November 25, 2008
Surprising numbers of healthy livestock carry bacteria that can be harmful, 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 and most of them were resistant to at least some antibiotics.
PLoS ONE journal published in their paper that the abundance of infected but asymptomatic hosts in all provinces represents a serious threat to food safety.
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.
Experts say developing vaccines for animals against common food-borne microbes, changing what livestock are fed and how they are raised are among the ways that infection could be cleared from even asymptomatic animals.
Mr. Perron, now a doctoral student in zoology at Oxford University, said asymptomatic animals have in the past generally been ignored by scientists, partly because it was easier to focus on clearly ill animals, rather than try to figure out which healthy livestock carried harmful bacteria.
His team, made up of researchers from McGill University and University of Montreal, tested more than 7,400 pigs in five provinces.
For the sake of the study, they focused on salmonella. They found that about 6 percent of the pigs carried the bacteria, although the range was from as little as about 1 percent in Saskatchewan to more than 9 percent in Ontario, much higher than earlier estimates.
The symptom-free hogs also carried several different strains of salmonella, including a number that were resistant to one or more antibiotics.
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, admitted Mr. Perron.
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 -- such as one for E. coli that VIDO helped develop recently -- changing what livestock are fed and keeping their living conditions more hygienic, he said.