Yellowstone National Park wolf pups face ‘particularly lethal’ virus

[Freija]

Yellowstone National Park wolf pups face ‘particularly lethal’ virus

A virus outbreak hit Yellowstone wolf pups hard in 2025.

www.sfgate.com

Yellowstone National Park’s famous wolf packs are facing an invisible threat that’s bringing the number of surviving pups to historic lows: a contagious viral disease called canine distemper.

At the end of 2025, Yellowstone National Park had 84 wolves dispersed among seven packs, according to spokesperson Linda Veress. This amounts to an approximately 16% decrease from the average end-of-year population over the past 15 years. Veress told SFGATE that the number of Yellowstone wolves fluctuates and that the park still supports a “healthy population,” but also acknowledged that the virus is likely taking a toll.

"A Wyoming Game and Fish Department report laid out declining numbers in more detail. Yellowstone wolf packs produced at least 36 pups in 2025. Only 17 of those pups — less than half — survived to the end of the year. The actual survival percentage may be even lower, “as several litters died before they could be counted ... or were almost certainly undercounted,” the report reads.

No pups survived until December in several Yellowstone packs, including the Bliss Pack, Cougar Creek, Mollie’s and Wapiti packs. The Junction Butte pack, one of the most viewed wolf packs in the world, also saw low survival, according to Veress.

The survival of only 17 pups is the lowest recorded number since wolves were reintroduced to Yellowstone in 1995 and the first litters were born in 1996. The report says the decline is likely a “natural variation due to a combination of factors including a possible disease outbreak.”

Biologists confirmed canine distemper virus in Wyoming from packs outside Yellowstone in 2025. Only 37% of wolf pups born in northwestern Wyoming’s “trophy game management area” survived, according to the report. More than half of the wolves captured here, or 64%, tested positive for the virus.

Veress wrote that the park is in the process of analyzing blood samples. “We have not yet confirmed CDV, but Yellowstone biologists predict similar findings of CDV exposure which is likely a contributor to poor pup production in 2025,” she wrote.

The article concludes saying Yellowstone experts aren't too worried about the disease and say “Past outbreaks show that survivors develop immunity and populations rebound,”

Isn't this what they call herd immunity?