General information
Hemorrhagic disease (HD) is a general term for illness caused by two related viruses: Epizootic Hemorrhagic Disease virus (EHD) and bluetongue virus (BT).HD primarily affects white-tailed deer and can cause significant mortality events, particularly in the northern United States. Mule deer and pronghorn antelope are also affected. Domestic ruminants (sheep, cows, goats) are also susceptible to HD. Cows typically do not show clinical signs, but sheep can suffer severe disease and death from BT infection. Neither EHD or BT are a disease of humans. Clinical signs of HD in white-tailed deer usually begin approximately 7 days after infection. Fever and edema are common, and deer often have a swollen head, neck, tongue, or eyelids. Animals may have reduced appetite, weakness, and loss of fear of humans. Deer die quickly within 8 to 36hours.
HD is transmitted to an animal host by Culicoides midges, tiny biting flies commonly known as “no-see-ums” or gnats that are smaller than mosquitoes and other flies. Diagnosis is based on the combination of clinical signs and virus testing on tissue and blood. Virus identification is essential to distinguish EHD and BT from other diseases of agricultural concern, such as Foot and Mouth Disease. There is no treatment for HD in wildlife populations, and no wildlife prevention plan currently exists.
Both EHD and BT are in the genus Orbivirus and family Reoviridae. There are 10 known serotypes (variations)of EHD and 24 serotypes of BT worldwide. Outbreaks of EHD in domestic ruminants are becoming more common.
HD produces lesions on the mouth and feet that resemble a serious viral disease of livestock called Foot and Mouth Disease which is not currently found in the US. BT can potentially infect domestic dogs. Clinical signs of HD infection in deer vary based on the serotype and whether or not animals have any preexisting immunity. The incubation period ranges from about 5 to 10 days. The acute form of HD has high mortality rates. Fever causes deer to seek out water, so dead deer may be found near or in water. Deer with chronic infections may show hoof abnormalities, including sloughing of hoof walls.
The viruses damage the endothelium, the lining of the blood vessels, causing small hemorrhages throughout the body. Hemorrhage in the heart and lungs can result in respiratory distress. There may be dental pad erosion or tongue ulcers as well as bloody discharge from the nasal cavity. Ulcers of the stomach (rumen and omasum) may also be present. HD causes high mortality in northern deer with little preexisting immunity. In domestic ruminants, such as cattle and sheep, HD is usually less severe but may cause fever, oral ulcers, and other mild signs. Sheep are especially susceptible to BT and can suffer severe disease and death.
Transmission of HD occurs when a female Culicoides midge picks up the virus from the blood of an infected host and then transmits the virus by biting another host.
Diagnosis: Laboratory tests that can confirm HD include immunofluorescence and reverse transcriptase polymerase chain reaction (RT-PCR) tests from tissues
or blood.
Epidemiology HD outbreaks occur in late summer and early fall when insect vectors are active. The midges and viruses are killed within two weeks of the onset of frost ending the transmission cycle. Although the vector is not found in all areas, it is easily transported to new areas by wind currents. EHD and BT have circulated in the southern United States for decades and outbreaks in deer there are typically mild or completely inapparent, detectable only by antibody titers. The periodic movement of infected midges in recent years to the northern United States where there is little history of previous exposure results in severe outbreaks with high mortality.