Abstract:

Strongyloides stercoralis is a parasitic helminth prevalent in tropical and subtropical areas globally, infecting 100 to 370 million people. This parasite can cause lifelong infection in immunocompetent individuals, and potentially death in immunocompromised patients. There is an increasing number of immunosuppressed people worldwide due to more cancers, chronic illnesses, and transplantations. The low sensitivity of stool microscopic examination hinders the diagnosis of strongyloidiasis; thus, serology plays an important complementary role to improve the detection rate. However, some individuals with confirmed infections were reported to be negative for Strongyloides-specific IgG, signaling the need to develop better serodiagnostic methods. To address this challenge, we have developed three new assays. The first is a lateral flow dipstick test (SsRapid™), which used two recombinant proteins as the test line and detected specific IgG4 antibodies. The diagnostic sensitivity and specificity were found to be 91.3% (n=23) and 100% (n=82), respectively. Its high diagnostic performance and rapid format are attractive for low resource settings. The second assay is an IgE-ELISA using a novel recombinant protein (rA133), and it showed diagnostic sensitivity and specificity of 100% (n=43) and 99% (n=94), respectively. This assay may potentially replace the current IgG-ELISA due to its excellent diagnostic performance. The third assay is an antigen detection ELISA (Ag-ELISA) using a recombinant monoclonal scFv antibody against rNIE antigen, which captures circulating immune complexes in infected individuals. The Ag-ELISA showed 100% diagnostic sensitivity (n=37) and specificity (n=101). Notably, comparison of optical density (OD) values of two groups of positive individuals, i.e., those who were ‘parasitology/stool-PCR positive and serology positive’ and those ‘stool-PCR positive but IgG negative,’ showed the latter had significantly higher OD values (p<0.05). A higher level of circulating antigen, positive stool-PCR, but negative specific IgG indicated that the latter group probably had early Strongyloides infection. In conclusion, the three new assays developed are useful additions to the available methods to improve the laboratory diagnosis of strongyloidiasis markedly.

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