State wildlife officials say that recent whitetail deer deaths west of Missoula were caused by epizootic (epi-ZOO'-ah-tic) hemorrhagic disease.
Fish, Wildlife and Parks officials said Monday that it was the first confirmed outbreak of the disease west of the Continental Divide in Montana.
Nearly 400 dead deer have been reported in the Clark Fork River valley since mid-September.
FWP wildlife biologist Vickie Edwards says samples from dead deer found farther away from the main outbreak are still being tested.
EHD is caused by a virus spread by biting midges or gnats. It causes bleeding that can kill the infected animal within a day or two after a six-day incubation period. Dead animals frequently are found near water, where they go to alleviate a high fever caused by the disease.