Epizootic Hemorrhagic Disease: Near but not yet here in Ontario
Epizootic Hemorrhagic Disease (EHD) is a viral disease of white-tailed deer, transmitted to them by biting midges. It is a common and important disease of white-tailed deer in the southeastern United States. Each summer, the midge species that acts as a carrier for the disease expands its range northward until it is stopped by the frosts of autumn. How far north the midge reaches will be determined by many factors, including the severity of the previous winter, the temperature and humidity in summer, etc.
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 it reaches these northern states, there is always concern among Ontario hunters, wildlife managers and disease specialists whether or not the potential exists for it to occur in Ontario deer.
The disease has been sporadically reported in Western Canada, but at the present time, the disease is restricted by the lack of competent carriers here in Ontario. The species of Culicoides midge that occurs in Ontario (Culicoides variipennis) is not capable of transmitting the virus. Thus, in order for the disease to occur here, it would require incursion of both midge and virus. This is not out of the realm of possibility, but it does give another reason to be grateful for the presence of the Great Lakes.
It is important, however, to remain vigilant for the possible occurrence of EHD in Canada. The clinical signs are highly variable in nature and severity. Here in Ontario, where the disease has likely never occurred, we might expect to see mostly severe, acute illness and death. The initial signs include fever, depression and respiratory distress. Tissues of the head and neck may be swollen. Deer are often found dead alongside of bodies of water, to which they are driven by thirst. Animals that survive this phase of the disease may develop inappetence, lameness and reduced activity. The lameness is due to damage to the hooves, in which the growing portion is affected. This may cause sloughing of the hoof or the development of horizontal bands, like tree rings, where the growth of the hoof has been retarded.
The onset of autumn frosts makes it unlikely that we will see this disease in Ontario this year, but with increasing evidence of global warming, it is a disease that may eventually reach Ontario.