Disease outbreak cuts Wyoming, Yellowstone wolf numbers to lowest level since reintroduction era

A flare up of a disease that’s especially lethal to wolf pups took a toll on Wyoming and Yellowstone National Park wolf numbers in 2025, reducing biologists’ counts to a level last seen when wolves were still reestablishing following the species’ historic 1995-96 reintroduction.

“It was the lowest number of wolves in 20 years,” Wyoming Game and Fish Department wolf biologist Ken Mills told WyoFile. “That was definitely during the population creep stage, so they were still establishing in the state.”

All signs point toward canine distemper being the primary reason Wyoming’s statewide wolf population plunged to a minimum count of 253 wolves and 14 breeding pairs at the end of 2025, Mills said.

Distemper was detected in 64% of animals in the northwestern Wyoming zone where wolves are classified as “trophy game.” While adults can survive the contagious virus, which is a measles-like affliction in canines, it’s “quite lethal” for pups and only an estimated “31 to 34” of the 87 documented born pups lived to the end of the year, a survival rate of just 37%, according to Game and Fish’s 2025 wolf monitoring report.

In the past, distemper was a density-dependent disease that surged when populations were high, Mills said. It last flared up in 2018, which wasn’t long after a two-year period where wolves were protected from hunting by the Endangered Species Act and populations — and conflict — were higher.

The 2025 flare up was the first time Mills documented lots of distemper when wolf numbers were not particularly high. The occurrence has him searching for alternative explanations.

“Could it be cyclical? Yeah,” the Pinedale-based biologist said. “However, these are potentially eight-year cycles, and it takes a lot of time to collect data and understand what’s going on.”

There’s cause to believe that distemper will abate in Wyoming wolves this year. When Yellowstone wolves have experienced outbreaks, the event lasts a year and then there’s recovery, Mills said. And the Wyoming population now has more antibodies and resistance built up and so is in good shape to recover itself, he said.

But in 2025, distemper hurt Wyoming wolf numbers, which was a first.

Before that, “we really haven’t had a canine distemper outbreak that has caused a population-level effect,” Mills said.

In 2024, Mills and his biologist counterparts detected 330 wolves and 24 breeding pairs statewide. The estimated 253 wolves and 14 breeding pairs in 2025 means the raw wolf count tumbled by 23% and the reproductive segment fell by 42%.

Of those, 132 wolves in 22 packs that included 10 breeding pairs dwelled in the mountainous portion of northwest Wyoming in the “trophy game” area. There were nine wolves in three packs and no breeding detected on the Wind River Indian Reservation. And in the zone where Wyoming manages wolves as predators — where they can be killed by any means without limit — there were 28 wolves in five packs, including one breeding pair.

The remainder of Wyoming’s wolves — 84 wolves running in seven packs that included three breeding pairs — dwelled in Yellowstone National Park, according to the state’s monitoring report.

The park’s public affairs officers, whose office has been inundated with inquiries about a recent grizzly bear attack, did not respond to WyoFile’s request for an interview before this story published.

The overall number of Yellowstone wolves has dipped into the 80s twice before, in 2012 and 2018. But by other measures, 2025 was a tough year that the park population had not experienced since the reintroduction era. The distemper outbreak appeared to be “synchronous” in Wyoming and Yellowstone, and pup production and survival was also dismal in the national park, Mills said.

“Seventeen pups survived in Yellowstone,” he said, “which was the lowest they ever recorded.”

Outside of Yellowstone, Game and Fish will consider the lower wolf population when its biologists and wardens are setting fall 2026 hunting seasons (hunting isn’t allowed in the park, though park wolves frequently leave ). That proposal isn’t public yet, but Mills anticipates that there will be a “surplus” of animals and a wolf hunting season, even if mortality limits are reduced.

Wyoming’s relatively few wolves have enabled the state to manage with precision and a degree of predictability, although the surge of distemper interrupted a long run of population stability. Still, the unexpected disease outbreak left Mills feeling good about Wyoming’s plan for managing its wolves.

“We set up the population objective of 160 wolves to be able to accommodate an event similar to what we experienced, and still meet our minimum recovery criteria,” Mills said.

That recovery criteria includes 10 breeding pairs outside of Yellowstone in Wyoming’s trophy game area. Mills’ 2025 surveys detected exactly 10 packs with pups in that zone.

“We met the minimum,” Mills said. “It actually worked exactly as we intended.”

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This story was originally published by WyoFile and distributed through a partnership with The Associated Press.