Pythiosis & Lagenidiosis in Dogs, Cats & Horses: 2025 Vet Insights by Dr Duncan Houston 🌾🩺

Hello, I’m Dr Duncan Houston, BVSc. Pythiosis—often called swamp cancer—is caused by the water mould Pythium insidiosum. It affects animals exposed to stagnant water or damp areas. Lagenidiosis, a related oomycete disease in dogs, spreads via the bloodstream and carries a worse prognosis.

🌍 Where & When It’s Found

Pythium insidiosum thrives in freshwater swamps, marshes, and flooded grasslands—especially in Gulf Coast states, but increasingly reported in temperate regions like Virginia and Kentucky :contentReference[oaicite:3]{index=3}. Infected animals often present during late summer into autumn.

🐶 Pythiosis in Dogs

• Cutaneous form: Non-healing, itchy ulcers or nodules with draining fistulas—common on limbs, tail, belly, neck.
• Gastrointestinal form: Masses in stomach/intestine cause vomiting, diarrhea, weight loss and poor appetite—often hidden until late :contentReference[oaicite:4]{index=4}.

🐱 Pythiosis in Cats

Rare, but when present, shows as ulcerated skin lesions or nasopharyngeal masses—less aggressive than in dogs.

🐴 Pythiosis in Horses (“Swamp Cancer”)

Cutaneous/subcutaneous “kunkers” appear as large, ulcerative, itchy nodules—often on limbs, abdomen, and chest. Chronic cases may invade bones or gut :contentReference[oaicite:5]{index=5}.

🦠 Lagenidiosis in Dogs

Starts as skin nodules and spreads via blood to lymph nodes, lungs, and large vessels—risking fatal aneurysms. Prognosis is worse than pythiosis :contentReference[oaicite:6]{index=6}.

🧪 Diagnosis

• Histopathology + culture/PCR/ELISA: Tissue biopsy with specific tests is key; routine fungal cultures often fail :contentReference[oaicite:7]{index=7}.
• Imaging: Ultrasound or CT for gut masses in dogs or deep lesions in horses.

🔧 Treatment Strategies

This infection demands aggressive, multi-modal therapy:

1. Wide surgical excision of lesions with clear margins—amputation may be necessary in distal cases.
2. Long-term antifungals (itraconazole + terbinafine) used for 6–9 months—response rates are low (~10–20%) :contentReference[oaicite:8]{index=8}.
3. Amphotericin B lipid complex may benefit some dogs (≈20% respond).
4. Equine-derived immunotherapy vaccine improves outcomes—35% cure in dogs, up to 75% in early-stage equine cases :contentReference[oaicite:9]{index=9}.

📉 Prognosis & Follow-Up

• Poor overall—only ~20–25% of dogs respond; better outcomes with early diagnosis and complete surgery :contentReference[oaicite:10]{index=10}.
• Horses fare better—timely excision plus immunotherapy yields higher success.
• Lagenidiosis carries a guarded to grave prognosis due to vascular spread and aneurysm risk.
• Monitor titers (ELISA) every 2–3 months—persistent elevation often means recurrence.

🛡️ Prevention Tips

• Avoid swampy or stagnant-water areas—especially during high-risk seasons.
• Protect open wounds—cover them and clean thoroughly after exposure.
• Fence off ponds or marsh paddocks; drain standing water when possible :contentReference[oaicite:11]{index=11}.
• Seek veterinary care for suspicious lesions—early biopsy is key to prognosis.

⚠️ Final Takeaways

• Pythiosis and lagenidiosis are aggressive “water mould” infections requiring fast, intense management.
• They often mimic skin or tumor-like diseases, making early diagnosis critical.
• Best outcomes when surgery & immunotherapy are promptly applied—medical-only therapy rarely suffices.

📲 Ask A Vet—Here to Help

Seeing suspicious non-healing lesions or gastrointestinal issues after marsh exposure? Download the Ask A Vet app for prompt, expert guidance—know when to act early. 🐾📞