Longitudinal studies of skin microfilaria and antibody conversion rates inchildren living in an endemic focus of onchocerciasis in Nigeria

We monitored skin microfilarial conversion and antibody (serum) conversion rates to Onchocerca volvulus recombinant antigens (OC3.6 and OC9,3) over a 2-year period, in a cohort of 208 children aged 9-11 years and living in a mesoendemic focus of savannah-type onchocerciasis in Nigeria in order to de tect prepatent onchocerciasis in children. Between time point A and time po int B (9 months apart) the seroconversion rate was 19.4 per cent, and betwe en time points B and C (15 months apart), a further 31.4 per cent of serone gative children had seroconverted. However, only a third of the seroconvert ed children became microfilaria-positive. From the time-lag between antibod y conversion and skin snip conversion, we estimated that a 9-12 month inter val was required for the detection of O. volvulus microfilariae following i nfection with viable third stage larva (L3s), The predictive value of antib ody detection as a measure of skin microfilarial conversion was >90 per cen t when the optical density (OD) at 490 nm was >0.80. We therefore propose a strategy for monitoring changes in transmission of onchocerciasis followin g vector control or chemotherapy through an evaluation of annual antibody s eroconversion rates in a standardized sentinel population of children.