The triad of warm temperatures, flooded lands, and aquatic vegetation highly favors the proliferation and maintenance of Pythium insidiosum, a pathogen of plants and, occasionally, mammals and birds. The disease, named pythiosis, has worldwide distribution in horses and has been known by different names according to geographical location: “bursatte” in India, “leeches” and “swamp cancer” in Australia and the United States, and “ferida da moda” in Brazil.

In horses, the agent causes ulcerative, granulomatous lesions mainly in the cutaneous and subcutaneous tissues. P. insidiosum also causes gastrointestinal, ocular, and disseminated forms of the disease in other animals.

In horses, skin lesions are more frequent in the limbs, chest and abdomen, parts which are in direct contact with water containing zoospores (the infective stage) of P. insidiosum. It is hypothesized that the zoospores are not able to penetrate the intact skin, but a small lesion such a mosquito bite is enough to allow entry. An association between lesion locations and areas of insect blood feeding has been demonstrated in horses.

While the pathogenesis of pythiosis is not fully understood, after an incubation time of 15 to 20 days, horses typically develop granulomatous, ulcerative lesions, marked by fistulas draining serosanguinous (blood and serous fluid) exudate. Lesions also contain firm and necrotic-gray material referred as “kunkers.” These kunkers contain viable hyphae (microscopic structures) that are important for differentiating the disease from equine sarcoid, squamous cell carcinoma, granu