Cause
Hemorrhagic disease (HD) in white-tailed deer can be caused by either of two closely related viruses, epizootic hemorrhagic disease (EHD) and bluetongue (BT) virus. Outbreaks of diseases similar to HD have been described since 1890, but the EHD virus was not isolated until an outbreak in New Jersey white-tailed deer in 1955. The BT virus was isolated following an outbreak in white-tailed deer and bighorn sheep in Texas in 1966.
Significance
Outbreaks of HD can cause mass mortalities at a local level, but the disease does not appear to have a significant long-term impact on Pennsylvania’s statewide deer population.
Species Affected
White-tailed deer and mule deer are the primary wildlife species affected by HD. The disease is occasionally associated with deaths of pronghorn antelope and bighorn sheep. Elk can become infected with this disease, but they do not seem to be nearly as susceptible as white-tailed deer. The EHD virus rarely causes disease in domestic animals, while the BT virus is a well-known disease of sheep, cattle, and goats and can also infect domestic dogs. The viruses are not known to cause disease in humans.
Distribution
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 and northwest. There have also been sporadic cases reported in British Columbia, Alberta, and Saskatchewan in Canada. Seasonal surveillance for the disease occurs throughout the United States and both viruses have been detected in Pennsylvania white-tailed deer. Neither virus has been detected in Pennsylvania elk.
Transmission
The EHD and BT viruses are transmitted by biting midges (Culicoides sp.), also known as “no-see-ums”. Female midges pick up the viruses by ingesting the blood of an infected animal and they transmit the viruses when they feed on an uninfected animal. Midges are found near mud, which is their preferred breeding habitat, so outbreaks usually occur when deer congregate in wet areas during the driest part of late summer and early fall when seasonal midge activity is at its peak. Outbreaks end when the first hard frost kills the midges and any viruses they may carry. How the viruses overwinter is not well understood, but theories include milder climates allowing for midge persistence and infected hosts surviving and serving as viral reservoirs.
Clinical Signs
Clinical signs of hemorrhagic disease are a result of the damage that the virus does to the walls of blood vessels and can range from sudden death to chronic disease. White-tailed deer usually develop clinical signs of disease about one week following infection with either virus, but some animals may never show any signs of infection. Acute clinical signs include swelling of the face or neck, loss of appetite, lethargy, weakness, lameness, respiratory distress, fever, and excessive salivation. Due to the fever, infected deer often seek out water. Deer with HD may have ulcers in the mouth and may bleed from the nose and/or mouth. Infected animals may also develop swollen, blue tongues. Deer with chronic disease may experience hoof overgrowth or have indentations or cracks in the walls of their hooves. Infected deer usually die within 8 to 36 hours following onset of clinical signs.
Diagnosis
Post-mortem examination of an animal that died of HD often reveals extensive hemorrhage from multiple internal organs, particularly the heart, liver, kidneys, lungs, spleen, and intestines. Laboratory tests can identify and isolate the EHD or BT virus from infected tissues, with lung and spleen being the preferred tissue type. There are multiple strains or serotypes of each virus.
Treatment
There is no treatment for hemorrhagic disease in wildlife populations.
Management/Prevention
Vector-borne diseases of wildlife are challenging to manage. Insect control could potentially decrease transmission of EHD and BT viruses in captive herds, but it is not feasible in wild populations. Regardless, seasonal surveillance is conducted to identify outbreaks of HD and potentially rule out other significant contagious diseases, such as Foot-and-mouth disease.