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.
Present address: Michigan State University, Department of Microbiology and Public Health, East Lansing, Michigan 48824.