Epizootic hemorrhagic disease virus, often abbreviated to EHDV, is a species of the genus Orbivirus, a member of the family Reoviridae. It is the causative agent of epizootic hemorrhagic disease, an acute, infectious, and often fatal disease of wild ruminants. In North America, the most severely affected ruminant is the white-tailed deer (Odocoileus virginianus), although it may also infect mule deer, black-tailed deer, elk, bighorn sheep, and pronghorn antelope. It is often mistakenly referred to as “bluetongue virus” (BTV), another Orbivirus that like EHDV causes the host to develop a characteristic blue tongue due to systemic hemorrhaging and lack of oxygen in the blood. Despite showing clinical similarities, these two viruses are genetically distinct.
Worldwide, eight serotypes of EHDV have been identified. Historically, only serotypes EHDV-1 and EHDV-2 have been found in North America, but recent research has discovered at least one more in the Midwest and Southern United States. EHDV can only be spread by an insect vector. In North America, the common vector is the biting midge (Culicoides variipennis). The first identified outbreak of EHDV in the United States in 1955 caused several hundred white-tailed deer to die in New Jersey and Michigan. Cases of EHDV-like die-offs have been reported prior to 1955 (as far back as 1890), but EHDV was not identified in these cases, as its existence was not yet known.
Viral classification
Epizootic hemorrhagic disease virus belongs to the family Reoviridae, a family of double-stranded RNA viruses that includes familiar genera Rotavirus (the most common cause of viral gastroenteritis in children), Coltivirus (causative agent of Colorado tick fever), and Orbivirus. Besides EHDV and BTV, other orbiviruses include equine encephalosis virus and African horse sickness virus.
Genome and structure
Epizootic hemorrhagic disease viruss genome is about 18-31 kDa in length and consists of 10 segments, where each segment encodes a single protein. These proteins could be one of the seven structural proteins (VP1-VP7) or one of the four non-structural proteins (NS1, NS2, NS3a, or NS3b). The nonstructural proteins are encoded by gene segments 6, 8, and 10, and have been found to be highly conserved. The segmented genome of EHDV facilitates reassortment within species, and may be why EHDV breakouts occur every year.
The virion outer layer is composed primarily of VP2 and VP5 trimers that are also involved in the virus's ability to attach to and penetrate a host cell. The outer core layer is formed by VP7, which provides a surface for VP2 and VP5 attachment, while the inner subcore layer is formed by VP3. The VP3 inner subcore layer self-assembles to control the size and organization of the capsid structure. The VP3 layer surrounds VP1, VP4, and VP6, in addition to the 10 linear double-stranded RNA segments.
Eight serotypes of EHDV are proposed. Serotypes EHDV-1 (New Jersey strain) and EHDV-2 (Alberta strain) are the dominant forms of EHDV in the U.S., except for in 2012, when an outbreak of EHDV-6 accounted for 63% of the collected isolates from moribund or dead white-tailed deer in the environment. EHDV-6 is thought to be a hybrid form where each of the collected type-6 viruses were all reassortants containing VP2 and VP5 derived from an exotic EHDV-6, while the remaining structural and nonstructural proteins were obtained from the indigenous EHDV-2. EHDV-6 was originally described as originating from Australia, but it has been recognized as an emerging pathogen among cattle in several countries, and is now being considered as an endemic strain in certain parts of the U.S.