Something was wrong with the foxes. That was what callers to the Dane County Humane Society in Wisconsin kept saying in April, as they reported fox kits, or young foxes, behaving in strange ways: shaking, seizing or struggling to stand. The kits, which were often lethargic and wandering by themselves, also seemed unusually easy to approach, showing little fear of humans.

“We just kept getting calls,” said Erin Lemley, a wildlife veterinary technician at the humane society’s wildlife center. “And the foxes started coming in.”

Some of the kits that were admitted for treatment were quiet and withdrawn, she said. Others stumbled around or had seizures, their heads ticcing, their eyes flicking rhythmically. After the staff ruled out rabies, low blood sugar and other potential causes, laboratory testing revealed a surprising culprit: a highly virulent strain of avian influenza.

“It was not 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, has been spreading rapidly in the United States this spring, infecting flocks of farmed poultry in 36 states and prompting mass culls of domestic birds.

But this version of the virus appears to be taking a much greater toll on wild birds than previous lineages have, finding its way into ducks, geese, gulls and terns, among many others. That, in turn, means that the virus poses an elevated danger to mammals that prey on those birds, including wild red foxes.

At least seven U.S. states have detected the virus in red fox kits, to which the pathogen appeared to be particularly lethal. 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 been reported as well, both of which were in people who had close contact with birds.)

There is no evidence that mammals play a significant role in spreading the virus, and the risk to humans remains low, experts said. “This is very much 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 pick up new mutations that could help it spread among foxes, bobcats or even humans.

“What it’s going to take for this virus to transition from being a duck or a chicken virus to being a mammalian virus is more chances to replicate in those mammalian hosts,” Dr. Webby said. “So that’s why when we see these mammals being infected by this virus, we do take notice.”

Twitching foxes

The new lineage of the virus spread through Europe, Africa, the Middle East and Asia last year, sparking outbreaks in wild and domestic birds. It also showed up in a handful of wild mammals, including fox kits in the Netherlands in the spring of 2021.

By the end of the year, the virus had made its way to North America. As it raced through the migrating American bird population this spring, reports began to emerge of infected fox kits — first in Ontario and subsequently in Wisconsin, Minnesota, Michigan, Iowa, Alaska, Utah and New York.  

In some bird species, the virus caused obvious neurological symptoms, and many infected foxes displayed abnormal behaviors, too. They twitched, walked in circles and salivated excessively. 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 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 microscopically,” she said. “The pattern of injury that 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, potentially because the young animals do not yet have fully developed immune systems, experts said.

But the overall infection and mortality rate is unknown. “We’re just getting kind of anecdotal reports in nature right now,” said Michelle Carstensen, the 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 showed reduced to no fear of humans,” Dr. Lindsey Long, wildlife veterinarian for the Wisconsin Department of Natural Resources, said in an email. “They were noted sitting on porches and in close proximity to human activity without the usual fear response.”

One bobcat seemed 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 “pretty much identical” to those in the affected foxes, Dr. Elsmo said.

The virus was also recently detected in a coyote pup in Michigan, said Dr. Megan Moriarty, the wildlife veterinary specialist at the state’s Department of Natural Resources.

Scientists suspect that the animals are acquiring the virus by eating infected birds. In a laboratory study, researchers had previously demonstrated that red foxes that were fed infected bird carcasses could contract, and then shed, the virus.

Although it is possible that the virus has evolved in ways that make it better at infecting mammals, scientists say that the most probable explanation for the sudden rise in infected mammals is that this lineage is infecting enormous numbers of wild birds, increasing the odds that hunters and scavengers might stumble across infected food sources.

So far, the virus does not appear to be causing enough illness or death in wild mammals to put these species at risk, experts said. And there is no evidence of sustained mammal-to-mammal transmission. “Mammals are generally considered to be dead-ends for highly pathogenic avian influenza,” Dr. Moriarty said.

An early analysis of viral genomes from the Wisconsin fox kits suggests that the infections are essentially a series of one-offs — the result of individual foxes coming into contact with infected birds rather than foxes transmitting the virus to each other. “The preliminary data that we have suggests that these are all independent spillover events,” Dr. Elsmo said.

But much remains unknown, including whether the virus will establish itself in wild birds for the long haul, which could pose a sustained risk to mammals.

And even isolated mammalian infections provide the virus with new opportunities to evolve. “There’s a risk of it adapting to and then transmitting 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 that they had started more routinely testing sick mammals for the virus, especially ones with neurological symptoms. Animals that test positive should also have samples of their virus sequenced so scientists can monitor for any potentially worrisome changes, Dr. Webby said.

Experts also encourage members of the public to report any wild animals that appear to be acting strangely. “That’s how all this started,” Dr. Elsmo said, “as citizens seeing abnormally behaving kits and reporting them.”