Last fall, a Clallam County, Washington, resident spotted a young male cougar walking slowly through a field on the northern edge of the Olympic Peninsula. It was the middle of the day — a clear sign that something was off — and he was also skinny and weak, dragging his matted tail in the mud.
The resident called a Washington Department of Fish and Wildlife (WDFW) game warden and Mark Elbroch, puma director of Panthera, a nonprofit dedicated to protecting wild cats globally. (Cougars are also called mountain lions or pumas.) Elbroch and the warden approached the cougar, yelling and clapping to gauge the animal’s alertness. But even when they were less than 20 feet away, the cougar didn’t respond. “The cat was on its last legs,” Elbroch said. “He literally couldn’t even get out from this pasture.”
The cougar was euthanized, and his tissue samples, which were tested for a number of diseases, revealed the presence of the H5N1 strain of avian influenza, or bird flu. Less than two weeks later, another infected cougar was found dead in Clallam County.
Mild forms of bird flu are common in wild and domestic birds, but sometimes a strain circulating on a poultry or waterfowl farm mutates into a more dangerous form and spills over into wild birds. The H5N1 strain, which was initially identified in domestic geese in China in the 1990s, had infected wild birds by 2002 and reached North America in 2021.
Since then, the virus has caused the deaths of tens of millions of domestic chickens, geese, ducks and turkeys in the U.S., contributing to the rise in egg prices. As of early March, it had killed at least 50,000 wild birds, according to estimates by the U.S. Geological Survey, and been detected in nearly 400 individual wild mammals, including felines, raccoons, rodents, seals and skunks. Globally, H5N1 has killed thousands of mammals in mass mortality events, including sea lions in Peru and Chile and elephant seals in Argentina.
The virus has caused the deaths of tens of millions of domestic chickens, geese, ducks and turkeys in the U.S., contributing to the rise in egg prices.
Over the past three years, more cases of H5N1 in wild mammals have been reported in Washington than in any other Western state except Colorado and New Mexico. Before H5N1 was identified in the two Olympic Peninsula cougars, sick animals had typically lived near wild bird populations infected with the virus. The 15 harbor seals that died in 2023, for example, frequented the same beach as a flock of infected Caspian terns. “The assumption was they were scavenging carcasses or even catching and eating sick birds,” said Katherine Haman, a WDFW veterinarian.
But the two cougars weren’t living near any known outbreaks in wild birds or other prey species, according to the agency. Their deaths illustrate how little we know about how the disease spreads in wildlife, and how far it may have already reached.
UNLIKE THE COUGAR in the cow pasture, Clallam County’s second victim showed no obvious signs of disease. He had been fitted with a GPS collar as part of Panthera’s Olympic Cougar Project, and Elbroch said that he looked to be “in perfect health.” Later analysis of movement data, though, revealed the cougar had started to behave differently, traveling shorter distances than previously.
Virus-carrying prey can also appear healthy. “We don’t really know which animals are carrying and which ones have been exposed and recovering,” Elbroch said.
While there were no documented H5N1 outbreaks in the area, the cougars may have eaten infected birds along the Pacific Flyway, a major north-south migration route that runs along the West Coast from Alaska to Mexico. Or they may have consumed a mammal — a raccoon, river otter, seal or sea lion — that had eaten an infected bird. Regardless of the cause, the two deaths in the same general area during the same short timespan suggest that bird flu is circulating undetected in the apex predators’ prey. “That just really highlights, to me, that this virus may be more widespread on our landscape than we know or think it is,” said Haman.
Although 70 H5N1 cases have been confirmed in humans — mostly poultry and dairy farmworkers — in the U.S., public health officials say the virus still poses a low health risk: It has not yet spread between people, and its symptoms are usually mild. But the further it spreads among other mammals, the more opportunities it will have to mutate into a form easily contracted by humans. If the virus acquires the ability to be more readily transmitted between mammals or between humans, Haman said, “we have a potential brewing pandemic.”
In December, the wildlife agency announced that the state was seeing an uptick in cases in both wild birds and mammals. So far this winter, it’s been detected in a long-tailed weasel, raccoon, harbor seal and bobcat.
If H5N1 continues to proliferate among wild birds and mammals, isolated or small populations — including threatened and endangered species, such as ferrets and California condors — will be particularly vulnerable. “Flu is … rarely considered a wildlife conservation issue,” said Justin Brown, a wildlife veterinarian and professor at Pennsylvania State University. “But with this virus, now that it causes disease in (other) wildlife, there are some conservation concerns.”
FULLY UNDERSTANDING any wildlife disease isn’t easy, due to the lack of monitoring and surveillance. “There’s just so much about this virus and the epidemiology and disease ecology of it that we don’t understand, especially in wild mammals right now,” Haman said. State wildlife agencies, which are chronically short of funding, primarily rely on the public to report sick and dead wildlife, meaning the numbers could be far greater than recorded.
Regular monitoring and sampling would help researchers better understand H5N1 and its spread. “We could get ahead of avian flu right now with the right resources,” Elbroch said. After cases among dairy cows and farmworkers rose in California last fall, Gov. Gavin Newsom declared a state of emergency to free up resources for monitoring; as of February, it’s the only state to do so. Though the federal government laid out a $1 billion plan to curb bird flu in domestic poultry in late February, the Trump administration is considering cutting funding for the development of a bird flu vaccine for people.
If the virus acquires the ability to be more readily transmitted between mammals or between humans. “We have a potential brewing pandemic.”
Brown, who is partnering with Washington wildlife officials to continue studying the disease, plans to test blood samples collected from living wild animals for H5N1 antibodies, an indication of recovery from infection. “I think that this is a disease that will be here for the foreseeable future,” Brown said. “The challenge then is: How do we tweak our research, our surveillance and our management to now deal with this new normal?”
This article by Kylie Mohr was first published by High Country News on 1 April 2025. Lead Image: A cougar infected with the H5N1 strain of avian influenza crosses a field in Clallam County, Washington, last fall. It was later euthanized. Courtesy of Mark Elbroch/Panthera.
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