Bluetongue is a viral, insect-borne disease common in sheep, deer and antelope. Cause by the Bluetongue virus (BTV), a widespread outbreak is predominant among white-tailed deer.

In the Idaho outbreak, BTV was linked in the infection of bighorn sheep and mule deer. Leaving hundreds dead during the attack in August 2015, experts expect more will die from the infection.

Bluetongue symptoms include a discharge from the animal's mouth or nose, after which infected animals can die from the infection. Bluetongue outbreaks occur almost every year in remote areas. The annual outbreaks are often limited to a considerably small group of animals.

However, the 2015 Bluetongue outbreak troubled many officials. Veterinarians and wildlife representatives are concerned that Bluetongue can affect a broader variety of animals. In the small town of Colville, Washington, around 68 white-tailed deer have died due to the Bluetongue virus, said Terry LeCaire, Colville's Street / Park Superintendent.

A landowner near the Palouse River in Washington and Idaho have reported around 30 dead white-tailed deer in his land, said Washington State University's College of Veterinary Medicine spokesperson Charlie Powell.

Epizootic hemorrhagic disease (EHD) is another fatal disease common in white-tailed deer. EHD is caused by the virus from Orbivirus genus. Symptoms include widespread hemorrhages that can effectively kill off various ruminants. EHD is also prevalent in white-tailed deer in southern Canada and the United States.

"The distinction between (Bluetongue) and EHD is important. Bluetongue can affect international trade (of livestock), while EHD is uncommon in cattle," added Powell.

Mark Drew, a wildlife veterinarian in Idaho, explained that Bluetongue prevails among the livestock in Great Basin every year. Some Bluetongue cases are fatal than others, he expressed.

Colville biologist Dana Base from the Washington Department of Fish and Wildlife added that majority of reported cases will come from highly populated area. Bluetongue-infected livestock in rural areas might be dying undetected, he added.

Kristin Mansfield, wildlife veterinarian who practices in Washington, explained that the Bluetongue outbreak will persist until winter when the biting cold will kill off the virus-laden gnats. Bluetongue virus is perpetuated by insects who transmit the virus from one animal to another.