Pythiosis in Horses (Equis) | Vetlexicon
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ISSN 2398-2977

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Synonym(s): Bursatti, Florida horse leeches, swamp cancer, phycomycosis, Gulf Coast fungus, espundia, cancer


  • Chronic subcutaneous fungal infection caused byPythium insidiosumin tropical and subtropical areas.
  • Cause:Pythium insidiosum.
  • Signs: Acute onset of rapidly enlarging ulcerated masses of granulation tissue.
  • Diagnosis: history, clinical signs, cytology, histopathology, fungal culture.
  • Treatment: surgery, immunotherapy, anti-fungal therapy.
  • Prognosis: good - if diagnosis is made early and treatment is immediate.
  • See also:
    • Subcutaneous fungal infection:Pythium.
    • Ulcerative, granulomatous lesions:Pythium.

Presenting signs

  • Acute onset of rapidly enlarging ulcerated masses of granulation tissue with sinus tracts that discharge a thick serosanguineous fluid   Skin: pythiosis 03 - face    Skin: pythiosis 04 - close-up  .
  • Lesions are severely pruritic. Foul odor is noted when necrosis is present.
  • Irregularly shaped yellow-tan, gritty masses called "kunkers" are seen in the sinus tracts. Kunkers are composed of sequestered vessels, eosinophils and Pythium hyphae.
  • Lesions are usually circular and vary in size according to the duration of infection.
  • Most commonly affected areas include distal extremities   Skin: pythiosis 01 - foot  , ventral abdomen and thorax.
  • Lymphangitis with edema and osteomyelitis may occur in chronic cases, especially on legs   Skin: pythiosis 02 - leg  .

Geographic incidence

  • Common in tropical and subtropical regions, particularly in states close to the Gulf of Mexico.
  • Reported also in Australia, Brazil, Columbia, Costa Rica, Argentina, Greece, France and Japan.

Age predisposition

  • All ages can be affected.

Breed/Species predisposition

  • No breed predilection.

Cost considerations

  • Expensive:
    • Surgery.
    • Anti-fungal therapy.

Special risks

  • Transmission ofPythiumfrom infected horses to humans or other horses has not been reported, but exudate may be a source of zoonotic infection.



  • Pythium insidiosum: plant parasite.
  • Classification ofPythium:
    • Kingdom: Protista.
    • Order: Peronosporales.

Predisposing factors

  • Presence of cutaneous wounds or scratches.
  • Access to swampy areas, especially in late summer and fall when high temperatures and heavy rainfall maintain the right environment forPythium.
  • Geographical area at risk (states close to the Gulf of Mexico).


  • Pythiumzoospores present in lakes, swamps, or ponds are attracted to open wounds and invade subcutaneous tissues causing tissue necrosis and swelling.
  • Zoospores are released in the water and attracted, via chemotaxis, to horse hair and tissues.
  • If open wound is present, zoospores can attach and penetrate tissues.
  • Zoospores loose the flagella, encyst, germinate and produce hyphae, which are responsible for tissue destruction.


  • Incubation time: few days.
  • Acute onset of rapidly enlarging granulomatous, ulcerative, pruritic lesions.


Client history

  • Access to swampy areas, in endemic areas.

Clinical signs

  • Acute onset of rapidly progressing pruritic ulcerative lesions, especially on ventral abdomen and legs.
  • Presence of kunkers in the draining tracts.

Diagnostic investigation

  • Cytologic examination of necrotic tissue shows irregularly branching hyphae (GMS stain).


  • Granulomatous, eosinophilic nodular to diffuse dermatitis, occasional vasculitis. Giant cells are seen in old lesions.
  • Histologically, kunkers present as large eosinophilic masses sequestered in neutrophils and surrounded by eosinophilic granulation tissue.
  • Hyphae are seen with special stains (GMS) at the periphery of kunkers. They are thick walled and irregularly branching.
  • Immunoperoxidase has been developed to stain hyphae in tissue sections.


  • Serologic diagnosis by complement fixation, immunodiffusion, have been reported.

Fungal culture

  • Pythiumgrows rapidly on sabouraud's dextrose agar, heart infusion agar and corn meal agar at 37°C.
  • Specimens should not be refrigerated, asPythiumis sensitive to low temperature.
  • Colonies are visible in 24 h, identified based on morphology.

Gross autopsy findings

  • Ulcerative, granulomatous skin lesions, edema of the limbs may be present.

Histopathology findings

  • Histopathology: granulomatous, eosinophilic, nodular to diffuse dermatitis. Osteomyelitis may be present in chronic cases.

Differential diagnosis

  • Basidiobolomycosis.
  • Conidiobolomycosis.
  • Cutaneous habronemiasis   Habronemiasis  .
  • Botriomycosis.
  • Nocardiosis.
  • Neoplasia (sarcoid, squamous cell carcinoma).
  • Mycetoma.
  • Exuberant granulation tissue.


Initial symptomatic treatment

  • Excision is the most common and successful treatment of early lesions.
  • Due to the potential of rapid expansion of the lesions, surgery should be done immediately.
  • Success is dependent on size and location of lesions.
  • Recurrence is common if lesions are not completely excised.

Subsequent management


  • Immunotherapy is most effective when preceded by surgical debridement.
  • Success of immunotherapy alone was 53%.
  • Success of immunotherapy following incomplete surgical excision (within 2 weeks) was 100%.
  • Two vaccines for pythiosis have been developed in the USA, but are not commercially available. Permit from United States Department of Agriculture is necessary to distribute the vaccine.
  • In horses responding to immunotherapy, first signs of improvement are seen within 7-14 days after the initial injection.
  • Amphotericin B   Amphotericin B  (IV and topically) can be used in combination with surgical excision. Expensive, potential for nephrotoxicity and anemia.



  • Prevent access to swampy areas in regions at high risk.



  • Success depends on duration of infection, location of lesions and general condition of the animal.
  • Recent infections may respond to immunotherapy alone, lesions older than 2 months are unlikely to respond.
  • Small lesions respond to surgical excision.
  • Older lesions with bone involvement may lead to death of the animal.

Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Chaffin M K, Schumacher J & McMullan W C (1995) Cutaneous Pythiosis in the horse. Vet Clin North Am Equine Pract 11 (1), 91-103 PubMed.
  • Eaton S A, Neuwirth L, Newton J C & Ross P S (1993) Equine Rounds on Pythiosis. Compendium 485-493.
  • Mendoza L, Hernandez F & Ajello L (1993) Life cycle of the human and animal oomycete pathogen Pythium insidiosum. J Clin Microbiol 31 (11), 2967-2973 PubMed.
  • Mendoza L, Villalobos J, Calleja C E & Solis A (1992) Evaluation of two vaccines for the treatment of pythiosis insidiosi in horses. Mycopathologica 119, 89-95 PubMed.
  • Brown C C, McClure J J, Triche P & Crowder C (1988) Use of immunohistochemical methods for diagnosis of equine pythiosis. Am J Vet Res 49 (11), 1866-1868 PubMed.
  • Miller R I (1981) Treatment of equine phycomycosis by immunotherapy and surgery. Australian Vet J 81, 377-382 PubMed.