Water Mold Infection (Pythiosis) in Dogs

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Last updated on
3 min read

Key takeaways

Pythiosis is an infection caused by Pythium insidiosum, an aquatic organism similar to a fungus. This organism is commonly found in stagnant water, and is particularly common in sub-tropical climates.

  • The infection is usually contracted through contamination of open wounds or through ingestion of contaminated water
  • There are two main forms of the disease: cutaneous and gastrointestinal
  • Affected dogs might present with hairless skin lesions, wounds that do not heal, ulcerations, weight loss, vomiting, and diarrhea
  • Diagnosing pythiosis involves a physical examination, bloodwork, diagnostic imaging, and specific tests to identify the fungus, such as tissue biopsy
  • The treatment of choice is usually surgical removal of affected tissues and antifungal therapy
  • The prognosis is guarded to poor, unless complete removal of all infected tissue is possible
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A closer look: Water Mold Infection (Pythiosis) in Dogs

The two main forms of pythiosis are cutaneous (skin) and gastrointestinal.

The skin infection occurs through contamination of open wounds, and presents with wounds that do not heal, and often drain or ooze fluid. These wounds are most commonly found on the tail, legs, stomach, face, and neck.

If the infection affects the stomach and intestine, symptoms include vomiting, diarrhea, weight loss, and reduced appetite. Severe abdominal pain and fever may develop in the later stages of the disease, due to obstruction or rupture of the intestine.

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Risk factors

Pythiosis in dogs is common, especially in high-risk areas. It is a life-threatening disease with a guarded to poor prognosis, particularly if complete surgical excision cannot be performed. As such, suspected cases of pythiosis require prompt veterinary attention.

Pythium insidiosum is typically found in swampy areas, particularly in subtropical climates. High-risk areas include Southeast Asia, eastern coastal Australia, South America, and parts of the USA (mostly in Gulf Coast states). In the USA, the infection is thought to be seasonal and is particularly active during the period between August and December.

Possible causes

Usually, dogs come in contact with the infectious organism through ingestion or through contamination of open wounds.

Main symptoms

Testing and diagnosis

Early diagnosis is fundamental for effective treatment and a better prognosis. The diagnostic process involves a physical evaluation, blood work, and diagnostic imaging of the abdomen, such as X-rays and ultrasound, to evaluate the spread of infection. Specific tests to identify the pathogenic organism include biopsy of tissues and/or identification of fungal proteins on blood work.

Steps to Recovery

Treatment involves surgical removal of the affected tissues. Surgery is the treatment of choice for both the cutaneous and the gastrointestinal form. In cases of skin infection, it may be necessary to amputate limbs to ensure complete removal of the infection. After surgery, antifungal medications are prescribed long-term, often for periods of up to nine months.

Prognosis is generally guarded to poor unless complete excision of all affected areas is possible. Therapy might be long and expensive, and only about 20% of affected dogs fully recover.


Pythiosis is not contagious. Prevention is possible by avoiding risk factors such as exposure to swampy areas, particularly in high-risk areas.

Is Water Mold Infection (Pythiosis) in Dogs common?

Pythiosis is common in dogs, especially in high-risk areas and during the fall and winter months.

Typical Treatment

  • Surgical removal of affected areas
  • Antifungal medications


Tamara Gull, DVM, PhD, DACVM, DACVIM (LA), DACVPM, - Writing for Merck Veterinary Manual
Amy M. Grooters - Writing for Veterian Key


Tamara Gull, DVM, PhD, DACVM, DACVIM (LA), DACVPM, - Writing for Merck Veterinary Manual
Amy M. Grooters - Writing for Veterian Key

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