Immunological Cross-Reactivity of Proteins Extracted from the Oomycete and the Fungus Compromises the Detection Specificity of Immunodiagnostic Assays for Pythiosis.

Pythiosis, a life-threatening disease caused by , has been increasingly diagnosed worldwide. A recently developed immunochromatographic test (ICT) enables the rapid diagnosis of pythiosis. During the 3-year clinical implementation of ICT in Thailand, we collected the laboratory reports of 38 animals with suspected pythiosis and detected ICT false-positive results in three horses and a dog with basidiobolomycosis. and cause infections with indistinguishable clinical and microscopic features. This study investigated cross-reactive antibodies by probing and crude extracts and cell-free synthesized I06 protein (encoded in genome, not other fungi) against a panel of pythiosis, basidiobolomycosis, rabbit anti-I06 peptide, and control sera by Western blot analyses. ICT false-positive results occurred from the cross-reactivity of anti- antibodies to the 15, 50, 60, and 120 kDa proteins of , not double infections caused by both pathogens. Notably, ICT could help to screen pythiosis, and the positive test requires confirmation by culture or molecular method. The detection specificity of ICT requires improvement. The crude extract containing multispecies antigens needs replacement with a refined -specific protein. We proposed that the 55 kDa I06 protein is an excellent candidate for developing a more specific serodiagnostic test for pythiosis.