INTRODUCTION

The genus Orbivirus, within the family Reoviridae, has a worldwide distribution and consists of14 groups or species of virus, plus a number of unclassified viruses. Each of the virus groups includes a variable number of serotypes and antigenic complexes, and reassortment is reported to be able to occur between at least some members of a group but not between members of different groups(¹).

Orbivirus virions have an indistinct outer capsid and a genome composed of 10 segments of dsRNA. The intact virions are about 80 nm in diameter and the core particles about 60 nm. Virus infectivity is stable at pH 8—9, but there is a marked loss of infectivity outside the pH range of 6.5—10.2. In sterile blood samples or when held in serum, viruses may remain infectious for decades at less than 15° C. They are rapidly inactivated on heating to 60° C but are resistant to treatment with solvents and detergents.

Depending on the virus, the vertebrate hosts of orbivi- ruses include ruminants (domestic and wild species), equids (domestic and wild species), rodents, bats, marsupi­als, sloths, birds and primates, including humans⁽¹⁾.

Orbiviruses replicate in, and are principally transmitted between, their vertebrate hosts by blood-sucking arthro­pods, which are biological vectors. Depending on the virus the vectors include mosquitoes, phlebotomines (sandflies), Culicoides biting midges and hard and soft ticks. Infection of vertebrate hosts i n uteri may also occur with some viruses⁽¹⁾. Orbivirus infection apparently has no ill effects on the arthropod vector, but in the vertebrate hosts the results of infection can range from subclinical to fatal depending on the virus and the host. Several orbiviruses cause significant diseases in domestic and/or wild live­stock, and probably the most important of these are African horse sickness (AHS), bluetongue (BT) and epiz­ootic haemorrhagic disease (EHD). BT has been attracting much international attention recently because of its massive and severe incursions into and through Europe.