Bluetongue (BT) is a non-contagious, infectious, arthropod transmitted viral disease of domestic and wild ruminants
that is caused by the bluetongue virus (BTV), the prototype member of the Orbivirus genus in the family Reoviridae.
Bluetongue was first described in South Africa, where it has probably been endemic in wild ruminants since antiquity.
Since its discovery BT has had a major impact on sheep breeders in the country and has therefore been a key focus of
research at the Onderstepoort Veterinary Research Institute in Pretoria, South Africa. Several key discoveries were made
at this Institute, including the demonstration that the aetiological agent of BT was a dsRNA virus that is transmitted by
Culicoides midges and that multiple BTV serotypes circulate in nature. It is currently recognized that BT is endemic
throughout most of South Africa and 22 of the 26 known serotypes have been detected in the region. Multiple
serotypes circulate each vector season with the occurrence of different serotypes depending largely on herd-immunity.
Indigenous sheep breeds, cattle and wild ruminants are frequently infected but rarely demonstrate clinical signs,
whereas improved European sheep breeds are most susceptible. The immunization of susceptible sheep remains the
most effective and practical control measure against BT. In order to protect sheep against multiple circulating
serotypes, three pentavalent attenuated vaccines have been developed. Despite the proven efficacy of these vaccines
in protecting sheep against the disease, several disadvantages are associated with their use in the field.