Pythiosis in Horses, Dogs & Cats by a Vet – 2025 Swamp Cancer Diagnosis & Treatment Guide 🌿🧬

By Dr Duncan Houston, BVSc

Introduction

Pythiosis—also known as swamp cancer, bursatti, leeches, or oomycosis—is a severe, potentially fatal disease caused by the aquatic organism Pythium insidiosum. It mimics a fungal infection, but it’s actually more closely related to algae. Pythiosis affects horses, dogs, and occasionally cats, especially in warm, wet regions like the Gulf Coast. Early surgical removal of lesions offers the best chance for survival. This 2025 guide covers symptoms, diagnosis, and treatment across species.

Where Pythiosis Thrives 🌍

• Common in warm, wet areas: Southeast Asia, Australia, South America, and the U.S.
• U.S. hot zones: Texas, Louisiana, Florida, Alabama, and Mississippi
• Reported as far north as Wisconsin, Indiana, and California
• Seasonal spikes from August through December

Pythiosis in Horses 🐴

Skin Form

• Causes large, ulcerated nodules or fistulated lesions on lower limbs, belly, chest, and genitals
• Lesions are itchy—self-trauma is common
• Yellow-gray necrotic “kunkers” or cores may be seen

Intestinal (Enteric) Form

• Leads to thickening, fibrosis, and narrowing of the gastrointestinal tract
• Symptoms: weight loss, diarrhea, colic, poor appetite

Pythiosis in Dogs 🐶

Skin Form

• Often enters through wounds exposed to swampy water
• Lesions: ulcerated, itchy, hairless lumps with draining tracts (fistulas)
• Common on tail, legs, face, and abdomen

Gastrointestinal Form

• Targets stomach, small intestine, colon, or esophagus
• Symptoms: vomiting, diarrhea, anorexia, rapid weight loss
• Often not systemic until late-stage

Pythiosis in Cats 🐱

• Rare, but reported in skin or nasal areas
• Lesions are less invasive than in dogs
• Include ulcerated wounds, draining tracts, and pruritus (itchiness)

Diagnosis 🧪

• Histopathology: Tissue biopsy with special stains
• PCR: Detects P. insidiosum DNA
• ELISA: Measures antibody response (used for monitoring after surgery)
• Culturing is difficult: Grows poorly and is often overrun by bacteria

Treatment Options 💊✂️

Surgical Removal – First Line of Defense

• Best prognosis if all infected tissue is removed with wide margins
• Amputation may be necessary in limb cases

Medical Therapy (Often Disappointing)

• Antifungals: Itraconazole + terbinafine for 6–9 months
• Success rate: only 20–25% of canine cases show improvement
• Amphotericin B (lipid complex) may be added—but with limited success

Immunotherapy (Experimental)

• Used in some horses and dogs to stimulate immune response
• Research ongoing—consult your vet for availability

Monitoring After Surgery 🧬

• ELISA titer: Used every 2–3 months post-op
• Falling titer = success; rising or persistently high = likely recurrence

Prognosis 📉

• Guarded to poor if surgery is not possible
• Survival is best when:
  • Lesions are small
  • Located away from vital organs
  • Infection duration is short
• Multisystemic cases are often fatal

Lagenidiosis: A Deadlier Cousin in Dogs

• Also aquatic; affects lymph nodes, lungs, large blood vessels
• Spreads via vasculitis (blood vessel inflammation)
• May cause sudden death from aneurysm rupture
• Treatment and diagnosis are similar to pythiosis—but prognosis is even worse

FAQs About Pythiosis

Q: Is pythiosis contagious?

A: No. It is acquired from the environment—particularly stagnant water—not between animals.

Q: Can it affect humans?

A: Very rarely. Human cases exist but are extremely uncommon. Horses and dogs are more commonly affected.

Q: How can I protect my animals?

A: Keep animals away from standing water, swamps, or flooded pastures—especially during late summer and fall.

Conclusion

Pythiosis is a devastating disease that strikes quickly in horses, dogs, and cats exposed to stagnant water. Because antifungal drugs are largely ineffective, early surgical removal is the most important step in survival. In 2025, with increased flooding and climate changes, awareness and prevention are your best tools. If you notice non-healing wounds, draining tracts, or GI symptoms in animals that have been near water, contact your vet immediately for testing and evaluation.

Need help diagnosing or managing pythiosis? Visit AskAVet.com or download the app 📱 to consult Dr Duncan Houston for case review, testing, and treatment planning. 🩺🐴🐶🐱