Yellowstone National Park\u2019s famous wolf packs are facing an invisible threat that\u2019s bringing the number of surviving pups to historic lows: a contagious viral disease called canine distemper.\u00a0<\/p>\n
At the end of 2025, Yellowstone National Park had 84 wolves dispersed among seven packs, according to spokesperson Linda\u00a0Veress. 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 \u201chealthy population,\u201d but also acknowledged that the virus is likely taking a toll.\u00a0<\/p>\n
\u201cPatterns suggest disease-related pup mortality,\u201d she wrote.\u00a0<\/p>\n
A Wyoming Game and Fish Department report<\/a> laid out declining numbers in more detail. Yellowstone wolf packs produced at least 36 pups in 2025. Only 17 of those pups \u2014 less than half \u2014 survived to the end of the year. The actual survival percentage may be even lower, \u201cas several litters died before they could be counted … or were almost certainly undercounted,\u201d the report reads.<\/p>\n No pups survived until December in several Yellowstone packs, including the Bliss Pack, Cougar Creek, Mollie\u2019s and Wapiti packs. The Junction Butte pack, one of the most viewed wolf packs in the world<\/a>, also saw low survival, according to Veress.\u00a0<\/p>\n 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 \u201cnatural variation due to a combination of factors including a possible disease outbreak.\u201d<\/p>\n Biologists confirmed canine distemper virus in Wyoming from packs outside Yellowstone in 2025. Only 37% of wolf pups born in northwestern Wyoming\u2019s \u201ctrophy game management area\u201d survived, according to the report. More than half of the wolves captured here, or 64%, tested positive for the virus.\u00a0<\/p>\n Veress wrote that the park is in the process of analyzing blood samples. \u201cWe 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,\u201d she wrote.\u00a0<\/p>\n Canine distemper, a measles-like virus, can infect domestic dogs, coyotes, foxes, raccoons, skunks and, of course, wolves. The Wyoming Game and Fish report calls it \u201ca common, naturally-occurring infection which cycles through areas with carnivore populations.\u201d The virus has been documented in Yellowstone four other times since 1995.\u00a0<\/p>\n