A quantitative approach to the epidemiology of onchocerciasis in West Africa

Abstract

The variables relating to the epidemiology of onchocerciasis were extracted from the literature and, where possible, statistically valid quantitative data were gathered. The variables affecting the transmission of the disease were identified as parameters: • relating to man as the source of infection; • relating to Simulium damnosum as the vector; • relating to Onchocerca volvulus. By using these parameters the number of infective larvae to which a population is exposed in unit time was calculated for the 3 major bioclimatic zones of West Africa. In the forest zone a person may receive 0·72 infective larvae per day (266 per year); in the guinea savanna zone 20·2 infective larvae per day (7422 per year); in the sudan savanna zone 58 infective larvae per day (14,184 infective larvae during the 8 months period of transmission in this instance). Thus the time taken for an individual to acquire 400 infective larvae, giving a probability of infection of 0·96, would be 1 year 3 months in the forest zone, 20 days in the guinea savanna zone and 7 days in the sudan savanna zone. The fate of the infective larvae is discussed and their relationship to the epidemiological and clinical patterns of the disease in the three major bioclimatic zones is postulated. A hypothesis which might explain the pathogenesis of the ophthalmic lesions is given. The differences between the epidemiological patterns of onchocerciasis in the 3 major bioclimatic zones of West Africa may thus be explained on the basis of different numbers of infective larvae challenging individuals in the populations at risk in unit time. Preliminary calculations made from data from Central America show that the theoretical number of infective larvae challenging individuals in the population is higher than in the sudan savanna zone of West Africa.