Two years after Congress removed gray wolves from the endangered species list in the northern Rockies, the animals are facing a new threat: disease. Outbreaks of infections such as sarcoptic mange, which is spread by mites, and canine distemper virus (CDV), have reduced wolf survival rates and contributed to an overall decline in Yellowstone National Park wolves.

Until recently, wolf populations in Yellowstone had been on a steady upswing. In 1995 park managers brought in 31 gray wolves from Canada to restore a population that had been virtually wiped out by hunting and other forms of depredation. (Montana veterinarians introduced the mange-carrying mite Sarcoptes scabiei to Yellowstone in 1905 in an effort to extirpate the wolves.) The most recent count put the regional wolf population at 1,727 in 2011, well above the lower limit set by federal agencies.

The Yellowstone wolves may be particularly susceptible to disease because, as transplants, they are relatively new to their environment. And wolf pups are the most at risk: only 16 survived this year, down from 34 in 2011.

Although scientists do not believe the illnesses pose a threat to the wolves' long-term survival, the new data may spur managers to tweak conservation plans. Wyoming has already reduced its hunting quota from 52 to 29, which experts say is a step in the right direction but may not be enough to shield the population from chance disturbances, be they trophy hunts or diseases. “Whether this is enough of a reduction will be evaluated following the next hunt,” which begins in December, says Emily Almberg, a graduate ecology student at Pennsylvania State University who studies the wolves.