Pythiosis is a form of infectious disease caused by Pythium insidiosum. It occurs most commonly in dogs and horses, but is also found in cats, cattle, and humans. The disease is typically found in young, large breed dogs.  Pythium occupies swamps in late summer and infects dogs who drink water containing it. Pythium insidiosum is different from other members of the genus in that human and horse hair, skin, and decaying animal and plant tissue are chemoattractants for its zoospores.

Pythiosis occurs in areas with mild winters because the organism survives in standing water that does not reach freezing temperatures.  In the United States it is most commonly found in the Gulf states, especially Louisiana, but has also been found in midwest and eastern states. It is also found in southeast Asia, eastern Australia, New Zealand, and South America.

It is suspected that pythiosis is caused by invasion of the organism into wounds, either in the skin or in the gastrointestinal tract.  The disease grows slowly in the stomach and small intestine, eventually forming large lumps of granulation tissue. It can also invade surrounding lymph nodes. Pythiosis of the skin in dogs is very rare, and appears as ulcerated lumps. Primary infection can also occur in the bones and lungs. Dogs with the gastrointestinal form of pythiosis will have severe thickening of one or more portions of the GI tract that may include the stomach, small intestine, colon, rectum, or in rare cases, even the esophagus. The resulting pathology will result in anorexia (no appetite), vomiting, diarrhea (sometimes bloody), and abdominal straining. Extensive weight loss may be evident.

In horses, subcutaneous pythiosis is the most common form and infection occurs through a wound while standing in water containing the pathogen.  The disease is also known as leeches, swamp cancer, and bursatti. Lesions are most commonly found on the lower limbs, abdomen, chest, and genitals. They are granulomatous and itchy, and may be ulcerated or fistulated. The lesions often contain yellow, firm masses of dead tissue known as kunkers.  It is possible with chronic infection for the disease to spread to underlying bone.  In humans it can cause arteritis, keratitis, and periorbital cellulitis.  In cats pythioisis is almost always confined to the skin as hairless and edematous lesions. It is usually found on the limbs, perineum, and at the base of the tail.  Lesions may also develop in the nasopharynx.