Salmonella prevalence in pigs reared on farms with and without antimicrobials

A convenience sample of farms using antimicrobials (antimicrobial-using, AMU) post- weaning for therapy and/or growth promotion (n=21) were contrasted to a convenience sample of farms not using antimicrobials post-weaning (antimicrobial-free, AMF, n=21) distributed across three US geographic areas. Fecal samples were screened for Salmonella enterica prior to slaughter and carcasses were tested after slaughter. Fecal samples were collected from 30 pigs per farm. At slaughter, 10 carcasses were swabbed at each of three locations on the processing chain. Salmonella spp. were isolated using conventional methods. The proportion of Salmonella positive fecal samples among AMF farms was higher (27.3%, CI 12.8-41.8%) when compared with AMU farms (12.3%, CI 5.0-19.5%). No differences were found among carcass swabs or antibiograms. It is concluded that AMF production may not enhance Salmonella food safety in the pork chain. Introduction Antimicrobials have been commonly used in commercial swine production. In the US, 88.5% of producers reported using antimicrobials in the feed at least once in the six months prior to the survey (USDA, 2002). Although commonly used, the effects of antimicrobials on the shedding of Salmonella enterica have not been quantified in commercial US settings. Logically, the application of antimicrobials could reduce the prevalence or duration of Salmonella shedding (Ebner, et al., 2000, Evangelisti, et al., 1975). Conversely, shedding of antimicrobial resistant Salmonella strains could be enhanced when antimicrobials are provided Salmonella-positive pigs. As part of a multi-state study to determine the prevalence, antimicrobial resistance and geno- typic diversity of three major foodborne pathogens in swine, Salmonella, Campylobacter and Yersinia, the goal of this report was to compare Salmonella spp. in market swine reared either on farms using or avoiding antimicrobials in growing pigs. The larger study includes 60 farms sam- pled in three distinct geographic areas in the US in North Carolina, Ohio/Michigan and the por- tions of the upper Midwest (South Dakota, Iowa, Minnesota and Wisconsin). Methods A total of 42 farms were selected, distributed over three geographic locations in the U.S: North Carolina, Ohio-Michigan, and South Dakota, Iowa, Minnesota and Wisconsin. Antimicrobials were provided in the feed on 21 farms for at least part of the post-weaning period, and 21 avoided antimicrobials post-weaning. In the latter group, pigs requiring antimicrobial thera- py were treated individually and were then removed from the population at the time of treatment and not re-introduced to the population. Fecal samples (10 g) were collected from each of 30 pigs on each farm within 72 hours of slaughter. In the case of small farms marketing less than 30 animals on a single day, all pigs des- tined for slaughter were sampled. At slaughter, sterile 8.9 x 12.7 cm cellulose swabs were used to sample skin surfaces. These were 3x larger area than the official USDA carcass swabs to allow the swabs to be divided after collection and distributed to the three participating laboratories. Shortly before evisceration 10 carcasses were swabbed (swab A) and the same 10 carcasses were sampled post-evisceration (swab B). After chilling, one swab was collected from each side of 10 carcasses on either side (i.e., left and right sides) of the carcass. These swabs were taken from carcasses not sampled previously to avoid sampling from the same surface. The first of these post-chilling samples, swab C, was collected in the same manner as swabs A and B. Swab D was collected from the opposite side using the published USDA: FSIS methods (Federal Register, 1996), except that both the swab and template were the larger size described above. A 300 cm 2 area was sampled in each of three locations, on the ham, lateral to the ventral midline, and on the neck using a sanitized template. In cases where the jowl area had less than 300 cm 2