#02-141

Complete

Date Full Report Received

08/27/2004

Date Abstract Report Received

08/27/2004

Investigation

Institution:
Primary Investigator:

Resistance to two antimicrobial drugs frequently used in swine production, tetracycline and sulfa drugs, was common among Salmonella isolates from commercial farms. Since these isolates were from apparently healthy pigs, this resistance may not be important for therapy of disease in pigs. However, administering these drugs could result in increased prevalence of these strains, and potentially increase the number of Salmonella in slaughtered pigs. The presence of antimicrobial resistant strains probably increases the risk of passing antimicrobial resistant strains down the food chain, especially if the farms administer antibiotics to which these strains are resistant. Very few isolates, less than one percent, were resistant to antimicrobials more important for human therapy, including ciprofloxacin, amikacin, ceftriaxone, and trimethoprim / sulphamethoxazole. Thus, the human infections by these strains should be responsive to therapy, should foodborne infection occur. The presence of one gene accounted for the majority of tetracycline resistance. However, this gene was apparently linked to resistance genes or factors for a number of antimicrobials. Therefore, the use of tetracycline has the potential to select for Salmonella strains resistant to multiple antimicrobials. Farmers could, therefore, inadvertently select for multi-resistant strains when using only one antimicrobial, such as tetracycline. This could potentially compromise therapy on farms and pose an increased risk to pork food safety. Increased mixing of pigs appears to increase the risk of resistant strains, since multiple nurseries flowing to a single grower / finisher increased the risk of finding resistant Salmonella strains. The biological reasons for this cannot be determined by this research method, but bears further research. One possible cause is simply from mixing of pigs with more diverse Salmonella strains from separate populations. Other causal factors, not recorded by the survey, may also be associated with these farm types. Increased barn size, but not increased farm size, was associated with increased risk of antimicrobial resistant Salmonella. The presence of larger numbers of animals in a single barn may result in an increased variety of strains of bacteria at placement, increasing the likelihood of starting with more resistant strains. In addition, larger barns may be managed in different ways from smaller barns. These associated management factors could cause increased spread of antimicrobial resistant strains. Feeding pelleted feed was associated with antimicrobial resistance. Pelleted feed has been also associated elsewhere with increased risk of Salmonella shedding. Coupled with the findings here, additional research on the mechanisms by which pelleted diets might cause (or be associated with) these increased risks is warranted.