NBC Montana is learning more as a deadly mystery has wildlife biologists tracking down leads in the Missoula Valley. Fish, Wildlife and Parks crews spent Thursday collecting dead deer samples that landowners had called in. A possible virus, called EHD (Epizootic hemorrhagic disease), is suspected in at least 150 White-tailed Deer deaths. That number is up from 103 counted by Tuesday.

EHD is transmitted by a biting midge, or gnat, and appears in late summer and early fall. A hard frost for about a couple weeks could bring the suspected virus to an end. F.W.P. biologists say the virus is not contagious from one animal to the other, but rather spreads through bug bites. The virus can not spread to humans, and when it spreads to livestock, the animals are often not symptomatic at all and do not die.

If the current outbreak is indeed EHD, it could be the first recorded outbreak west of the continental divide. Cases have been documented in Eastern Montana.

“It’s really sad to see these populations that were thriving die off,” said biologist Vickie Edwards.

Biologists say the death zone in Missoula is slowly spreading. The original outbreak was centered between Harper’s Bridge Road and the Erskine Fishing Access site. Now, more carcasses are being found east of that area, toward the Kona Bridge fishing access site. There are also reports of dead deer in the Mill Creek Road area near Frenchtown.

On Thursday, a wildlife veterinarian arrived from Bozeman to Missoula to perform animal autopsies. Samples have already been sent in for lab testing, and wildlife workers hope to have a cause of death over the next week.