A virus highly lethal to wolf pups drove down Wyoming and Yellowstone National Park wolf populations to their lowest levels in two decades, leaving biologists with a minimum count of 253 wolves and 14 breeding pairs at the end of 2025. The drop represents a 23 percent decline in the raw wolf count and a 42 percent drop in the reproductive segment compared to the 330 wolves and 24 breeding pairs recorded in 2024. “It was the lowest number of wolves in 20 years,” Wyoming Game and Fish Department wolf biologist Ken Mills said. “That was definitely during the population creep stage, so they were still establishing in the state.”
All signs point to canine distemper as the primary driver of the decline, Mills said. The contagious virus was detected in 64 percent of animals tested in northwestern Wyoming’s trophy game zone. While adult wolves can typically survive the measles-like affliction, it proved highly lethal to newborns this year. State records show that only an estimated 31 to 34 of the 87 documented pups born in Wyoming survived to year’s end, a 37 percent survival rate.
The outbreak extended simultaneously into Yellowstone National Park, where managers counted 84 wolves across seven packs, including three breeding pairs. Mills said pup survival in the park was equally stark, with only 17 juveniles making it through the season. “Seventeen pups survived in Yellowstone,” he said. “Which was the lowest they ever recorded.”
Distemper flare-ups have historically followed density-dependent patterns, surging when wolf populations grow large enough to facilitate transmission. The virus last spread widely in 2018, shortly after a period of federal protections that drove numbers higher. The 2025 outbreak is unusual because it depressed a population that was already relatively low. Mills said the pattern has him weighing alternative explanations, including whether the virus operates on a longer cycle. “Could it be cyclical? Yeah,” he said. “However, these are potentially eight-year cycles, and it takes a lot of time to collect data and understand what’s going on.”
The outbreak also distributed wolves unequally across Wyoming’s management zones by year’s end. The northwest trophy game area held 132 wolves in 22 packs, including 10 breeding pairs. The predator management zone, where wolves can be killed by any means without limit, contained 28 wolves across five packs with one breeding pair. Nine wolves in three packs occupied the Wind River Indian Reservation, where no breeding was detected.
Despite the steep drop, state officials project a swift rebound. Historical data suggest distemper outbreaks in wolf populations typically last one year before subsiding, allowing numbers to climb as antibodies build across surviving adults. Mills said the sudden decline did not derail Wyoming’s management framework, which sets a baseline population objective of 160 wolves to absorb environmental shocks while maintaining minimum federal recovery standards.
The 2025 surveys recorded exactly 10 packs with surviving pups in the northwest trophy game area, hitting the exact minimum threshold required outside the national park. “We met the minimum,” Mills said. “It actually worked exactly as we intended.” Game and Fish officials will factor the reduced population into fall 2026 hunting season proposals, though Mills anticipates a surplus of wolves will still support a regulated season, potentially with adjusted mortality limits.