Nancy Cornick, the ISU associate professor of veterinary microbiology who conducted the study, noted a 2001 survey that showed 80% of producers treated their swine with antibiotics, mostly for disease prevention and growth promotion. In her study, Cornick examined the usage of three particular antibiotics - tylosin, chlorotetracycline, and bacitracin methylene disalicylate - that are generally used at dosages to encourage growth promotion.


Supplemented diet
Cornick's project showed that the pigs that were fed the diet supplemented with chlorotetracycline and tylosin shed significantly less E. coli O157:H7 than did pigs that were fed antibiotic-free diets. "The antibiotics I chose were the ones that were most commonly added at subtherapeutic doses, which is what they're usually looking for with growth promotion," Cornick said.


Cornick noted that many veterinarians favor an end to administering subtherapeutic antibiotics because they aren't used for disease treatment or disease prevention. The problem, she explained, is that incidents of disease in swine may increase when producers stop using the subtherapeutic antibiotics.


E. coli O157:H7 is well known as a significant cause of foodborne illness in meat that comes from cattle, but the pathogen is not as prevalent in swine. Studies in recent years have found reports linking pork products to outbreaks of human disease caused by E. coli O157:H7. Cornick acknowledged that such incidents are rare, but the potential problem is worth keeping on food producers' radar.


Feral pigs
Cornick pointed to the case of feral pigs in California that were suspected along with cattle of contributing to E. coli O157:H7 contamination in a vegetable field in the Salinas Valley in 2006. "I would argue that those feral pigs were probably exposed to fewer antibiotics than conventionally raised swine," Cornick said. "That may be a reason that they were colonised by the E. coli O157:H7."


Even without E. coli O157:H7 being a widespread occurrence in pigs, Cornick believes the potential makes it a problem worth investigating. With low level fecal shedding, the pigs can transmit the pathogen among each other. If usage of antibiotics drops off, Cornick wonders if there would be a corresponding increase of E. coli O157:H7. "Maybe there would be," she said, "or if I can find another reason why E. coli O157:H7 isn't in swine then maybe that's something cattle producers can use as a management strategy." 


Related website:
• Iowa State University