Pythiosis is a severe, invasive fungal infection of the skin in which a fungus (usually found on aquatic plants) invades and infects a pre-existing wound. This causes the wound to develop into an angry, rapidly growing red mass that often oozes a blood-tinged fluid.

Pythiosis is found worldwide in horses but is most common in swampy, tropical climates where the parasite lives in stagnant pools of water. There is some speculation that this infection is on the rise, and is spreading into cooler and drier regions. It mostly occurs summer and fall months.

Areas affected include the lower abdomen, lower limbs and chest, but infections can occur anywhere on the body. This infection can look like other deep fungal infections, non-healing wounds, or skin cancer. Left untreated, these lesions become large and ugly masses of angry, red tissue. Horses tend to bite or kick at the sores.

Diagnosis requires biopsy for confirmation. In areas where the condition is common, diagnosis is often presumed by appearance. A blood antibody test can be used to confirm the diagnosis.

Treatment involves surgical removal or debridement, along with the use of systemic antifungal medications. Pythiosis can be difficult and costly to treat.