SAEVA Proceedings 2016 | Page 97

  Figure 1. A map of the zones within the African horse sickness (AHS) Controlled Area of South Africa. AHS is transmitted by small (1-3mm in size) biting midges (Culicoides spp.), which are most active at dawn and dusk. In endemic areas the most significant vectors are Culicoides imicola and C. bolitinos. The midges become infected by taking blood meals from viraemic animals. The incubation period in the midge (time between biting an infected horse and being able to transmit to another horse) is around 8 to 10 days at around 25°C, after which the virus localizes in the salivary glands of the midge and can then be transmitted to another horse. The incubation period can be prolonged in lower temperatur es. As the ambient temperature drops, the midge lifespan is extended, but this drop in temperature also will decrease infection rate as well as viral multiplication. At temperatures below 15°C AHSV does not replicate, however, it can resume if temperatures rise again within the life span of the midge. The exact time limit on this process is still unknown. Wind-borne spread of infected midges may play a role in local spread of AHS. Long-distance spread of AHS is usually the result of inadvertent movement of Equidae infected with AHSV. The incubation period in horses following natural infection with AHSV is between 5 and 9 days. In susceptible horses mortality is 70 to 95% and the prognosis is extremely poor. In mules the mortality is approximately 50% and in European and Asian donkeys it is 5-10%. Mortality is very low in African donkeys and zebra. 15-­‐18  February  2016      East  London  Convention  Centre,  East  London,  South  Africa     96