Penn Vet | Fact Sheet Detail



Hemorrhagic Disease

By: Wildlife Futures Team Date: Feb 18, 2021

Other names: Epizootic Hemorrhagic Disease Virus (EHD), Bluetongue Virus (BT)


Hemorrhagic Disease (HD) is caused by two closely related viruses; Epizootic Hemorrhagic Disease Virus (EHD) or Bluetongue Virus (BT). The disease features produced by both viruses are indistinguishable; therefore, the term hemorrhagic disease is used when the specific virus is not known.

Outbreaks of disease similar to HD have been described since the late 1880’s, but the EHD virus was not isolated until an outbreak in New Jersey white-tailed deer in 1955. The BT virus was first isolated from white-tailed deer and bighorn sheep with HD in Texas in 19661. There is recent evidence that new variants of the EHD virus are now present in North America and that the viruses, and possibly their vectors, are expanding their ranges2,3.


Hemorrhagic disease is the most common infectious disease of white-tailed deer in the eastern United States and can cause a significant number of deaths during an outbreak2.  While endemic in the eastern US, it is considered an emerging disease in other parts of North America.

Species Affected

Both wild and domestic ruminants can be infected by EHD and BT viruses but neither of these viruses is known to cause disease in humans. BT is generally more common in domestic species and EHD is more common in wild species1.

White-tailed deer and mule deer are the primary wildlife species affected by EHD. It has caused severe mortality in at least one outbreak in bighorn sheep. Elk can become infected with this virus, but they do not seem to be as susceptible as white-tailed deer1. Moose are not thought to be susceptible.

BT is a well-known disease of sheep, cattle, goats, and can also infect domestic dogs. It is occasionally associated with deaths of pronghorn antelope2.


EHD and BT viruses are found worldwide in temperate and tropical climates, but they have only been reported in free-ranging wildlife in North America. In the United States, HD has been confirmed in most eastern and southeastern states as well as several states in the midwest, the plains states, and the northwest2. Sporadic cases have also been reported in British Columbia, Alberta, Saskatchewan, and Ontario, Canada4.


The EHD and BT viruses are both transmitted by direct contact through biting flies or midges from the genus Culicoides. Female midges take up the viruses when they ingest the blood of an infected animal and then transmit the virus when they feed on a naive susceptible animal1. Midges preferentially breed in mud; therefore, outbreaks of hemorrhagic disease usually occur when deer congregate at water sources during the driest part of late summer and early fall when seasonal midge activity is at its peak and water levels are at or near their lowest points5. Dead or dying deer near water in later summer or early fall are a common characteristic of an HD outbreak. Outbreaks end when the first hard frosts kill the midges1.

Though these insects do not occur naturally in all parts of the United States, they are known for their ability to be transported to new areas on wind currents. Animals that live in areas where this primary vector is usually absent do not have any built-up immunity to the virus. As a result, when outbreaks of HD do occur, the mortality rates among white-tailed deer populations in these areas are high. In areas where the vector is found normally, immunity is more prevalent and mortality rates are lower6.

Clinical Signs

Clinical signs of hemorrhagic disease can be highly variable and range from very rapid illness resulting in death within 36 hours, to a longer, chronic course where animals remain ill for several weeks or months. Variation in clinical signs between individuals is not fully understood, but it appears that maternal antibodies, innate host resistance, the particular type of HD, and previous immunity may all be contributing factors. White-tailed deer usually develop clinical signs about 5-10 days following infection with the EHD or BT virus, but some infected animals that have previously conferred immunity may remain asymptomatic.

EHD and BT viruses cause disease by damaging the lining of blood vessels, making them weak or leaky, or destroying them. Clinical signs include swelling of the face or neck, loss of appetite, lethargy, weakness, lameness, respiratory distress, high fever, and excessive salivation. Infected animals may develop swollen, bluish tongues. Others will bleed from ulcers in the mouth and may also bleed from the nose. Frequently, deer will go into shock and die within 8 to 36 hours of the onset of clinical signs. Examination of animals that die of HD will often reveal extensive hemorrhage of the internal organs, including the heart, liver, kidneys, lungs, spleen, and intestines1. Those that survive may exhibit hoof overgrowth and have indentations or cracks in the walls of their hooves (see photos).


Diagnosis is based on clinical signs and laboratory tests, particularly virus isolation or PCR from infected tissue. Blood, spleen, and lung are preferred samples for virus isolation, with bone marrow recently identified as an additional tissue with sampling potential7.


There is currently no treatment for hemorrhagic disease in wildlife populations.


Hemorrhagic disease can cause high local mortality rates and is considered the most important viral disease of white-tailed deer in the United States7. Both free-ranging and captive deer and elk are at risk of contracting HD and transporting infected animals to areas where HD is not yet present has spread the disease. The impact of this disease on local deer populations is not thought to be long lasting6, but it is important to test suspect cases in order to identify outbreaks of HD. Insect control could theoretically decrease transmission of EHD and BT viruses in captive herds, but it has not proven to be feasible or e­ffective in the past. An autogenous vaccine has been developed for use in captive white-tailed deer populations, but its efficacy is poor8.

Literature Cited

1. Howerth, E.W., D.E. Stallknecht, and P.D. Kirkland. 2001. Bluetongue, epizootic hemorrhagic disease, and other orbivirus-related diseases. Pages 77-97 in E. S. Williams and I. K. Barker, editors. Infectious Diseases of Wild Mammals. Iowa State University Press.

2. Ruder, M.G., Lysyk, T.J., Stallknecht, D.E., Foil, L.D., Johnson, D.J., Chase, C.C., Dargatz, D.A. and Gibbs, E.P.J., 2015. Transmission and epidemiology of bluetongue and epizootic hemorrhagic disease in North America: current perspectives, research gaps, and future directions. Vector-Borne and Zoonotic Diseases15(6), pp.348-363.

3. Samy, A.M. and Peterson, A.T., 2016. Climate change influences on the global potential distribution of bluetongue virus. PloS one11(3), p.e0150489.

4. Allen, S.E., Rothenburger, J.L., Jardine, C.M., Ambagala, A., Hooper-McGrevy, K., Colucci, N., Furukawa-Stoffer, T., Vigil, S., Ruder, M. and Nemeth, N.M., 2019. Epizootic hemorrhagic disease in white-tailed deer, Canada. Emerging Infectious Diseases25(4), p.832.

5. Christensen, S.A., Ruder, M.G., Williams, D.M., Porter, W.F. and Stallknecht, D.E., 2020. The role of drought as a determinant of hemorrhagic disease in the eastern United States. Global Change Biology26(7), pp.3799-3808.

6. Christensen, S.A., Williams, D.M., Rudolph, B.A. and Porter, W.F., 2021. Spatial variation of white-tailed deer (Odocoileus virginianus) population impacts and recovery from epizootic hemorrhagic disease. The Journal of Wildlife Diseases57(1), pp.82-93.

7. Becker, M.E., Healy, S., Forbes, W., Roberts, J., LaCour, J. and Foil, L.D., 2020. Postmortem detection of bluetongue and epizootic hemorrhagic disease viruses in the bone marrow of white-tailed deer (odocoileus virginianus). Journal of Wildlife Diseases56(1), pp.58-65.

8. Wisely, S.M. and Sayler, K. 2016. Autogenous vaccine field trial for epizootic hemorrhagic disease virus and bluetongue virus does not result in high titer to homologous virus serotypes. International Journal of Infectious Disease. Vol 53. Pages 149-150.

Suggested Reading



EHD and BT Distribution

WF-EHD and BT distribution in the US 

Known distribution of EHD and BT in the United States, 2014.

Clinical signs of EHD

WF-Clinical Signs of EHD  

Clinical signs of EHD in white-tailed deer. Photograph courtesy of the Southeastern Cooperative Wildlife Disease Study.