Canine Pythiosis

Pythiosis is a severe and often fatal disease caused by the water mold Pythium insidiosum. When animals with damaged skin come into contact with contaminated water or wet soil, zoospores enter the exposed tissue and encyst. In widespread disease, the organism usually penetrates the gastrointestinal tract and can spread to adjacent tissues such as pancreas, uterus, and lymph nodes. Other organisms with similar structure to Pythium species include fungi classified in the class Zygomycetes, which cause Zygomycosis. Zygomycosis can affect the alimentary tract of the dog. Young, male large-breed dogs are most often affected, with reports of predilection in the German Shepherd Dog and in outdoor working breeds (Labrador Retriever).

Human pythiosis
Source:BMC Infect Dis. 2011; 11: 33. under the terms of the Creative Commons Attribution License

Regardless of species and age, animals exposed to warm, standing fresh water are more likely to be in contact with the infectious zoospores and may have increased risk for the disease. Reports of animals being infected with no known history of being near water suggests that animals can be inoculated by contact with resistant spores that form in wet soil and on grass. In the United States, the infection is seen in warmer to temperate areas. Canine pythiosis most often occurs along the Gulf coast and parts of California. There also have been reports from Missouri, Georgia, North Carolina, South Carolina, and Tennessee. Dogs become infected by ingesting, or swimming in contaminated water.

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There are 85 species of Pythium, most of which are parasites of plants, fish, and crustaceans. Pythium species belong to the class Oomycetes. The oomycetes, often referred to as water molds, have many characteristics in common with the fungi, but are more closely related to algae. Chitin, an essential component of the fungal cell wall, is generally lacking in oomycetes. Water lilies and other aquatic plants and submerged grasses, including rice plants, are thought to be normal hosts. The organisms have a strong attraction for horse skin and hair, human hair, water-lily, and grass leaves.

Canine pythiosis most commonly occurs as gastrointestinal disease. Affected dogs will have a history of upper gastrointestinal tract obstruction and may have a palpable abdominal mass. The stomach and small intestine are the most frequent locations of the lesion, although any region of the alimentary tract can be affected. Clinical signs include loss of appetite, weight loss, vomiting, and diarrhea.3 The diarrhea may become bloody due to intestinal necrosis and ulceration. Cutaneous pythiosis in dogs typically causes nonhealing wounds and invasive masses, which contain ulcerated nodules and draining tracts, most often involving the extremities, tail head, and neck.

The cutaneous/subcutaneous form of pythiosis affects skin and is characterized by non-healing, tumor-like nodules with ulceration and pus-filled discharge. Lesions can be solitary or multiple and are usually confined to one area of the body, especially the legs, face, and tail head. Early lesions on the limbs may resemble acral lick dermatitis. Rarely, dogs develop locally invasive skeletal or disseminated infection, which is thought to be triggered by use of corticosteroids or weakened state of the immune system. The diagnosis of oomycete infections can be difficult due to the clinical and histological similarity to fungal infections. Special expertise and considerable time are required for specific laboratory procedures. Two ELISA tests are available for detection of antibodies in animals. Unfortunately, even with treatment, most cases are fatal. Antifungal chemotherapy agents such as amphotericin B, ketoconazole, flucytosine have been disappointing in the treatment of this disease, although a newly formulated vaccine has been successful in treating cutaneous pythiosis in horses and dogs.


  1. Oomycosis: Pythiosis in the Dog, Horse and Cat and Lagenidiosis in the Dog
  2. Muller & Kirk's small animal dermatology. George H. Muller, Danny W. Scott, Robert Warren Kirk, William Howard Miller, Craig E. Griffin
  3. Todd R. Tams. Handbook of small animal gastroenterology
  4. Stephen J. Ettinger, DVM, DACVIM and Edward C. Feldman, DVM, DACVIM. Textbook of Veterinary Internal Medicine

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