‘We haven’t come across a virus like this’: Could dabbling ducks be the link in avian flu’s spread?

Hundreds of seabirds found dead on Africa's coast. Remains of endangered condors discovered in California. Dozens of dead Canada geese removed from a Markham park.

Around the world, a highly pathogenic H5N1 strain of avian influenza is killing wild birds in numbers never seen before.

This strain is unfortunately affecting wild birds quite severely and many, many species are getting severely ill and dying from this virus," said Brian Stevens, the wildlife pathologist for the Ontario and Nunavut branch of the Canadian Wildlife Health Cooperative, a group of scientific experts that monitor the health of wildlife.

It's very different from most other avian influenza viruses that we've come across."

The current outbreak of avian flu, which began in Europe in 2020 and reached North America a year later, is said to be one of the biggest in history.

It's estimated 200 million birds have died worldwide.

In Canada, the virus has killed an estimated 46,000 wild birds since monitoring began here in December 2021, although many more mortalities have likely gone undetected, said Christopher Sharp, a population management biologist with Canadian Wildlife Service, a branch of Environment and Climate Change Canada.

Wild birds pick up the virus from other birds when migrating in large numbers, but they don't typically die from it.

The key difference from earlier outbreaks is that HPAI (highly pathogenic avian influenza) is circulating broadly in wild birds," said Sharp in an email. In addition, the current virus has led to significantly greater mortality in wild birds than what was previously experienced."

The impact of the virus could be lessened somewhat this year, he said, because it's now been circulating for a year, which could give wild birds some immunity. There will, however, likely be infection in young birds that have never encountered the virus before or in older birds whose immunity has waned with time, said Sharp.

Canada has been hit with outbreaks of HPAI before.

But in the past, the virus has been most dangerous for domestic poultry, where it spreads through contact with wild birds or indirectly through fecal matter and contaminated water, soil and feed, according to the Canadian Food Inspection Agency, which monitors for the virus on farms.

It can have devastating consequences," according to the agency. Millions of poultry were culled on farms in Canada last year to stop the spread of the virus, which attacks the intestinal and respiratory tracks of the animals and spreads quickly among chickens and turkeys and is more lethal."

Nearly 7.5 million poultry have been affected in the current outbreak, according to the Canadian Food Inspection Agency, although the number of domestic birds affected this year is down more than a third compared to the same time period last year.

However the federal agency notes that Canada is still in the midst of the spring migration period, which lasts until June.

This means that there is still a possibility for the number of affected birds to increase in the coming months," said the agency. It's important to note that the ongoing response is unique in the number of detections, the geographical distribution of outbreaks and its long duration, which has extended beyond typical migratory periods."

Stevens said that avian influenza viruses, like human flu virus, mutate and change.

Presumably at some point this virus is going to change and that is going to lessen the virulence," said Stevens. And it's going to be taken over by another strain of the virus that will then move to the population and hopefully not be as severe a strain.

But as I said, we haven't come across a virus like this," he said, one that sticks around in the wild population as long as it has.

So I don't know when that is going to happen or how long we're in for. But presumably at some point in the future this will lessen and we won't have to worry about it as much."

Scientists don't know the rate that the virus is killing wild birds because they don't know how many are infected.

For instance, more Canada geese died in the GTA this year compared to last year, including hundreds of dead geese that were removed from a Markham park this spring. The city hired a contractor to collect the birds, which were brought to a landfill and incinerated.

Canada geese seem to be more susceptible," said Stevens. We have people that contact us and say there's a handful of Canada geese that have died from this. But there's 50 others in the pond that seem to be fine.

So we don't know exactly how susceptible Canada geese are," said Stevens. But because they are out in parks where people are seeing them, they're reported to us more than many other species."

There is no evidence that the geese are carriers. Instead, scientists believe dabbling ducks such as mallards are the carriers, because they don't appear to be symptomatic or die from infections.

This makes them more likely to be a key species group for moving the virus around on the landscape," said Sharp.

The strain has however killed other species including snow geese, tundra swans and trumpeter swans. And scavenger birds that feed on remains have also fallen prey to the virus, including bald eagles, red tailed hawks, great horned owls, turkey vultures and crows, said Stevens.

In May, the Canadian Wildlife Health Cooperative diagnosed the death of the first mammals in Ontario, red fox kits.

Across the country, the virus has been found in 110 mammals across 10 species, including scavenging species such as the red fox, striped skunk and mink, as well as in opportunistic marine mammals such as harbour seal and the Atlantic white-sided dolphin, according to Sharp.

Mammals infected with HPAI have been found in all provinces and territories, except for New Brunswick, Northwest Territories, and Nunavut, he said.

It's extremely rare for the virus to infect humans, or pets for that matter, although a domestic dog in Canada was confirmed in April this year to have died from the virus.

Pet owners are advised to keep cats indoors and dogs on a leash to limit contact with potentially infected wild birds or carcasses, and to minimize exposure to outside environments such as ponds where birds congregate. Pets shouldn't be given any raw meat from game birds or poultry. And people shouldn't feed or handle any wild bird by hand.

The Canadian Food Inspection Agency said dead, injured, or sick birds should be reported immediately to the relevant provincial or territorial authority, or the Canadian Wildlife Health Cooperative information line at 1-800-567-2033 or by using their online reporting tool.

Patty Winsa is a Toronto-based data reporter for the Star. Reach her via email: pwinsa@thestar.ca