Mechanical transmission of Trypanosoma congolense in cattle by the African tabanid Atylotus agrestis.

The trypanosomes pathogenic to livestock in Africa (Trypanosoma congolense, Trypanosoma vivax, and Trypanosoma brucei) are mainly cyclically transmitted by tsetse (Glossina). However, T. vivax, can also be mechanically transmitted by haematophagous insects. Laboratory studies have demonstrated the mechanical transmission of T. congolense, but confirmation of this under natural conditions was necessary. An experiment was therefore carried out in Lahirasso, Burkina Faso, in a corral completely covered by mosquito net, to avoid exposure to tsetse. Eight receiver heifers, free of trypanosome infection, were kept together with two donor heifers, experimentally infected with local stocks of T. congolense. On average, 291 Atylotus agrestis, freshly captured in Nzi traps, were introduced into the mosquito net daily for a period of 20 days to initiate mechanical transmission among cattle. Daily microscopical observation of their blood indicated that two of the eight receiver heifers became infected with T. congolense from days 42 and 53. Mechanical transmission of T. congolense by A. agrestis was demonstrated unequivocally with a 25% incidence over a 20-day period of exposure under a mean challenge of 29 insects/animal/day. These results, in addition to previous reports, demonstrate the ability of A. agrestis to transmit T. vivax and T. congolense to cattle in Africa by mechanical means. Efforts to eliminate cattle trypanosomosis should therefore consider the eventual persistence of disease as a result of mechanical transmission of trypanosomes by tabanids. Index descriptor and abbreviations: Trypanosoma congolense (Trypanosomatidae) is a pathogenic trypanosome found in wild and domestic herbivores, principally in cattle (Bos taurus, Bos indicus, and cross-breds), in Africa. It is cyclically transmitted by tsetse (Glossina, Diptera); however, mechanical transmission by biting insects may also occur. The present study demonstrates unequivocally the mechanical transmission of T. congolense to cattle by one of the most common African tabanids, A. agrestis. The main conclusion is that tabanids are able to transmit T. congolense; however, the incidence of transmission was lower than in studies carried out under the same conditions with T. vivax. Better models of mechanical transmission are required to understand why, on the one hand, epidemiological studies support the mechanical transmission of T. vivax but not T. congolense, and, on the other hand, experimental studies confirm that both species can be mechanically transmitted. Our studies suggest that the epidemiology of trypanosomosis in cattle involves tabanids, and hence, the eradication of tsetse-flies in Africa will not necessarily lead to the eradication of trypanosomosis in domestic livestock. ADT, apparent density of insects per trap per day (mean number of insects caught in one type of trap per 24h of trapping); D, day; NS, not statistically significant