Ocular onchocerciasis and intensity of infection in the community. I. West African savanna.

A method is introduced for the analysis of community patterns of ocular onchocerciasis in relation to the intensity of infection as measured by the Community Microfilarial Load (CMFL). Specific features of this method are the clear definition of ocular lesions and their separation into early and advanced stages, and the estimation of the prevalence of onchocercal blindness after exclusion of other causes of blindness. The method is applied to the ophthalmological and parasitological data from 33 villages from the West African savanna in order to obtain a reference pattern for subsequent analyses of ocular onchocerciasis patterns from other bioclimatic zones. In the savanna, there exists a clear linear relationship between most indices of ocular onchocerciasis and the CMFL. Mean ocular microfilarial loads, prevalences of the advanced lesions of the anterior and posterior segment of the eye and prevalences of different classifications of blindness show a high degree of correlation with the CMFL, as does also early sclerosing keratitis. The correlation is poor for the other early ocular lesions. All relationships are similar for the two sexes with the exception of posterior segment lesions which remain more common in males after correction for intensity of infection. The CMFL is superior to the prevalence of microfilariae in the skin as an index of endemicity. It allows a good prediction of the severity of onchocercal ocular disease in savanna communities using parasitological information only.