Research at the University of Guelph and Health Canada shows that hogs do carry the E. coli strain after all.

Canadian scientists have discovered that hogs carry the same deadly strain of E. coli bacteria that killed and sickened people in Walkerton, disproving a theory that cattle spread the microbes but hogs were somehow immune.

There are many types of E. coli, distinguished by different genes. The most dangerous by far, called O157:H7, entered the water supply of Walkerton in May 2000 after heavy rains washed contaminated manure from a dairy farm.

Since then, hog producers have fended off concerns about contamination from factory hog farms by claiming that hogs are totally free of this strain of E. coli.

New hog farms, often with several thousand hogs in each barn, are springing up all across southern Ontario, as well as Quebec, Manitoba, New Brunswick and Saskatchewan.

But now research at the University of Guelph and Health Canada shows that hogs do carry the E. coli strain after all. It's not yet known how many hogs carry the strain of bacteria and "shed" them in manure.

The discovery "is very recent," said Carlton Gyles, a professor of microbiology at Guelph, which is Canada's biggest agricultural school.

In June he announced his work to a conference of the International Pig Veterinary Society in Ames, Iowa, and also presented results at a meeting in Centralia, Ont., north of London.

"For a long time we believed that they did not (spread O157:H7) because there have not been a lot of studies looking at it," Mr. Gyles said. One large study in the mid-1990s, where U.S. Department of Agriculture scientists looked at 4,000 hog manure samples, appeared to show none of them had the dangerous E. coli strain.

But that study did not use the most sensitive techniques.

"We used the best current techniques and we could find them (E. coli bacteria)," Mr. Gyles said.

They are in lower concentrations in hogs than in cattle, he said. As well, it's not yet known how many hogs are infected.

But at the same time, he said even even a low concentration may turn out to be dangerous if it is found in millions of litres of liquid manure, which giant hog farms often store in vats and spray on farm fields all at once.

"They tend to be in low numbers, but we certainly found them... If they are excreting the organism they certainly could contaminate the environment."

E. coli infection often causes severe bloody diarrhea and abdominal cramps, and can cause kidney failure and death.

In Sarsfield, where a Quebec company plans to convert a dairy farm to intensive hog production, one pig farm opponent says the discovery "will have major implications because of Walkerton."

"We're trying to get all that type of information together," said Cumberland councillor Phil McNeely. "It's certainly something that should create concern."

He said the presence of E. coli in pig manure should be considered as the province draws up its new regulations on manure management, to be issued next spring.

In Guelph, Mr. Gyles's work is not over. He's trying to pin down the numbers, and also to learn more characteristics of the bacteria.

The remaining mystery is that even O157:H7 has two different sub-types, or "lineages." One is found in humans and cattle, the other in cattle but not in humans.

"We don't know whether a pig will carry strains of human lineage or bovine (cattle) lineage." Mr. Gyles said. "We believe -- we don't know for sure -- that it's only the human one that makes you sick. But clearly cattle carry human-type strains as well."

Genetic researchers in Nebraska and at Health Canada are currently working to interpret the entire genome, or full set of 4,000 genes, in O157:H7.

So far pigs have never been shown to cause E. coli poisoning in humans, he said, "perhaps because pork tends to be cooked thoroughly.

He's currently trying to find out how many hogs carry the bacterium, and how many bacteria a single pig will carry and excrete.

"We still don't have a good handle on the level of the risk, basically."

In cattle, infection comes and goes unpredictably, like a cold bug spreading in an office.

"You test a herd this week and you get one cow positive. You test next week and that cow will be negative and maybe another one will be positive," he said.

"We believe what is happening is they become infected, they contaminate the environment and they become reinfected."