EHD (Blue Tongue)Description
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 throughout the U.S. and Canada. The two diseases are antigenically different.
Distribution
Since 1890, deer die-offs from diseases which might have been EHD have occurred in various parts of North America. These early die-offs were variously diagnosed as blackleg, blacktongue, bluetongue, mycotic stomatitis or hemorrhagic septicemia or they were undetermined. The causative agents were never confirmed. A review of the case histories, signs and lesions, seasonal occurrence, and lack of a bacterial agent suggests that they might have been EHD.
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.
Additional die-offs attributed to EHD occurred in Michigan in white-tailed deer in 1974 and 2006. The 1974 die-off occurred in several counties and resulted in approximately 100 deer dying. The 2006 die-off occurred in the southwestern portion of the state in Allegan county and involved 50-75 animals.
Transmission and Development
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 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 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.
Pathology
The gross and histological lesions of EHD have been characterized by, as its name implies, extensive hemorrhage. The hemorrhages range from pinpoint 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.
Generalized edema and increased pericardial fluid are consistently found in EHD. These changes also reflect the widespread interference with normal blood circulation.
The virus can be recovered from a variety of tissues of animals which have succumbed to EHD. These include blood, liver, spleen, kidney, lung, heart and muscle.
Diagnosis
A combination of case history, characteristic signs and lesions, and the isolation of the virus is necessary for a diagnosis of EHD. Useful aids in obtaining a diagnosis are the epizootic nature of the disease, its seasonal occurrence, and its spectacular hemorrhagic lesions. Because of the similarity of its symptoms to other diseases, such as bluetongue and malignant catarrhal fever, the isolation and identification of the virus is essential.
Methods to be used for virus isolation are: (1) inoculation of cell cultures; (2) inoculation of susceptible sheep or deer combined with serologic monitoring; and (3) intravenous inoculation of embryonating chicken eggs.
Treatment and Control
There is no known effective treatment or control of EHD. Theoretically, an oral vaccine could be developed for administration through a supplementary winter feeding program, but this is presently impossible, impractical and unwarranted.
Significance
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.
Nationwide distribution of acute or chronic hemorrhagic disease in wild deer.
