Onchocerca antigens in protection, diagnosis and pathology

Abstract

Characterization of the immune response to Onchocerca volvulus is important for the diagnosis, control and understanding of the disease it causes. The antibody response to surface, secreted and somatic antigens of the worm has therefore been examined at an individual immunoglobulin (Ig) class level, by using a panel of different human sera. Onchocerca-specific antigens tend to be of low molecular mass and preferentially recognized by IgG4 and IgE. There is considerable cross-reaction between O. volvulus and O. gibsoni, so that the latter may be an alternative source of material for use in diagnosis. A surface-enriched fraction of low molecular mass appears to be a most promising diagnostic tool. Amongst somatic antigens, two were uniquely recognized by IgG3 antibodies in sera from sowda patients, thereby providing a molecular correlate for a recognized pathological condition. Improved diagnosis is needed for detecting infection in both humans and the vector. Our target for detection in humans is a continuously released, nonimmunogenic product, which is ideally stage and parasite specific. The excretions of adult worms do contain components not recognized by antibodies in infected serum, but we cannot rule out that these are of host, rather than parasite origin. Excretions of Litomosoides carinii contain both host and parasite molecules and, in addition, stage-specific and sex-specific components. Unfortunately, however, the rate of production of excretions varies during the life of L. carinii. This finding may be relevant to the detection of Onchocerca excretions if they are produced at a similarly uneven rate. Finally, for detecting infective larvae in the vectors, we are currently screening a genomic library of O. volvulus for an appropriate probe. To date, one DNA sequence has been cloned that shows promising specificity.