Pythiosis, a life-threatening disease caused by Pythium insidiosum, 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. P. insidiosum and Basidiobolus ranarum cause
infections with indistinguishable clinical and microscopic features. This study investigated cross-reactive
antibodies by probing P. insidiosum and B. ranarum
crude extracts and cell-free synthesized I06
protein (encoded in P. insidiosum 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-B. ranarum
antibodies to the 15, 50, 60, and 120 kDa
proteins of P. insidiosum, 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 P. insidiosum-specific
protein. We proposed that the 55 kDa I06
protein is an excellent candidate for developing a more specific serodiagnostic test for
pythiosis.