HD generally occurs in late summer and early fall, probably relating to the seasonal patterns of vector activity, as illustrated in this graph of HD cases in the southeastern U.S. Early summer transmission has been reported.
Frequency and severity of disease vary. The frequency of HD reports decreases with latitude while the probablity that these reports will involve mortality increases. In the southern United States, although reports of disease are more frequent, mortality is lower with numerous animals surviving infection. In some cases, e.g. in Texas, high exposure rates with little or no disease are observed. Factors associated with variation in both frequency of reported disease and clinical outcome are not understood but may include acquired and innate herd immunity, virulence factors associated with the virus, and vector species composition and activity patterns.
Different patterns of EHDV and BTV transmission occur throughout the United States, ranging from cyclic to sporadic. Antibody prevalence to these viruses is highest in the southern part of the United States where it can approach 100% (top illustration). Antibody prevalence is age dependent and this should be considered in surveillance strategies (lower illustration).
Limited work in the southeast suggests that specific EHDV and BTV serotypes change over time and their distribution varies. Serotype diversity also varies with area as seen in this serologic survey from Georgia. In the accompanying figure, note that in northeastern Georgia only antibodies to EHDV 2 were found while in southeastern Georgia two serotypes of EHDV and three serotypes of BTV were found.
The high prevalence of antibodies in wildlife species indicates that infection does not always result in disease. This model has been proposed to explain why infection does not always result in disease in white-tailed deer.
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