Disease Outbreak Drives Wyoming and Yellowstone Wolf Counts to Lowest Point in Two Decades

A canine distemper outbreak devastated wolf populations across Wyoming and Yellowstone National Park in 2025, dropping total counts to levels not seen since wolves were still working to establish a foothold in the region following their reintroduction in the mid-1990s.

Wyoming Game and Fish Department wolf biologist Ken Mills said the state’s minimum count fell to 253 wolves and 14 breeding pairs by year’s end. That is down sharply from 330 wolves and 24 breeding pairs recorded in 2024, representing a 23% drop in overall numbers and a 42% decline in breeding pairs.

“It was the lowest number of wolves in 20 years,” Mills said.

Pups Hit Hardest

Canine distemper, a highly contagious measles-like virus, proved especially deadly for wolf pups. While adult wolves can survive exposure, the disease is, in Mills’ words, “quite lethal” for young animals. Of 87 pups documented in Wyoming, only an estimated 31 to 34 survived to year’s end, a survival rate of roughly 37%.

The virus was detected in 64% of wolves captured in the northwestern Wyoming trophy game zone, where wolves hold that management classification.

Yellowstone fared similarly. The park recorded only 17 surviving pups in 2025, the lowest figure in its documented history. Eighty-four wolves in seven packs, including three breeding pairs, were counted within park boundaries at year’s end. The park’s total wolf count has dipped into the 80s before, in 2012 and 2018, but by other measures 2025 was the hardest year the Yellowstone population had faced since the reintroduction era.

An Unusual Outbreak

What puzzled Mills most was the timing. Historically, distemper has behaved as a density-dependent disease, surging when wolf populations are relatively high. The last major flare-up came in 2018, not long after a stretch when wolves were federally protected under the Endangered Species Act and numbers were elevated. The 2025 outbreak was the first Mills had documented occurring when wolf numbers were not particularly high.

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

Mills is based in Pinedale, Wyoming, and has tracked the state’s wolf population closely. He noted the outbreak’s apparent synchrony between Wyoming’s trophy game zone and Yellowstone, with both areas experiencing poor pup survival at the same time.

It was also the first time in Mills’ experience that distemper produced a measurable, population-level effect across Wyoming. “We really haven’t had a canine distemper outbreak that has caused a population-level effect,” he said.

Recovery Expected

Despite the setback, Mills expressed confidence the population can rebound. Historical patterns from Yellowstone show distemper outbreaks typically last one year before recovery begins. Wyoming wolves now carry higher levels of antibodies and natural resistance built up through the 2025 exposure, which Mills said puts the population in a strong position going forward.

The state’s wildlife management structure also appears to have held up under the stress. Wyoming has set a population objective of 160 wolves specifically to maintain a buffer against events like a distemper outbreak while still meeting minimum federal recovery requirements. Those requirements include 10 breeding pairs outside Yellowstone in Wyoming’s trophy game area.

Mills’ 2025 surveys counted exactly 10 qualifying packs in that zone.

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

Hunting Season Under Review

Game and Fish biologists and wardens will factor the reduced population into proposals for fall 2026 wolf hunting seasons. No proposal has been made public yet. Hunting is not permitted inside Yellowstone, though park wolves regularly move outside its boundaries.

Mills indicated he still anticipates a surplus of animals and expects a hunting season to proceed, though mortality limits may be scaled back to reflect current population levels.

Outside Yellowstone, the state’s wolves were distributed across several management zones at year’s end. The trophy game area held 132 wolves in 22 packs, including 10 breeding pairs. The Wind River Indian Reservation held nine wolves in three packs with no confirmed breeding. The predator zone, where wolves may be killed by any means without restriction, held 28 wolves in five packs, including one breeding pair.

Byline: North Idaho Republican Staff