Epizootic hemorrhagic disease (EHD) is one of the most important diseases of deer in North America. The viruses that cause EHD are widespread in whitetail deer and cause serious epidemics in wild populations.
Some of the EHD virus strains causing disease in deer can also affect cattle. While EHD is rarely fatal in cattle and usually associated with epidemics in deer, the confusing array of lesions in cattle can be mistaken for foot-and-mouth disease (FMD) — a potentially serious mistake, especially if inadvertently made during large runs of cattle in the fall of the year. Even the temporary suspension of commerce in the cattle business because of mistaken identity could wreak havoc. The cattle industry in Canada is all too familiar with how quickly U.S. partners can slam the door on north-south beef trade, and how stubborn the resumption of trade can be.
The significance of EHD sits on two fronts: First is the close relationship the EHD virus maintains with bluetongue virus in the large genus of insect-transmitted viruses known as Orbiviruses. Two consecutive outbreaks of EHD Type 2 and bluetongue Type 11 occurred in the Okanagan Valley in 1987 and 1988, the last time the diseases were identified since a major incursion in 1975. The EHD virus is also closely related — some consider it identical — to the Ibaraki virus that causes extensive disease outbreaks in cattle across Japan, Korea and Taiwan.
In North America EHD has been identified in white-tailed deer, mule deer, and pronghorn antelope. Sheep can be experimentally infected, but rarely show clinical signs, and goats do not seem to be susceptible to infection. Exposure to the EHD virus and midges capable of carrying the virus appears extensive. EHD outbreaks in deer are regularly reported from Michigan, Virginia, New Jersey, Pennsylvania and New York. Outbreaks have also occurred in Kansas, Montana and California.
Three syndromes may be seen in deer:
- Peracute disease and rapid death with swelling of the head and neck. Deer are often found dead alongside of bodies of water, to which they are driven by thirst.
- Acute form with extensive hemorrhage in many tissues and blood-tinged salivation and nasal discharge from ulcerations.
- Chronic form with sloughing of hooves and extreme lameness. Deer may be seen crawling on their knees and chest. Some animals recover. Hoof deformities may linger through the year.
Outbreaks of EHD in deer typically occur in late summer and early fall. The onset of freezing weather usually stops the appearance of new cases.
Biting midges primarily responsible for transmitting EHD belong to the genus Culicoides. Some species of gnats and mosquitoes can also transmit EHD virus. The biological range of midges capable of transmitting EHD virus is creeping northward. Factors like the severity of winters, ambient temperatures during vector season and summer humidity will govern how far north midge vectors penetrate into Canada.
The second reason EHD is a significant issue for the cattle industry is the confusing array of lesions EHD causes when clinical signs emerge in cattle. Fever, anorexia (off feed) and difficulty swallowing are seen initially. Hemorrhages, erosions and ulceration develop in the mouth, on the lips and around the coronary band (horn-hoof junction). Animals frequently drool and appear stiff and lame. Abortions and stillbirths have been reported. Some affected cattle die. These are all symptoms that can be easily confused with foot-and-mouth disease.
Confirmed cases of epizootic hemorrhagic disease in cattle through the summer and fall of 2013 have occurred in Iowa, Wisconsin, Nebraska and North Dakota. This summer has seen significant EHD activity in the northern reaches of its known range. The disease has been confirmed in white-tailed deer from Wyoming, South Dakota, Nebraska, Iowa, Wisconsin, Indiana, Illinois, Michigan, Ohio, Pennsylvania and New Jersey. As the disease reaches northern states, there is always concern among hunters, wildlife managers and disease specialists whether the potential exists for outbreaks in deer north of the 49th, and subsequent infections in cattle.
Through September and October of 2013, the Nebraska Department of Agriculture investigated multiple cases of cattle with severe oral, nasal and ocular lesions. Because the lesions look very similar to vesicular stomatitis and foot-and-mouth disease, foreign animal disease investigations were initiated with submissions to the Foreign Animal Disease Diagnostic Laboratory at Plum Island, N.Y. The investigation revealed 10 confirmed cases of EHD.
Preferred tissues for laboratory diagnostics in deer include spleen, liver, lung, lymph nodes, and unclotted whole blood in EDTA or heparin. Both fresh and fixed (formalin) tissues should be collected if possible. Samples for virus isolation should be transported under refrigeration. For suspect cases in cattle, blood should be collected into anticoagulant (calcium citrate, EDTA, or heparin) and sent chilled for virus isolation or RT-PCR. Paired serum samples (during clinical signs and through recovery stages) should also be collected if possible. To rule out FMD and vesicular stomatitis, fluid from intact vesicles as well as aggressive swabs or biopsy samples near lesions are also suggested.
While EHD is a disease of great concern for deer in North America, the association of disease in cattle with EHD outbreaks in deer is troubling. The role of insect vectors, especially biting midges, gnats, and mosquitoes and the emergence of significant changes in their distribution induced by climate change always needs to be a factor in our thinking. Both veterinarians and producers can make mistakes should signs suddenly appear in cattle.
Knowing a disease imposter might be among us is critically important so appropriate diagnostic steps can be taken. It’s also important that veterinarians, producers and wildlife officials remain aware of what’s happening in wildlife. Regulatory and political intemperance with far-ranging consequences could be the result of inadvertent oversight in the field.
— Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]). This article originally appeared in the November 2013 issue of Cattlemen (page 22).