Prevalence and Antimicrobial Susceptibility Patterns of Salmonella from Beef Cows

October 2001

Salmonellae have been isolated from nearly all vertebrates, and infection has been associated with both animal and human disease. Human Salmonella infections in the United States have been estimated in the millions annually. In some of these cases, foods of animal origin have been implicated as the source of the pathogen. In addition to being a food-safety concern for humans, Salmonella can cause illness or death among cattle. There is worldwide concern that many bacteria, including Salmonella, are becoming resistant to antimicrobial agents. Trends in antimicrobial susceptibility patterns of Salmonella isolates are being monitored.

The USDA:APHIS:VS conducted a study of health and management of cattle and calves on cow-calf operations as part of the National Animal Health Monitoring System's (NAHMS) Beef '97 study. This study was designed to determine the prevalence of Salmonella shedding in feces of beef cows on cow-calf operations throughout the U.S., and to determine antimicrobial susceptibility patterns for all Salmonellae isolated. A stratified random sample of cow-calf operations from the 23 major cow/calf states1 was selected for the Beef '97 study. A total of 2,713 producers participated.

A convenience sample of 187 operations in 21 states was selected to submit fecal samples (operations in Arkansas and Wyoming were not tested). Fecal samples were collected between August 1997 and January 1998. Culture work was performed at the Richard Russell Research Center in Athens, Georgia, and the serotyping was done at the National Veterinary Services Laboratories in Ames, Iowa. Up to 40 fecal samples were collected from the ground on each operation, depending on herd size. All Salmonella isolates were serotyped. In addition, all Salmonella isolates were screened for resistance against a panel of 17 antimicrobials2.

A total of 5,049 fecal samples on 187 operations were cultured for Salmonella. Salmonella spp. were recovered from 21 operations (11.2 percent). Seventy samples (1.4 percent) tested positive for Salmonella, and from these samples, 78 Salmonella isolates were identified. Most of the isolates (64.1 percent) were recovered from two operations (46.2 percent from one operation and 17.9 percent from another). Only one serotype was recovered from the majority of operations. The five most common serotypes of Salmonella isolated from samples were S. oranienburg, S. cerro, S. anatum, S. bredeney, and S. mbandaka (Table 1).
 

Table 1. The Five Most Common Salmonella Serotypes Isolated from Cattle Feces

Serotype

Number of 

Positive Isolates

Percent Isolates

Number of Positive Operations

Percent Positive Operations

Oranienburg

17

21.8

3

14.3

Cerro

17

21.8

1

4.8

Anatum

8

10.3

2

9.5

Bredeney

7

9

3

14.3

Mbandaka

4

5.1

2

9.5

Herds were classified into one of five geographic regions (Figure 1). Herds also were classified into one of five herd size categories based on the number of beef cows reported on the date of sample collection. The herd prevalence of fecal shedding of Salmonellae was compared by geographic region and herd size using chi-square analysis.

Herd prevalence was related to geographic region (Table 2). The largest proportion of operations tested was positive in the Southcentral region followed by the Central region. More herds with 1 to 25 beef cows or 26 to 49 beef cows had positive samples, though prevalence by herd size was not significantly different (Table 3). Most of the isolates (87.2 percent) were sensitive to the entire panel of antimicrobials tested. The most common resistance identified was to Sulfamethoxazole and Streptomycin (11.5 percent of isolates each) followed by Tetracycline and Gentamicin (2.6 percent of isolates each). For all other antimicrobials, fewer than 1.5 percent of isolates were considered resistant.

Table 2. Number of Operations with at Least One Positive Sample for Salmonella, by Region

 

Region

Number Operations Tested

Number Positive Operations

Percent Positive Operations

West

26

1

3.80

Northcentral

37

1

2.70

Southcentral

42

9

21.40

Central

39

6

15.40

Southeast

43

4

9.30

Total

187

21

11.20


 

Table 3. Number of Operations with at Least One Positive Sample for Salmonella, by Herd Size

Herd Size (Cows)

Number Operations Tested

Number Positive Operations

Percent Positive Operations

1-25

46

7

15.20

26-49

42

7

16.70

50-99

41

3

7.30

100-249

40

2

5.00

250 or more

18

2

11.10

Total

187

21

11.20

While the prevalence of Salmonella within beef cow-calf operations appears to be variable, in general it is very low. Only 1.4 percent of samples and only 11.2 percent of the operations were cultured positive for Salmonellae. Factors that may have influenced the outcome of these results include the time of year samples were collected, quality and number of samples collected, and the presence or lack of environmental stressors affecting shedding by the cattle. A higher proportion of the herds that tested positive came from the Southcentral, Central, and Southeast regions, which could be related to weather conditions at the time of sampling that were more conducive to detecting Salmonella. Determination of the full significance of the antimicrobial susceptibility patterns for Salmonella isolates will have to await further studies. Prudent use of all antimicrobics by producers and veterinarians will help ensure that these products remain effective and available.


For more information, contact:

Centers for Epidemiology and Animal Health
USDA:APHIS:VS, attn. NAHMS
2150 Centre Ave., Bldg. B, MS 2E7
Fort Collins, CO 80526-8117
Telephone: (970) 494-7000
E-mail: NAHMSweb@usda.gov
www.aphis.usda.gov/vs/ceah/ncahs
N300.1001

Alabama, Arkansas, California, Colorado, Florida, Georgia, Illinois, Iowa, Kansas, Kentucky, Mississippi, Montana, Nebraska, New Mexico, Missouri, North Dakota, Oklahoma, Oregon,   South Dakota, Texas, Tennessee, Virginia, and Wyoming.
Amikacin, Amoxicillin/Clavulanic Acid, Ampicillin, Apramycin, Ceftiofur, Ceftriaxone, Cephalothin, Chloramphenicol, Ciprofloxacin, Gentamicin, Kanamycin, Nalidixic Acid, Streptomycin, Sulfamethoxazole, Tetracycline, Ticarcillin, and Trimethoprim/Sulfamethoxazole.