Hemorrhagic Disease
Hemorrhagic Disease is the most important infectious disease of white-tailed deer, and outbreaks occur almost every year in the Southeast. It is caused by either of two closely related viruses, epizootic hemorrhagic disease (EHD) virus or bluetongue virus. Because disease features produced by these viruses are indistinguishable, a general term, hemorrhagic disease, often is used when the specific virus responsible is unknown. Because EHD and bluetongue viruses are transmitted by biting flies, hemorrhagic disease is seasonal and occurs in late summer and early fall.
 |
Nationwide
distribution of acute or chronic hemorrhagic disease in wild deer. |
The Viruses
There are 2 subtypes of EHD virus and 5 subtypes of bluetongue virus in North America. It is suspected that there is variation in ability among these subtypes or even with strains of each subtype to cause illness in deer. Furthermore, it is unlikely that recovery from infection with one virus subtype will protect the deer from infection with another. EHD and bluetongue viruses are not new to North America, as infections in deer were first reported in 1955 and 1968, respectively. Even earlier, deer die-offs resembling hemorrhagic disease were described as "blacktongue" by several naturalists in their writings.
Hemorrhagic disease viruses cannot survive outside the host animal or biting fly vector. When deer die, EHD or bluetongue virus will deteriorate rapidly in the carcass, and virus isolations are seldom obtained beyond 24 hours after death. Therefore, a live sick deer or a freshly dead deer is needed to make a virus isolation and determine which virus is present.
The Vectors
 |
| Adult female biting midge |
Neither EHD nor bluetongue virus is spread by direct contact. Both are transmitted by tiny biting flies in the genus Culicoides. The best documented vector in North America is Culicoides variipennis, although other species of Culicoides probably transmit the viruses. These flies are commonly known as biting midges but also are called local names such as sand gnats, sand flies, no-see-ums, and punkies. Hemorrhagic disease characteristically occurs from mid-August through October, and this seasonality is related to the abundance of biting midges. The onset of freezing temperatures, which stops the midges, brings a sudden end to the outbreaks. How the viruses persist through the winter when midges are not active is not clear. Possibly, viruses could overwinter in a few surviving midges but it also is known that some ruminants can carry virus for several weeks.
The Victims
Hemorrhagic disesase viruses are infectious to a wide range of wild ruminants. In addition to white-tailed deer, hemorrhagic disease has occurred in mule deer, black-tailed deer, bighorn sheep, and pronghorn antelope. Infections in these wild ruminants have ranged from inapparent to episodes of high mortality. In the Southeast, silent infections in white-tailed deer are common and are evidenced only by antibodies to the viruses in serum of normal, healthy deer. EHD and bluetongue viruses also infect domestic ruminants. Cattle and goats generally show no clinical illness or only mild disease when infected. In contrast, domestic sheep may develop severe illness when infected with bluetongue virus but apparently do not get sick when infected by EHD virus.
Information and photos reprinted with permission from the Southeastern Cooperative Wildlife Disease Study (SCWDS) group.