In the past three years, scientists have documented a slow but steady increase in the number of Pacific Northwest species that are dying from avian influenza, commonly known as bird flu.

Wild birds have long carried low pathogenic strains of avian influenza. When it spread to domestic birds, the virus sometimes mutated into more severe strains, called high pathogenic avian influenza—or HPAI.

For decades, HPAI was a disease that largely affected domestic poultry, and sometimes the people who handle them. But in a paradigm shift, the strain started spreading back to wild bird populations in 2002.

Now, the latest strain of HPAI H5N1—emerging in 2021—is proving to be especially worrisome to wild birds, and potentially to other wildlife.

In 2022, some migratory birds that spend time in Washington and Oregon started getting sick and dying—sometimes by the hundreds, and in at least one case, more than a thousand. And, for the first time, wild mammals ranging from cougars and bobcats to weasels, skunks and raccoons began perishing from the virus in the Pacific Northwest and across the country.

But HPAI is also a serious threat to certain types of wild birds, despite their having a long history of exposure to bird flu. Scientists believe that most other wildlife is catching the disease after scavenging on wild birds that died of bird flu.

Excepting poultry-farm workers, the disease is still not a large risk to people.

But the extent of bird flu in wild mammals is understudied.

How bad is it?

The Washington Department of Fish and Wildlife has reported that since April 2022, when a snow goose in Walla Walla County was confirmed to have died from a strain of HPAI, more than 400 wild animals are known to have died from the disease in the state.

But these are just confirmed cases.

“The tip of the iceberg,” WDFW Veterinarian Katie Haman tells Columbia Insight.

In the spring and fall of 2023 and 2024, WDFW documented large die-offs of Canada geese, snow geese, swans and other birds.

So, what does “large die-off” mean in terms of actual numbers?

“That’s the million dollar question,” says Haman.

One of the most well documented die-offs occurred in 2023 on Rat Island, off the Washington coast near Port Townsend.

Volunteers helped count nearly 1,600 dead Caspian terns, including 1,101 adults. But only a small fraction of those birds were tested and confirmed to have died from the disease.

Additionally, the total estimate doesn’t include birds that may have died but weren’t counted because they washed out to sea.

“That confirmed number is a very gross underestimate of how many birds died (in this die-off),” says Haman.

Further study found that 56% of the Caspian tern colony on the island died from the 2023 outbreak. Since then, no birds have successfully bred there.

Because it was such a large colony, that die-off is estimated to have wiped out 10-14% of the entire Caspian tern population in the Pacific Flyway.

It’s catastrophic—especially since older birds that should have had some immunity died along with chicks that had never been exposed.

At the same time that WDFW was collecting dead Caspian terns near Port Townsend, a separate outbreak ravaged Caspian terns at the mouth of the Columbia River.

Other regional outbreaks have been documented.

One of the largest was north of Salem, Ore., at Statts Lake, where about 300 snow geese died in 2023.

As with other mass die-offs, the state confirmed the disease in a small number of the birds, and assumed the rest also died from the flu.

Oregon Department of Fish and Wildlife Veterinarian Dr. Julia Burco Speten calls the small lake “an early warning system.” That’s because it’s surrounded by homes, and is a documented stopover for migratory ducks, geese and swans. Residents report dead birds.

“It’s pretty publicly visible,” says Speten.

At other stopover sites, significant numbers of cackling geese—a subspecies of Canada geese—have died from the disease.

A cluster of roughly seven sandhill cranes died last fall from HPAI in eastern Oregon’s Malheur County.

Mammals infected

In the last three years, state wildlife agencies have received reports of dead mammals and birds of prey—including bald eagles and red-tailed hawks—though numbers are relatively small.

Oregon and Washington have confirmed HPAI in skunks, raccoons, red fox, cougars, bobcats and weasels.

In 2024, 20 big cats—including cougars, lynx and bobcats—died of HPAI at a big cat sanctuary in Shelton, Wash. Investigators believe the outbreak was associated with contaminated food.

Excepting harbor seals, Haman believes other wildlife and raptors that have died from avian flu probably died after scavenging on infected birds.

The major die-off of Caspian terns in Washington in 2023 resulted in a study that also found the H5N1 virus was transmitted to harbor seals (15 of them) for the first time on the northeastern Pacific Coast.

“It is unlikely that multiple seals acquired the viruses through predation or scavenging of an infected source (such as an infected bird) at this scale, as birds are not a typical food source for harbor seals,” NOAA Fisheries reported.

Exactly how the seals contracted the disease is being investigated. But scientists believe it initially came from exposure to infected Caspian terns through respiratory droplets or exposure to their feces.

“Once a new variant of influenza has entered into a seal population, it is then often able to spread from seal to seal,” according to the NOAA Fisheries statement.

Cloudy numbers

State and federal agencies rely on the public to report wildlife fatalities.

Most animals that die in the wild are probably not found or reported by the public.

As a result, government researchers don’t have accurate numbers of how many animals are dying—or the impacts to populations—of avian influenza.

Since state agencies don’t have the resources to conduct widespread surveys across many species, they don’t know if any wild mammals are catching the disease and surviving.

The upshot?

“We really don’t have a good idea of the impact on a lot of species,” says Haman.

Positive trend?

Despite an uptick in cases in December 2024, Haman and Speten say they have had fewer reports and confirmed deaths in wild birds or mammals so far this year,

“I would say this spring was a little better” compared with 2024 or 2023, says Haman.

However, she says, there’s usually a lull in reported cases in the summer, since migrating birds in the Pacific Flyway generally stop in this region in the spring and fall.

“We’re just not getting a lot of reports of dead birds right now,” she says.

“The promising news is, we really have had a decrease” in reported cases, says Speten. She believes this could be evidence that wild populations are developing a resistance.

“With any pathogen you would expect birds to start mounting an immune response. Hopefully, that’s what we’re beginning to see,” she says.

The real test will be if the drop-off continues through this fall, when wild ducks, geese, swans, shorebirds, terns and seabirds most susceptible to the disease stop at Pacific Northwest lakes and estuaries for extended time.

With bird migration season approaching, wildlife agencies in Oregon and Washington will be relying on the public to report wildlife fatalities.

“The information goes into a database,” says Haman. “By reporting wildlife mortality events or even single cases, that’s really useful information for us.”

“How it evolves in wildlife is going to impact whether or not it’s going to spill over into humans,” says Haman. “I think that’s the big part we’re going to be missing with not doing surveillance for it.”

Haman says unlike in domestic poultry—where infected flocks would be euthanized to prevent continued spread and mutations—HPAI is now being maintained in wildlife populations.

“It’s just kind of always here,” she says. “I think people should be prepared for it being on our landscape for the foreseeable future.”

Speten advises that despite the unsettling progression of the disease, people shouldn’t be fearful of HPAI as a human health risk. However, common precautions—like not handling dead wildlife—should be taken.

She, too, urges residents to report wildlife fatalities.

ODFW dead bird reporting hotline: 866-968-2600 (10)

WDFW has an online link for reporting sick, injured or dead wildlife.