Germ Warfare: Monitoring hogs to protect food supply
FoodNet monitors eight bacterial foodborne pathogens in 10 states representing 44.5 million people or 15 percent of the U.S. population. In 2005, FoodNet recorded a 49 percent decrease in human yersiniosis cases from 2002 to 2005. This decrease may be due to the overall decline in foodborne illnesses linked to pork.
Pigs are the major animal reservoir for Y. enterocolitica, which can be isolated from either the tonsils or feces. Pigs harbor serotypes O:3; O:5, 27; O:8; and O:9, which infect humans.
Y. enterocolitica virulent strains carry a gene, the adherence and invasion locus (the ail gene), which encodes a virulence protein. Thus an isolate which carries the ail gene is considered a potential human pathogen.
Because swine are the major animal reservoir strains for pathogenic for humans, there is a need to monitor hogs for the ail-positive Y. enterocolitica.
In collaboration with the USDA-APHIS-National Animal Monitoring System (NAHMS) Swine 2000 study, we screened U.S. hogs in the top 17 pork producing states. State veterinarians collected fecal and tonsilar swabs. We detected Y. enterocolitica in 13.1 percent of individual hogs and on 45.1 percent (55 of 122) of premises.
We also serotyped and determined virulence attributes for a subset of porcine fecal isolates (n=109) and found Y. enterocolitica serotypes O:3 (75.2 percent) and O:5 (24.8 percent). All of these isolates harbored the ail-gene, indicative of their potential to infect humans.
With NAHMS epidemiologists, we correlated the presence of Y. enterocolitica with 40 on-farm management practices, based on questionnaires given to the hog producers. Some of the factors analyzed included the presence of rodents, cats or dogs on the premise as well as other livestock, presences of roundworms, gastric ulcers, and age at which pigs experience respiratory illness. Ultimately, four risk factors were identified with their accompanying odds ratio (OR): location in a central state, (OR=0.3), vaccination for E. coli (OR=3.0), percent deaths due to scours (OR=3.5), and presence of meat/bone meal in grower-finisher diet (OR=4.1). A full report can be found in our article published in the Journal of Food Protection, June 2008 issue.
Continued surveillance will show the decline of potential human foodborne pathogens in the U.S. pig herd. Correlation of on-farm prevalence with management practices will identify risk factors as well as factors that protect against the introduction of Y. enterocolica in the grower/finisher pig.