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Conservation > Fish & Wildlife Diseases Bluetongue - Epizootic Hemorrhagic Disease

For questions/concerns about this disease in humans, please call your doctor or the Montana Department of Public Health and Human Services  (DPHHS).

For questions about this disease/parasite in wildlife, please call the FWP Wildlife Health Lab at (406) 577-7882.


Hemorrhagic diseases (Epizootic Hemorrhagic Disease –EHD and Bluetongue Virus – BTV) are common viral diseases primarily of white-tailed deer. They sometimes cause disease in other species such as pronghorn antelope and mule deer. EHD and BTV are caused by different but closely related viruses that produce nearly identical symptoms (Adrian, 1981).


In Montana, EHD and/or BTV occur in late summer and early fall when conditions are suitable for the small biting gnats (Culicoides) that carry and transmit these viruses. The Culicoides reproduce in moist muddy areas, often around the edges of ponds, creeks and other bodies of water. The first severe frost of the year destroys the biting gnats and abruptly ends the spread of disease for the year (Adrian, 1981). Because the incubation period (time from infection until symptoms develop) for these viruses may be as long as 10 days, cases of EHD or BTV may be detected for a couple weeks after a hard frost.

Montana regularly experiences EHDV-2 outbreaks in white-tailed deer beginning in the summer and lasting through early fall. Generally, outbreaks have occurred in eastern Montana, with varying degrees of mortality from year to year. Most of these white-tailed deer EHD outbreaks have not been considered to have long term negative impacts on white-tailed deer populations.

In 2007, Montana experienced a Bluetongue virus outbreak in pronghorn antelope in eastern Montana. This outbreak caused very high mortality in antelope, and some of these populations have still not recovered. The BTV outbreak is likely only one factor that has inhibited recovery of the antelope populations. The outbreak was followed closely by severe winter weather that resulted in high antelope mortality. Other factors affecting these antelope populations are not clearly understood.

In 2013, Montana experienced an unusually wide distribution of hemorrhagic disease. In addition to widespread EHD mortalities in eastern Montana, EHDV-2 was documented in white-tailed deer in FWP Regions 2 and 3 for the first time. This was the first year that FWP documented EHD virus west of the continental divide in Montana. BTV-17 was detected in one white-tailed deer from Yellowstone County, and one pronghorn from Bighorn County.

Species affected

EHD virus is probably the most important virus of white-tailed deer. It also may infect mule deer, elk, and bighorn sheep, and although these species may occasionally develop symptoms, they are much less susceptible to EHD virus. Domestic cattle, goats and sheep may also be bitten by midges that are carrying EHD virus, but these species rarely develop disease.

In Montana, BTV primarily affects white-tailed deer and pronghorn antelope. Domestic sheep are also very susceptible to BTV infection and disease.


Both BTV and EHD are transmitted by a small biting gnat/ midge (Culicoides variipennis). This gnat breeds in damp muddy areas and reaches its yearly population peak in late summer and fall (Adrian, 1981). The gnat transfers the virus to the new animal host via its bite. A hard frost will end the spread of the virus from midges for the season; however, cases of EHD or BTV may be detected for up to two weeks after the frost because the incubation period for these viruses can be up to 10 days.


Clinical signs of EHD and BTV are very similar. Hemorrhagic disease outbreaks are characterized by their late summer-through fall occurrence and an almost explosive onset with large numbers of animals reported sick or dead in a short period of time. These animals are usually in good nutritional condition, with most having signs of varying degrees of hemorrhage throughout the body. Hemorrhages range from pinpoint to massive in size, and involve different tissues and organs in individual animals. No organs appear to be exempt from hemorrhage, with the most regularly involved being the heart, liver, spleen, kidney, lung and intestinal tract. The hemorrhage is due to the BTV or EHD virus producing severe damage to the delicate lining of small blood vessels (Adrian, 1981).

Signs are variable, ranging from none, to sickness, or sudden death. Some animals die quickly once symptoms develop (acute form), but many will survive for a time and develop lameness and weight loss or emaciation (chronic form). In the acute form of the disease, deer pass into a shock-like state, become prostrate and die within 8 to 36 hours following the onset of observable signs (Michigan, 2010). Animals initially lose their appetite and fear of man, grow progressively weaker, often salivate excessively, and develop a rapid pulse and respiration rate. Because affected animals become dehydrated and develop high fever, deer suffering from EHD or BTV often seek out bodies of water to lie in to reduce their body temperature. Sick and dead animals are often found near water. Hemorrhage and lack of oxygen in the blood may result in a blue appearance of the oral mucosa, hence the name 'bluetongue'. Other common signs include swelling of the face, tongue, neck and conjunctiva of the eyes, lack of coordination, excessive salivation (often blood tinged), nasal discharge (often blood tinged), and bloody diarrhea. Animals that are more chronically infected will often develop lameness with overgrown or cracked hooves, and ulcers in the tissues of the mouth, tongue and stomachs (Miller et al, 2003) .

Public Health Concerns

EHD virus is not associated with human disease (Miller et al, 2003) .

Is it safe to eat the meat?

An animal that is symptomatic with EHD or BTV may not be fit for consumption due to the hemorrhages within the body. However, the viruses are not transmitted to humans. The meat should not be fed to dogs (Miller et al, 2003) . If you harvest an animal and are unsure whether it is safe to eat, contact your local FWP staff for guidance soon after the animal is harvested.


Adrian, W.J., editor. 1981. Manual of Common Wildlife Diseases in Colorado. Denver: Colorado Division of Wildlife.

Michigan Wildlife Disease Manual. 2010. Michigan Department of Natural Resources. Website

Miller, M.J.R., R.D. Dawson, and H. Schwantje. 2003. Manual of Common Diseases and Parasites of Wildlife in Northern British Columbia. University of Northern British Columbia.