Hemorrhagic disease is a viral disease caused by either the epizootic hemorrhagic disease virus or the bluetongue virus. These viruses are closely related, but genetically distinct.
In North America, hemorrhagic disease is seen primarily among white-tailed deer and occasionally in mule deer, black-tailed deer, elk, pronghorn antelope, and bighorn sheep. Wild deer as well as pen-raised deer are susceptible. Cattle and sheep are occasionally affected with varying severity.
The clinical disease produced by the two viruses is indistinguishable and highly variable. Thus, the term “hemorrhagic disease” is often used when the specific virus has not yet been identified. Three forms of clinical disease are recognized:
- Peracute – affected deer develop high fever, weakness, difficulty breathing and swelling of the head, tongue, and eyelids. Death may occur within 8 to 36 hours with few symptoms.
- Acute – Extensive hemorrhage under the skin, heart, gastrointestinal tract, and other tissues and in some cases, ulcers of the mouth or tongue may develop. Mortality is high among deer with this form.
- Chronic – Breaks or rings in the hooves caused by interruptions in growth, which may result in lameness; ulcers, scars or erosions in the rumen, which may lead to emaciation. Deer with the chronic form are ill for several weeks, but may gradually recover.
Affected deer are drawn to streams, lakes, ponds and other water sources as they seek relief from high fever and dehydration.
Hemorrhagic disease does not spread directly from deer to deer, but depends on a biological vector for transmission.
Both the epizootic hemorrhagic disease virus and the bluetongue virus are transmitted by Culicoides, a genus of insects that includes biting midges and no-see-ums. The most commonly incriminated species is C. variipennis, a biting midge found in moist areas and low wetlands. Midges breed in moist, muddy areas.
The disease is seasonal, occurring from late summer into fall. The first good frost or freeze will usually result in an abrupt end to an outbreak, as winter weather will halt Culicoides activity.
Diagnosis
An accurate diagnosis for hemorrhagic disease requires a case history, clinical signs, and tissues/blood samples submitted for laboratory testing. These are also needed to determine which virus is responsible for the disease.