Visceral lesions, or visible signs of infection or organ damage, can help predict Salmonella contamination on pig carcasses, according to new research published in the American Journal of Veterinary Research this month.

Researchers at Iowa State University’s College of Veterinary Medicine found that Salmonella contamination was 90 percent more likely to occur in carcasses with lesions that were visibly identifiable, when compared to carcasses without visible lesions.

The study analyzed 202 conventionally raised pigs and 156 antimicrobial-free pigs in a Mid-western processing plant during December 2005 and January 2006. All carcasses were swabbed to check for Salmonella contamination and then both nonexperts and three veterinary pathologists examined the carcasses for lesions.

U.S. Department of Agriculture inspectors are on hand at every slaughter facility to inspect animals for visual health defects before and after slaughter. If during antemortem inspection, animals are deemed sick, they are kept out of the abattoir, or are tagged as suspect and then inspected postmortem, or after slaughter, which should, keep sick animals out of the food supply.

But as the researchers note, animals without overt symptoms can slip through inspections:

“Subclinically ill animals may pass antemortem examination. Such illness can manifest as chronic, residual lesions from a prior disease process (ie, fibrous adhesions) or active, low-grade infection or as inflammation (eg, adhesions, abscesses, arthritis, or injection-site lesions).”

Subclinical illness in food animals is a public health issue because it often means more Salmonella and Campylobacter contamination.

“Animals with long-term, low-grade illness are more likely to shed foodborne pathogens, particularly salmonellae, upon arrival at the abattoir than our healthy animals,” reads the study. “Such animals are likely to shed pathogens in the holding pens, providing an infection source for animals subsequently held here. Additionally, carcasses of animals with abscesses or other clinically important lesions require additional trimming or further handling during evisceration, increasing the likelihood of cross-contamination between carcasses.”

Researchers also found that the pigs that were not raised on antimicrobials were significantly more likely to be contaminated with Salmonella (17.3 percent), compared to pigs raised conventionally (4.5 percent). Conventionally raised pigs were found to have a slightly higher — though not significant — probability of enterococcus contamination.

The study authors note that there has not been a lot of research that has quantitatively demonstrated the relationship between veterinary health and human health as it relates to meat products. They suggest that more research should be conducted and that lesion evaluating systems aimed at reducing foodborne illness — even using nonexperts to evaluate — be considered in the development of public health practices and policies.