Visual impairment and incidence of blindness in Liberia and their relation to onchocerciasis.

The results of the epidemiological survey of 10,860 inhabitants of 121 Liberian communities from 1969-1971 were analysed on the prevalence, incidence and risk of blindness associated with onchocerciasis infection. The overall prevalence of economic blindness in the population sample was 3.24%, the total rate of measured visual damage in both eyes was 5.37% (impairment of vision etc.). Onchocerciasis was prevalent in a very high proportion of those having visual defects. Causes of blindness as by frequency were cataract in 45%, lesions of the anterior segment, other than onchocerciasis-related in 14%, phthisis bulbi (9%) and optic atrophy (6%). The importance of onchocerciasis as a cause of blindness and as a public health problem in Liberia is examined by the comparison of 5 categories of village populations, classified by their adjusted onchocerciasis prevalence rates. The risk of exposure to onchocerciasis increases gradually from nil to 100%, providing one infection-free control population and increasing degrees of prevalence in the next categories, dependent on duration of exposure and intensity of transmission. The attempt to estimate the risk of blindness attributable to onchocerciasis from this data reveals a 3-fold risk in people exposed to the infection in their habitat, whereas a 2,5-fold risk is found from the evaluation of findings in people classified by mf-prevalence in their skins only. Considering a proportion of at least 65% of the rural population to be living under onchocerciasis exposure, the attributable risk amounts to more than 50%, which suggests that this population suffers from 50% more blindness than would be expected without exposure to onchocerciasis sometime in life.