How is bird flu spreading in dairy cattle? Here’s where the clues are pointing — and what it could mean to Canada

Whether Canada can expect bird flu to hit dairy cattle here could hinge on the answer.

“That first transmission in cattle would have come from a bird,” said Scott Weese, a veterinarian and professor at the University of Guelph, noting that cows come in contact with wild birds in barns or the pasture. “We don’t really know what happened after that. That’s the challenge at this point.”

The U.S. Department of Agriculture is tracking the situation but has not yet confirmed the virus’s transmission route.

”Is this being spread from cow to cow or is this continually being reintroduced from birds?” said Weese. “Or is there some other source we don’t know. And that has big control implications for us in Canada.

“If it’s just cow-to-cow transmission in the U.S., we can keep it out of the country by not bringing U.S. dairy cows into Canada,” said Weese. “If it’s introduced repeatedly by wild birds, well, we’re going to have the same wild birds they have. So we’ve got more risk.”

The spring migration of wild birds to Canada has already begun.

In recent years, the highly pathogenic avian influenza (HPAI) has been circulating widely in wild birds around the world, often causing the culling of millions of chickens and turkeys when it spreads to domestic poultry. In Canada, the virus has also been found in mammals, many of them scavenging species that may have fed on infected birds. 

But the virus has never been transmitted to cattle.

Avian flu causes inflammation in the udder of dairy cows, causing reduced milk production and milk that appear thick and curdled, said Weese. Milk from infected cows is thrown out.

If some of the virus does make it into the milk, it would not survive processing.

“The virus is not very resistant to heat so pasteurization should be more than enough to kill it,” said Lawrence Goodridge, a professor of food safety at the University of Guelph and director of the Canadian Research Institute for Food Safety.

“It also has to go through a stomach, which has a low PH, which would also destroy the virus,” said Goodridge.

Even so, scientists are monitoring the situation closely not only to protect the industry, but to ensure that as the virus picks up steam, and moves through herds, that it doesn’t mutate and add factors that could help it infect humans.

“Any time it moves into a different species, you’re creating more of an opportunity for that virus to change,” said Weese. “And what we don’t want to do is to see it accumulate more and more factors that make it able to infect mammals, including us.”

One farm worker in Texas has already been infected with bird flu, but Weese said the case was mild.

The Canadian Food Inspection Agency said in an email that it has not detected the virus in dairy cattle in Canada. The agency is monitoring the situation closely. Dairy farmers are being asked to watch their cows for signs of the virus and to contact their vet if they suspect a case.

In the U.S., at least 17 states have limited cattle importation from the eight states where the virus is known to have infected dairy cows.

Canada could do the same, said Weese, limiting imports of dairy cattle into the country if it turns out the virus is being spread by the cows or the equipment used to milk them.

Weese said there is limited data on infections.

But he said samples from the respiratory secretions of cows have rarely tested positive for the virus, whereas the milk seems to have a lot of virus in it, which could point to the virus being passed through milking equipment that is being used on multiple animals on dairy farms.

Dairy Farmers of Canada said it is in close communication with the Canadian Food Inspection Agency.

“Canadian dairy producers already adhere to some of the highest biosecurity standards in the world,” said the organization in an email. “It should be noted that only milk from healthy animals is authorized for distribution and for human consumption.  Additionally, pasteurization, a requirement in Canada, kills harmful bacteria and viruses while retaining the nutritional properties of milk.”

Both Weese and Goodridge note that drinking unpasteurized milk should be avoided.

Avian flu is ”just one more reason to consider avoiding raw milk,” said Weese. “We know raw milk is associated with lots of infectious disease risks.”