Antibodies to Strongyloides stercoralis larval surface antigens in chronic strongyloidiasis

Antibodies to intact rhabditiform and filariform larvae of Strongyloides stercoralis were measured by indirect immunofluorescence using quantitative fluorescence microscopy. Forty-four of 48 sera from infected patients (92 per cent) were positive for IgG antibodies specific for surface antigens of filariform larvae. None of 30 North American adult control sera were positive. All sera from patients with Schistosoma mansoni, Loa loa, or hookworm infections or with idiopathic hypereosinophilia were negative. Three of nine sera from patients with Bancroftian filariasis were weakly positive. Pre- and posttreatment sera from seven patients were tested, and there was a marked decrease in antibody titers in all posttreatment sera. Twelve sera from infected patients were also tested for the presence of IgA and IgM antibodies specific for surface antigens of filariform larvae; all sera were negative for specific IgA, and only one exhibited a weak positivity for specific IgM. Nine of the IgG-positive sera were tested for the presence of IgA and IgG antibodies directed against surface antigens of S. stercoralis rhabditiform larvae. No parasite-specific IgA was detected in any of the sera, but most sera contained small amounts of parasite-specific IgG. We conclude that the majority of patients with chronic uncomplicated strongyloidiasis develop specific IgG antibodies directed against the surface of filariform larvae, the invasive stage of the parasite. such antibodies may be involved in host defenses that prevent fatal hyperinfection in immunocompetent individuals. In addition, our results confirm that the indirect immunofluorescence employing filariform larvae as antigen is a sensitive and specific test that may be a useful adjunct to stool examination in the diagnosis of strongyloidiasis.