Seroepidemiological investigations of onchocerciasis in a hyperendemic area of West Africa.

Immunological study of individuals (aged 4 to 70 years) living in an area of Mali hyperendemic for onchocerciasis revealed an 83% prevalence of skin microfilariae (mf). Microfilariae counts from skin snips were highly age-dependent. Screening for concomitant helminth infections showed a low prevalence of hookworms and Mansonella (Dipetalonema) perstans, but neither schistosomiasis nor bancroftian filariasis. Immunological results revealed strong correlation between radioallergosorbent test (RAST) and skin test (5 and 50 ng adult O. volvulus extract), between RAST and total IgE, and between IFAT and ELISA. A negative correlation exists between mf counts and skin tests and between mf counts and RAST; the lowest median values were obtained in the group with high mf counts. Skin sensitizing antibodies were detected in most locally born children aged 4-5 years. Intradermal tests showed a high rate of sensitization to O. volvulus antigen in mf-negative children, whereas ELISA and IFAT values were significantly lower in these children than in mf-positive children. Increasing concentrations of circulating IgE antibodies were found in children aged 4-11 years by RAST, and, in individuals aged 12-19 years (age group for which mf counts sharply increase), skin testing revealed a state of anergy. In long lasting infections (adults greater than 20 years) skin reactivity was comparable to that of young children or was depressed. ELISA and IFAT achieved similar results in each age group.