistance

Monitoring System (NARMS) guidelines. The

antimicrobials included for all studies included

amikacin, amoxicillin-clavulanic acid,

ampicillin, ceftiofur, ceftriaxone,

chloramphenicol, ciprofloxacin, gentamicin,

kanamycin, nalidixic acid, streptomycin,

tetracycline, and trimethoprimsulfamethoxazole. Apramycin was included in

Dairy 1996 only. Cefoxitin was included in

Dairy 2002 and 2007 but not Dairy 1996.

Cephalothin was included in Dairy 1996 and

2002 but not 2007. Ticarcillin was included in

Dairy 1996 only. Sulfamethoxazole was

included in Dairy 1996 and 2002, and then a

similar sulfa antimicrobial, sulfisoxazole,

replaced it in Dairy 2007.
USDA APHIS VS / 57

Section III: Sampling and Diagnostic Testing—C. Laboratory Methods

Breakpoints used for susceptibility testing of Salmonella1,2

Breakpoints (μg/mL)

Antimicrobial 

Class 

Antimicrobial 

Agent 

Susceptible 

(less than 

or equal) Intermediate 

Resistant 

(greater than 

or equal) 

Aminoglycosides 

Amikacin 16 32 64 

Gentamicin 4 8 16 

Kanamycin 16 32 64 

Streptomycin 32 NA 64 

β-lactam/βlactamase 

inhibitor 

combinations 

Amoxicillinclavulanic acid 8/4 16/8 32/16 

Cephems 

Cefoxitin 8 16 32 

Ceftiofur 2 4 8 

Ceftriaxone2 8 16–32 64 

Folate pathway 

inhibitors 

Sulfamethoxazole/ 

sulfisoxazole3 256 NA 512 

Trimethoprinsulfamethoxazole 2/38 NA 4/76 

Penicillin Ampicillin 8 16 32 

Phenicols Chloramphenicol 8 16 32 

Quinolones 

Ciprofloxacin 1 2 4 

Nalidixic acid 16 NA 32 

Tetracyclines Tetracycline 4 8 16 

1

Breakpoints were adopted from CLSI (Clinical and Laboratory Standards Institute), except for streptomycin, 

which has no CLSI breakpoints. 

2

CLSI revised the breakpoints for ceftriaxone in its M100-S20 document published in January 2010. The old 

breakpoints were used for the data in this report. 

3

Sulfamethoxazole was tested from 1996 through 2003 and was replaced by sulfisoxazole in 2004. 
58 / Dairy 2007

Section III: Sampling and Diagnostic Testing—C. Laboratory Methods

4. Campylobacter

testing of fecal

samples

Various diagnostic testing methods are available

for detecting Campylobacter. Culture is the

traditional identification method, but PCR is

also commonly used. Culture methods are

required in order to perform antimicrobial

susceptibility testing. Specific PCR and culture

methods differed among the dairy studies, and

diagnostic procedures follow.

Dairy 1996

For Campylobacter testing, a multiplex PCR

was used that allowed for the simultaneous

identification of C. jejuni and C. coli (Harmon

et al., 1997). The assay targeted the flA genes of

C. jejuni and C. coli, which yielded a 460-bp

product. A second set of primers identified a

nucleic acid sequence unique to C. jejuni and

yielded a 160-bp product. Within 36 h of sample

collection, approximately 1 g of feces was

diluted (10 percent wt/vol) in buffered peptone

water (9 mL). An aliquot (0.4 mL) of the fecal

suspension was plated to the surface of modified

blood-free charcoal, cefoperazone deoxycholate

agar (CM 739; Oxoid Ogdensburg, NY) and

incubated microaerobically for 2 to 3 d at 42°C

(Ono et al., 1995). After incubation, bacterial

growth from the first quadrant was harvested

with a bacteriological loop, placed in TrisEDTA buffer (pH 7.4, 200μL) and frozen

(-20°C) prior to PCR analysis. The bacterial

suspension in Tris-EDTA (200 μL) was boiled

for 5 min prior to PCR analysis and centrifuged

(13,000 x g, 1 min at room temperature), and a

5-μL aliquot was used as the PCR template.

Samples were subjected to an initial

denaturation step (94°C for 4 min), followed by

25 amplification cycles. Each amplification

cycle consisted of denaturation (1 min at 94°C),

primer annealing (1 min at 45°C), and primer

extension (1 min at 72°C). Final primer

extension (7 min at 72°C) followed the last

amplification cycle. PCR products were

electrophoretically separated (120 V, 45–55

min). C. coli were identified by the appearance

of a 460-bp product, and C. jejuni was identified

by presence of both a 460-bp and a 160-bp

product. Antimicrobial susceptibility testing was

not performed on Campylobacter isolates from

the Dairy 1996 study.

Dairy 2002

Fecal samples were diluted 1:4 and 1:40 in

phosphate-buffered saline. 100-μL aliquots of

each dilution were spread uniformly on

duplicate Campy-Cefex plates (Stern et al.,

1992). The plates were placed in zip-top bags

and incubated microaerobically (5 percent O2

,

10 percent CO2, and 85 percent N2) for 48 h at

42°C. Campylobacter was presumptively

identified from microscope wet mounts of cells

using phase contrast optics at 100x. Samples

from 97 operations were tested for

Campylobacter, and antimicrobial susceptibility

testing was performed on isolates from

94 operations. For 26 operations, all available

isolates were tested for antimicrobial

susceptibility. For cost reasons, 5 isolates (or as

many as were available for operations with

fewer than 5 isolates) were randomly chosen for

antimicrobial susceptibility testing from the

remaining 68 operations. From each sample with

Campylobacter growth, a single colony was

selected for antimicrobial susceptibility testing.

The isolates were identified to the species level
USDA APHIS VS / 59

Section III: Sampling and Diagnostic Testing—C. Laboratory Methods

using the Campylobacter BAX® PCR (DuPont

Qualicon, Wilmington, DE), a multiplex assay

specific for C. coli and C. jejuni (Englen and

Fedorka-Cray, 2002). A total of 532 isolates,

including 473 C. jejuni and 59 C. coli, were

selected for susceptibility testing to

8 antimicrobials. The Etest® method (ABBiodisk, Piscataway, NJ) was used according to

the manufacturer’s directions as described by

Englen et al. (2005). Briefly, 150-mm Mueller

Hinton plates containing 5 percent lysed horse

blood (B-D Biosciences, Sparks, MD) were

inoculated with 100 μL of a cell suspension

equal to a 1.0 McFarland standard. The

inoculum was swabbed evenly across the entire

plate surface, and four Etest strips were laid at

right angles onto each plate. The plates were put

into zip-top bags and incubated in a

microaerobic atmosphere (5 percent O2

,

10 percent CO2, and 85 percent N2) for 48 h at

42°C. Following incubation, the point at which

the zone of growth inhibition intersected the

strip was read as the MIC of the antimicrobial in

μg mL-1. Quality control ATCC strains C. jejuni

33560, Escherichia coli 25922, and

Staphylococcus aureus 25923 were tested

biweekly to confirm susceptibility to all eight

antimicrobials. The antimicrobial resistance

break points (MICs) used were those established

by NARMS in accordance with Clinical and

Laboratory Standards Institute guidelines:

azithromycin, >2 μg mL-1; chloramphenicol,

>32 μg mL-1; ciprofloxacin, >4 μg mL-1;

clindamycin, >4 μg mL-1; erythromycin,

>8 μg ml-1; gentamicin, >16 μg ml-1; nalidixic

acid, >32 μg ml-1; tetracycline, >16 μg ml-1.

Dairy 2007

Fecal samples were diluted 1:10 in phosphatebuffered saline before being enriched in

Bolton’s enrichment broth for 48 h at 42°C

under microaerophilic conditions (5 percent O2,

10 percent CO2, 85 percent N2). Aliquots

(10 μL) were spread onto Campy-Cefex plates

(Stern et al., 1992) which were incubated as in

2002. Presumptive Campylobacter colonies

were selected by observation of cellular

morphology and motility using a wet mount

under phase-contrast microscopy. Isolates were

identified using the Campylobacter BAX PCR

(DuPont Qualicon, Wilmington, DE), a

multiplex assay specific for C. coli and

C. jejuni. The assay was performed according to

manufacturer directions as previously described

(Englen and Fedorka-Cray, 2002). If a PCR

product was not obtained using the BAX PCR, a

traditional PCR was used as previously

described (Wang et al., 2002). This traditional

PCR can identify C. jejuni, C. coli, C. lari,

C. fetus, and C. upsaliensis. Campylobacter

isolates were susceptibility tested using broth

microdilution in a custom panel of nine

antimicrobials: azithromycin, ciprofloxacin,

clindamycin, erythromycin, florfenicol,

genamicin, nalidixic acid, telithromycin, and

tetracycline. The semi-automated SensititreTM

System (TREK Diagnostic Systems, Inc.,

Cleveland, OH) was used per manufacturer’s

instruction. MICs were determined for each

isolate and classified as susceptible,
60 / Dairy 2007

Section III: Sampling and Diagnostic Testing—C. Laboratory Methods

intermediate, or resistant according to Clinical

and Laboratory Standards Institute standards,

where available. Otherwise, breakpoint

determinations were based on those used by

NARMS (FDA, 2009).

Breakpoints used for susceptibility testing of Campylobacter1

Breakpoints (μg/mL)

Antimicrobial 

Class 

Antimicrobial 

Agent 

Susceptible 

(less than 

or equal) Intermediate 

Resistant 

(greater than 

or equal) 

Aminoglycosides Gentamicin 2 4 8 

Ketolides Telithromycin 4 8 16 

Lincosamides Clindamycin 2 4 8 

Macrolides 

Azithromycin 2 4 8 

Erythromycin 8 16 32 

Phenicols Florfenicol2 4 NA 8 

Quinolones 

Ciprofloxacin 1 2 4 

Nalidixic acid 16 32 64 

Tetracyclines Tetracycline 4 8 16 

1

Breakpoints were adopted from CLSI (Clinical and Laboratory Standards Institute) when available. 

2

For florfenicol, only a susceptible breakpoint (=4 μg/mL) has been established. In this report, isolates with an 

MIC =8 μg/mL are categorized as resistant. 
USDA APHIS VS / 61

Section III: Sampling and Diagnostic Testing—D. Testing Methods Overview

D. Testing Methods Overview

The table below presents a synopsis of the

testing methods used on each NAHMS dairy

study, by organism and type of sample.

c. Testing method by NAHMS study, organism, and type of sample 

Study Year 

1996 2002 2007 

Organism/ 

Sample Type Culture PCR Culture PCR Culture PCR 

Salmonella

 Individual cow 

fecal samples X X X 

Composite fecal 

(environmental) 

samples 

 X 

Bulk-tank milk 

sample X X X 

Milk filter samples X 

Campylobacter 

Individual cow 

fecal samples X X X 

Listeria 

Bulk-tank milk 

samples X X 

Milk filter samples X 
62 / Dairy 2007

Appendix I: NAHMS Study Methodology—Phase II

Appendix I: N Appendix I: NAHMS Study

Methodology—Phase II hodology—Phase II hodology—Phase II*

*

For more detailed information about the methodology for

each study, see methodology section of each descriptive

report at: http://nahms.aphis.usda.gov

NAHMS Dairy Studies 

1996 2002 2007 

Data collection dates 2/26–7/10 3/27–9/25 2/28–8/30 

Minimum number of dairy cattle 30 30 30 

Number of States 20 21 17 

Data collectors State and Federal veterinary medical officers 

and animal health technicians 

Participating States as a percentage of U.S. population coverage

Operations 85.6 86.9 84.7 

Cows 82.7 85.7 82.5 

Respondent sample profile (herd size)

Small (fewer than 100 cows) 630 400 233 

Medium (100–499 cows) 502 392 215 

Large (500 or more cows) 87 221 134 

Respondent sample profile (region)

East 931 805 474 

West 288 208 108 

Response category

Survey complete 1,219 1,013 582 

Percent of total 76.0 70.4 54.0 

Refused 340 335 380 

Did not contact 16 76 111 

Ineligible 29 14 4 

Total 1,604 1,438 1,077 
USDA APHIS VS / 63

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K, Green A. 2001. Fecal shedding of Salmonella

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Schroeder-Tucker L, Glover M, Siddique I.

2000. Fecal shedding of Campylobacter and

Arcobacter spp. in dairy cattle. Appl Environ

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Blau DM, McCluskey BJ, Ladely SR, Dargatz

DA, Fedorka-Cray PJ, Ferris KE Headrick M.

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United States: prevalence and antimicrobial

drug susceptibility. J Food Prot 68(4):696–702.

Englen MD, Hill AE, Dargatz DA, Ladely SR,

Fedorka-Cray PJ. 2007. Prevalence and

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102(6):1570–1577.

Karns JS, Van Kessel JS, McCluskey BJ, Perdue

ML. 2005. Prevalence of Salmonella enteric in

bulk tank milk from U.S. dairies as determined

by polymerase chain reaction. J Dairy Sci

88(10):3475–3479.

USDA. 2003. Salmonella and Campylobacter

on U.S. dairy operations. USDA–APHIS–VS,

CEAH. Fort Collins, CO. Available at: http://

nahms.aphis.usda.gov/dairy/index.htm

USDA. 2003. Salmonella and Listeria in bulk

tank milk on U.S. dairies. USDA–APHIS–VS,

CEAH. Fort Collins, CO. Available at: http://

nahms.aphis.usda.gov/dairy/index.htm

USDA. 2005. Salmonella on U.S. dairy

operations: prevalence and antimicrobial drug

susceptibility. USDA–APHIS–VS, CEAH. Fort

Collins, CO. Available at: http://

nahms.aphis.usda.gov/dairy/index.htm

Van Kessel JS, Karns JS, Gorski L, McCluskey

BJ, Perdue ML. 2004. Prevalence of

Salmonella, Listeria monocytogenes, and fecal

coliforms in bulk tank milk on U.S. dairies. J

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Ruzante JM, Lombard JE, Wagner B, Fossler

CP, Karns JS, Van Kessel JA, Gardner IA. 2010.

Factors associated with Salmonella presence in

environmental samples and bulk tank milk from

U.S. dairies. Zoonoses Pub Health. 57:e217-

e225
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Appendix III: Previously Published Material

USDA. 2009. Prevalence of Salmonella and

Listeria in bulk tank milk and in-line filters on

U.S. dairies, 2007. USDA–APHIS–VS, CEAH.

Fort Collins, CO. Available at: http://

nahms.aphis.usda.gov/dairy/index.htm

USDA. 2009. Salmonella and Campylobacter

on U.S. dairy operations, 1996–2007. USDA–

APHIS–VS, CEAH. Fort Collins, CO. Available

at: http://nahms.aphis.usda.gov/dairy/index.htm