YELLOWSTONE NATIONAL PARK, Wyo. » A new study says canine distemper appears to be the cause of periodic increased death rates among wolf pups in Yellowstone National Park.

Since wolves were reintroduced in Yellowstone in the mid-1990s, wolf pup survival rate has been extremely low in 1999, 2005 and 2008.

Canine parvovirus was believed to be the cause of the wolf pup deaths in 1999 and 2005.

But new research finds that parvovirus appears to be chronic in the park's wild canines.

Researchers believe that when distemper showed up in wolf populations, it weakened the immune system of wolf pups and made them more susceptible to other infections, such as parvovirus.

The research also indicates that Yellowstone wolves seem to fare well despite some chronic infections and rebound well from periodic exposure to distemper.

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On the Net:

Yellowstone wolf research: http://www.plosone.org/article/info:doi/10.1371/journal.pone.0007042 .

Yellowstone National Park: http://www.nps.gov/yell