Tsetse and trypanosomiasis

Can other insects transmit trypanosomiasis?

Yes! The form of trypanosomiasis caused by T. vivax occurs in South America and Asia, where there are no tsetse flies. In these places, other biting insects such as stable flies (Stomoxys) and horse flies transmit the disease. In Africa, the camel disease 'surra' caused by Trypanosoma evansi is transmitted mechanically by horseflies. However, the general consensus is that the trypanosome species causing disease in cattle (T. vivax, T. congolense) and humans (T. brucei spp.) are transmitted by tsetse only. The most compelling evidence for this is that when tsetse were eliminated from large areas of Zimbabwe, Botswana and South Africa, trypanosomiasis ceased, even though potential mechanical vectors such as stable flies and horse flies were still abundant.

Horse fly (Philoliche elongata) (photograph: ICIPE)

Tsetse workers in some parts of Africa have reported the presence of trypanosomiasis in the apparent absence of tsetse. These observations have led some people to conclude that the disease is transmitted mechanically by biting flies, such as Stomoxys, in Africa. Tsetse can however survive at very low densities (<10 per square kilometre) and a single trap catches only ~1% of a local tsetse population per day. Thus even the most effective traps need to be operated continuously for many weeks to provide compelling evidence that there are no tsetse in the vicinity.

Past studies purporting to show the apparent presence of trypanosomiasis in the absence of tsetse were often undertaken with inferior designs of trap. If the density of tsetse were low, then it would be very likely that no tsetse would be caught by such traps. Not catching any tsetse does not mean that there is none!