Pythiosis

Pythiosis is a rare and deadly tropical disease caused by the oomycete Pythium insidiosum. Long regarded as being caused by a fungus, the causative agent was not discovered until 1987. It occurs most commonly in horses, dogs, and humans, with isolated cases in other large mammals. The disease is contracted after exposure to stagnant fresh water such as swamps, ponds, lakes, and rice paddies. P. 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. Additionally, it is the only member in the genus known to infect mammals, while other members are pathogenic to plants and are responsible for some well-known diseases in plants.

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 Southern Gulf states, especially Louisiana, Florida, and Texas, but has also been reported as far away as California and Wisconsin. It is also found in southeast Asia, eastern Australia, New Zealand, and South America.

Pythiosis is suspected to be 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.

The species are listed in decreasing frequency of infection.

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.

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