[Onchocerciasis control program in West Africa: socioeconomic development and risk of recrudescence of transmission. 2. Experimental study of the transmission of Onchocerca volvulus strains from Southwestern Sierra Leone by Simulium yahense and Simulium squamosum].

As part of the Onchocerciasis Control Program in West Africa an experimental study was conducted to ascertain the degree of transmission likely to occur when the more pathogenic strains of Onchocerca volvulus from southwest Sierra Leone are carried by infected persons to the mining zones of northern Sierra Leone. Intense migration to the mining zones will likely result in increased contact between carriers of the strain more associated with river blindness in southwest Sierra Leone and the vectors Simulium yahense and Simulium squamosum which account for most transmission in the mining areas. Cutaneous biopsies were taken of two men from the southwest and two from the northern mining zone. One subject from each zone had a heavy and one a light infection. Wild females of S. yahense and S. squamosum were gorged on the four carriers after which the insects were captured and studied. Analysis of the data collected during dissection permitted a reliable interpretation of the variability of the parasitism of the female as a function of vector species duration of parasite development origin of the onchocerciasis patient in southwest or northern Sierra Leone and whether the skin microfilarial load was considered heavy with more than 100 microfilaria per biopsy or light with fewer than 50. The results suggest that transmission may be significant only when heavily infected onchocerciasis carriers come in contact with S. yahense. The probability of transmission by S. yahense was greatly reduced after contact with lightly infected Onchocerciasis patients. S. squamosum had a limited capacity to transmit O. volvulus strains from southwest Sierra Leone regardless of the microfilarial load of the patients. A high risk of intensive transmission would thus result only from contact between S. yahense and Onchocerciasis patients with high skin microfilarial loads. The probability that this will occur is greatly reduced because of the decrease in skin microfilarial loads in communities undergoing mass treatment with ivermectin as have those of southwest Sierra Leone since 1989.