Veterinary Services 

Centers for Epidemiology and Animal Health October 2005 

_________________________________________________________________________________________________________________________ 

Salmonella on U.S. Dairy 

Operations: Prevalence and 

Antimicrobial Drug 

Susceptibility 

Background 

In 2002, there were 16,580 cases of laboratorydiagnosed foodborne illnesses attributed to 10 

organisms under surveillance by the Centers for Disease 

Control and Prevention (CDC). Salmonella was the most 

common bacterial pathogen identified, accounting for 

36.4 percent of the cases.1

 In a CDC analysis of the 

impact of foodborne illnesses, nontyphoidal Salmonella

accounted for 25.6 percent of hospitalizations and 30.6 

percent of deaths due to known foodborne pathogens.2

Salmonella causes fever, abdominal cramping, and 

diarrhea in humans. Severe cases can result in systemic 

infections and even death. 

 Foodborne Salmonella infections have been 

attributable to raw, undercooked, or contaminated 

poultry and poultry products, eggs, meat and meat 

products, dairy products, vegetables, and other 

agriculture sources.3

Salmonella organisms can be transmitted from dairy 

cattle to humans through several routes, including the 

consumption of contaminated milk and ground beef, and 

direct contact with infected animals.4 5 6 7 On-farm 

management practices can help control the transmission 

of foodborne pathogens in dairy cattle. Reducing and 

controlling pathogens within herds may reduce the risk 

of human exposure, while benefiting the health and 

productivity of dairy cows. 

Salmonella on U.S. Dairy Operations 

Clinical signs of salmonellosis in cattle include fever, 

diarrhea, anorexia, abortion, and decreased milk 

production. The effects of infection can range from 

subclinical to endotoxemia and death.8

 Cows can shed 

Salmonella in their feces during transportation, lactation, 

and calving—without showing clinical signs typically 

associated with infection. 

 Although over 2,000 serotypes of Salmonella have 

been identified, most laboratory-confirmed Salmonella

infections in cattle are due to a small number of 

serotypes. Multidrug resistant strains of Salmonella 

Typhimurium have been recognized for years as 

important cattle and human pathogens. Recently, a 

drug-resistant form of Salmonella enterica Newport 

(S. Newport) has emerged. These multidrug-resistant 

isolates have caused disease in humans, adult cattle, 

and in calves, and are of concern to both veterinary and 

public health officials. 

In 2002, the U.S. Department of Agriculture’s 

(USDA) National Animal Health Monitoring System 

(NAHMS) conducted the Dairy 2002 study, NAHMS’ 

third national study of U.S. Dairy operations. During 

Dairy 2002, fecal samples were collected via rectal 

retrieval on approximately five operations in each of the 

21 States* participating in the study. On each operation, 

fecal samples were collected from approximately 40 

cows. The testing scheme was estimated to detect 

disease on operations where the within-herd prevalence 

was at least six percent. From March 25 through 

September 25, 2002, 3,709 samples were collected on 

97 operations and cultured for Salmonella. Each sample 

was tested for resistance to a panel of 16 antimicrobial 

drugs.** Resistance break points used by the National 

Antimicrobial Resistance Monitoring System were used 

to classify isolates as susceptible, intermediate, or 

resistant. 

Of the fecal samples collected and cultured, 7.3 

percent (269) were positive for Salmonella. From these 

269 positive samples, 294 isolates were recovered. 

There were 28 different serotypes identified, of which 

62.9 percent of isolates were represented by five 

serotypes (table 1). S. Newport isolates were only 2.7 

percent of the isolates recovered and were present on 

5.2 percent of operations. 

*

California, Colorado, Florida, Idaho, Illinois, Indiana, Iowa, Kentucky, 

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

Pennsylvania, Tennessee, Texas, Vermont, Virginia, Washington, 

Wisconsin 

**amikacin (Am), amoxicillin/clavulanic acid (Amo), ampicillin (Amp), 

cefoxitin (Cefo), ceftiofur (Ceft), Ceftriaxone (Ceftri), cephalothin 

(Ceph), chloramphenicol (Chlor), ciprofloxacin (Cip), gentamicin (Gen), 

kanamycin (Kan), naladixic acid (Nal), streptomycin (Str), 

sulfamethoxazole (Sulf), tetracycline (Tet), and 

trimethoprim/sulfamethoxazole (Tris). 
Table 1. Percentage of Isolates and Percentage of 

Herds for the Five Most Common Salmonella

Serotypes Isolated from Cattle Feces Collected 

During Dairy 2002 

Salmonella 

Serotype Percent Isolates Percent Herds 

Meleagridis 24.1 5.2 

Montevideo 11.9 8.3 

Typhimurium 9.9 3.1 

Kentucky 9.5 8.2 

Agona 7.5 3.1 

Overall, 30.9 percent of herds had at least one 

positive culture for Salmonella. This herd prevalence is 

slightly higher—although not statistically significant—

than the prevalence reported in NAHMS Dairy ‘96 study 

where 27.5 percent of U.S. dairy operations had at least 

one milk cow shedding Salmonella spp.9

 

There was a significant association between herd 

size and the percentage of herds positive for Salmonella. 

Large operations (501 or more cows) had the highest 

percentage of herds positive for Salmonella (52.0 

percent) compared to medium (100 to 500 cows) and 

small (1 to 99 cows) operations, 28.2 percent and 18.2 

percent, respectively (figure 1). 

0

20

40

60

18.2

28.2

52.0

30.9

Small

(1 to 99)

n=33

Medium 

(100 to 500)

n=39

Large 

(501 or more)

n=25

All 

Operations

n=97

Herd Size (Number of Cows)

Figure 1. Percentage of Herds Positive for 

Salmonella, by Herd Size

Percent

There also was a significant association between region 

and the percentage of herds positive for Salmonella. The 

percentage of herds positive for Salmonella was higher 

in the Southeast and West regions (50.0 percent and 

42.9 percent, respectively) compared to the Midwest and 

Northeast regions (25.0 and 6.7 percent, respectively) 

(figure 2). 

No association was found between reported cases 

of diarrhea within herds and Salmonella-positive fecal 

cultures. Of the operations with positive fecal cultures, 

26.7 percent reported no cases of diarrhea in any dairy 

cow, suggesting that cattle in these herds had subclinical 

infections. There was no apparent relationship between 

Salmonella infections within herds and milk production, 

as evaluated by rolling herd averages of pounds of milk 

produced. However, two of the three Salmonella-positive 

operations that had a rolling herd average of less than 

16,000 pounds had the highest within-herd prevalence 

levels of Salmonella (97.5 percent and 82.1 percent). 

0

20

40

60

42.9

25.0

6.7

50.0

West

(n=28)

Midwest

(n=40)

Northeast

(n=15)

Southeast

(n=14)

Region

Percent

Figure 2. Percentage of Herds Positive for 

Salmonella, by Region

Antimicrobial Susceptibility 

Salmonella isolates showed relatively little 

resistance to a number of antimicrobial agents, with 83.0 

percent susceptible to all antimicrobial drugs tested. All 

isolates were susceptible to amikacin, ciprofloxacin, 

naladixic acid, and trimethoprim/sulfamethoxazole. 

Overall, 17.0 percent of the isolates were resistant to at 

least one antimicrobial drug, and 4.8 percent were 

resistant to more than one antimicrobial drug. 

Resistance to tetracycline was most common (11.9 

percent of all isolates) followed by resistance to 

streptomycin (9.6 percent). Table 2 depicts the 

resistance patterns from all isolates. 
Table 2. Resistance Patterns Among Salmonella

Isolates From Dairy Operations

 Salmonella Isolates

Resistance Pattern Number Percent 

Susceptible to all antimicrobials* 244 83.0 

Tet 21 7.1 

Str 15 5.1 

Amo, Amp, Cefo, Ceft, Ceph, 

Chlor, Str, Sulf, Tet 7 2.4 

Amo, Ampicillin, Cefo, Ceft, Ceph, 

Chlor, Gen, Kan, Str, Sulf, Tet 2 0.7 

Amo, Amp, Cefo, Ceft, Ceph, 

Chlor, Str, Tet 2 0.7 

Amo, Amp, Ceft, Ceph, Chlor, Str, 

Sulf, Tet 2 0.7 

Amo, Ceph, Tet 1 0.3 

Total 294 100.0 

*Intermediate isolates were classified as susceptible 

Multidrug resistance was observed among certain 

serotypes, including S. Mbandaka, S. Newport, S. 

Reading and S. Typhimurium. Of isolates resistant to 

more than one antimicrobial drug, 50.0 percent were S. 

Newport. These isolates were found in four different 

herds. Currently, there is concern in the United States 

about the emergence of a strain of multidrug resistant S. 

Newport and the potential impact on human health. 10 11

The multidrug resistant form of S. Newport was relatively 

uncommon, accounting for only 2.4 percent of isolates. 

Table 3 depicts serotypes with at least one resistant 

isolate and the resistance patterns for those isolates. 

Again, it is apparent that for some serotypes most 

isolates are pan-susceptible, whereas for other 

serotypes most isolates are resistant to some 

antimicrobials. This appears to be especially true for S.

Newport. 

Table 3. Serotypes of Resistant Salmonella Strains and 

resistance Patterns from Cattle Feces 

Serotype 

Number 

Isolates

Percent 

Serotype 

Isolates 

Resistance 

Pattern 

Number 

Herds 

Agona 22 9.1 Str 1 

Anatum 6 33.3 Tet 1 

Kentucky 28 53.6 Tet 2 

Mbandaka 12 8.3 Amo, Ceph, Tet 1 

Monte- 34.3 Str 1 

video 35 

8.6 Tet 1 

 12.5 

Amo, Amp, Cefo, 

Ceft, Ceph, Chlor, 

Str, Tet 

2 

 Newport 8 62.5 

Amo, Amp, Cefo, 

Ceft, Ceph, Chlor, 

Str, Sulf, Tet 

3 

 12.5 

Amo, Amp, Cefo, 

Ceft, Ceph, Chlor, 

Gen, Kan, Str, 

Sulf, Tet 

1 

Reading 1 100.0 

Amo, Amp, Cefo, 

Ceft, Ceph, Chlor, 

Gen, Kan, Str, 

Sulf, Tet 

1 

 3.4 

Amo, Amp, Cefo, 

Ceft, Ceph, Chlor, 

Str, Tet 

1 

29 6.9 

Amo, Amp, Ceft, 

Ceph, Chlor, Str, 

Sulf, Tet 

2 

Typhimurium 

 

 6.9 

Amo, Amp, Cefo, 

Ceft, Ceph, Chlor, 

Str, Sulf, Tet 

2 

Un- 7.1 Str 1 

typeable 14 7.1 Tet 1 

Conclusions 

The prevalence of Salmonella on dairy operations in 

the United States continues to remain relatively low with 

7.3 percent of cows and 30.9 percent of herds having 

one or more positive fecal cultures. Little change has 

occurred in Salmonella prevalence compared to a similar 

study conducted in 1996.9

 Resistance of Salmonella to 

antimicrobial drugs on dairy operations in the United 

States also continues to be relatively low. 
References 

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Bresee, C. Shapiro, P.M. Griffin, and R.V. Tauxe. 1999. 

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3 Chin, J. 2000. Communicable Diseases Manual. 

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