Planning the control of sleeping sickness

Abstract

Attendances at well established treatment centres give a reliable index of the incidence of sleeping sickness within the area served, and have been used for planning control experiments in an epidemic in the Lawra and Wa districts in the north-west corner of the Gold Coast and for studying the effects of the measures employed. Glossina palpalis and G. tachinoides are the main vectors of the T. gambiense form of sleeping sickness present and also of animal trypanosomiasis. G. morsitans occurs only in thinly populated regions and is chiefly concerned with cattle trypanosomiasis. Sleeping sickness, present along the Black Volta river prior to the beginning of the century, had by 1938 developed into a serious pandemic covering over 30,000 square miles of the Upper Volta territory. Depopulation followed in the most heavily infected localities and, by concentrating the population on the uplands between infected river valleys, was giving rise to serious secondary evils arising from water shortage and soil erosion. The extension of G. morsitans into depopulated areas was also taking place. A striking feature of the distribution of the disease in Lawra district in 1938 was the relatively small amount of infection along the Black Volta, where fly-belt is heavy and continuous, and the concentration of infection on the tributaries and especially their headwaters, where fly-belt is lighter and less continuous or even reduced to isolated groves. Inferences from this distribution led to planning a concentration of attack, by tsetse eradication, on the main epidemic area (the Volta tributaries) while the Volta fly-belt was to be left untouched. Eradication of G. palpalis and G. tachinoides was effected by selective clearing, a method involving the removal over each complete river system of only certain species of trees and shrubs which are essential constituents of the dry-season habitat of these tsetse. Between 1940 and 1945 1,100 square miles of Lawra district were freed from fly at a total cost of £4,500. A population of 90,000 is affected. The permanent tsetse communities disappeared on each stretch of river as soon as clearing was completed. The high degree of control obtained has been shown by continuous observation on three of the cleared rivers, where 0 to 9 flies have been caught per year in places where the pre-clearing catches were 2,000 to 7,500 flies per year. The catch at a control point on the uncleared Volta was 20,127 flies in 1947. An intrusion of G. morsitans was brought under complete control by game reduction backed by settlement. In the area of tsetse eradication a 97 per cent. reduction in the incidence of sleeping sickness took place between 1938 and 1947. In Wa district systems of protective clearing were applied over two blocks of country and effected reductions of 80 and 50 per cent. of pre-clearing incidence of trypanosomiasis, the reduction being proportionate to the number and length of clearings. In south-west Wa, where no control measures had been applied, an increase of over 100 per cent. in the number of cases was observed between 1940 and 1947. Tsetse eradication is considered essential for the complete control of epidemic sleeping sickness and has the added advantage of controlling animal trypanosomiasis. A modification of selective clearing leading to the eradication of the fly-belt vegetation has been applied throughout the Lawra area of reclamation and has given such a degree of stability that maintenance can be taken over by the local natives even at such low population densities as 20 per square mile. This is important because sleeping sickness does not appear as a serious problem below this density. Full advantage of the reclamation of the river valleys is being taken by the people, about 1,500 having settled voluntarily along the Kamba and over 4,000 acres of new farms having been broken since clearing was finished. Conclusions on the epidemiology of sleeping sickness show that a very exacting combination of factors, bringing the tsetse into close and continuous contact with man, is required for epidemics to arise. Without the full set of factors the disease is absent or present at low infection rates only. The factors are analysed and discussed. The discussion leads to the conclusion that the most rapid and complete control of sleeping sickness can be obtained by concentrating attack on the true epidemic centres which lie commonly on tributaries of the main Volta rivers. Eradication of the tsetse and fly-belt vegetation throughout these natural land units, followed by their fuller development, affords a permanent solution to both human and animal trypanosomiasis and can, at the same time, be of economic benefit to the community as a whole.