ONCHOCERCA OCHENGI - EPIDEMIOLOGIC EVIDENCE OF CROSS-PROTECTION AGAINST ONCHOCERCA-VOLVULUS IN MAN

In North Cameroon, the vector of Onchocerca volvulus (causative agent of human onchocerciasis) also transmits 2 filariae of animals: O. oche ngi from cattle and O. ramachandrini from wart hogs. In order to asses s the qualitative and quantitative roles of these 'animal filariae' in the epidemiology of O. volvulus, the transmission of the 3 parasites was measured in 2 villages and related to the endemicity of human onch ocerciasis. In Galim, a cattle-farming Guinea savanna village where wi ld animals are rare, the overwhelming majority of all filarial infecti ons found in the Simulium damnosum s.l. vectors throughout the year we re O. ochengi (89%). The remaining infections were mainly O. volvulus (10.5%), and a few O. ramachandrini (0.5%). In Karna, a crop-farming S udan savanna village where cattle are rare, but wild animals common, f lies were also more frequently infected with animal filariae than with the human parasite. In the dry season, when nomadic cattle are presen t, 54% of all infections were O. ochengi, 36% O. volvulus and 10% O. r amachindrini. In the rainy season, when the cattle move away, flies we re mainly infected with O. ramachandrini (52% of all infections) and s econdly with O. volvulus (48%). In Karna, the relationship between the Annual Transmission Potential (ATP) of O. volvulus and its prevalence in the human population conformed to other onchocerciasis foci, in th at a moderate ATP led to hyperendemic onchocerciasis. In Galim, howeve r, a 7-fold higher O. volvulus-ATP (caused by a very high biting rate of the flies) contrasted with a strikingly low endemicity of onchocerc iasis. Since, at the same time, in Galim the transmission of O. ocheng i (measured on man) was very high (15 000 L3/fly collector/year), we h ypothesize that the reduced endemicity of onchocerciasis in Galim is d ue to 'natural heterologous vaccination' by the large annual number of O. ochengi-L3, inoculated into man by anthropo-boophilic S. damnosum s.l. The importance of micro-epidemiology for the understanding of the interlinkage of human and animal onchocerciasis is discussed.