There was something wrong with the fox. Callers at the Dan County Human Society in Wisconsin said in April that they were behaving strangely in fox kits, or young foxes: struggling to shake, hold or stand. The kits, which are often lazy and wandering around on their own, seemed unusually easy to come by, scaring people a little.
“We just kept getting calls,” said Erin Lemley, a wildlife veterinary technician at the Human Society’s Wildlife Center. “And the fox started coming.”
Some of the kits admitted for treatment were calmed and withdrawn, he said. Others stumbled or stumbled around, their heads tilted, their eyes shaking rhythmically. After staff ruled out rabies, low blood sugar and other possible causes, laboratory tests revealed a surprising culprit: Avian influenza, a highly lethal strain.
“It wasn’t a fun surprise,” said Dr. Shawna Hawkins, a zoo and wildlife veterinarian at the University of Wisconsin-Madison.
The virus, known as the Eurasian H5N1 bird flu, has spread rapidly in the United States this spring, infecting flocks of poultry farmed in 36 states and causing widespread outcry against domesticated birds.
However, this version of the virus seems to be doing more harm to wild birds than previous breeds, and has found its way into many more, including ducks, geese, roses and terns. This means that the virus poses a high risk for mammals that prey on those birds, including wild red foxes.
At least seven states in the United States have identified the virus in Red Fox kits, where the pathogen appeared to be particularly lethal. Two bobcats from Wisconsin, a coyote dog from Michigan, and a Canadian skunks have also tested positive for the virus, including foxes, otters, a lynx, a polka dot and a badger in Europe. (In the case of two humans, one in the United States and one in Britain, as well as reported, both were among people in close contact with birds.)
Interesting world of birds
There is no evidence that mammals play a significant role in spreading the virus and that the risk to humans is low, experts say. “It’s still pretty much an avian virus,” said Richard Webby, an influenza virologist at St. Jude Children’s Research Hospital in Memphis.
But evolution is a game of numbers, he said, and the more mammals transmit the virus, the more opportunities there are to pick new mutations that could help it spread to foxes, bobcats or even humans.
“Anything that takes the virus to transform from a poultry virus to a mammalian virus is more likely to be replicated in that mammal host,” said Dr. Webby. “That’s why when we see these mammals being infected by this virus, we notice.”
The shining fox
New strains of the virus spread to Europe, Africa, the Middle East and Asia last year, with outbreaks among wild and domesticated birds. It was also spotted in the spring of 2021 in several wild mammals, including the fox kit.
Towards the end of the year, the virus entered North America. As migratory American bird populations began to emerge this spring, reports of infected fox kits began to emerge – first in Ontario and then in Wisconsin, Minnesota, Michigan, Iowa, Alaska, Utah and New York.
In some bird species, the virus causes obvious neurological symptoms and many infected foxes show abnormal behavior. They twist, walk in circles and salivate excessively. In the most severe cases, the fox produces convulsions; Deaths often follow later, experts say.
An autopsy 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 he examined the animal’s brain tissue under a microscope, Dr. Elsmo saw clear signs of damage.
“Microscopically there was a lot of inflammation in the brain,” he said. “The type of injury I saw was consistent with a viral injury.”
So far, the virus seems to be taking a higher toll on fox kits than on adult foxes, possibly because young animals have not yet fully developed immune systems, experts say.
But the overall infection and death rate is unknown. “We’re now getting a kind of anecdotal report on nature,” said Michelle Carstensen, overseer of the Minnesota Department of Natural Resources’ Wildlife Health Program.
Wisconsin officials also identified the virus in two adult bobcats this spring. Dr. Lindsay Long, a wildlife veterinarian at the Wisconsin Department of Natural Resources, said in an email that “both bobcats have lost their fear of humans.” “They have been spotted sitting on the porch without the usual fear response and have been spotted in close proximity to human activities.”
One bobcat seems to be trembling, the other seems to be having trouble breathing, he added. Bobcats, who were euthanized, had microscopic lesions of the brain that were “very similar” to those of infected foxes, Dr. Elsmo said.
The virus was also recently detected in a coyote dog in Michigan, said Dr. Megan Moriarty, a wildlife veterinarian at the state’s Department of Natural Resources.
Scientists suspect that animals are transmitting the virus by eating infected birds. In a laboratory study, researchers previously showed that red foxes that were fed the carcasses of infected birds could contract and then spread the virus.
While it is possible that the virus has evolved in a way that makes it better at infecting mammals, scientists say the most likely explanation for the sudden increase in infected mammals is that the lineage is infecting large numbers of wild birds, increasing the hostility of predators. And scavengers can stumble across infected food sources.
So far, the virus does not appear to be causing enough illness or death in wild mammals to endanger these species, experts say. And there is no evidence of mammalian infection from permanent mammals. “Mammals are generally thought to be highly endangered by the highly pathogenic avian influenza,” said Dr. Moriarty.
A preliminary analysis of viral genomes from the Wisconsin Fox Kit suggests that the infections are essentially a series of one-offs – the result of foxes coming into contact with infected birds rather than transmitting the virus to each other. “The basic information we have is that these are all independent spillover events,” he said. Elsmo says 6
Much remains unknown, however, including whether the virus will establish itself among wild birds in the long run, which could pose a permanent threat to mammals.
Even isolated mammalian infections provide new opportunities for the virus to develop. “In mammals there is a risk of adaptation and then infection, and then you have a new problem,” said Dr. Julian Rizkos, a veterinarian at the Dutch Wildlife Health Center.
Some state officials say they have begun testing sick mammals regularly for the virus, especially those with neurological symptoms. Animals that test positive should also have sequential samples of their virus so that scientists can monitor for potentially alarming changes, Dr. Webby said.
Experts encourage members of the public to report on any wildlife that seems to be working strangely. “That’s how it all started,” said Dr. Elsmo.