A Comparative study of the Epidemiology of Endemic Rhodesian Sleeping Sickness in different parts of Africa

Abstract

In a history of Rhodesian sleeping sickness ORMEROD [this Bulletin, 1962, v. 59, 339] has suggested that the disease arose from a single focus in the Zambezi area and spread northwards in epidemics which left residual foci behind them. The present work was done to study the epidemiological conditions in 4 areas: -The Zambezi river system, where the disease was first recognized but no epidemic occurred; Ngamiland, where cases have recently become more numerous; and 2 areas in Tanganyika which had a big epidemic in 1928-32. In the Gokwe District of Southern Rhodesia (Zambezi area) 5 cases have occurred in the last 20 years, and they have been relatively chronic and sporadic. In Ngamiland, the disease is increasing and 248 cases were diagnosed in 1957-60. The disease, however, is relatively chronic, so that some cases might almost be considered as "healthy carriers", and there is little tendency for it to become epidemic. In the Kahama and Tabora districts of Tanganyika there was an epidemic in 1928, and in 1939; in 1957-59 84 cases occurred. In this area the cases occur at particular foci and the disease is acute. The occupations specially exposed to risk are honey-gathering, cultivation in the bush, and fishing, all of which take men into contact with fly. In the Kasulu district of Tanganyika, there was an epidemic in 1930-32 and another in 1957-60 with 244 cases. The disease was acute and widespread and it affected mainly travellers and those collecting firewood or honey in the bush. Several kinds of game can maintain infection with Trypanosoma rhodesiense, but owing to their habits and habitats most of them do not come into close contact with men and so they are unimportant. The animal which is most likely to act as a reservoir of infection is the bush-buck; it remains in a limited area and returns to its habitat even when disturbed; it often lives near men; it tolerates T. rhodesiense for long periods and on one occasion T. rhodesiense has been isolated from a wild bush-buck [HEISCH et al., ibid., 1959, v. 56, 698]. T. rhodesiense infection of men is most likely to occur when there is sustained triple contact of man, fly and bush-buck (or other game). Glossina pallidipes is the most effective vector because it stays mostly in one small area; G. morsitans is less effective because it ranges more widely, but if infection builds up to a certain level, then G. morsitans readily spreads an epidemic. If this triple contact can be effectively broken, then epidemics can be controlled and prevented.