A South African overview of the virus, vectors, surveillance and unique features of bluetongue.
The origin of bluetongue (BT) is probably African and the disease was first recognised in South Africa in Merino sheep in the late 18th Century. Diagnostic and research findings for a number of years have been summarised to obtain data relevant to the distribution of BT and its serotypes in the country. The role of ruminant game and cattle as maintenance hosts for BT virus (BTV) is mentioned although cattle appear to have largely replaced antelope in this role. Only about 30% of over 1,000 game animals tested for export were found to be BT-antibody positive. An outbreak of a bluetongue-like disease in cattle is mentioned as are the BT and epizootic haemorrhagic disease of deer (EHD) isolates in the outbreak. A summary by serotype and province of sheep isolates is given and it is pointed out that the sheep population in a province does not reflect the number of isolates made and the province with the largest sheep population has almost the smallest number of BTV isolates and vice-versa. South Africa currently has 21 of the 24 BTV serotypes with 17, 20 and 21 being exotic to the country. The recent retrospective typing of serotype 17 in South Africa is being investigated, as type 17 crosses strongly with type 20, which is absent and also with type 4 which is present. 1, 3, 4 and 2 were the most common serotypes while 18, 19, 22 and 23 were not found among the isolates. Mention is made of BTV isolates made from Culicoides bolitinos catches during two devastating outbreaks of African horse sickness in an unvaccinated population. A six-year Culicoides monitoring project is mentioned and the many BTV isolates made of a variety of serotypes. BTV is endemic in Africa and in South Africa unvaccinated indigenous breeds appear to have achieved a balance with the virus. Indeed, it is possible to find virus, antibody and lesions in asymptomatic animals in different situations. Bluetongue creates a significant trade barrier but the virus remains interesting among a number of other uniquely African viruses.
Gerdes
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Agricultural Research Council, Onderstepoort Veterinary Institute, Onderstepoort, South Africa.