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Water Mold Infection (Pythiosis) in Dogs

Pythiosis in Dogs

Pythiosis, a condition affecting dogs and cats, is caused by Pythium insidiosum, a parasitic spore belonging to the phylum Oomycota. This spore, capable of spontaneous movement, enters the body through the nose/sinuses, esophagus, or skin. Once inside, it tends to settle in the lungs, brain, sinuses, gastrointestinal tract, or skin of the host.

Dogs affected by pythiosis typically display subcutaneous or cutaneous masses, which manifest as lesions on various parts of the body including the legs, tail, head, neck, perineum, and inside of the thigh.

Pythiosis is commonly associated with swampy regions in the southeastern U.S., earning it the moniker “swamp cancer.” Signs of the infection typically emerge in the fall or early winter months. Although the organism thrives in tropical and subtropical waters such as ponds, wetlands, and swamps, cases have been reported as far west as the central valley of California.

Symptoms and Types

Pythiosis infection in dogs can affect various parts of the body, each presenting distinct symptoms:

  1. Lungs, Brain, or Sinuses: Dogs may exhibit symptoms like stuffiness, head pain, fever, coughing, and sinus swelling when infected with pythiosis in these areas.
  2. Digestive Tract: Infection in the digestive tract leads to chronic disease, causing severe thickening of the stomach and/or intestines. Additional symptoms of gastrointestinal (GI) pythiosis include:
    • Fever
    • Vomiting
    • Diarrhea
    • Regurgitation
    • Persistent weight loss
    • Abdominal mass
    • Abdominal pain
    • Enlarged lymph nodes
  3. Skin (Cutaneous Pythiosis): Cutaneous pythiosis presents with swollen, non-healing wounds, and invasive masses of ulcerated pus-filled nodules along with draining tracts. Tissue death (necrosis) ensues, leading to affected skin turning black and wasting.


The infection stems from direct contact with water containing Pythium insidiosum, a waterborne fungal parasite. Dogs typically ingest or inhale the parasite, which then travels to the animal’s intestinal tract.


Your veterinarian will conduct a comprehensive physical examination of your dog, along with performing various diagnostic tests including a chemical blood profile, complete blood count, urinalysis, and electrolyte panel. A blood sample will be sent to the Pythium Laboratory at Louisiana State University for serological testing using an Enzyme-Linked Immunosorbent Assay (ELISA).

It’s important to provide a detailed history of your dog’s health, symptoms onset, and recent activities, including any exposure to water in the past few months.

For dogs with gastrointestinal (GI) pythiosis, abdominal radiographs may reveal signs such as intestinal blockage, intestinal wall thickening, or abdominal mass. Ultrasound imaging of the abdomen may show thickening of the stomach or intestine walls, along with enlarged lymph nodes indicating infection.

While biopsy can suggest a diagnosis of pythiosis, a positive culture is necessary for confirmation. An immunohistochemical stain that specifically attaches to P. insidiosum hyphae in tissue sections is also available.

Another definitive diagnostic method involves testing tissue samples and cultured isolates using nested Polymerase Chain Reaction (PCR), which examines the dog’s DNA.


For optimal prognosis, it’s crucial to seek treatment for your dog promptly upon observing the initial symptoms.

All dogs will require surgery to remove as much affected tissue as possible. Subsequently, the remaining tissue post-surgery will undergo laser treatment (photoablation) to eradicate any fungal filaments in the surrounding tissue. Biopsies of enlarged lymph nodes in the abdominal cavity are necessary for examination, involving surgical removal of tissue. Medical therapy should be sustained for at least six months.

Living and Management

Your veterinarian will arrange follow-up appointments every two to three months post the initial surgery for ELISA serological tests. Abdominal X-rays will be retaken during each visit to reassess intestinal signs of the disease. A chemical blood profile should be repeated at each check-up to monitor your pet for liver toxicity while undergoing treatment with Itraconazole, the preferred drug for treating pythiosis.


In 2004, a novel immunotherapeutic vaccine for dogs was introduced to combat pythiosis. Upon diagnosis, prompt vaccination of your dog with the pythiosis vaccine is recommended to diminish the size of the lesion. This proactive measure aims to facilitate easier and more successful surgery thereafter.

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