Veterinary Services 

Centers for Epidemiology and Animal Health January 2009 

_________________________________________________________________________________________________________________________ 

Salmonella on U.S. Swine 

Sites—Prevalence and 

Antimicrobial Susceptibility 

Background 

In 2006, there were 17,252 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 38.6 percent of the cases.1

 In humans, 

Salmonella causes fever, abdominal cramping, and 

diarrhea. Severe cases can result in systemic infections 

and even death. 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

 

 Foodborne Salmonella infections have been 

attributed 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 pigs 

to humans through consumption of contaminated pork 

products and direct contact with infected animals.4 5 

Pigs can shed Salmonella in their feces during all 

stages of pork production without showing any clinical 

symptoms. Shedding is also common during 

transportation and at lairage (holding pens) in slaughter 

houses. Clinical signs of salmonellosis in pigs include 

fever, diarrhea, anorexia, abortion, and decreased 

weight gain.6

 

Although over 2,500 serotypes of Salmonella have 

been identified, most laboratory-confirmed Salmonella

infections in pigs are due to a small number of 

serotypes. Antimicrobial resistance is serotype 

dependent. Multidrug-resistant strains of Salmonella, 

such as S. Typhimurium, have been recognized for 

years as important swine and human pathogens and are 

of concern to both veterinary and public health officials. 

Salmonella on U.S. swine sites 

In 2006, the USDA’s National Animal Health 

Monitoring System (NAHMS) conducted a study on 

swine health and management practices from a random 

sample of swine production sites with 100 or more pigs 

in 17 States.* These States represented approximately 

94 percent of U.S. pig inventory and 94 percent of U.S. 

pork producers with 100 or more pigs. 

 As part of Swine 2006, fecal samples were 

collected from different areas of pen floors on 135 sites. 

On each site, up to 60 fecal samples were collected from 

pens containing grower/finisher pigs and cultured for 

Salmonella. From September 5, 2006, through March 

15, 2007, 7,788 samples were cultured for Salmonella. 

Overall, at least one sample was found culturepositive for Salmonella on 52.6 percent of sites, 

43.5 percent of barns, and 18.4 percent of pens. Of the 

fecal samples cultured, 564 (7.2 percent) were positive 

for Salmonella. From these samples, 584 isolates were 

recovered (20 samples had 2 isolates). Twenty-seven 

different serotypes were identified; however, 4 serotypes 

accounted for 70.5 percent of isolates (table 1). 

Table 1. Number and Percentage of Isolates and 

Number and Percentage of Sites by Salmonella 

Serotypes Isolated, 2006 

Salmonella 

Serotype

Number 

Isolates 

Percent 

Isolates 

Number 

Sites 

Percent 

Sites 

Derby 173 29.6 31 23.0 

Typhimurium 

(Copenhagen) 132 22.6 21 15.6 

Agona 63 10.8 13 9.6 

Anatum 44 7.5 7 5.2 

All others 172 29.5 63 46.6 

Total 584 100.0 NA NA 

 

* States 

Arkansas, Colorado, Illinois, Indiana, Iowa, Kansas, Michigan, 

Minnesota, Missouri, Nebraska, North Carolina, Ohio,

Oklahoma, Pennsylvania, South Dakota, Texas, and Wisconsin.
The 584 Salmonella isolates were tested for 

resistance to a panel of 15 antimicrobial drugs.**

Resistance break points used by the National 

Antimicrobial Resistance Monitoring System were used 

to classify isolates as susceptible, intermediate, or 

resistant. 

The percentage of Salmonella-positive sites was 

higher in Swine 2006 (52.6 percent) than in the NAHMS 

Swine 1995 and Swine 2000 studies (38.2 and 32.8 

percent of sites, respectively). A comparison of the 

ranking of frequently isolated serotypes in 1995, 2000, 

and 2006 is shown in table 2. Derby, Agona, and 

Typhimurium Copenhagen were the top three serotypes 

in all three studies.

Table 2. Ranking of Most Common Salmonella 

Serotypes Identified from Three NAHMS Studies 

Rank 1995 2000 2006 

1 Derby Derby Derby 

2 Agona Agona Typhimurium 

Copenhagen 

3 Typhimurium 

Copenhagen 

Typhimurium 

Copenhagen Agona 

4 Brandenberg Anatum 

5 

Heidelberg/ 

Brandenberg 

(tie) 

6 

Mbandaka/ 

Typhimurium 

(tie) Anatum 

Meleagridis/ 

Typhimurium 

(tie) 

7 Worthington 

8 

Heidelberg/ 

Anatum (tie) 

Typhimurium/ 

Worthington 

(tie) 

9 Enteriditis Infantis 

Barranquilla/ 

Johannesburg 

(tie) 

10 Worthington Uganda Muenchen 

Antimicrobial susceptibility 

Table 3 shows that all isolates were susceptible to 

amikacin, ceftriaxone, ciprofloxacin, and naladixic acid. 

Resistance to tetracycline was most common 

(78.6 percent of Salmonella isolates), followed by 

resistance to sulfisoxazole (67.3 percent of isolates). 

 

** Amikacin, amoxicillin/clavulanic acid, ampicillin, cefoxitin, ceftiofur, 

ceftriaxone, chloramphenicol, ciprofloxacin, gentamicin, kanamycin, 

naladixic acid, streptomycin, sulfisoxazole, tetracycline, and 

trimethoprim/sulfamethoxazole.

Table 3. Number and Percentage of Salmonella 

Isolates Resistant* to the Following Antimicrobials 

Antimicrobial Number Percent 

Tetracycline 459 78.6 

Sulfisoxazole 393 67.3 

Streptomycin 311 53.3 

Ampicillin 226 38.7 

Chloramphenicol 194 33.2 

Amoxicillin/ 

clavulanic acid 88 15.1 

Cefoxitin 88 15.1 

Ceftiofur 85 14.6 

Kanamycin 59 10.1 

Trimethoprim/ 

sulfamethoxazole 44 7.5 

Gentamicin 12 2.1 

Naladixic acid 0 0.0 

Ciprofloxacin 0 0.0 

Ceftriaxone 0 0.0 

Amikacin 0 0.0 

* Intermediate isolates were classified as not resistant.

Table 4 shows the multi-drug resistance of the 

584 isolates tested. About 20 percent of Salmonella

isolates were susceptible to all antimicrobial drugs 

tested. Overall, 79.6 percent of the isolates were 

resistant to at least one antimicrobial drug, 73.9 percent 

were resistant to more than one antimicrobial drug, and 

8.7 percent were resistant to nine or more drugs. 

Table 4. Number of Antimicrobials by Number and 

Percentage of Salmonella Isolates Showing 

Resistance* 

Number 

Antimicrobials 

Number 

Salmonella

Isolates 

Percent 

Salmonella

Isolates 

0 119 20.4 

1 33 5.6 

2 95 16.3 

3 116 19.9 

4 42 7.2 

5 92 15.7 

6 8 1.4 

7 14 2.4 

8 14 2.4 

9 or more 51 8.7 

Total 584 100.0 

*Intermediate isolates were classified as not resistant.
Conclusions ______________ 

For more information, contact: Although the prevalence of Salmonella in swine 

feces in the United States remains low (7.2 percent of 

fecal samples tested culture positive), 52.6 percent of 

sites had one or more positive fecal cultures. The top 

three Salmonella serotypes isolated from swine feces 

have remained the same since 1995. Overall, 85.1 

percent of isolates were resistant to 5 or fewer 

antimicrobials. Over 20 percent were not resistant to any 

15 antimicrobials evaluated. 

USDA:APHIS:VS:CEAH 

NRRC Building B, M.S. 2E7 

2150 Centre Avenue 

Fort Collins, CO 80526-8117 

970.494.7000 

E-mail: NAHMS@aphis.usda.gov 

http://nahms.aphis.usda.gov 

N536.0109 

______________ 

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

all its programs and activities on the basis of race, color, national 

origin, sex, religion, age, disability, political beliefs, sexual orientation, 

or marital status or family status. (Not all prohibited bases apply to all 

programs.) Persons with disabilities who require alternative means for 

communication of program information (Braille, large print, audiotape, 

etc.) should contact USDA’s TARGET Center at (202) 720-2600 . 

1 Centers for Disease Control and Prevention. 2007. 

Preliminary FoodNet data on the incidence of infection 

with pathogens transmitted commonly through food-10 

States, 2006. MMWR Morb Mortal Wkly Rep; 56:336-

339. To file a complaint of discrimination, write USDA, Director, Office of 

Civil Rights, Room 326-W, Whitten Building, 1400 Independence 

Avenue, SW, Washington, DC 20250-9410 or call (202) 720-5964 

(voice and TDD). USDA is an equal opportunity provider and employer. 

2 Mead, P.S., L. Slutsker, V. Dietz, L.F. McCaig, J.S. 

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

Food-related illness and death in the United States. 

Emerg Infect Dis; 5:607-625. Mention of companies or commercial products does not imply 

recommendation or endorsement by the USDA over others not 

mentioned. USDA neither guarantees nor warrants the standard of any 

product mentioned. Product names are mentioned solely to report 

factually on available data and to provide specific information.

3 Heymann, D. 2004. Control of Communicable Disease 

Manual. 18th ed. American Public Health Association, 

Washington D.C., p. 469-473. 

4 Bahnson, P.B., P.J. Fedorka-Cray, S.R. Ladely, and 

N.E. Mateus-Pinilla. 2006. Herd-level risk factors for 

Salmonella enterica subsp. enterica in U.S. market pigs. 

Prev Vet Med; 76:249-62. 

5 Hendriksen, S.W.M, K. Orsel, J.A. Wagenaar, A. Miko, 

and E. van Duijkeren. 2004. Animal-to-human 

transmission of Salmonella Typhimurium DT104A 

variant. Emerg Infect Dis; 10:2225-2227. 

6 Schwartz, K.J. 1999. In: Diseases of Swine. 8th ed. 

Ames, Iowa State University Press. p. 538.