The parasite. Trypanosoma gambiense, T. rhodesiense and T. brucei are morphologically identical trypanosomes which differ, however, in their behaviour in the vertebrate host. T. gambiense is transmitted by flies of the Glossina palpalis group from man to man in whom it produces a chronic disease. No important wild animal reservoir has so far been demonstrated. T. rhodesiense is transmitted by flies of the G. morsitans group. The strain is maintained in wild animals; man is an accidental host. The trypanosome is far more virulent in man and in animals than T. gambiense. T. brucei is transmitted also by flies of the G. morsitans group. The strain is maintained in wild animals; man cannot be infected, it seems, by this trypanosome.
The distribution of the three trypanosomes is related to temperature, vector characteristics, infectivity in the vertebrate host, and the behaviour of the hosts. Most experiments show that the three trypanosomes retain their specific character when cross-inoculated into foreign hosts or tsetse flies. The conversion of one type of trypanosome into the other remains a possibility, however.
The three strains could have evolved from a single “stock” strain. The behaviour of the different strains can be explained by studying their ecology in the field. T. gambiense is believed to remain confined to the forest belts because of its inability to produce a potent infection in animals and because of the behaviour of its vector, G. palpalis. T. rhodesiense, at present scattered within the savannah fly belts, might well invade the forest belts because of its ability to infect animals and also to develop in G. palpalis. This constitutes a serious danger. It is indeed possible that in certain areas, the relatively mild T. gambiense could be replaced by T. rhodesiense. G. palpalis has been found to carry T. rhodesiense in Uganda. It seems that the trypanosome was, in this case, brought into the palpalis habitat by first having been established in dense thicket vegetation by G. pallidipes.
The staple food pattern of tsetse flies is regarded as the most important factor regulating the epidemiology of the trypanosomiases. Examples are cited to explain sudden Rhodesian trypanosomiasis flareups, one factor being the modification in the composition of the wild animal population, the other the modification of the vegetation. In the relationship between the T. brucei group trypanosomes and G. morsitans, the role of certain animals could be of great importance. Bushbuck (Tragelaphus scriptus) and common duiker (Sylvicapra grimmia) could be responsible for bringing polymorphic trypanosomes from the savannah to the forest belts when the main host of G. morsitans, the warthog (Phacochoerus aethiopicus), disappears from its feeding grounds.
The fly. Because of their medical and veterinary importance tsetses are a serious hindrance to the economic and social development of the continent. The environmental conditions suitable for fly are governed by three basic factors—climate, food, and vegetation. The various relationships of these factors can be represented schematically as a biological triangle.
Tsetse feeding habits, which play a dominant part in the spread and maintenance of trypanosomiasis, can be explained by the interplay of various factors. The shift of feeding habits to secondary hosts may explain sudden outbursts of Rhodesian trypanosomiasis. Host relationship may result in bringing trypanosomes from one environment into another.
The habitat of the fly is constituted of a combination of vegetation types and the animals therein. The distribution of species is explained by the tolerance of the larval and pupal stages towards certain physical conditions and the ability of the adult fly to feed on the animals found in the habitat. The frequency with which different plants are rested upon by G. morsitans is given for mixed woodland in the Bugesera region of the Rwands. Sex ratios and hunger stages of hand-caught G. morsitans vary according to the types of vegetation communities, as observed in three different regions in Rwanda-Burundi. Vegetation composition and densities are expressed in simple descriptive formulas which may help characterise fly habitats. There is a need for quantitative indication of each factor in a formula defining the fly habitat.
Open grassland and abandoned fields are in danger of reverting to woodland and thornbush, which, in turn, may become potent fly belts.
Fly control. Of the many theoretically possible ways to control tsetse flies, only a few are of practical value in the African environment. Three methods offer the best chances of success: the modification of fly habitats by various degrees of bush clearing, the use of insecticides, and game control. No universal fly control method can be applied, even to the same species of tsetse. Each region will call for a method adapted to the local conditions, biological and social. In many instances, the association of two or more control methods will be preferable from the point of view of both effectiveness and cost.
Great advantages may be derived from subdividing a large fly belt into smaller areas partitioned off by barrier clearings. In the islands thus created, various fly control measures can be applied, each island being an independent unit. Treated islands can be settled while reclamation work goes on in the remaining ones. Other advantages of this sectional clearing are discussed and a hypothetical example of the method is described.
Fly control should be planned for the protection of man and his cattle either in areas already settled or in areas where more agricultural land is needed. Soil specialists, agronomists, hydrologists and population experts should be consulted before any action is taken.
Land use should form the basis of any antitsetse plans. These plans should not only consider the problem from the point of view of soil and agriculture, pastures and cattle, but should also include serious consideration of whether and how the local game population might be kept and used to advantage. The areas where favorable conditions permit agriculture and mixed farming should be separated from the much wider areas where the soils are marginal, the water scarce, and the economic outlook poor. Fly control should be envisaged only for the former areas and isolation with proper game management achieved in the latter. A pilot survey should permit an accurate appraisal of both cattle breeding and game management in marginal lands.
Fly surveys and research remain an essential part of tsetse control. Aerial photographs and correlation maps are basic tools for the planning of anti-tsetse work. A new type of fly round is proposed which would fill several important gaps in our present understanding of the epidemiology of Rhodesian trypanosomiasis.
In order that research ability, equipment, and funds for control programs be not wasted and made available to all African countries, there is need for the establishment of an inter-African central body sponsored by the specialized international developmental agencies, to act on an interterritorial basis and be responsible for the control of trypanosomiasis in both cattle and man.
Department of Tropical Medicine and Public Health, Tulane University of Louisiana, New Orleans.
George Williams Hooper Foundation, University of California Medical Center, San Francisco.