Onchocerciasis in Sudan: the distribution of the disease and its vectors.

The distribution of onchocerciasis and its vectors in Sudan has been reviewed with special emphasis on the hyperendemic foci where there is serious visual handicap caused by the disease. These blinding foci lie primarily in the south-west of the country, along the rivers flowing north and east from the borders with Central African Republic and Zaire, though at least one such focus is known from the eastern part of the country close to the Ethiopian border. In the blinding foci, often small villages localized to short stretches of the rivers, ocular onchocerciasis is as severe as that found in any other African foci. Only the S. damnosum s.l. species complex has been implicated in disease transmission and only the two dangerous, morphologically similar, savanna cytospecies, S. damnosum s.s. and S. sirbanum, have been identified from breeding sites close to known onchocerciasis foci. Near to the Uganda and Zaire borders it is very likely that other, less anthropophilic, cytospecies occur. Different Onchocerca-Simulium complexes (different strains of parasite with different pathogenicities transmitted by different vector species) may be responsible for the different severities of disease found in the 3 main areas of onchocerciasis in North, East and South-West Sudan. The localized, focal distribution of the communities seriously blinded by onchocerciasis, suggests that a strategy of tackling the disease on a focus basis may prove optimal. A control scheme, planned to treat all the vector breeding sites with insecticide, as in West Africa, would receive extensive invasion from the Zaire/Congo River Basin and the headwaters of the White and Blue Niles in neighbouring countries. While some foci are situated beside major river rapids, with vector breeding only controllable by regular insecticide treatments, several foci have been identified as lying close to removable man-made objects which provide excellent breeding sites at certain water levels e.g. causeways. The destruction of such breeding sites should be considered since localized vector control may produce substantial reductions in onchocerciasis transmission.