The geographical and climatic distribution of trypanosome infection rates in G. morsitans group of tsetse-flies (Glossinawied., diptera)

Estimates of trypanosome infection rates in Glossina morsitans and G. pallidipes in the Zambezi Valley are compared with similar data obtained elsewhere in the G. morsitans group of tsetse flies. It is demonstrated that a positive correlation exists between infection rates and distance from the median (7°S) of the Glossina belt in Africa. Infection rates in G. pallidipes in Zululand do not fit this pattern. Experimental evidence exists to show that temperature exerts a marked control over the ability of a tsetse fly to acquire an infection. It is demonstrated that the positive correlation with latitude is associated with increasing mean annual temperature. At higher altitudes or, as in Zululand, a situation approaching the temperate zone, low infections occur as a response to cooler climates. The regression of proboscis-type infections on mean annual temperature is much more marked than that of gut and/or gut and proboscis-type infections. It is suggested that the mean temperature at which the wild tsetse exists, possibly associated with the range of temperature experienced between seasons, may account for the general level of the infection rate, but that within the same neighbourhood variations may be produced as a result of differences in local fauna providing the trypanosome reservoirs.