Bird flu keeps spreading beyond birds. Scientists worry it signals a growing threat to humans, too

As a deadly form of avian influenza continues rav‐ aging bird populations around much of the world, scientists are tracking in‐ fections among other ani‐ mals — including various types of mammals more closely related to humans.

Throughout the last year, Canadian and U.S. officials de‐ tected highly pathogenic H5N1 avian flu in a range of species, from bears to foxes. In January, France's national reference laboratory an‐ nounced that a cat suffered severe neurological symp‐ toms from an infection in late 2022, with the virus showing genetic characteristics of adaptation to mammals.

Most concerning, multiple researchers said, was a large, recent outbreak on a Spanish mink farm.

Last October, farm work‐ ers began noticing a spike in deaths among the animals, with sick minks experiencing an array of dire symptoms like loss of appetite, excessive saliva, bloody snouts, tremors, and a lack of muscle control.

The culprit wound up be‐ ing H5N1, marking the first known instance of this kind of avian influenza infection among farmed minks in Eu‐ rope, notes a study published in Eurosurveillance this month.

"Our findings also indicate that an onward transmission of the virus to other minks may have taken place in the affected farm," the re‐ searchers wrote.

Eventually, the entire pop‐ ulation of minks was either killed or culled — more than 50,000 animals in total.

That's a major shift, after only sporadic cases among humans and other mammals over the last decade, accord‐ ing to Michelle Wille, a re‐ searcher at the University of Sydney who focuses on the dynamics of wild bird viruses.

"This outbreak signals the very real potential for the emergence of mammal-tomammal transmission," she said in email correspondence with CBC News.

It's only one farm, and no‐ tably, none of the workers — who all wore face shields, masks, and disposable over‐ alls — got infected.

But the concern now, said Toronto-based infectious dis‐ ease specialist Dr. Isaac Bo‐ goch, is if this virus mutates in a way that allows it to be‐ come increasingly transmissi‐ ble between mammals, in‐ cluding humans, "it could have deadly consequences."

"This is an infection that has epidemic and pandemic potential," he said. "I don't know if people recognize how big a deal this is."

WATCH | Explosive avian flu hits global bird popula‐ tions:

H5N1 has high mortality rate

Among birds, the mortality rate of this strain of highly pathogenic avian influenza can be close to 100 per cent, causing devastation to both wild bird populations and poultry farms.

It's also often deadly for other mammals, humans in‐ cluded.

The World Health Organi‐ zation (WHO) has document‐ ed 240 cases of H5N1 avian in‐ fluenza within four Western Pacific countries — including China, Cambodia, Laos, and Vietnam — over the last two decades. More than half of the infected individuals died.

Global WHO figures show more than 870 human cases were reported from 2003 to 2022, along with at least 450 deaths — a fatality rate of more than 50 per cent.

Bogoch said the reported death toll may be an overesti‐ mate, since not all infections may be detected, though it's clear people can "get very, very sick from these infec‐ tions."

Most human infections al‐ so appeared to involve peo‐ ple having direct contact with infected birds. Real-world mink-to-mink transmission now firmly suggests H5N1 is now "poised to emerge in mammals," Wille said — and while the outbreak in Spain may be the first reported in‐ stance of mammalian spread, it may not be the last.

"A virus which has evolved on a mink farm and subse‐ quently infects farm workers exposed to infected animals is a highly plausible route for the emergence of a virus ca‐ pable of human-to-human transmission to emerge," she warned.

Louise Moncla, an assis‐ tant professor of pathobiolo‐ gy at the University of Penn‐ sylvania school of veterinary medicine, explained that hav‐ ing an "intermediary host" is a common mechanism through which viruses adapt to new host species.

"And so what's concerning about this is that this is exact‐ ly the kind of scenario you would expect to see that could lead to this type of adaptation, that could allow these viruses to replicate bet‐ ter in other mammals — like us."

Surveillance, vaccines both needed

What's more reassuring is the ongoing development of influenza vaccines, giving hu‐ manity a head start on the well-known threat posed by bird flu.

Wille noted the earlier spread of H7N9, another avian influenza strain which caused hundreds of human cases in the early 2010s, prompted similar concern that the virus would acquire the mutations needed for on‐ going human-to-human transmission.

"However, a very aggres‐ sive and successful poultry vaccination campaign ulti‐ mately stopped all human cases," she added.

But while several H5N1 avian influenza vaccines have been produced, including one manufactured in Canada, there's no option approved for public use in this country.

To ward off the potential threat this strain poses to hu‐ man health, Bogoch said on‐ going surveillance and vaccine production needs to remain top-of-mind for both policy makers and vaccine manufac‐ turers.

Dr. Jan Hajek, an infectious diseases physician at Vancou‐ ver General Hospital, also questioned whether it's time to wind down global mink farming, given the spread of various viruses, from avian in‐ fluenza to SARS-CoV-2, the virus behind COVID-19.

"We're closely related to minks and ferrets, in terms of influenza risks … if it's propa‐ gating to minks, and killing minks, it's worrisome to us," he said.

In 2021, B.C. officials an‐ nounced an end to mink farming across the province, saying the farms can be reser‐ voirs for viruses and repre‐ sent an ongoing danger to public health. All mink farm operations must be shut down, with all of the pelts sold, by April 2025.

However, other provinces — and plenty of countries — do intend to keep their mink farms operating.

"Is it responsible to have these kinds of farming condi‐ tions where these types of events can occur?" ques‐ tioned Moncla. "If we're going to keep having these types of farms, what can we do to make this safer?"