Bird flu keeps spreading beyond birds. Scientists worry it signals a growing threat to humans, too
As a deadly form of avian influenza continues ravaging bird populations around much of the world, scientists are tracking infections among other animals — including various types of mammals more closely related to humans.
Throughout the last year, Canadian and U.S. officials detected highly pathogenic H5N1 avian flu in a range of species, from bears to foxes. In January, France's national reference laboratory announced that a cat suffered severe neurological symptoms 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 workers 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 being H5N1, marking the first known instance of this kind of avian influenza infection among farmed minks in Europe, 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 researchers wrote.
Eventually, the entire population 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, according to Michelle Wille, a researcher 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-to-mammal transmission," she said in email correspondence with CBC News.
It's only one farm, and notably, none of the workers — who all wore face shields, masks, and disposable overalls — got infected.
But the concern now, said Toronto-based infectious disease specialist Dr. Isaac Bogoch, is if this virus mutates in a way that allows it to become increasingly transmissible between mammals, including 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 populations:
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 included.
The World Health Organization (WHO) has documented 240 cases of H5N1 avian influenza 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 overestimate, since not all infections may be detected, though it's clear people can "get very, very sick from these infections."
Most human infections also appeared to involve people 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 instance of mammalian spread, it may not be the last.
"A virus which has evolved on a mink farm and subsequently infects farm workers exposed to infected animals is a highly plausible route for the emergence of a virus capable of human-to-human transmission to emerge," she warned.
Louise Moncla, an assistant professor of pathobiology at the University of Pennsylvania school of veterinary medicine, explained that having 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 exactly the kind of scenario you would expect to see that could lead to this type of adaptation, that could allow these viruses to replicate better in other mammals — like us."
Surveillance, vaccines both needed
What's more reassuring is the ongoing development of influenza vaccines, giving humanity 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 ongoing human-to-human transmission.
"However, a very aggressive and successful poultry vaccination campaign ultimately 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 human health, Bogoch said ongoing surveillance and vaccine production needs to remain top-of-mind for both policy makers and vaccine manufacturers.
Dr. Jan Hajek, an infectious diseases physician at Vancouver General Hospital, also questioned whether it's time to wind down global mink farming, given the spread of various viruses, from avian influenza 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 propagating to minks, and killing minks, it's worrisome to us," he said.
In 2021, B.C. officials announced an end to mink farming across the province, saying the farms can be reservoirs for viruses and represent 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 conditions where these types of events can occur?" questioned Moncla. "If we're going to keep having these types of farms, what can we do to make this safer?"