Bird flu has spread far and wide, infecting a range of species like chickens, skunks, cows, foxes and polar bears and renewing questions about what level of risk it poses to people as a B.C. teenager remains in critical condition in hospital after being infected.

But it’s the recent jump to pigs that has experts on high alert, as swine offer the perfect conditions for the virus to mutate, making it a potential threat to human health.

Last month, United States health officials reported the first case of avian influenza A(H5N1) in a pig on a backyard farm in Oregon, marking the first time the virus had been detected in pigs in the country. Days later, officials confirmed a second pig on the farm had also tested positive.

“With every species it jumps to, it elevates the risk,” said Kerry Bowman, a professor of bioethics and global health at the University of Toronto. “But pigs are particularly worrisome species. The risk has risen yet again.”

Pigs represent a particular concern for the spread of bird flu because they can become co-infected with bird and human viruses, which could swap genes to form a new, more dangerous virus that can more easily infect humans.

“Pigs can work as a mixing vessel, as they can have both bird flu and human flu simultaneously. And these things could recombine,” Bowman said, adding that this could result in the emergence of a new influenza A virus with different properties.

These “mixing vessel” events have happened in pigs in the past; it is believed to have caused the 2009 influenza A(H1N1) pandemic, Bowman said.

Currently, the risk of bird flu remains low, but Bowman said every time the virus jumps to a new species, it raises the risk.

Earlier this month, British Columbia health officials announced the first case of a human avian flu infection acquired in Canada, with the teenage patient still in critical condition in hospital. It is still not known how the teenager was exposed, but the strain is related to viruses found in flocks in an outbreak at B.C. poultry farms.

Most recently, the U.S. Centers for Disease Control and Prevention (CDC) on Nov. 15 confirmed a highly pathogenic form of bird flu in a person in Oregon. The infected person is linked to a previous outbreak tied to a commercial poultry operation in the state, where the virus has been confirmed in 150,000 birds.

And on Monday, U.S. health officials confirmed the presence of bird flu in Hawaii, the first case of the virus in a domestic flock in the state since the current outbreak began in 2022.

Human infection with avian influenza is rare and usually occurs after close contact with infected birds, other infected animals or highly contaminated environments.

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But that could change rapidly if the avian virus finds a suitable host in which to mutate, Bowman warned.

Because the virus was detected in a backyard pig, it doesn’t pose a significant threat at the moment, but the real concern lies with its potential spread in commercial pig farms, Bowman said.

“Industrial livestock is a nightmare; animals are kept in incredibly tight conditions. So it would spread very quickly in a commercial endeavour. And the other thing that goes on with commercial farming is pigs are moved around and sold, so there’s more cause of spread there,” he said.

Levon Abrahamyan, a virologist at the University of Montreal, echoed Bowman’s worries.

“Fortunately it was not a commercial pig farm,” he said. “The pig was probably infected by a wild bird. The risk is low at this moment.”

He explained that it would be very concerning if a large pig farm were found to be infected with avian influenza. In that case, every effort should be made to localize the outbreak and eliminate the virus to prevent further spread.

Abrahamyan said it’s usually difficult for avian flu to jump from birds to humans or from pigs to humans.

This is because viruses have surface proteins (a key) that must match specific receptors on the surface of host cells (the lock) to enter and infect the cell.

If the virus’s key doesn’t fit the cell’s lock, it can’t infect the cell, Abrahamyan said.

In the case of avian and pig flu strains, when two different strains (from birds and pigs) infect the same host, their genetic material can mix, he explained. This can create a new virus with a combination of “keys” that might fit locks on human cells more effectively.

“And that can be a dramatic change, and then the human immune system is not ready for that type of change,” he added.

This is exactly what happened in 2009, during the H1N1 pandemic, also known as the “swine flu.”

In this case, Abrahamyan said there was genetic mixing from human, bird and pig viruses. The pigs acted as mixing vessels, leading to the creation of a new subtype that could infect humans, and created a global pandemic.

There has been no evidence of person-to-person spread of bird flu so far. But if that were to happen, the ingredients for a potential pandemic could be there, scientists have said.

Bowman said he’s concerned that Canada is not doing enough in terms of testing and surveillance.

“Bird flu is not going away and it’s going to keep spreading in more species,” Bowman said. “If we look at this within Canada, a lot of species have bird flu now, a lot of our wildlife has it and domestic species and wild birds … and with every species this elevates.”

He stressed the need for more surveillance and timely testing and reporting.

The Canadian Food Inspection Agency has also been testing milk for signs of H5N1 in dairy cows.

There has not yet been any indication of the virus in Canadian cows, but bird flu has plagued many herds in the United States.

The Public Health Agency of Canada (PHAC) does wastewater testing for seasonal flu in several cities and towns across the country, including Toronto. But it doesn’t check specifically for H5N1 bird flu because it’s “not possible to differentiate positive wastewater signals that are due to wildlife versus human or livestock sources,” the agency told The Canadian Press in an email.

“We also need incentives for people working with livestock, as many may not want to report it because they don’t want to deal with the problem. So we need more reporting and we need more surveillance,”  Bowman said.

Concerns about the availability of bird flu vaccines are also growing, experts say. Currently, there is no avian influenza vaccine available for public use in Canada, though there are some available globally.

Angela Rasmussen, a virologist at the University of Saskatchewan’s Vaccine and Infectious Disease Organization (VIDO), told The Canadian Press that Canada should consider stockpiling H5N1 vaccines, similar to the U.S., rather than relying on agreements with manufacturers to supply them on demand.

This is because deploying an H5N1 vaccine under existing contracts could take three to six months.

However, PHAC stated in an email to The Canadian Press that it is not stockpiling H5N1 vaccines due to their limited shelf life, is only up to two years.

In an email to Global News in July, PHAC stated it has “proactively met with pandemic influenza vaccine suppliers (e.g., GSK, Seqirus and Sanofi) with whom we have an agreement for domestic or off-shore vaccine manufacturing to discuss pandemic influenza vaccine preparedness activities in order to inform steps that could be taken against avian influenza.”

–With files from Reuters and The Canadian Press