Commonly held views that antibiotic resistance problems in humans can virtually all be traced back to overuse of drugs in livestock feed do not hold up to scientific review of the facts.

For years, scientists and policymakers have followed the assumption that farm animals are to blame for rising levels of antibiotic resistance causing illness in humans.

Following that assumption, last fall the Food and Drug Administration (FDA) issued a draft plan proposing that the safety of approved animal antibiotics be assessed based on antibiotic resistance. (See “Critics Resist FDA's Drug Plan,” page 12, Nov. 15, 2002 issue of National Hog Farmer.)

Gay Miller, DVM, professor of veterinary pathobiology and agricultural economics, University of Illinois, says repetition of that assumption has given it scientific validation that should be questioned.

Salmonella Research Project

Miller is part of a University of Illinois research team that is in the final year of a three-year project funded by the Illinois Department of Agriculture.

The team is looking at the farm-to-fork risk of pork production with regard to salmonella and antibiotic resistance.

While searching for facts on antibiotic resistance to use in the salmonella risk model, Miller made a startling discovery. There was no concrete evidence to back up assumptions from the U.S. Department of Agriculture (USDA) and other research centers that 87% to 96% of human salmonellosis cases were foodborne.

In a general article on antimicrobial resistance and foodborne illness recently published in the Journal of Food Protection, Miller and the Illinois team (David Barber, currently with the Wyoming Department of Health and Paul McNamara, Illinois Department of Agricultural and Consumer Economics,) reported that there were important antimicrobial resistance risks to humans from salmonellosis other than from livestock farms.

“We suggest that the role of food-producing animals in the origin and transmission of antimicrobial resistance and foodborne pathogens has been overestimated and overemphasized in the scientific literature; consequently, non-foodborne transmission, including pet-associated human cases, has been underemphasized,” the team concludes.

Figure 1 schematically represents the risk pathways connecting swine production and human health. “Although the most prominent pathway may be the diagonal pathway beginning on the farm and proceeding through the transport, lairage, slaughter, processing, distribution and consumption stages, alternative reservoirs of antibiotic-resistant bacteria and their possible interaction with the pork system are also considered,” the report points out.

In actuality, from 1993 to 1997, fruits and vegetables were identified as vehicles in more human salmonellosis outbreaks (13) than pork (5), chicken (6), turkey (6) or milk (3). The true origin of the vast majority of cases was never identified.

Underreported means of possible spread of salmonella bacteria, according to the Illinois researchers, include:

  • Human-to-food transmission: An outbreak of salmonellosis after a recent conference was traced to salsa that had been contaminated by an infected foodservice worker. This outbreak affected more than 650 people and was believed to be the first outbreak of a foodborne illness to sicken people from all 50 states.

  • Human-to-food animal transmission: “We don't think about microbial ecology, but there are actually microbes that are passed between humans and animals and animals to humans,” reports Miller. Most public health agencies exclude the importance of human-to-animal salmonella transmission.

    “It is reasonable to expect that pet owners or herdsmen who fail to wash their hands could transmit salmonella to animals just as the food worker in the salsa example above transmitted salmonella to humans,” the report explains.

    Moreover, the spread of antibiotic-resistant organisms from humans to animals could spread resistant organisms among food-producing animals, even without the use of antibiotics in the animals.

    In a 2000 study by North Carolina State University researchers, swine herds that didn't provide workers with access to toilet facilities on the farm were 11 times more likely to have a high prevalence of salmonella in hogs.

  • Nonfoodborne transmission: Evidence suggests that interpersonal transmission of salmonella is important in the U.S. and other countries. Infected humans shed salmonella from five weeks to three months, and 1% of cases can shed the bacteria for a year or more after remission of clinical signs.



Household pets are known risk factors for salmonella; antibiotic-resistant zoonotic pathogens, including salmonella, have been isolated from both dogs and cats.

Research Confirms Value of Antibiotics

A preliminary report of the salmonella research project confirms that hygiene and handling during processing and at home are more important in the fight against food contamination than what happens at the farm level, says Miller.

Another research paper she will publish in December relates antibiotic use to pig production, using National Animal Health Monitoring System data published by USDA.

“We demonstrated in the paper that profits were enhanced by 9% with antibiotic use overall. With small margins, right now the profit is less than $1/pig,” reports Miller.

“Still, it represents 9% of the producer's profits. If I were a producer and the government was telling me I was going to have to get rid of feed grade antibiotics, I would first want some pretty concrete evidence that antibiotic use is causing some major problems someplace,” she declares.

Miller grants there is some portion of antibiotic resistance that is farm-associated.

But for an antibiotic-resistant organism from the farm to survive and have an influence on human illness is a whole pathway “that I am convinced is only a small part of the problems that are seen,” she comments.

Miller hopes her research ignites a scientific inquiry into what has been written about antibiotics, and what is really known about antibiotics used in food animal production. Then decisions on their future use can be made based on the facts and not out of fear.