Studies on an outbreak of Wesselsbron virus in the Free State Province, South Africa.

In early March 1996, Wesselsbron (WSL) virus caused mortality among lambs on a farm near Bultfontein in the northern Free State Province, South Africa. Mosquito collections were therefore undertaken from 27 March to 1 April to collect floodwater Aedes mosquitoes for attempts at virus isolation. In all, 4,732 floodwater Aedes were tested; 5 WSL, 1 Middelburg (MID), and 5 unidentified viruses were isolated from 3,052 Aedes (Neomelaniconion) mcintoshi/luridus (minimum infection rate [MIR] for WSL = 1.63) and 5 WSL, 1 MID, and 3 unidentified viruses from 1,478 Aedes (Ochlerotatus) juppi/caballus (MIR for WSL = 3.38). One of the authors developed WSL fever on 3 April; WSL virus was isolated from his serum, and he developed a titer of 1:640 in the hemagglutination inhibition (HI) test and became IgM positive against WSL virus. Among a sample of 44 sheep bled on 4-5 September, 59% were antibody positive by the HI test against WSL and 48% against MID viruses. Mosquito collecting was restricted to 2 discrete, shallow, grassy depressions that were the main floodwater Aedes breeding sites on the farm so they will be investigated further as possible foci of transovarial transmission of WSL and MID viruses.