Epizootic haemorrhagic disease of deer

Epizootic haemorrhagic disease of deer

N J MACLACHLAN AND B I OSBURN

Introduction

Epizootic haemorrhagic disease (EHD) of deer is a Culicoides-transmitted haemorrhagic viral disease of wild ruminants, particularly white-tailed deer (Odocoileus virginianus).19, 36 The causative agent, EHD virus (EHDV), is an orbivirus that is very closely related to bluetongue virus (BTV) and both EHDV and BTV cause a similar disease in susceptible ruminants.11, 15, 19, 21, 28, 29, 31, 33, 36 Epizootic haemorrhagic disease was first described in North America in 1955 and substantial epidemics have been described among populations of white-tailed deer and, to a lesser extent, other wild ruminant species including pronghorn antelope (Antilocapra americana) and bighorn sheep (Ovis canadensis).6, 7, 11, 19 Infection with EHDV has also been documented in other regions of the world.3, 9, 14, 17, 18, 26, 36 Although infection of cattle and sheep is often common in endemic areas, EHDV is generally not regarded as a pathogen of domestic ruminants.1, 4, 8, 16, 17, 35 A notable exception is Ibaraki virus, a virus closely related to EHDV that caused an extensive outbreak of disease in cattle in Japan in 1959, and continues to cause disease amongst cattle in the Far East and perhaps elsewhere (see Ibaraki disease in cattle). Similarly, although Koch’s postulates are yet to be fulfilled, EHDV has recently been implicated as the cause of a poorly defined syndrome of oral ulceration and coronitis in cattle in both the USA and South Africa.5, 18, 20

Aetiology

Epizootic haemorrhagic disease virus is a member of the family Reoviridae, genus Orbivirus. 31 It is very closely related to BTV, the prototype virus of the genus Orbivirus, and to African horse sickness virus.24 Viruses within the EHDV serogroup all share group-specific serologic cross-reactivity, and some nine distinct serotypes are distinguished on the basis of neutralization tests with type-specific antisera.31 The particles of EHDV are non-enveloped and include a genome of ten double-stranded RNA segments. The virion consists of two protein shells, with an inner ‘core’ that consists of viral proteins (VP) VP1, VP3, VP4, VP6 and VP7 and an outer shell of VP2 and VP5. Four nonstructural (NS) proteins (NS1, NS2, NS3/3A) also are produced in EHDV-infected cells. The coding assignments and functions of each protein are identical to those of BTV, which are described in detail in Bluetongue.

Ibaraki virus is serologically related to the prototype strain of EHDV serotype 2 that was originally isolated in Alberta in 1964, although Ibaraki virus is also currently classified as EHDV serotype 7 (see Ibaraki disease in cattle). The serotype-specific L2 gene of Ibaraki virus shares only partial homology (72 per cent) with isolates of EHDV serotype 2 from the USA,27 whereas the sequence of genes encoding core proteins (VP3, VP7) of Ibaraki virus are more conserved and very similar to those of Australian strains of EHDV.23 The similarity of the core protein genes of Australian strains of EHDV and Ibaraki virus suggests that they are included in an Asian ‘topotype’ of EHDV.

Epidemiology

Epizootic haemorrhagic disease virus infection has been described in North America, Africa and Asia. Its distribution is probably similar to that of BTV and includes tropical and temperate regions throughout the world between latitudes of approximately 40 °N and 35 °S.2, 3, 9, 12, 14, 17, 19, 26, 29, 36 The infection in ruminants is not contagious; the virus is transmitted between susceptible ruminants by haematophagous midges of the genus Culicoides that serve as biological vectors of the virus.19 The specific vector species differ throughout the world; Culicoides imicola is a vector of EHDV in South Africa, Culicoides brevitarsis in Australia, and Culicoides variipennis in the USA.36 Serological evidence of EHDV infection has been reported in many ruminant species, both wild and domestic, including sheep, cattle, various species of Asian and North American deer, elk (Cervus canadensis), moose (Alces alces), pronghorn antelope, bighorn sheep, black (Diceros bicornis) and white (Ceratotherium simum) African rhinoceros, American black bears (Ursus americanus), and Florida panthers (Felis concolor) among others.2, 12, 13, 14, 16, 19, 25, 28, 29, 36, 37 In temperate regions infection is most common in the late summer and autumn when vector populations peak, whereas infection occurs year-round in tropical regions of the world.

Female Culicoides midges become persistently infected with EHDV and can transmit the virus to susceptible ruminants after an external extrinsic period of approximately 10 to 14 days. As in BTV infection, viraemia can be prolonged beyond 50 days in EHDV-infected ruminants because of a novel association of the virus with ruminant erythrocytes.30, 32 Truly persistent infection of ruminants does not occur with either BTV or EHDV, although cattle probably serve as temporary reservoir hosts of both viruses.1, 4, 8, 17, 35 The cycle of infection is completed when uninfected female Culicoides midges feed on viraemic ruminants.

Pathogenesis

The pathogenesis of EHDV infection of ruminants is probably very similar or identical to that caused by BTV, and has been most comprehensively described in white-tailed deer.15, 19, 21, 22, 30, 32, 34 After infection has occurred by the feeding of infected Culicoides, the...

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