Investigations into the transmission of bluetongue in sheep during the season 1931/1932

Abstract

During the summer 1931/32 transmission experiments with bluetongue in sheep were carried out at Onderstepoort. Owing to lack of rain, the season was very unfavourable for our work. The result of a mosquito survey had pointed to Aedes species as being very suitable transmitters from an epizootological point of view, and some species of this genus were used for our experiments. Three strains of virus were used, the vaccine strain of the laboratory and 2 strains from natural cases, obtained towards the end of the season. Nearly 3,000 clean mosquitoes were fed on 18 infected sheep, the majority belonging to Aedes caballus and A. lineatopennis, and only relatively small numbers of 5 other species including A. hirsutus, A. vittatus and A. dentatus. In 22 experiments, 324 specimens were injected into sheep after periods ranging between ½ and 45 days, 127 A. caballus, 125 A. lineatopennis, the remainder belonging to A. vittatus, A. hirsutus, A. dentatus and A. punctothoracis. Infected mosquitoes were refer 346 times after 14-29 days on susceptible sheep at periods varying between 14 and 29 days, viz. , 281 A. caballus, 61 A. lineatopennis and 4 A. hirsutus and A. durbanensis. Three positive results were obtained. In the first case 5 mosquitoes were injected shortly after their infection, showing only that sufficient virus was taken up. In the second case 60 A. lineatopennis were injected after 17-19 days, and in the last experiment 15 specimens of the same species refed after 14-15 days. In the last case no immunity was acquired but the presence of virus could be ascertained by subinoculation. All the other experiments were either negative or the results doubtful. Aedes lineatopennis seems to be a transmitter of bluetongue, very adapted for this purpose from an epizootological point of view owing to its breeding habits and behaviour in the adult stage. It could not be ascertained if this species is an important or only a more or less accidental transmitter. Aedes caballus does not seem capable of transmitting the disease. The work carried out up to now does not present a solution of the problem of the natural transmission of bluetongue.