Detection of circulating parasite antigen and specific antibody in Toxocara canis infections.

Serological surveys, measuring humoral antibody responses, have indicated significant levels of human infection with the zoonotic nematode Toxocara canis, and raised concern about the resultant risk of ocular and neurological damage. Such measurements do not distinguish with certainty current infection from past exposure. Thus, we have developed a test for circulating Toxocara antigen released by parasites in the host. This monoclonal antibody-based two-site 'sandwich' assay discriminates between T. canis and the related feline ascarid T. cati, and has been used, in tandem with the standard ELISA, to examine experimental and human infections. In experimental animals, antigen is transiently detectable, disappearing when immunocomplexed with host antibody. Antigen was also found in sera from UK patients diagnosed with visceral or ocular toxocariasis, and in four asymptomatic Papua New Guinean children. In the latter population, individuals positive for parasite antigens were not necessarily positive for antibody, implying that some infected cases may be negative in the current diagnostic ELISA. The antibody test was also adapted to measure host antibody directed to single monoclonal antibody-defined epitopes, revealing evidence of differential temporal regulation of distinct antibody specificities.