A disease of horses and mules, dogs, cattle, and cats (rarely of humans) caused by the fungus Pythium insidiosum (Hyphomyces destruens); most common in tropic or subtropic environmens, characterized by granulomatous and necrotic lesions that appear on the head and lower legs, ulcerate, and enlarge by subcutaneous extension. Also causes gastrointestinal tract granulomas, particularly in dogs. Rarely, ophthalmic, nasopharyngeal, or disseminated forms occur in animals. Risk of infection increases with exposure to swampy, warm environments.