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Natural vectors of onchocerciasis (S. yahense) were collected. Ninety-eight were intrathoracically inoculated with 30 freshly obtained skin-dwelling microfilariae. Of the 82 flies dissected, 56 were infected and 28 harbored infective larvae. A total of 193 infective O. volvulus larvae were obtained. The mean infective worm burden was 6.9.

A survey for the prevalence of onchocerciasis was conducted on selected divisions of the Firestone Rubber Plantation, Harbel , Liberia, in November 1980. Divisions 19, 22, 23, and 36 are located in the north-central portion of the plantation, and were selected to complement transmission studies which were performed there in 1979 and 1980. Biopsy of the calf, iliac crest, and shoulder was done on a total of 583 individuals. No lateral difference in microfilarial density was observed. A prevalence of 80.8% was found. Prevalence appeared to be equal between the sexes, and to plateau above 35 years of age. Of those presenting positive skin biopsies, 20.2% had palpable onchocercomata . The geometric mean wet weight microfilarial density/mg of skin for the population sampled was 19. Onchocerciasis is considered hyperendemic in the Harbel area. Further investigations are warranted to define its dimensions in relation to clinical sequelae on the plantation.

A quantitative description of the transmission dynamics of onchocerciasis on the Firestone Rubber Plantation, Harbel, Liberia is presented. The putative vector, Simulium yahense, comprised 98.5% of the flies examined. Diurnal vector-host contact was unimodal, expressing a distinct peak between 0800 and 1100 hours, particularly during the wet season. Perennial breeding occurs in the Firestone biotope, and is manifested by a mean annual daily landing rate of 75 flies per person per day. S. yahense is considered an efficient vector by nature of its anthropophily, moderate infective worm burden (4.3 infective larvae per infective fly) and its spatial and temporal ubiquitousness. Onchocerciasis transmission peaked during the dry season, when 73.5% of the annual transmission occurred. The mean annual transmission potential for the study area was estimated to be 1,425 infective larvae per person. The significance of onchocerciasis as a public health problem on the Plantation is discussed, and the potential for limiting transmission is considered.