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A female adult northern harrier feeds on one of at least five dead birds found on the ice at Stearns Lake near Broomfield on Jan. 4, 2023. It's not clear if these dead birds had avian flu, but scenes like these and the potential for spreading the devastating virus are playing out across Colorado, wildlife officials and raptor watchers warn. (Dana Bove, Front Range Eagle Studies, Special to the Colorado Sun)

Colorado’s worst-ever avian flu outbreak has crossed over into more mammals, including a black bear put down in Huerfano County and a mountain lion found dead in Gunnison County, state wildlife officials said Thursday. 

While the number of mammal cases of the bird-spread flu remains low, Colorado Parks and Wildlife said, the recent deaths confirm national warnings that the massive current outbreak will have an impact beyond domestic chickens and wild waterfowl. 

More than 6 million chickens have been killed on commercial egg farms in Colorado during the latest outbreak, which accelerated in the spring and fall of 2022. Thousands more wild birds have been killed by the fast-working disease, including large flocks of snow and Canada geese near Eastern Plains reservoirs. A handful of raptors have also been felled by the disease, including bald eagles, which total only a few hundred nesting pairs across Colorado.

Wildlife officials and researchers believe the mammals usually come in contact with avian flu by feeding on bird carcasses. A few cases have been reported in smaller mammals, and there are worries in other states that mink farms will further spread the disease. 

Crossover to humans for the current strain of avian flu so far is extremely rare, with one case in Colorado from a worker in close contact with large chicken populations. 

“CPW continues to stress the importance of keeping your distance from wildlife, and specifically not handling sick or dead birds. Although rare, some HPAI strains can infect people,” a state news release said. “The main protection for the general public is to avoid handling sick or dead birds and keep your distance from wildlife.”

A state wildlife officer killed the black bear in Huerfano County in October after it showed signs of the disease, which include circling, odd behavior, confusion and no fear of humans. The bear’s remains were frozen until lab tests could be conducted that confirmed the avian flu. 

“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.”

The mountain lion was found just outside Gunnison city limits in mid-January, the release said, in an area common for mountain lions, and the carcass was tested. A Weld County skunk’s carcass was also found to be positive in November for the disease, known as Highly Pathogenic Avian Influenza or HPAI.

Wildlife and agriculture officials in Colorado encourage the public to stay well away from suspected infected animals or carcasses, but to report them to local or state wildlife authorities. Updates to statewide avian flu cases and protocols are posted regularly on websites including the state Department of Agriculture.

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Michael Booth is a Colorado Sun reporter covering health, health policy and the environment. Email: Twitter: @MBoothDenver