Something was wrong with the foxes. That’s what callers from the Dane County Humane Society in Wisconsin kept saying in April, when they reported fox kits, or young foxes, behaving strangely: shaking, grabbing or struggling to stand. The kits, which were often lethargic and wandering alone, also seemed unusually easy to approach, showing little fear of humans.
“We kept getting calls,” said Erin Lemley, wildlife vet tech at the Humane Society Wildlife Center. “And the foxes started coming.”
Some of the kits admitted for treatment were silent and withdrawn, she said. Others stumbled or convulsed, their heads twitching, their eyes blinking rhythmically. After staff ruled out rabies, hypoglycemia and other potential causes, lab tests revealed a surprising culprit: a highly virulent strain of bird flu.
“It wasn’t a fun surprise,” said Dr. Shawna Hawkins, a zoo and wildlife veterinarian at the University of Wisconsin-Madison.
The virus, a type of bird flu known as Eurasian H5N1, spread rapidly across the United States this spring, infecting domestic poultry flocks in 36 states and causing mass cullings of domestic birds.
But this version of the virus appears to wreak far greater havoc on wild birds than previous lines, finding its way into ducks, geese, gulls and terns, among others. This, in turn, means the virus poses a high danger to mammals that feed on these birds, including wild red foxes.
At least seven US states have detected the virus in red fox kits, for which the pathogen appeared particularly deadly. Two bobcats in Wisconsin, a coyote pup in Michigan and skunks in Canada have also tested positive for the virus, as have foxes, otters, a lynx, a polecat and a badger in Europe. (Two human cases, one in the United States and one in Britain, have also been reported, both in people who had close contact with birds.)
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There is no evidence that mammals play a significant role in spreading the virus, and the risk to humans remains low, experts said. “It’s still an avian virus,” said Richard Webby, an influenza virologist at St. Jude Children’s Research Hospital in Memphis.
But evolution is a numbers game, he said, and the more mammals the virus infects, the more opportunities it has to detect new mutations that could help it spread among foxes, lynxes redheads or even humans.
“What it will take for this virus to go from a duck or chicken virus to a mammalian virus is more of a chance of replicating in those mammalian hosts,” Dr Webby said. “That’s why when we see these mammals infected with this virus, we take notice.”
The new lineage of the virus spread across Europe, Africa, the Middle East and Asia last year, triggering outbreaks in wild and domestic birds. It also appeared in a handful of wild mammals, including fox kits in the Netherlands in spring 2021.
By the end of the year, the virus had made its way to North America. As it swept through the population of migrating American birds this spring, reports began of infected fox kits – first in Ontario, then in Wisconsin, Minnesota, Michigan, Iowa, L Alaska, Utah and New York.
In some species of birds, the virus has caused obvious neurological symptoms, and many infected foxes have also exhibited abnormal behaviors. They were shaking, spinning around and salivating excessively. In the most severe cases, the foxes developed convulsions; death often followed soon after, experts said.
Post-mortem examinations revealed many kits had pneumonia, said Dr. Betsy Elsmo, a diagnostic pathologist at the Wisconsin Veterinary Diagnostic Laboratory who performed the necropsies. When she examined the animals’ brain tissue under a microscope, Dr. Elsmo saw clear signs of damage.
“There was a lot of inflammation in the brain under the microscope,” she said. “The injury pattern I saw was consistent with viral injury.”
So far, the virus appears to be wreaking greater havoc on fox kits than on adult foxes, potentially because young animals do not yet have fully developed immune systems, experts said.
But the overall infection and death rate is unknown. “We’re just getting anecdotal reports in the wild right now,” said Michelle Carstensen, wildlife health program supervisor for the Minnesota Department of Natural Resources.
Wisconsin officials also detected the virus in two adult bobcats this spring. “Both bobcats have shown themselves to be unafraid of humans,” Dr. Lindsey Long, wildlife veterinarian for the Wisconsin Department of Natural Resources, said in an email. “They have been noted sitting on porches and in close proximity to human activity without the usual fear response.”
One bobcat appeared to be shivering, while the other appeared to be struggling to breathe, she added. The bobcats, which were euthanized, had microscopic brain lesions that were “roughly identical” to those of the affected foxes, Dr Elsmo said.
The virus was also recently detected in a coyote pup in Michigan, said Dr. Megan Moriarty, wildlife veterinary specialist with the state Department of Natural Resources.
Scientists suspect that animals contract the virus by eating infected birds. In a laboratory study, researchers had previously demonstrated that red foxes fed the carcasses of infected birds could contract, and then excrete, the virus.
While it’s possible the virus evolved to better infect mammals, scientists say the most likely explanation for the sudden increase in the number of infected mammals is that this lineage infects huge numbers of birds. wild, increasing the chances that hunters and scavengers might stumble upon infected food sources.
So far, the virus does not appear to cause enough illness or death in wild mammals to put those species at risk, experts said. And there is no evidence of sustained mammal-to-mammal transmission. “Mammals are generally considered dead ends for highly pathogenic avian influenza,” Dr Moriarty said.
An early analysis of virus genomes from Wisconsin fox kits suggests infections are essentially a series of point infections – the result of individual foxes coming into contact with infected birds rather than foxes passing the virus on to each other. “The preliminary data we have suggests that these are all independent overflow events,” Dr Elsmo said.
But much remains unknown, including whether the virus will establish itself in wild birds long-term, which could pose a lasting risk to mammals.
And even isolated mammalian infections offer the virus new opportunities for evolution. “There is a risk that it will adapt and then be transmitted between mammals and then you have a new problem,” said Dr Jolianne Rijks, a veterinarian at the Dutch Wildlife Health Center.
Some state officials said they have begun testing sick mammals more regularly for the virus, especially those with neurological symptoms. Animals that test positive should also have samples of their virus sequenced so scientists can monitor any potentially concerning changes, Dr Webby said.
Experts are also encouraging members of the public to report any wildlife that appears to be acting strangely. “That’s how it all started,” Dr. Elsmo said, “as citizens seeing misbehaving kits and reporting them.”
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