Equine pythiosis is a chronic granulomatous, infectious disease caused by the oomycete Pythium insidiosum. It affects horses living in tropical and subtropical regions worldwide. P. insidiosum also infects humans and other animals (such as cattle and sheep).
Equine pythiosis occurs in three distinct forms in horses: cutaneous, gastrointestinal, and rhinofacial. The cutaneous form of the disease is generally associated with the onset of large, ulcerative, proliferative, pyogranulomatous lesions affecting the outer layers of the skin. It is most commonly found along the horse's legs, belly, and face.
Where found P. insidiosum is an opportunistic pathogen that lives in warm stagnant water and usually most abundant during the rainy season. Most cases of equine pythiosis have been associated exposure to swamps or marshes, where horses either habitually graze in water, or are flood-bound, standing in water for long periods.
A definite diagnosis of equine pythiosis is obtained through cultivation and identification of P. insidiosum from the affected horse. This is accomplished through several different tests, including immunohistochemistry, serological testing by ELISA and polymerase chain reaction (PCR).