Human subcutaneous pythiosis.

Two cases of subcutaneous infection caused by the primitive aquatic hyphal organism Pythium are described. Pythium is an important pathogen of horses in the U.S.A. and Australia. Cases of human subcutaneous pythiosis have been cited in the literature, but clinical and histopathological features have not been described previously. Both cases occurred in young immunocompetent males in the periorbital region and showed rapid growth, clinically mimicking a tumor and requiring operative biopsy. In both cases there was a history of exposure to either swampy water or horses. The tissue reaction was distinctive, closely resembling that seen in equine pythiosis, comprising well-defined granular eosinophilic islands bordered by macrophages, multinucleate giant cells, fibrosis and numerous eosinophils. Hyphae were well demonstrated with the Grocott stain but only poorly with the PAS method. Identity of the organisms was confirmed with an immunoperoxidase technique employing a polyclonal antiserum to Pythium. Both patients responded well to amphotericin B.