There was something wrong with the foxes. That’s what callers at the Dane County Humane Society in Wisconsin kept saying in April, when they reported that fox kits, or young foxes, were behaving in strange ways: trembling, grabbing or struggling to get up. Often lethargic and wandering on their own, the kittens also seemed unusually easy to approach and showed little fear of humans.
“We kept getting calls,” said Erin Lemley, a wildlife veterinarian at the Humane Society Nature Center. “And the foxes started to come in.”
Some kits admitted for treatment were quiet and withdrawn, she said. Others stumbled or had seizures, their heads trembled, their eyes flashed rhythmically. After the staff ruled out rabies, low blood sugar and other possible causes, lab tests revealed a surprising culprit: a highly virulent strain of bird flu.
“It wasn’t a nice surprise,” says 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, has spread rapidly in the United States this spring, infecting flocks of farmed poultry in 36 states and causing massive culls of domestic birds.
But this version of the virus seems to take a much greater toll on wild birds than previous genera, making its way onto ducks, geese, gulls and terns, among others. That, in turn, means the virus poses an increased danger to mammals that prey on those birds, including wild red foxes.
At least seven US states have detected the virus in Red Fox kits, for which the pathogen has been found to be 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 have also been reported, one in the United States and one in Britain, both in people who had close contact with birds.)
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There is no evidence that mammals play a major role in the spread of the virus, and the risk to humans remains low, experts say. “This is still a bird virus,” said Richard Webby, a flu 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 chances it has to pick up new mutations that could help it spread among foxes, bobcats or even humans.
“What it takes for this virus to move from a duck or chicken virus to a mammalian virus is more chances to replicate in those mammalian hosts,” said Dr. webby. “So that’s why we notice when we see these mammals being infected by this virus.”
The new lineage of the virus spread through Europe, Africa, the Middle East and Asia last year, leading to outbreaks in wild and domestic birds. It also showed up in a handful of wild mammals in the spring of 2021, including fox kits in the Netherlands.
By the end of the year, the virus had made its way to North America. As it swept through the migrating U.S. bird population this spring, reports of infected fox kits came in — first in Ontario, then in Wisconsin, Minnesota, Michigan, Iowa, Alaska, Utah and New York.
In some bird species, the virus caused marked neurological symptoms, and many infected foxes also exhibited abnormal behavior. They trembled, ran in circles and drooled profusely. In the most severe cases, the foxes developed seizures; death often followed shortly after, experts said.
Post-mortem examinations revealed that many of the kits had pneumonia, said Dr. Betsy Elsmo, a diagnostic pathologist at the Wisconsin Veterinary Diagnostic Laboratory who performed the autopsies. When examining the animals’ brain tissue under a microscope, Dr. Elsmo clear signs of damage.
“There was microscopic inflammation in the brain,” she said. “The pattern of injury I saw was consistent with a viral lesion.”
So far, the virus appears to be taking a greater toll on fox kits than adult foxes, possibly because the young animals don’t yet have fully developed immune systems, experts said.
But the overall infection and death rate is unknown. “We’re just getting anecdotal reports in nature right now,” said Michelle Carstensen, supervisor of the wildlife health program at the Minnesota Department of Natural Resources.
Wisconsin officials also discovered the virus in two adult bobcats this spring. “Both bobcats showed reduced to no fear of humans,” said Dr. Lindsey Long, wildlife veterinarian for the Wisconsin Department of Natural Resources, in an email. “They were noticed 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 having trouble breathing, she added. The bobcats, which were euthanized, had microscopic brain lesions that were “nearly identical” to those of the affected foxes, said Dr. elsmo.
The virus was also recently discovered in a coyote pup in Michigan, said Dr. Megan Moriarty, the wildlife veterinarian at the state’s Department of Natural Resources.
Scientists suspect that the animals contract the virus by eating infected birds. In a lab study, researchers had previously shown that red foxes fed infected bird carcasses could contract the virus and then shed it.
While it’s possible the virus evolved to better infect mammals, scientists say the most likely explanation for the sudden increase in infected mammals is that this lineage infects vast numbers of wild birds, increasing the likelihood of hunters and scavengers. may encounter contaminated food sources.
So far, the virus doesn’t seem to cause enough disease or death in wild mammals to endanger these species, experts said. And there’s no evidence of continued mammal-to-mammal transmission. “Mammals are generally considered dead ends for highly pathogenic avian flu,” said Dr. moriarty.
An early analysis of viral genomes from the fox kits in Wisconsin suggests the infections are essentially a series of one-time actions — the result of individual foxes coming into contact with infected birds rather than foxes transmitting the virus to each other. “The preliminary data we have suggests that these are all independent spillover events,” said Dr. elsmo.
But much is still unknown, including whether the virus will settle long-term in wild birds, which could pose an ongoing risk to mammals.
And even isolated mammalian infections offer the virus new opportunities to evolve. “There is a risk that it will adapt and then be transferred between mammals, and then you have a new problem,” says Dr Jolianne Rijks, veterinarian at the Dutch Wildlife Health Center.
Some state officials said they had begun more routine testing of sick mammals for the virus, especially those with neurological symptoms. Animals that test positive should also have samples of their virus sequenced so scientists can monitor for potentially worrisome changes, said Dr. webby.
Experts also encourage the public to report wild animals that appear to be behaving strangely. “That’s how this all started,” said Dr. Elsmo, “as citizens who saw and reported abnormally-behaving kits.”