The mode of transmission of EHD in nature is via a Culicoides biting fly or midge. 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 which 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.
Experimentally, the disease can be transmitted to susceptible deer by the inoculation of virus-laden material from infected deer by subcutaneous, intramuscular, intravenous or oral routes.
Clinical Signs
Clinical signs of EHD and bluetongue are very 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 fever (affected animals frequent bodies of water to lie in to reduce their body temperature) and finally become unconscious. Hemorrhage and lack of oxygen in the blood results in a blue appearance of the oral mucosa, hence the name 'bluetongue'. Eight to 36 hours following the onset of observable signs, deer pass into a shock-like state, become prostrate and die.