Epizootic Hemorrhagic Disease (EHD)
Epizootic Hemorrhagic Disease (EHD) is a virus that affects many species of deer throughout the world. Here in North America we primarily find it in the white-tailed deer population where mortality rates are usually high. We most commonly see outbreaks in late summer and early fall. Once the weather starts to turn cold, we usually stop seeing the number of cases. This disease is an important one because it has the potential to have serious consequences on the wild deer populations. Just because though we primarily see deaths in the white-tail population does not mean that it does not infect other species of deer. Other deer, including mule deer and pronghorn antelope will show symptoms but at a lesser extent. Numerous other deer will be seropositive upon testing. Cattle are rarely reported of being infected with EHD, but if they do occur, it is usually seen during high outbreaks in the white-tailed deer populations. Sheep have been infected experimentally but they do not show any clinical symptoms. Humans and goats on the other hand do not seem to be susceptible to infection.....
The transmission of EHD to other susceptible animals through biological vectors (insects) is usually by biting midges of the Culicoides genus. The midge most commonly transmitting the virus is the C. variipennis. Along with this species, some gnats and mosquitoes can also transmit EHD. Once a deer has been infected with the virus, the incubation period (time before clinical symptoms are present) is 5 to 10 days in deer.
There are three different syndromes that the deer can exhibit ranging from peracute, acute to chronic forms of the virus. In the peracute form of the disease the animal will exhibit a high fever, anorexia, weakness, respiratory distress and severe and rapid edema of the head and neck areas. Individuals might also notice a swelling of the tongue and conjunctivae in the infected deer. Animals that become infected with the peracute form of the disease usually die quickly (8-36 hours). The clinical symptoms for the acute form of EHD include the same symptoms as the peracute form as well as extensive hemorrhages in tissues including the skin, heart and gastrointestinal tract. The deer may also show signs on excessive salivation and nasal discharge which is tainted with blood as well as erosions which can be found on the tongue, dental pad, palate, rumen and omasum. Both the peracute and acute forms result in high mortality losses in the deer populations that they infect. The third form is the chronic form where the deer appear ill for several weeks, but eventually recover. The problem with this form is that the animal will usually slough the hoof wall and leave the deer unable to walk normally. Deer with a sloughed hoof wall may be seen crawling on its knees or its belly. If by chance an animal survives the disease, they will have a natural immunity to EHD for awhile.
EHD is a reportable disease to state regulatory agencies even though it is endemic to the United States. It is an important disease to monitor because some of the clinical symptoms resemble those of foot-and-mouth disease, blue tongue and plant photosensitization. The state agencies will be able to run the correct tests to determine what the animals are infected with.
It is important to prevent animals from becoming infected with EHD. There are several ways to go about doing this. As previously mentioned, the virus is transmitted by vectors (biting midges, possibly mosquitoes and gnats) so controlling these vectors will reduce the risk of infection. To prevent this disease you might think about controlling vectors by reduce the breeding areas for the vectors, applying insecticides and larvicide’s and using insect repellents. There are no vaccines available for EHD so prevention is the best method.
Resources:
Fact Sheet-Diseases Caused by the Epizootic Hemorrhagic Disease Virus Serogroup http://www.cfsph.iastate.edu/DiseaseInfo/
International Veterinary Information Services (IVIS)
http://www.ivis.org
