The demand for “natural” and organic meat products is growing, and most often consumers will cite antibiotic use as the reason. However, a recent study shows that more of the pigs raised without antibiotics carry bacteria and parasites associated with food-borne illnesses than their conventionally reared counterparts.

Researchers at The Ohio State University, University of Wisconsin and IowaStateUniversity studied hogs raised according to antibiotic-free protocols and in conventional pork production systems, where they received preventative doses of antimicrobial drugs. (See sidebar for a brief definition of the two systems.)

The outdoor farms “tend to have a higher occurrence of Salmonella, as well as higher rates of parasitic disease,” says Wondwossen Gebreyes, DVM, Ohio StateUniversity and lead project researcher. 

The scientists tested pigs on farms located in North Carolina, Ohio and Wisconsin. Of the total animals studied, 324 were raised in antibiotic-free systems and 292 lived on conventional farms. The researchers took blood samples to test for the presence of antibodies against both bacterial and parasitic infections.

More than half of the pigs on antibiotic-free farms tested positive for Salmonella, compared to 39 percent of conventionally raised pigs infected with the bacterial pathogen. The Toxoplasma gondii parasite was detected in 6.8 percent of antibiotic-free pigs, compared to 1.1 percent of conventionally raised pigs. Gebreyes points out that of the 616 pigs sampled, two “naturally” raised pigs tested positive for Trichinella spiralis, a parasite considered to be virtually eradicated from conventional U.S. pork operations.

The higher infection rates reported on the natural farms were consistent in all three geographic regions.

Of the three pathogens detected, the positive tests for the Trichinella spiralis surprised researchers the most. USDA’s last two National Animal Health Monitoring System surveys (2000 and 2006) had no Trichinella-positive tests, so the two associated with the antibiotic-free systems in this study of about 600 pigs were significant.

The industry has worked long and hard to eliminate concerns and build consumer confidence at home and abroad that U.S. pork is free of Trichinella. It remains a significant obstacle, keeping some consumers from purchasing pork at all or at the very least continuing to overcook the product.

Even though pork continues to carry the stigma associated with trichinosis, cases of any origin are rare today and are more likely to be associated with exposure to infected wild animals than domestic hogs. But should a case occur in humans that could be traced back to any type of pork product, it would create a major market challenge, especially for exports.

The nearly 7 percent of naturally raised pigs infected with Toxoplasma is a relatively small number but still represents a significantly higher infection rate than that found in the conventional herds, Gebreyes notes. Most people with a functioning immune system can resist symptoms associated with a Toxoplasma infection, which is considered most risky for pregnant women and those with compromised immune systems.

Salmonella is a more common food-borne illness concern. Exposure can come from many sources. The illness in humans typically causes diarrhea, fever and abdominal cramps that resolve within a week and rarely require treatment in healthy people. According to the World Health Organization, more than 1 million people are infected by Salmonella in the United States each year.

Routine antibiotic use does not fully prevent Salmonella occurrence even in conventional herds. The study shows that 39 percent of those pigs also tested positive. But antibiotics appear to have some benefits, Gebreyes points out. By comparison, 54 percent of antibiotic-free pigs tested positive for Salmonella.

“The advantage of using antibiotics is to prevent these infections from occurring. The disadvantage is it appears to create a favorable environment for bacteria strains that are resistant to antibiotics,” Gebreyes says. “On the other hand, when antibiotics are not used, the pigs tend to get less-resistant bugs but have higher rates of common bacteria that present food-safety concerns. The prevalence of Salmonella was significantly higher in the antibiotic-free herds than in the conventional herds. That could cause concern down the road about eating this (natural) product.”

The researchers theorize that the naturally raised pigs’ exposure to moisture, vegetation and other animal species could contribute to their higher pathogen rates. This study, which was sponsored by the National Pork Board, is part of a comprehensive examination of food-safety issues related to pork production that includes testing pigs for a broader range of disease-causing organisms.

“Does having an antibiotic-free and animal-friendly environment cause the re-emergence of historically significant pathogens? That is an extremely important question for consumers, policymakers and researchers to consider,” Gebreyes says.

At the very least, the study’s results raise significant questions about reality, perception and expectations.

Editor’s note: The study’s co-authors include Peter Bahnson, University of Wisconsin School of Veterinary Medicine; Julie Funk and James McKean, Iowa  StateUniversity, Collegeof Veterinary Medicine; and Prapas Patchanee, The Ohio State University, Department of Veterinary Preventive Medicine.

A Tale of Two


  • Antibiotic-free farms: On these farms, pigs were reared in open fields with free access to soil and water. They were given antibiotics only for treatment against active infections, and once the sick pigs were treated, they were separated from the herds and no longer marketed as naturally raised pork.
  • Conventional farms: On these farms, pigs remained indoors in ventilated barns and had free movement within pens. Antibiotics were added to their feed to promote growth and protect against infections. This was followed by a withdrawal period before slaughter to ensure that the meat contained no potential antibiotic residue.