A second case of a devastating pig virus has been found on a Manitoba farm, one month after the federal government made a controversial change to the rules governing the washing of hog-transport trucks.

Veterinarians and provincial agriculture officials are tracking movements of trucks, farm staff, feed and equipment in an effort to learn how porcine epidemic diarrhea, a piglet-killing disease known as PED, took root in two hog farms in the southeast part of the province in recent days.

Manitoba's first two new cases of PED in about 16 months follow a move by the Canadian Food Inspection Agency, effective May 1, to reinstate a rule requiring pig transport trucks to use U.S. truck washes that the Canadian pork industry says are contaminated with the virus.

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Andrew Dickson, general manager of Manitoba Pork, urged the CFIA to force returning trucks to be sanitized in the province, where wash facilities are more rigorous than those across the border.

"It's really unfortunate," Mr. Dickson said. "We fully expected we were going to get one or two cases; I just hope this is it."

Egan Brockhoff, a veterinarian who has worked with the virus in Canada and around the world, said it is too soon to name the new source of the virus in Manitoba, but dirty trucks are a likely cause.

"There's a short list of how the virus could have entered the farm, and transport would always be at the top of our list – contaminated transports and a concurrent biosecurity breakdown," Dr. Brockhoff said by phone.

PED is transmitted through manure and spread among farms, abattoirs and assembly yards by dirty boots and trucks. A 2014 outbreak in Ontario was linked to feed containing infected pigs' blood.

The disease has killed more than eight million pigs in the United States, where it remains widespread, causing pork prices to soar. The disease does not affect humans or food safety.

In Canada, the virus has been centred in Southern Ontario. Western Canada has seen only a few cases, a situation the industry says is due to heightened biosecurity efforts that include a temporary measure enacted by the CFIA to allow pig trucks returning from the United States to use truck washes in Manitoba.

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Unlike many washes in the United States, the domestic facilities do not recycle water, and they use high-temperature drying systems that are effective ways to kill the virus.

But the CFIA has declared the worst of the outbreak over and reinstated a rule that forces trucks to be cleaned before returning to Manitoba. The agency will not be re-establishing the domestic truck-washing rule, it said in an e-mail to The Globe and Mail.

For pigs raised in Alberta, Saskatchewan and Manitoba, Emerson, Man., is the main entry point into the United States. About 120 trucks and 65,000 pigs – mostly weanlings – cross the border every week.

The Western Canadian pig industry has been urging truck operators and farmers to continue to use the more effective Canadian washing sites in a bid to stave off the outbreak that can cost a farmer several months' production.

Dr. Brockhoff said the disease is devastating for the animals and the people who care for them.

"It's so frustrating that a herd became infected," he said. "That virus is such a horrible virus when it enters a pig barn. All the piglets less than 10 days of age are going to die from that virus. There's very high piglet mortality. All the piglets from 10 to 21 days of age are going to get severe diarrhea and really go through a difficult disease syndrome. The sows are going to be vomiting. The sows are going to get diarrhea."

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"And then there's the people who have to look after those animals," he added. "It's got to be disheartening going into those barns … and seeing animals suffering like that. And for the family that owns the farm, it's an emotionally taxing thing. There's pressure in the community to do the right thing and keep the virus from spreading. There's a social stigma: 'Where was your breakdown?'"