Water Mold Infection in Dogs (Pythiosis)

  • Occurrence: rarer
  • Location of disease: Abdomen/pelvis
Author: Dr. med. vet. Gisa Löwe | Last modified: 07.08.2026
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When to visit the vet?

Non-urgent see a veterinarian within 2–3 days

?

If the condition worsens / symptoms persist, consult a veterinarian.

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Definition

Pythiosis is a rare but serious infectious disease in dogs caused by the water mold Pythium insidiosum. This disease can affect various tissues, particularly the skin and gastrointestinal tract.

The most important facts at a glance

Pythiosis in dogs is caused by the water mold Pythium insidiosum, which thrives in warm, humid environments such as stagnant water. Dogs that swim in such water are at increased risk of infection. The pathogen enters the body through small skin injuries or mucous membranes and causes granulomatous inflammatory conditions that are difficult to treat. Symptoms include fever, vomiting, diarrhea, weight loss, and enlarged lymph nodes. Diagnosis requires special laboratory tests, imaging, and biopsies, which is often delayed due to the rarity of the disease.

Treatment is complex and combines surgical interventions with the administration of antifungals such as itraconazole or terbinafine, as conventional agents are often ineffective. Immunotherapy may help in some cases but is expensive and not always successful. The prognosis is guarded to poor, especially with late diagnoses or systemic spread. Early intervention can improve survival chances. Prevention includes avoiding contaminated water, regular skin checks, and, if necessary, prophylactic immune strengthening. Avoiding contact with potentially contaminated water remains the most effective prevention strategy.

Causes

Pythium insidiosum belongs to the class of oomycetes, which despite their fungal-like properties are more closely related to algae. This water mold thrives in warm, humid environments, particularly in stagnant or slow-moving water. Dogs that frequently swim in or spend time near such water are at higher risk.

Infection occurs primarily through contact with contaminated water, with the water mold entering the body through small skin injuries or mucous membranes. The high prevalence of the disease in tropical and subtropical regions is due to optimal climatic conditions for pathogen growth.

Pythium insidiosum is able to survive in host tissue and trigger strong inflammatory reactions. This leads to granulomatous lesions that are difficult to treat. The exact pathogenesis is complex and involves a variety of immunological reactions that ultimately lead to tissue damage and loss of function.

Symptoms

Rare symptoms:

The symptoms of pythiosis in dogs vary depending on the affected tissue. In skin infections, dogs can develop nodular, ulcerating lesions that are often severely itchy and painful. These lesions tend to spread rapidly and can lead to secondary bacterial infections.

If the gastrointestinal tract is affected, symptoms such as vomiting, diarrhea, weight loss, and loss of appetite can occur. The extensive tissue damage can lead to thickening of the intestinal walls and, in severe cases, to intestinal obstruction. These symptoms are often nonspecific and can be confused with those of other gastrointestinal diseases.

In some cases, the infection can also become systemic, leading to Fever, general weakness, and signs of sepsis. The clinical picture can be highly variable, making diagnosis difficult.

Therapy

Treatment of pythiosis is challenging and often requires a combination of surgical and medical approaches. For cutaneous lesions, early surgical removal of infected tissue is crucial to prevent the spread of infection. In gastrointestinal infections, resection of affected intestinal segments may also be necessary.

Medically, the infection is often treated with a combination of antifungal and immunomodulatory medications. Since Pythium insidiosum does not respond to conventional antifungals, specific agents such as itraconazole or terbinafine are used. These medications must be administered over extended periods to achieve complete elimination of the pathogen.

In some cases, immunotherapy may be considered to strengthen the dog’s immune system and improve defense against the pathogen. However, these treatment methods are often expensive and not always successful.

Prognosis and follow-up care

The prognosis for dogs with pythiosis is generally guarded to poor, especially if the disease is diagnosed late or has already taken on systemic proportions. Early diagnosis and aggressive treatment can improve survival chances, but the risk of recurrence of the infection remains.

Dogs that receive complete surgical removal of infected tissue and respond well to medical treatment have the best chances of recovery. Nevertheless, the long-term prognosis depends on the severity of the disease and the response to therapy.

Without treatment, the disease can progress rapidly and lead to death within a few months. Careful monitoring of dogs living in high-risk areas or frequently swimming in stagnant waters is crucial for rapid intervention and treatment.

Prevention

Since pythiosis is mainly prevalent in certain geographic regions, an essential prevention measure is to keep dogs away from potentially contaminated water, especially during the warm and humid months of the year.

Regular skin checks for injuries or unusual changes can help detect early infections and respond more quickly. For dogs living or working in risk areas, good hygiene should be maintained to minimize the risk of infection.

In some cases, prophylactic treatment with immune-boosting supplements may be considered, especially for dogs with known risk or in areas with high disease prevalence. Nevertheless, avoiding contact with contaminated water sources remains the most effective prevention strategy.

Outlook on current research

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Pythiosis in dogs is a severe, often rapidly progressive disease caused by Pythium insidiosum. Despite the name, it is not a true fungus but an oomycete. This is crucial for research because many classic antifungals are only limitedly effective against it. Particularly common in dogs are the gastrointestinal and cutaneous/subcutaneous forms, and the prognosis depends heavily on how early the disease is detected. (merckvetmanual.com, pubmed.ncbi.nlm.nih.gov)

The most important research outlook today lies in earlier and more precise diagnostics. Clinically, pythiosis can deceptively mimic other inflammatory or neoplastic processes. Therefore, serology, PCR, histopathology, and immunohistochemical methods are being further improved to distinguish the disease more quickly and reliably. (pubmed.ncbi.nlm.nih.gov, merckvetmanual.com)

A second focus is therapy development. Since standard antifungals are often insufficient, radical surgical removal remains central, provided it is anatomically possible. In parallel, research is investigating combined medical strategies and immunotherapies to reduce relapses and better treat inoperable cases. (pubmed.ncbi.nlm.nih.gov, pubmed.ncbi.nlm.nih.gov)

Overall, the outlook is cautiously optimistic: progress is expected primarily through faster diagnostics, better understanding of oomycete biology, and more targeted combination treatments.

Frequently Asked Questions (FAQs)

1. What is pythiosis?
Pythiosis is an infection caused by the water mold-like pathogen Pythium insidiosum. Pythium insidiosum is not a true fungus but belongs to the oomycetes. In dogs, it primarily affects the gastrointestinal tract, less commonly the skin. The disease is widespread in tropical and subtropical regions but also occurs in temperate zones.
2. How do dogs become infected?
Infection occurs through contact with warm stagnant freshwater (swamps, ponds, ditches). The infectious zoospores penetrate through small skin lesions or via the intestinal mucosa. Young, large breeds that enjoy swimming or playing in water are particularly at risk.
3. Which breeds are particularly affected?
Large, young dogs are predisposed, particularly Labrador Retrievers, Pointers, and other hunting dogs. Dogs affected in the gastrointestinal tract are usually one to three years old. Males are slightly more frequently affected than females.
4. What symptoms occur?
In the gastrointestinal form, vomiting, chronic diarrhea, weight loss, and a palpable abdominal mass are typical. In the cutaneous form, non-healing, granulomatous skin changes develop on paws, limbs, or trunk with fistula formation. Both forms progress without treatment.
5. How is the diagnosis made?
Clinical presentation, imaging (ultrasound, CT), and cytology provide clues. Crucial is the histopathological examination with special staining to detect the broad, rarely septate hyphae, as well as serological tests (ELISA) for antibody against Pythium insidiosum. PCR on tissue confirms the diagnosis.
6. How is pythiosis treated?
The most effective treatment is complete surgical removal of the affected tissue, wherever this is anatomically possible. Medical therapy with itraconazole plus terbinafine is standard but only responds in a portion of patients. Immunotherapy with Pythium vaccines has been attempted; results are inconsistent.
7. Why is medical therapy often ineffective?
Pythium insidiosum is not a true fungus and does not possess ergosterols in the cell membrane, which is why classic antifungals work significantly less effectively than against true fungi. This explains the limited efficacy of itraconazole, amphotericin B, and similar substances.
8. What is the prognosis?
The prognosis depends crucially on whether complete surgical removal is successful. With radical resection, the long-term survival rate is up to 60 percent. Without surgery, the prognosis is poor; the majority of dogs die within a few months.
9. Is pythiosis contagious?
Direct transmission from dog to dog or to humans is not described. Infection occurs exclusively from the environment via contaminated water. Humans can also become ill in rare cases, which primarily affects farmers in endemic areas.
10. How can it be prevented?
In endemic areas, dogs should not bathe in or drink from warm, stagnant water. Alternative water sources, avoidance of flooded pastures, and immediate care of open skin wounds reduce the risk. A vaccination is not available.

Literature

  • Cridge, H. (2025). Pythiosis in dogs. Veterinary Clinics of North America: Small Animal Practice, 55(2), 225–236. https://doi.org/10.1016/j.cvsm.2024.11.008
  • Billings, P., Walton, S., Shmalberg, J., & Santoro, D. (2023). The use of mefenoxam to treat cutaneous and gastrointestinal pythiosis in dogs: A retrospective study. Microorganisms, 11(7), Article 1726. https://doi.org/10.3390/microorganisms11071726
  • Hamlin, A. N., Locker, S., Huguet, E., Berry, C. R., Cole, R., Griffin, J. F., IV, Gremillion, C., Hecht, S., & Vilaplana Grosso, F. R. (2024). Computed tomographic characteristics of confirmed and presumed noncutaneous pythiosis in 25 dogs. Veterinary Radiology & Ultrasound, 65(2), 87–98. https://doi.org/10.1111/vru.13326
  • Yolanda, H., & Krajaejun, T. (2022). Global distribution and clinical features of pythiosis in humans and animals. Journal of Fungi, 8(2), Article 182. https://doi.org/10.3390/jof8020182
  • Reagan, K. L., Marks, S. L., Pesavento, P. A., & Grooters, A. M. (2019). Successful management of 3 dogs with colonic pythiosis using itraconazole, terbinafine, and prednisone. Journal of Veterinary Internal Medicine, 33(3), 1434–1439. https://doi.org/10.1111/jvim.15506
  • Berryessa, N. A., Marks, S. L., Pesavento, P. A., Krasnansky, T., Yoshimoto, S. K., Johnson, E. G., & Grooters, A. M. (2008). Gastrointestinal pythiosis in 10 dogs from California. Journal of Veterinary Internal Medicine, 22(4), 1065–1069. https://doi.org/10.1111/j.1939-1676.2008.0123.x
  • Grooters, A. M. (2003). Pythiosis, lagenidiosis, and zygomycosis in small animals. Veterinary Clinics of North America: Small Animal Practice, 33(4), 695–720. https://doi.org/10.1016/S0195-5616(03)00034-2