By
MIKE KOSHMRL/WyoFile
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.
