Project Summary

Source and Prevalence of Multi-Drug Resistant Salmonella in Dairy Cattle

Principle Investigator(s):
Tom Edrington, Ph.D.
Food and Feed Safety Research Unit, USDA-ARS
Completion Date:
June 2006



Dairy cattle serve as an important reservoir for Salmonella and have been implicated in cases of human salmonellosis. Cattle typically show no symptoms while shedding Salmonella into the environment. Previous research conducted by USDA-ARS showed the prevalence of mature dairy cattle shedding this pathogen in their feces varies greatly but can be as high as 100%. Further, serotype prevalence and diversity varies among farms and season. 

The development of antimicrobial resistance in pathogenic bacteria species, including Salmonella, has been the subject of considerable research in the past decade. Salmonella Newport gained attention in 1983 when a multi-drug resistant strain was isolated from several infected people that apparently acquired this Salmonella from ground beef originating from beef cattle. In 1985, Salmonella Newport was isolated from human patients in California with an unusual resistance pattern (including chloramphenicol) that was traced through hamburger to dairy farms. Other researchers have reported the prevalence of Salmonella Newport appeared to be increasing in California and that a chloramphenicol resistant strain was the most common serotype identified in 1986. Most recently, in 2002, numerous people in the Eastern United States were made ill and one later died as a result of a multi-drug resistant (MDR) strain of Newport which was later implicated as coming from ground beef from dairy cattle. Research conducted previously identified antimicrobial resistant Salmonella isolates in dairy cattle, although most resistance patterns were to antimicrobials commonly used in veterinary medicine. However, subsequent dairy research identified multi-resistant Salmonella isolates, including serotype Newport, with resistance patterns similar to those reported in Newport isolates in the 2002 outbreak. 

The occurrence of MDR Salmonella in dairy cattle and the link to human cases of illness in the United States is cause for concern. However, while it is generally recognized that dairy cattle are reservoirs for Salmonella, some of which may be MDR, the prevalence and source of MDR Salmonella in dairy cattle has not been extensively studied. The hypothesis in this study is that MDR Salmonella will comprise a small proportion of all dairy Salmonella isolates and they are associated with certain classes of cattle within the dairy operation 

The objectives of this study were as follows: 

  1. Determine prevalence of multi-resistant Salmonella in dairy cattle (calves, heifers, lactating cattle, dry cows, and cull cows).
  2. Determine if co-mingling of calves from multiple farms at a heifer feedlot serves as a transmission vector for multi-resistant Salmonella back to the dairy.
  3. Examine waste milk for multi-resistant Salmonella prior to use as calf feed. 

This research was conducted on four large commercial dairies (> 2000 head each) in the southwestern United States. In previous studies, some of the dairies sampled had significant Salmonella prevalence in the lactating cattle. All four dairies are located within a small geographical region (approximately 5 miles) and are managed similarly. Mature animals (lactating and dry cows, 12 and 24 month old heifers, and sick/fresh cows) are maintained in large dry-lot pens with shade provided. Calves from each dairy are transported to a central heifer raising facility located within one of the dairies and raised there until just prior to freshening, at which point they return to their farm of origin. Newborn calves are maintained in individual calf hutches until 2-3 months of age at which time they are group penned according to age and size. 

Fecal pat samples were collected from various groups of cattle on one or two occasions, October 2005 and/or March 2006. The following cattle groups were sampled: Hutch calves (sampled twice); 12 and 24 month heifers (sampled once in October); lactating cows (sampled twice); dry cows (sampled once in March); and cows in sick/fresh pens (sampled once in October). 

All fecal samples were tested for the prevalence of Salmonella and resistance to multiple antimicrobial drugs. The following antimicrobials were tested: NARMS panel (cefoxitin, amikacin, chloramphenicol, tetracycline, ceftriaxone, amoxicillin/clavulanic acid, ciprofloxacin, gentamicin, nalidixic acid, ceftiofur, sulfisoxazole, trimethoprim/sulphamethoxazole, kanamycin, ampicillin and streptomycin) and Bovine/Porcine panel (ceftiofur, erythromycin, chlortetracycline, oxytetracycline, penicillin, tiamulin, gentamicin, florfenicol, ampicillin, danofloxacin, sulphadimethoxine, neomycin, sulphachloropyridazine, tylosin, sulphathiazole, spectinomycin, tilmicosin, clindamycin, trimethoprim/sulfamethoxazole and enrofloxacin.


Fecal Salmonella prevalence varied among groups of cattle and between the two collection periods. Hutch calves and cattle in the sick/fresh pen were more likely to harbor MDR Salmonella than heifers or lactating and dry cows. However, a significant number of MDR Salmonella were detected in lactating cattle on one farm during the October collection. The MDR Salmonella belonged to serogroup B and were predominantly of the serotypes Reading and Typhimurium. No Salmonella Newport was identified. A small portion of the isolates examined for antimicrobial resistance were MDR and 30 isolates were resistant to 9 or 10 antibiotics on the NARM’s panel.


While the presence of MDR Salmonella is a cause for concern, all isolates examined were susceptible to ciprofloxacin and ceftriaxone, two antibiotics used in the treatment of severe cases of human salmonellosis.