A recent survey for onchocerciasis conducted among the Yanomami Indians living on the middle reaches of the Mucajaí and Catrimâni Rivers (Territory of Roraima) has failed to show any alteration in the local pattern of the disease. Infact, after a decade from the first surveys in these areas located at the periphery of the Yanomami focus, the prevalence and the intensity of infection have not experienced a significant change. Considering only the residents in the villages inside the surveyed areas, the prevalence varied from zero in the neighbourhood of the Catrimâni mission to 3.1 per cent near the Mucajai mission. Had there been a competent vector of Onchocerca volvulus in the region higher rates might be expected due to the Yanomami custom of periodic visits among the groups of the tribe. In some of these visits, numbers of highly infected Indians from the central and mountainous part of the Yanomami territory - where more than 90 per cent of the adults have onchocerciasis - come to the villages in the lowland area, attracted by the facilities offered by the mission posts. As the visitors stay in the host villages for several days or weeks, the residents could become exposed to the disease. Simulium oyapockense s.l., a man-biting species of black fly, widely distributed in Northern Brazil, is the only possible vector of O. volvulus in the investigated areas, on the grounds of its local abundance. However, it must be a poor vector (as has already been demonstrated for Mansonella ozzardi), since the prevalence rates in the lower part of the Yanomami territory have not changed for the past ten years, in spite of the constant presence of infected Indians visiting the villages. In order to explain the much higher prevalence in the central part of the Indian territory, we have to admit the presence there of another vector, extremely efficient, whose habits would be linked to the montainous region of the border between Brazil and Venezuela.
onchocerciasis; Yanomami Group; Simulium oyapockense sp. 1; Territory of Roraima