© 2000 by Oxford University Press
Longitudinal studies of skin microfilaria and antibody conversion rates in children living in an endemic focus of onchocerciasis in Nigeria
A Molecular and Biochemical Parasitology Programme, Department of Biochemistry, University of Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa B Department of Veterinary Microbiology and Parasitology, University of Ibadan, Ibadan, Nigeria C Department of Medicine and Molecular Microbiology, Washington University, St Louis, USA Z Corresponding author Tel: )11 716 3287 Fax: 011 339 8215
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 detect prepatent onchocerciasis in children. Between time point A and time point B (9 months apart) the seroconversion rate was 19.4 per cent, and between time points B and C (15 months apart), a further 31.4 per cent of seronegative children had seroconverted. However, only a third of the seroconverted children became microfilaria-positive. From the time-lag between antibody conversion and skin snip conversion, we estimated that a 9-12 month interval was required for the detection of O. volvulus microfilariae following infection with viable third stage larva (L3s). The predictive value of antibody detection as a measure of skin microfilarial conversion was >90 per cent when the optical density (OD) at 490 nm was >0.80. We therefore propose a strategy for monitoring changes in transmission of onchocerciasis following vector control or chemotherapy through an evaluation of annual antibody seroconversion rates in a standardized sentinel population of children.