Entomological aspects of the epidemiology of sleeping sickness
Bursell, E.
Central African Journal of Medicine 19(9): 201-204
1973
ISSN/ISBN: 0008-9176 PMID: 4762458 Document Number: 63519
The reasons for the lack of importance of sleeping sickness in Rhodesia in spite of the presence of the virulent Trypanosoma rhodesiense are discussed. Although the unsuitability for agriculture of the areas with the greatest populations of Glossina may play a part and the African population may have some kind of resistance to the disease, it is concluded that one of the most important factors is the lack of attractiveness of man for G. morsitans Westw. and G. pallidipes Aust., the two species found in Rhodesia. Tsetse flies are selective feeders, and the distribution of feeds between the large mammals available is generally related not to their relative abundance but their powers of resistance to attack. Suids provide many feeds whereas baboons and excitable, long-tailed species such as the impala do not. Man, like the baboon, would constitute a great threat to a feeding fly. Only very hungry flies feed on man.The possibility that man was repellent was investigated, and it was found that the presence of a man depressed the catch on a cow or a warthog [60, 1259, etc.]. Man seemed to be least repellent to males of G. morsitans, most repellent to females of G. pallidipes, with males of G. pallidipes and females of G. morsitans intermediate. However, man was followed by a swarm containing all elements of the population. It appears that all components are subject to long-range attraction but only males of G. morsitans overcome the short-range repulsion that characterises man and includes both visual and olfactory elements. This repulsion explains why man can walk about in areas heavily infested with Glossina and rarely get bitten but is badly bitten when his characteristic shape and smell are masked as when he is in a Landrover. Here, when the fly has settled on the skin by chance, the powerful contact stimulus of high surface temperature becomes over-riding.