Hidden diseases

Healthy livestock can carry harmful bacteria

Tom Blackwell, National Post


Published: Monday, November 24, 2008

Source of Article:  http://www.canada.com/vancouversun/health/story.html?id=987983

Surprising numbers of seemingly healthy livestock carry bacteria that can be harmful to humans, representing a "hidden reservoir" of disease that poses a serious risk to public health, a new Canadian study has concluded.

Montreal-based researchers sampled thousands of "asymptomatic" pigs -- those that showed no signs of illness and would likely end up slaughtered and sold as meat products -- in what they called the first such research of its kind.

They found that many carried strains of salmonella that can make humans sick, and most of the bacteria were resistant to at least some antibiotics.

"The abundance of infected but asymptomatic hosts in all provinces represents a serious threat to food safety," said their paper, just published in the journal PLoS ONE.

"Asymptomatic carriers can [also] have a significant role in the contamination of the environment and other animals, since large volumes of the bacterium can be excreted during fattening, transport and slaughter."

To try to keep tainted meat off the market, farmers and veterinarians tend to single out animals that appear ill, then either treat or cull them.

But as authorities fight to curb Canada's continuing food-safety problem, the new findings suggest they should put more focus on healthy animals, said Gabriel Perron, one of the study's authors. That involves developing vaccines for animals against common food-borne microbes, changing what livestock are fed and how they are raised.

Mr. Perron, now a doctoral student in zoology at the University of Oxford, said asymptomatic animals have in the past been generally ignored by scientists, partly because it was easier to focus on animals that were clearly sick themselves.

His team, made up of researchers from McGill University and the University of Montreal, tested more than 7,400 pigs in five province.

For the sake of the study, they focused on salmonella, cause of many human disease outbreaks, which trigger symptoms ranging from vomiting to fever and bloody diarrhea. They found that about 6% of the pigs carried the bacteria, though the range was from as little as about 1% in Saskatchewan to more than 9% in Ontario, much higher than earlier estimates, their paper said.

The symptom-free hogs also carried several different strains of salmonella, including a number that were resistant to one or more antibiotics. Treating people who contracted one of the strains from an asymptomatic pig with ampicillin, a "broad-spectrum" antibiotic, would result in treatment failure in 50% of patients, the study says.

The study's findings would likely hold true for other bacteria and animals, Mr. Perron said. He noted that E. coli 0157-H7, the bug that caused the Walkerton outbreak and so-called hamburger disease, does not cause illness in the cattle that carry it.

While the study suggests that healthy farm animals should be a greater concern for food-safety experts, tackling the issue will not be easy, Mr. Perron admitted.

"It is really difficult to identify asymptomatic hosts and it would be logistically impossible to test every single animal to see if they are asymptomatic carriers," he said.

Andrew Potter, head of the University of Saskatchewan's Vaccine and Infectious Disease Organization (VIDO), said such livestock are, in fact, getting more attention now.

The options to clear them of infection include vaccines, changing what livestock are fed and keeping their living conditions more hygienic, he said. "We need to look at the whole food chain a bit differently," he said. "Disease [in animals] isn't necessarily the end point here."



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