The Control of Trypanosomiasis by entomological Means

Extract

The investigations were carried out in the Inland Savanna Forest Zone of the Northern Territories of the Gold Coast, the climate of which is characterised by extreme variations in rainfall and relative humidity between the wet and dry seasons.

Glossina palpalis and G. tachinoides are widely distributed along the waterways throughout the zone. They come into intimate contact with the human population and are, in consequence, the most important vectors of human and animal trypanosomiasis.

G. morsitans submorsitans occupies unpopulated and thinly populated regions and so, coming more rarely into contact with man and cattle, is a less important vector of trypanosomiasis.

A pandemic of sleeping sickness, with 30,000 square miles of heavily infected country, lies across the upper reaches of the Volta rivers and involves parts of the north of the Gold Coast. In the areas studied high rates of infection (4–7 per cent.) were found associated with serious population declines which have given rise to secondary evils, the invasion of G. morsitans into depopulated parts and the concentration of populations on the watersheds between infected river valleys, with resultant over-farming, erosion and lowering of the standard of living.

The results of two methods of controlling G. palpalis and G. tachinoides were studied : (1) “ Protective Clearings ”, aimed at breaking the contact between the tsetse and man or cattle at certain points only ; (2) “ Eradicative Clearing ”, aimed at the complete removal of tsetse throughout an area.

The effects of protective clearings on the fly were as follows : Clearings less than 300 yards in length are considered useless, and may cause an increase in fly-incidence by forming artificial feeding grounds. Clearings of 440 yards can effect up to 70 per cent, reduction in fly-incidence measured over a long period, but for several months of the year show no reduction or may even show an increased incidence. Clearings up to 880 yards long can bring about 60–90 per cent, fly reduction, but at two periods during the year, at the end of the dry season and in the middle of the rains, their effectiveness is considerably lowered by the invasion of hungry flies into the clearings. Clearings over a mile in length exclude all but a few vagrant flies, but even five miles of clearing are traversed by occasional flies.

A study of the effects of various applications of protective clearings on the incidence of sleeping sickness led to the following conclusions. To have any value protective clearings must be made in as many places as possible over a wide area. They effect no general reduction in the disease if applied at a few places only. Small clearings, averaging 400 yards in length, if made consistently over a large area, can effect up to 40 per cent, reduction in sleeping sickness in 4–5 years, after which the disease tends to become stabilised at the lower level. Long clearings, averaging 1,000 yards, almost double the rate and amount of reduction, effecting 70 per cent, reduction in three years, but the tendency for the disease eventually to become stabilised remains. Clearings over a mile in length at each village caused a reduction of 85 per cent, in one area.

Protective clearings alone are considered unlikely to be able to effect complete control in areas of serious endemic sleeping sickness. They do not entirely exclude the fly, and they do not touch all the points where the transmission of infection takes place. Further disadvantages arise from the fact that, to attain maximum fly reduction, all vegetation, including tall trees, should be cut. This is expensive and difficult and may lead to erosion and the silting up of water-holes, thus driving the people back to uncleared parts of the river. Finally, animal trypanosomiasis can never be controlled by localised clearings, therefore improvement in animal husbandry, and consequently in agricultural practice and nutrition cannot be fully realised.

Eradication of G. palpalis and G. tachinoides is based on the consideration of the tsetse community of a complete river system as a natural biological unit. During the prevalence of the adverse climatic conditions of the dry season, the breeding and survival of tsetses are possible only in certain restricted habitats from which the flies spread extensively along the water courses during the rains. The dry-season habitats are confined to definite plant associations that are characterised by a limited number of species of trees and shrubs. The removal of only these species throughout a river system renders the habitats untenable during 4–6 months of the dry season and results in the disappearance of the whole fly community.

By the method of selective clearing, 1,050 square miles of country in the northwest corner of the Gold Coast were freed from tsetse between December, 1940 and March, 1945. On the main river of this region, the Kamba, which was cleared by 1942, routine catching after clearing showed 1–8 flies per year in places where the pre-clearing averages had been 2,700–7,500 flies per year. After the clearing of some neighbouring fly-belts, these occasional captures ceased. Wet-season migration from the uncleared Volta, however, brings small numbers of flies 5–9 miles up the lower reaches of the cleared river.

The incidence of sleeping sickness for the Kamba area fell by 92 per cent, between 1938, the year before the start of control measures, and 1944.

Along the Kamba, 160 square miles of land which had become depopulated through trypanosomiasis is being developed voluntarily by the natives, by farming, grazing, etc. Approximately 1,000 people have settled in parts of this area since clearing was finished.

In 1939 G. morsitans invaded the depopulated area, and increased greatly in numbers in the following years. The spread was checked, and finally a high degree of control was established by the disturbance of big game and its reduction in numbers. Settlement of the thinly populated country is needed to consolidate this control.

Selective clearing can be followed by the eradication of essential dry-season habitat vegetation which will enable maintenance to be put on a long-term rotation of about ten years. Permanent reclamation is a possibility that is now being studied.

The relative merits of medical and entomological methods of controlling sleeping sickness are discussed. Mass treatment can have great success, but fails to control under difficult conditions, and has not yet been found able to eliminate the disease. The addition of adequate tsetse control increases the effect of mass treatment. Eradication of the fly is capable of eliminating the disease and does not require the addition of mass treatment to bring this about.

The removal of tsetse throughout the river system, as in selective clearing, has the following additional advantages : It makes possible a sound agricultural development based on the possession of good quality livestock; this reflects also on nutrition. It enables the population to live with impunity in the vicinity of permanent water and, by spreading out at a density optimum for their type of agriculture, to avoid concentrations at local clearings or on dry, hilly country away from rivers; thus the possibilities of over-farming and erosion are avoided and the watersheds and headwaters can be reserved for afforestation. By these means, instead of ground being abandoned to the tsetse, the fly is replaced by a healthy agricultural population.