Conservation Wildlife Management

Deadliest Bird Flu Outbreak in U.S. History Is Killing Bears, Mountain Lions, and Marine Mammals

If we've learned anything about the H5N1 virus lately, it's that it doesn't discriminate against mammals
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dead duck with avian influenza

Ducks, like many bird species, are struggling to survive the recent avian flu outbreak. Some mammals are getting sick after eating dead birds. Carsten Koall / Getty Images

On Thursday, Feb. 9, Colorado Parks and Wildlife published a press release detailing three recently-confirmed cases of highly-pathogenic avian influenza (HPAI or “avian flu”) in mammals; one black bear, one skunk, and one mountain lion. These cases join a growing list of instances in which the latest bird flu outbreak has infected wildlife other than birds. The U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) is maintaining this list in a database on their website. So far, avian flu has infected at least 121 wild mammals in 21 states (that we know of).

Here are the current numbers on the 16 non-bird species that have contracted avian flu since March 2022:

  • American black bear (2)
  • Amur leopard (1)
  • Amur tiger (1)
  • Bobcat (4)
  • Bottlenose dolphin (1)
  • Coyote (1)
  • Fisher (1)
  • Grey seal (1)
  • Grizzly bear (3)
  • Harbor seal (16)
  • Kodiak bear (1)
  • Mountain lion (2)
  • Raccoon (8)
  • Red fox (57)
  • Skunk (8)
  • Striped skunk (13)
  • Virginia opossum (3)

Of course, the next question is whether this strain of bird flu will jump to humans. Less than 10 human cases of H5N1 have occurred globally since December 2021, according to the Centers for Disease Control and Prevention. The first case in the U.S. was reported on April 28, 2022. That person made a full recovery after a few days of fatigue.

But while the World Health Organization considers the risk to humans low, that doesn’t mean things couldn’t change in the future.

“Since H5N1 first emerged in 1996 we have only seen rare and non-sustained transmission of H5N1 to and between humans,” WHO Director General Tedros Adhanom Ghebreyesus said during a virtual press conference on Feb. 8. “But we cannot assume that will remain the case and we must prepare for any change in the status quo.”

Bird Flu Outbreak in Colorado Wildlife

On Oct. 8, 2022, a CPW official euthanized a black bear in Huerfano County, Colorado after he saw the bear having seizures, the press release reads. Officials froze the remains and sent them to a lab for a necropsy, which revealed signs of HPAI.

“The decision to humanely euthanize the animal by our wildlife officer was made following the abnormal behavior and knowledge that numerous infectious diseases cause neurological symptoms,” CPW area wildlife manager Mike Brown said. “Extremely ill animals have difficulty moving and often act abnormally. While clinical signs of numerous diseases may be observed, diagnostic laboratory testing and necropsy services help determine the actual cause of death.”

A month later, a striped skunk from Weld County tested positive for HPAI. Then, a dead mountain lion was discovered just outside Gunnison on Jan. 15, 2023. The mountain lion had necrosis, or excessive cell death, in its liver. It also had bronchointerstitial pneumonia, meaning its respiratory system was in bad shape. Domestic cats with HPAI showed both these symptoms. This prompted CPW to test the big cat, according to the press release.

How Wildlife Get Infected

Even though the list of infected mammal species is long and varied, they all share one common trait: birds and/or scavenging are part of their diet. That helps explains why some aquatic mammals are getting sick. As one example, a marine animal rescue team from the University of Florida discovered an infected bottlenose dolphin in Horseshoe Beach, Florida, in March 2022. UF veterinary students collaborated with state and federal labs to test the dolphin for avian flu, and they believe it must have interacted with a dead, infected bird at some point.

“Although avian flu infection had never been documented in a dolphin, the high incidence of the virus in wild birds within the state in the spring—specifically aquatic bird species such as ducks, gulls, terns and herons—suggested that encounters between dolphins and dying or dead birds near the shoreline was not out of the realm of possibility,” UF assistant veterinary virology professor Andrew Allison said in a UF news release.

avian flu outbreak in mammals
The H5N1 virus is infecting mammals around the world. Smith Collection/Gado / Getty Images

CPW points out that not all animals that consume an infected bird will get sick. But the Centers for Disease Control and Prevention highlight direct consumption as the primary way other animals become victims of the current avian flu outbreak. This is also why the CDC has cautioned pet owners against letting dogs, cats, and other critters come in close contact with dead animals, especially birds.

Current Bird Flu Outbreak Numbers

According to the CDC, as of Feb. 8, the virus has infected 6,111 wild birds and 58,349,754 poultry birds. All 50 states have infected wild birds and 47 states have infected poultry. (In case you were curious why eggs are so hard to come by right now, the avian flu outbreak is the culprit.)

Read Next: We’re Experiencing the Deadliest Bird Flu Outbreak in History. Here’s What All Bird Hunters Need to Know

Of course, these numbers dwarf the current tally for infected non-avian wildlife species. But as more birds become sick, their position in the food chain spells trouble for all the species that feed on them.