Hemorrhagic Diseases of White-tailed Deer
Overview
Epizootic hemorrhagic disease virus (EHDV), bluetongue virus (BTV) and adenovirus are possible virological causes of a hemorrhagic disease in White-tailed deer (WTD). EHDV and BTV are two closely related, but genetically distinct, Orbivirus that can cause a severe disease in ungulates. In North America, WTD is the most commonly affected host species. Whereas many serotypes of EHDV and BTV can be distinguished worldwide, most cases of hemorrhagic disease observed in WTD can be attributed to EHDV serotype 1 and 2. However, the current prevalence of EHDV or BTV serotypes in WTD populations is not clear. In addition, recently these viruses has been isolated and/or detected by molecular techniques from diseased WTD without clear definition of their serotype.
Adenovirus hemorrhagic disease (AHD) was identified in California in 1993 and caused high mortality in mule deer. Pulmonary edema and intestinal luminal hemorrhage were seen in mule deer with systemic infection. Experimental infection of WTD with this adenovirus showed similar lesions to some of the lesions described in WTD naturally infected with BTV and EHDV. Since gross lesions of systemic AHD and the orbivirus hemorrhagic disease (BTV and EHDV) are similar, all three should be considered in the differential diagnosis.
EHDV and BTV are thought to be transmitted by species of Culicoides midges and this vector dependency has been considered to limit the geographical distribution of the disease. For the disease to be transmitted, the ambient temperature must be appropriated to allow for vector activity and for the virus to replicate within the vector. The disease characteristically occurs in late summer and early fall, and this seasonality is related to the occurrence of midge vectors.
Clinical signs of BTV in deer are similar to those observed in sheep; however, the presence of BTV in deer can often be confused with that of EHDV. Recently, eight serotypes of BTV previously exotic to US and a new serotype of the related orbivirus, EHDV, have been identified in the United States. The presence of these new serotypes raises the concern that the epidemiology of these viruses in North America may be changing and could result in more extensive disease in U.S. livestock and wildlife.
Hemorrhagic disease is by far the most significant endemic disease of WTD. The disease occurs annually, but distribution and severity of occurrence are highly variable. Morbidity and mortality rates during outbreaks usually are less than 25% but can be 50% or more. Disease symptoms can range from mild to severe debilitation and death, and there is evidence that pathogenicity and disease prevalence differ among geographic areas.
Bluetongue (BTV) has historically been a disease of sheep, although infection can occur in many other ruminant species. In WTD, the described lesions stem primarily from the viral replication in and destruction of the vascular endothelial cells. This leads to thrombosis, edema, coronitis and hemorrhage (serosal and mucosal surfaces). Animals are frequently chronically ill for several weeks. In some cases, the virus may persist in infected hemopoietic cells for many weeks.
Epizootic hemorrhagic disease (EHDV) of deer primarily causes severe disease in WTD but also infects domestic livestock as well as native and exotic wildlife species.
As with BTV, clinical signs of EHDV are highly variable. This variation may be associated with viral virulence and host resistance. Severe cases of the EHDV or BTV are presented as a classical hemorrhagic disease with some animals showing edema in the head, neck, tongue, conjunctiva and lung.