The birds are back in town. Spring migration could fuel spread, evolution of avian flu, scientists warn

Across Canada, an estimated seven million birds and counting have been infected with highly pathogenic avian influenza (HPAI) during a devastating global outbreak that shows no signs of winding down.

The virus is also striking and killing other species, from farmed mink to wild sea lions to a domestic dog in Ontario, federal officials announced last week.

Now, with millions of birds crossing the continent and flying north to Canada in the months ahead, scientists warn it's yet another opportunity for this virus to spread and evolve.

"It gives more opportunity for transmission," said veterinarian Dr. Scott Weese, a professor at the Ontario Veterinary College. "And if we have birds coming from different areas where the virus has evolved differently, it creates more opportunities for this virus to change further."

Canadian researchers are watching this year's migration season closely. They aim to gauge how it impacts ongoing virus transmission, and to determine which species are being hit hardest by HPAI — and which ones are, somehow, beating the odds.

WATCH | Scientists say bird migration routes help spread deadly avian flu: 

Able to infect mammals

This fast-spreading H5N1 strain has already caused mass mortality among waterfowl, raptors, and poultry flocks.

Deaths among mammals appear largely relegated to close interactions with infected birds — the Ontario dog that died, for instance, had been "chewing on a wild goose," as noted in a government release — though a large outbreak on a Spanish mink farm also suggested possible mammal-to-mammal transmission.

What's particularly concerning for Dr. Samira Mubareka, a clinician-scientist with Sunnybrook Health Sciences Centre in Toronto, are the early signals that this virus appears to be evolving in ways that make it more adept at infecting mammals, with heightened virulence.

One recent study from Canadian researchers, which looked at sporadic H5N1 cases among 40 different species including red foxes, skunks, and mink, found the disease presented as a central nervous system infection, with 17 percent of the virus samples showing "mammalian adaptive mutations."

"I'm definitely more concerned now than I was ten years ago…. We're seeing neurological illness, we're seeing [viral] reassortment," Mubareka said.

Migratory birds are particularly at risk of HPAI, and the mutations they bring back from southern climates this spring and summer could be different from those currently circulating in Canada, she added, potentially posing new threats at a time when scientists and farmers are already struggling to track and contain the virus.

Hundreds of Canadian poultry farms have battled outbreaks so far, which often involve culling vast numbers of birds. There's also growing concern that reported deaths among wildlife — which are tougher to track — are just the tip of the iceberg.

"Is it a handful of mammals affected?" questioned Weese. "Or is it happening more in the background, and they die and just don't get noticed?"

'Some immunity is built up'

Andrew Lang, a professor in Memorial University's biology department, said there were waves of infections among seabirds on the East Coast last year, and various species fared differently, potentially impacting their subsequent migration patterns.

"Some birds seem to migrate normally while infected," he said, "while other birds get extremely sick and are almost guaranteed to die."

Those with prior exposure to milder strains of avian influenza likely built up some level of cross-protective immunity, while other species hadn't.

"That's what I think happened with northern gannets," Lang said, speaking of a species native to Newfoundland whose death toll is likely in the thousands. "Avian influenza typically doesn't circulate within those birds, because of their biology and limited interactions with other birds. When this strain came along, it devastated them."

WATCH | 'Explosive' avian flu hits global bird populations:

Jennifer Provencher, a research scientist with Environment and Climate Change Canada's (ECCC) wildlife health and ecotoxicology division, said European researchers have seen similar patterns, with blood testing showing that there are birds who are getting the H5N1 strain and surviving.

"And so we would expect that some immunity is built up at least in some of the populations," she said.

Going forward, Provencher stressed the need for ongoing surveillance, and said members of the public can report sightings of dead or dying wildlife to ECCC through the Canadian Wildlife Health Cooperative.

"That will actually help us predict where we need to be paying attention," she said.

Beyond the deaths of poultry and wildlife, Lang said it's also important to find out which bird species in Canada are developing antibodies against the virus.

"If we can sample as many birds as possible this coming year — the ones alive this year — can we find evidence that they were infected last year and survived?"