Water Mold Infection (Pythiosis) in Dogs

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Pythiosis in Dogs

Belonging to the phylum Oomycota, Pythium insidiosum is a parasitic spore that is capable of spontaneous movement (or a motile zoospore) that enters the body through the nose/sinuses, esophagus, or through the skin. Infection then usually settles in the dog's lungs, brain, sinuses, gastrointestinal tract, or skin.

Affected dogs will exhibit subcutaneous or cutaneous masses, which develop as lesions on the legs, tail, head, neck, perineum, and/or the inside of the thigh.

Pythiosis is typically thought of as occurring in swampy areas in the southeastern U.S., and has thus been nicknamed “swamp cancer.” Signs of pythiosis usually appear in the fall or early winter months, and while this organism does typically thrive in tropical and subtropical waters, such as ponds, wetlands, and swamps, it has been found to occur as far west as the central valley of California.

The condition or disease described in this medical article can affect both dogs and cats. If you would like to learn about how pythiosis affects cats, please visit this page in the PetMD health library.

Symptoms and Types

Pythiosis of the lungs, brain, or sinus will manifest in the dog as stuffiness, head pain, fever, coughing, and swelling of the sinuses. Infection of the dog's digestive tract leads to a chronic disease, which causes the tissue of the stomach and/or intestines to become severely thick. Other symptoms of gastrointestinal (GI) pythiosis include:

  • Fever
  • Vomiting
  • Diarrhea
  • Regurgitation
  • Long-term weight loss
  • Abdominal mass
  • Abdominal pain
  • Enlarged lymph nodes

Pythiosis of the skin (or cutaneous pythiosis) results in the development of swollen, non-healing wounds, and invasive masses of ulcerated pus-filled nodules and draining tracts. Tissue death (necrosis) follows, with the affected skin eventually turning black and wasting.


This infection is caused by direct contact with water that accommodates Pythium insidiosum, a water borne fungal parasite. It is usually swallowed or inhaled by the dog, and from there makes its way to the animal's intestinal tract.


Your veterinarian will perform a complete physical exam on your dog, with a chemical blood profile, a complete blood count, a urinalysis and an electrolyte panel. A blood sample will be sent for serological testing (via an Enzyme-Linked Immunosorbent Assay, called ELISA) to the Pythium Laboratory at Louisiana State University.

You will need to give a thorough history of your dog's health, onset of symptoms, and recent activities, including any exposure your pet may have had to water in the last several months.

Abdominal radiographs in dogs with GI pythiosis may show an intestinal blockage, intestinal wall thickening, or an abdominal mass. An ultrasound image of the dog's abdomen will tend to show thickening of the wall of the stomach or intestine. Enlarged lymph nodes may also be evident, as it is an indication of an infection.

While biopsy can suggest a diagnosis of pythiosis, a positive culture will be needed for a definitive diagnosis. There is also an immunohisto-chemical stain, which specifically attaches to P. insidiosum hyphae in thin sections of tissue.

Another method for definitively diagnosing pythiosis is to test tissue samples and cultured isolates with nested Polymerase Chain Reaction, a test of dog's deoxyribonucleic acid (DNA).


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