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Pythiosis (Oomycosis, Lagenidiosis, Swamp Cancer, Bursatti, Leeches) in Dogs, Cats and Horses
Published: November 22, 2010

Pythiosis is caused by a fungal-like aquatic organism called Pythium insidiosum. This organism is not a true fungus, but behaves somewhat similarly. It exists in stagnant water, or on grasses that have been exposed to the stagnant water.

The disease is most common in tropical and sub-tropical climates such as Southeast Asia, eastern coastal Australia, South America, and the USA. (In the USA, it is seen mostly in the Gulf Coast states, although it has been found in more diverse areas such as Wisconsin, Kentucky, Missouri, Indiana, North Carolina, California, and Washington.)

In the USA, the disease tends to be seasonal, and most cases occur between August and December.

Pythiosis in Dogs

In dogs, pythiosis can occur as the cutaneous (skin), gastrointestinal (stomach and intestine), or multisystemic form. The skin form occurs when the organism comes in contact with an open wound. The gastrointestinal form occurs when the dog ingests the organism.

In the skin form of the disease, signs include wounds that won’t heal, ulcerated small lumps, and fistulas (small tunnel-like passages in body tissue). Hairless lesions are found most commonly on the tail, legs, stomach, neck, and face. These hairless lesions are perforated by draining fistulas. The lesions are itchy and damage from self-mutilation is common.

Gastrointestinal pythiosis occurs most often in the stomach, proximal small intestine, and ileocolic junction, but any part of the intestine, esophagus, and colon can be diseased. Clinical signs include anorexia, vomiting, diarrhea, and weight loss. The weight loss can be severe, but affected dogs usually do not appear systemically ill until late in the disease.

Pythiosis in Cats

Pythiosis in cats is rare, and usually occurs as either cutaneous or nasopharyngeal lesions.

The skin lesions in cats are less invasive and destructive than in dogs, but do include the non-healing wounds, ulcerated skim lumps, draining fistulas, and pruritus.

Pythiosis in Horses

In horses, the lesions are large nodules (roughly circular, granulomatous, ulcerated, and fistulated) or subcutaneous swellings with yellow-gray necrotic cores. The lesions are most common on the legs - especially the lower limbs - abdomen, chest, and genitalia, all of which are areas of the body that would come in contact with infected water or grass. The lesions are itchy, so self-trauma is common.

The horse’s bones may become infected as the disease becomes chronic.

Enteric (intestinal) pythiosis in horses is characterized by fibrosis and narrowing of the gastrointestinal tract.


Early diagnosis is necessary for effective treatment and survival.

Diagnosis is most often based on histopathology tests, as well as PCR and ELISA assays. It can be difficult to culture Pythium. It does not grow well on routine fungal culture media, and rapid bacterial overgrowth often occurs on bacterial culture media, skewing the results.

Treatment in Dogs

Once the correct diagnosis is made, surgery to remove affected tissue, leaving wide margins, can be effective for both the skin and gastrointestinal forms. The surgery will not cure the dog if it does not completely eliminate all of the lesions. Limb amputation may be necessary. After amputation, antifungal drugs should be given for two to four months.

With gastrointestinal pythiosis, complete surgical excision is the treatment of choice, but the disease is often too extensive at the time of diagnosis to allow complete resection. After resection, medical therapy for gastrointestinal pythiosis should include itraconazole and terbinafine given for six to nine months. However, only 10 – 20 percent of dogs respond.

Treatment with amphotericin B lipid complex can also be attempted. Approximately 20% of dogs will respond to long-term therapy.

Treatment in Cats

Complete surgical excision of small or accessible lesions can be effective. Medical therapy rarely works.

Treatment in Horses

Complete surgical excision of small or accessible lesions can be effective.


Prognosis for most cases of pythiosis is poor, and the relative newness of the disease and lack of early testing for P. insidiosum is probably responsible for that. Medical therapy has been frustrating, with only 20 - 25% of canine cases showing a positive response. A positive response to treatment is usually defined as a resolution of clinical signs and weight gain. However, caught early enough, the disease is treatable.

Factors that influence the prognosis include size and site of lesion and duration of infection. Small lesions of short duration that have not invaded critical structures usually respond best to treatment.

Following amputation of an infected limb or excision of a gastrointestinal mass, the ELISA assay may be performed every 2 to 3 months to help detect recurrence. A dramatic decrease in the titer is usually noted 2 to 3 months after successful surgical resection. Persistently high titers often indicate a strong possibility of recurrence.

The disease is often fatal in animals that have lesions that cannot be completely excised or that have multisystemic disease.

Lagenidiosis in Dogs

Lagenidiosis is characterized by cutaneous lesions and enlarged lymph nodes. The infection causes inflammation of the blood vessels, allowing the disease to spread via the bloodstream. The lymph nodes, lungs, and large blood vessels may then become affected. The vasculitis may result in aneurysms that can suddenly rupture, causing sudden death.

Diagnosis, treatment, and prognosis are similar to those for pythiosis, but prognosis currently is worse than that for pythiosis.

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