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, mule deer, and bighorn sheep. EHD and BTV are caused by different but closely related viruses that produce nearly identical symptoms (Adrian, 1981).

Hemorrhagic Disease outbreaks in the United States occur over a large part of the country, and the geographic range of the virus appears to be expanding (SCWDS).

In Montana, outbreaks of EHD and BTV have historically occurred east of the continental divide, and while most deer and pronghorn mortality in Montana resulting from hemorrhagic disease infection still occurs in the eastern part of the state, EHD was documented west of the continental divide in Montana for the first time in 2013.

While mortality rates vary from year to year, they can be quite high, however, most EHD outbreaks have not been considered to have long term negative impacts on white-tailed deer populations.

In 2007, Montana experienced a BTV outbreak in pronghorn antelope in eastern Montana. This outbreak caused high rates of pronghorn mortality. Additionally, the outbreak was followed closely by severe winter weather that resulted in even more pronghorn mortality. Other factors affecting these antelope populations are not clearly understood.

Outbreaks of EHD and 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.

Although EHD and BTV are infectious to a wide range of wild ruminants, susceptibility varies among species. Clinical disease due to EHDV has been reported in white-tailed deer, mule deer, bighorn sheep, elk, and pronghorn, and clinical disease due to BTV has been reported in these species, as well as in black-tailed deer. Infections in these wild ruminants can range from mild or no disease to episodes of high mortality. Antibodies or virus also have been detected in bison and mountain goats; however, these infections were not associated with disease (SCWDS). Domestic cattle, goats and sheep may also be bitten by midges that are carrying EHD virus, but these species rarely develop disease. BTV is a well-known disease of domestic sheep, cattle, and goats, in addition to affecting deer (WDFW).

The viruses that cause HD are transmitted by biting flies in the genus Culicoides. The best documented vector in North America is Culicoides sonorensis although other Culicoides species may play a role in local transmission in certain regions. These flies are commonly known as biting midges but also are called sand gnats, sand flies, no-see-ums, and punkies (SCWDS). These flies breed in damp muddy areas and reach their yearly population peak in late summer and fall (Adrian, 1981). The gnat transfers the virus to the new animal host via its bite.  The seasonal occurrence of hemorrhagic disease coincides with periods of biting midge abundance. The onset of freezing temperatures in late fall affects vector populations and usually brings a sudden end to hemorrhagic disease outbreaks. How the viruses persist through the winter when midges are not active is not clear. However, it is believed that in areas with a mild climate, vector populations may remain active and locally support year-round virus transmission (SCWDS).

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) .

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.

Southeastern Cooperative Wildlife Disease Study (SCWDS), University of Georgia. Hemorrhagic Disease of White-tailed deer brochure. https://vet.uga.edu/wp-content/uploads/2019/07/Hemorrhagic_Deer_Brochure_V2.pdf 

Washington Department of Fish and Wildlife (WDFW). Website. https://wdfw.wa.gov/species-habitats/diseases/hemorrhagic