Where, when and why do tsetse contact humans? Answers from studies in a national park of Zimbabwe

Abstract

Background: Sleeping sickness, also called human African trypanosomiasis, is transmitted by the tsetse, a blood-sucking fly confined to sub-Saharan Africa. The form of the disease in West and Central Africa is carried mainly by species of tsetse that inhabit riverine woodland and feed avidly on humans. In contrast, the vectors for the East and Southern African form of the disease are usually savannah species that feed mostly on wild and domestic animals and bite humans infrequently, mainly because the odours produced by humans can be repellent. Hence, it takes a long time to catch many savannah tsetse from people, which in turn means that studies of the nature of contact between savannah tsetse and humans, and the ways of minimizing it, have been largely neglected.

Methodology/principal findings: The savannah tsetse, Glossina morsitans morsitans and G. pallidipes, were caught from men in the Mana Pools National park of Zimbabwe. Mostly the catch consisted of young G. m. morsitans, with little food reserve. Catches were increased by 4-8 times if the men were walking, not stationary, and increased about ten times more if they rode on a truck at 10 km/h. Catches were unaffected if the men used deodorant or were baited with artificial ox odour, but declined by about 95% if the men were with an ox. Surprisingly, men pursuing their normal daily activities were bitten about as much when in or near buildings as when in woodland. Catches from oxen and a standard ox-like trap were poor indices of the number and physiological state of tsetse attacking men.

Conclusion/significance: The search for new strategies to minimize the contact between humans and savannah tsetse should focus on that occurring in buildings and vehicles. There is a need to design a man-like trap to help to provide an index of sleeping sickness risk.

Figure 1. Wing fray classes of male G. m. morsitans caught from men and ox.

Percent distribution of wing fray classes of male G. m. morsitans caught from men (N = 211) and ox (N = 100).

Figure 2. Ovarian categories of female G. m. morsitans from (A) men, (B) ox and (C) traps.

Percent distribution of ovarian categories of female G. m. morsitans from (A) men, (B) ox and (C) traps. Sample sizes of 257, 94 and 283 for men, ox and traps, respectively.

Figure 3. Uterine contents of female G. m. morsitans from (A) men, (B) ox and (C) traps.

Percent distribution of uterine contents of female G. m. morsitans in ovarian categories>0, from (A) men, (B) ox and (C) traps. Sample sizes of 189, 78 and 270 for men, ox and traps, respectively.

Figure 4. Uterine contents of female G. pallidipes from (A) men, (B) ox and (C) traps.

Percent distribution of uterine contents of female G. pallidipes in ovarian categories>0, from (A) men, (B) ox and (C) traps. Sample sizes of 16, 115 and 1498 for men, ox and traps, respectively.