A virus causing epizootic hemorrhagic disease (EHD) is the culprit in the deaths of almost 400 dead white-tailed deer west of Missoula, Montana.

The first dead deer were reported in early September and the count has been going up since then. Samples of the dead deer were tested by the Montana Fish, Wildlife and Parks. The results showed EHD as the cause. The disease had been found east of the Continental Divide, but these are the first white-tailed deer deaths west of the divide. They were found mainly west of Missoula, between Harper's Bridge on the Clark Fork River and Mill Creek in the Frenchtown area.

The virus is spread by an insect called the biting midge - also known as sand gnats or "no-see-ums." The virus causes internal hemorrhaging and can kill an infected deer within two days, after an initial week-long incubation period. The deer are often found near water, because they are affected by a high fever.

EHD is not contagious and is not transmissible to humans, according to Jennifer Ramsey, FWP wildlife veterinarian in Bozeman.

In a news release, officials said they expect freezing temperatures, which kills the midges, to stop the outbreak. However, with the incubation period, more deaths could be seen. Those who find dead animals are asked to contact the Montana Fish, Wildlife and Parks.