Epizootic Hemorrhagic
Disease (EHD) Killing Idaho Deer

From Patricia Doyle, PhD

A ProMED-mail post ProMED-mail, a program of the International Society for Infectious Diseases
Deer Are Dying In North Idaho
Biologists Aren't Sure Why - But They've Ruled Out CWD
The Idaho Statesman 8-9-3
An epidemic in the Woodland area of the Clearwater River breaks has killed at least 100 whitetail deer. Dead deer are scattered in draws and near water sources. On [one] ranch, at least 40 have died. The smell of death is prevalent as he drives an all-terrain vehicle around his place.
The good news is that Idaho Department of Fish and Game (F&G) officials say the disease is not chronic wasting disease (CWD), and herds should not be affected for hunting. F&G biologists say the deer are being killed by a disease spread by gnats near watering holes. Washington State University veterinarians are running tests.
Behavior displayed by the dying deer is consistent with epizootic hemorrhagic disease or one similar to it, F&G conservation officer Dave Cadwallader said. Personnel at the Washington State University veterinary school have not been able to confirm that the virus is the cause. Tissue samples from the dead deer have been sent to other laboratories in California and Georgia. Results from those tests should be back early next week, according to F&G information officer Jack Trueblood. "These signs are not consistent with the signs of CWD," he said.
Epizootic hemorrhagic disease is common in southern states with high summer temperatures, he said. It does not spread to other animals or humans and cannot be spread between deer.
[The rancher involved also] raises sheep on his ranch near Kamiah in the same places deer are dying, and none of his animals have become ill.
Outbreaks of the disease are usually geographically isolated and occur during the hottest part of the summer when deer gather around water sources.
"We think it´s probably drought-related, where deer are drawn to water and gnats are drawn to water," Trueblood said. The Kamiah area has high concentrations of deer that could make the herd more susceptible to the ailment. The epidemics usually stop when wetter, cooler weather spreads the deer out or a hard frost kills the gnats.
Trueblood said if the outbreak remains localized, it won´t affect the deer population in the Clearwater Region, which supports the state´s largest whitetail herd. "We´re not going to have a shortage of deer," Trueblood said. "We don´t think it´s going to have an impact on the hunting season."
(Epizootic hemorrhagic disease (EHD) is an acute, infectious, often fatal viral disease of some wild ruminants. This malady, characterized by extensive hemorrhages, has been responsible for significant epizootics in deer in the northern United States and southern Canada. A similar hemorrhagic disease called bluetongue also occurs in wild ruminants. The 2 diseases are antigenically different, although there are similar clinical signs.
The first occurrence and subsequent identification of EHD occurred in 1955 when several hundred white-tailed deer (_Odocoileus virginianus_) succumbed in both New Jersey and in Michigan. It was considered a new disease of deer, and the name "epizootic hemorrhagic disease" was suggested to describe its main clinical and pathological features.
Since these initial confirmed outbreaks of EHD, documented epizootics have occurred in white- tailed deer in South Dakota, North Dakota, Wyoming, and Alberta, Canada. Suspected EHD outbreaks have occurred in Missouri, Washington, Nebraska, Iowa, and British Columbia. South Dakota, Missouri, and Nebraska have experienced periodic outbreaks of EHD, and the disease might be considered enzootic in these areas.
Since the initial 1955 outbreak, this malady has occurred primarily among white-tailed deer, although occasionally mule deer (_O. hemionus_) and pronghorn antelope (_Antilocapra americana_) have succumbed.
The mode of transmission of EHD in nature is via a _Culicoides_ biting fly or gnat. _Culicoides variipennis_ is the most commonly incriminated vector in North America. A common observation in outbreaks involving large numbers of deer (as in Michigan, New Jersey, and Alberta) is that they are single epizootics that do not recur. Die-offs involving small numbers of deer (as experienced in South Dakota and Nebraska) occur almost annually, and the disease appears to be enzootic in these areas. All documented outbreaks of EHD have occurred during late summer and early fall (August-October) and have ceased abruptly with the onset of frost.
Clinical signs of EHD and bluetongue are similar. White-tailed deer develop signs of illness about 7 days after exposure. A constant characteristic of the disease is its sudden onset. Deer initially lose their appetite and fear of man, grow progressively weaker, often salivate excessively, develop a rapid pulse and respiration rate, and finally become unconscious. Hemorrhage and lack of oxygen in the blood result in a blue appearance of the oral mucosa, hence the name "bluetongue." Between 8 and 36 hours following the onset of observable signs, deer pass into a shock-like state, become prostrate, and die.
The gross and histological lesions of EHD are characterized by extensive hemorrhage, ranging from pin-point to massive in size, and involve different tissues and organs in individual animals. No organs appear to be exempt from hemorrhage, with the most regularly involved being the heart, liver, spleen, kidney, lung, and intestinal tract. Extensive hemorrhaging is the result of interference with the blood-clotting mechanism together with degeneration of blood vessel walls.
Because of its very high mortality rate, EHD can have a significant effect upon the deer population in a given area, reducing numbers drastically. Hemorrhagic disease can be transmitted to other wild ruminants. The EHD virus can infect domestic animals but rarely causes disease. In all probability the virus does not infect humans. Presently there is no evidence that crosses into humans. (Portions extracted from Roselake Wildlife Disease Laboratory site at:
- Mod.TG)
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