Pythiosis in Horses: Signs, Diagnosis and Treatment

By Dr Duncan Houston

A wound that will not heal is always worth taking seriously, but pythiosis is in a different category.

Pythiosis, often called swamp cancer, is an aggressive infection caused by Pythium insidiosum. It is not a true fungus, even though it looks fungus-like under the microscope. It is an oomycete, a water-loving organism that thrives in wet, warm, stagnant environments and can invade through small skin wounds. (Pavlab)

The danger is that pythiosis can look like other equine skin problems at first: a wound, proud flesh, summer sores, bacterial infection, fungal infection or a tumor-like mass. The difference is speed and severity. A lesion that grows quickly, drains blood-tinged fluid, contains firm yellow-gray “kunkers” and fails to respond to normal wound care should trigger urgent veterinary attention. (MSD Veterinary Manual)

Quick Answer

Pythiosis in horses is a serious, fast-moving infection caused by the water-associated organism Pythium insidiosum. It commonly causes large, itchy, ulcerated, draining skin lesions, often on the lower limbs or abdomen, and may contain firm coral-like material called kunkers. Early veterinary diagnosis and aggressive treatment give the best chance of success. Waiting, applying routine wound creams or assuming it is just proud flesh can allow the lesion to become much harder to treat. (MSD Veterinary Manual)

What Is Pythiosis?

Pythiosis is an infection caused by Pythium insidiosum, a fungus-like organism that belongs to the oomycetes rather than the true fungi. It produces motile spores that move through standing water and can invade animals through breaks in the skin. (Pavlab)

In horses, pythiosis most often causes cutaneous or subcutaneous lesions. These are usually ulcerated, granulomatous, draining wounds or masses. They may look like a tumor, proud flesh or a severe wound infection. The lower limbs are common sites because they are more likely to contact contaminated water, wet grass and muddy ground. The abdomen, chest, lips and genital region can also be affected. (MSD Veterinary Manual)

Pythiosis is often associated with tropical, subtropical and wet environments, but it is no longer something to think about only in one region. Pan American Veterinary Laboratories notes that cases have been reported beyond traditional tropical areas, including parts of the Midwest and Northeast United States. (Pavlab)

Is Pythiosis Contagious?

Pythiosis is generally acquired from the environment, not from another horse.

The organism is linked to standing water, wet soil and moist vegetation. Pan American Veterinary Laboratories and Today’s Veterinary Nurse both note that animal-to-animal and animal-to-human transmission has not been documented. (Pavlab)

That said, a yard with one case may have an environmental risk. If several horses share the same stagnant water, flooded pasture, marshy turnout or contaminated wet areas, more than one horse could be exposed.

What Does Pythiosis Look Like in Horses?

Pythiosis lesions are often dramatic once established.

Common signs include:

• A wound that grows quickly

• Ulcerated or tumor-like tissue

• Thick granulation tissue

• Draining tracts or fistulas

• Blood-tinged or mucous-like discharge

• Strong odor

• Severe itching

• Repeated self-trauma

• Swelling around the lesion

• Tissue loss

• Firm yellow-gray or coral-like material inside the wound, called kunkers

• Poor response to normal wound treatment

MSD Veterinary Manual describes equine lesions as large, roughly circular, granulomatous, ulcerated or fistulated nodules or swellings, often containing yellow-gray necrotic mineralized masses known as kunkers. It also notes that lesions are often extremely itchy and may discharge copious mucosanguineous fluid. (MSD Veterinary Manual)

That combination matters clinically: a fast-growing, itchy, draining wound with kunkers is pythiosis until proven otherwise in a horse from a wet-risk environment.

What Are Kunkers?

Kunkers are firm, gritty, yellow-gray or coral-like pieces of necrotic material found within many equine pythiosis lesions.

They are not ordinary scabs. They are intralesional masses made from necrotic debris, inflammatory material and viable Pythium elements. MSD Veterinary Manual describes kunkers as necrotic coral-like debris found in lesions caused by Pythium insidiosum. (MSD Veterinary Manual)

If an owner or vet finds kunkers in a rapidly growing wound, pythiosis becomes a major concern.

Why Pythiosis Is So Dangerous

Pythiosis is dangerous because it can expand quickly and become difficult or impossible to remove completely.

The organism can invade tissue deeply. Lesions may extend beyond what is visible at the surface, especially in chronic cases. MSD Veterinary Manual notes that prognosis is influenced by lesion size, site and duration of infection, and that small lesions of short duration respond best. It also states that timely recognition and treatment are essential. (MSD Veterinary Manual)

In practice, the cases that become heartbreaking are often the ones treated for weeks as ordinary wounds, proud flesh, summer sores or fungal infections before pythiosis is considered.

Where Do Horses Get Pythiosis Lesions?

Common locations include:

MSD Veterinary Manual lists the legs, abdomen, chest, lips and genitalia as common lesion sites, with the distribution linked to the aquatic nature of the organism. (MSD Veterinary Manual)

Severity Guide: How Worried Should You Be?

The key decision point is this: a wound that is getting bigger despite treatment should not be managed as a routine wound.

What Else Can Look Like Pythiosis?

Pythiosis can mimic several conditions.

Important rule-outs include:

Summer sores, or cutaneous habronemiasis
These can also cause ulcerated, itchy, granulomatous wounds, especially in warm seasons.

Proud flesh
Pythiosis may look like excessive granulation tissue, but it tends to be more aggressive, itchy, draining and destructive.

Bacterial wound infection
Infected wounds can drain and smell, but kunkers and rapid granulomatous growth raise concern for pythiosis.

Fungal infection
Pythiosis is fungus-like but not a true fungal infection, so routine antifungal assumptions can be misleading.

Sarcoids
Sarcoids can look warty, ulcerated or proud-flesh-like. They may also develop at wound sites.

Squamous cell carcinoma
A non-healing ulcerated mass can be cancer, especially around the eye, penis, sheath, vulva, anus or poorly pigmented skin.

Mucormycosis or entomophthoromycosis
MSD notes that oomycosis lesions can resemble other granulomatous fungal-like diseases and may be confused with habronemiasis, exuberant granulation tissue and equine tumors. (MSD Veterinary Manual)

The practical message is simple: do not diagnose pythiosis by appearance alone, but do not ignore the pattern.

When Is This an Emergency?

Call your vet urgently if your horse has:

• A wound that is rapidly enlarging

• A wound that fails to improve with normal care

• A lesion after exposure to stagnant water, flooded pasture or marshy ground

• Yellow-gray, gritty, coral-like kunkers

• Blood-tinged or mucous-like discharge

• Severe itching or self-trauma

• Strong odor from the wound

• Deep tissue loss

• Swelling around the lesion

• Lameness

• A lesion near a joint, tendon sheath, tendon or hoof

• Weight loss, dullness or reduced appetite

• A wound that has been treated as proud flesh or summer sores but keeps worsening

This is one of the skin conditions where waiting can dramatically change the outcome. Small, early lesions are much more treatable than large, chronic, deep lesions. (MSD Veterinary Manual)

How Vets Diagnose Pythiosis

A vet may suspect pythiosis from the history and lesion appearance, especially in a horse with wet-environment exposure and a fast-growing draining lesion.

Diagnosis may involve:

• Careful wound examination

• Collection of kunkers or tissue samples

• Histopathology

• Culture

• PCR testing

• ELISA blood testing for Pythium insidiosum antibodies

• Differentiation from related oomycete infections

• Assessment for deeper tissue involvement

MSD Veterinary Manual states that diagnosis is supported by culture and molecular methods, ELISA for Pythium insidiosum antibodies and histopathology combined with culture or PCR. Pan American Veterinary Laboratories offers ELISA diagnostics for pythiosis and related oomycete infections such as lagenidiosis and paralagenidiosis. (MSD Veterinary Manual)

Fresh kunkers can be especially valuable for culture. A PubMed-indexed study found that isolation rates were highest when fresh kunkers were cultured on selective media. (PubMed)

Why Early Diagnosis Matters So Much

Early diagnosis matters because treatment success depends heavily on size, location and duration.

MSD Veterinary Manual notes that the prognosis is poor if wide surgical excision cannot be done and that small lesions of short duration, without invasion of critical structures, respond best to treatment. (MSD Veterinary Manual)

In practice, that means a suspicious lesion should be sampled early rather than treated with repeated wound creams, antibiotics, antifungals or proud flesh products for weeks.

How Is Pythiosis Treated?

Treatment depends on lesion size, location, depth, duration and whether complete removal is possible.

1. Surgical removal

When possible, aggressive surgical excision is usually the most important treatment.

MSD Veterinary Manual describes complete surgical excision as the treatment of choice for pythiosis, with wide margins recommended when feasible. Pan American Veterinary Laboratories also describes surgical removal as the most common treatment in horses, while noting that recurrence can happen when infected tissue is not completely removed. (MSD Veterinary Manual)

The difficulty is that pythiosis often affects the lower limbs, where wide removal may be hard without damaging tendons, joints, ligaments or other vital structures.

2. Immunotherapy

Immunotherapy uses antigens from Pythium insidiosum to help redirect the immune response.

MSD Veterinary Manual notes that immunotherapy in horses may consist of intradermal or subcutaneous injections of killed, sonicated whole-cell hyphal antigens or precipitated soluble antigens. A PubMed-indexed review explains that immunotherapy has been used in horses and humans and is thought to work by shifting the immune response away from a Th2-dominant pattern toward a Th1 response. (MSD Veterinary Manual)

Pan American Veterinary Laboratories states that an immunotherapy product derived from P. insidiosum antigens has been used successfully to treat pythiosis in horses and people. (Pavlab)

The important limitation is that immunotherapy is generally more successful when used early, before the lesion is large, chronic or associated with severe tissue loss. (Pavlab)

3. Combination therapy

Many cases need more than one approach.

A practical treatment plan may include surgery plus immunotherapy, especially when the lesion is large, difficult to remove completely or likely to recur. PAVL notes that surgical treatment can recur if infected tissue is incompletely removed, and MSD notes that surgical excision, immunotherapy or a combination may be effective in horses. (Pavlab)

4. Medical therapy

Because Pythium insidiosum is an oomycete and not a true fungus, standard antifungal treatment is often less reliable than owners expect.

MSD Veterinary Manual lists itraconazole, terbinafine and a tapering dose of prednisone as a medical option for unresectable lesions, but it also emphasizes that the disease is poorly responsive to medical treatment and that wide surgical excision is recommended. PAVL notes that medical therapy for non-resectable pythiosis is usually unsuccessful, although some drugs have been described in cutaneous cases. (MSD Veterinary Manual)

So the clinically honest message is:

Do not rely on antifungal creams or routine wound medication as the main plan for suspected pythiosis.

5. Supportive wound care

Supportive care may include:

• Pain control

• Anti-inflammatory medication

• Wound cleaning under veterinary guidance

• Bandaging where appropriate

• Fly control

• Preventing self-trauma

• Treating secondary bacterial infection if present

• Monitoring anemia, weight loss or systemic compromise in severe cases

Supportive care matters, but it does not replace definitive control of the infected tissue.

What Should You Do Right Now?

1. Stop treating it like a routine wound

If the wound is expanding, draining, itchy or failing to improve, change the question.

Instead of asking, “What can I put on this?” ask, “What diagnosis are we missing?”

2. Photograph the lesion

Take clear photos every few days.

Include:

• A close-up

• A wider image showing location

• A ruler or object for scale

• Notes on discharge, odor, itchiness and growth

This helps your vet judge progression.

3. Do not dig out kunkers yourself

Kunkers are diagnostically useful, but the wound should not be aggressively picked or excavated at the yard.

Rough handling can worsen bleeding, pain and contamination. If kunkers are present, your vet can collect appropriate samples for testing.

4. Do not apply caustic proud flesh products

Caustic products can damage tissue and make later surgery or assessment harder.

If the lesion is pythiosis, burning the surface is not a cure.

5. Keep the horse away from wet-risk areas

Move the horse out of stagnant water, marshy pasture, flooded fields, wet grass and muddy low-lying turnout where practical.

Environmental exposure is central to the disease ecology of Pythium insidiosum. (Pavlab)

6. Call your vet promptly

Tell your vet if the wound:

• Is in a wet-exposure region

• Is growing quickly

• Has draining tracts

• Contains firm gritty material

• Is very itchy

• Has failed routine treatment

• Looks like proud flesh but is worsening

Those details matter.

Common Mistakes Owners Make

Waiting too long

This is the big one. Pythiosis is much easier to manage when caught early.

Assuming it is proud flesh

Pythiosis can look like exuberant granulation tissue, but routine proud flesh treatment does not address the organism.

Treating it as ordinary fungus

Pythium insidiosum is not a true fungus, so standard antifungal assumptions can fail. (MSD Veterinary Manual)

Ignoring the water exposure history

Recent turnout in stagnant water, marshy areas, flooded fields or wet vegetation is a major clue.

Using repeated home remedies

Every week spent applying random creams is another week the lesion can enlarge.

Not sending tissue or kunkers for testing

The diagnosis needs confirmation. Proper sample collection can save time and guide treatment.

Can Pythiosis Be Prevented?

You cannot eliminate all risk, but you can reduce exposure.

Helpful prevention steps include:

• Avoid turnout in stagnant water

• Fence off swampy, flooded or low-lying pasture areas where possible

• Limit access to wet, marshy vegetation

• Check horses daily after wet-weather turnout

• Treat small wounds promptly

• Keep lower-limb cuts clean and protected

• Monitor non-healing wounds closely

• Control flies around open wounds

• Photograph wounds that are not improving

• Call your vet early for lesions that grow instead of heal

There is no routine preventive vaccine for pythiosis. The immunotherapy discussed for pythiosis is therapeutic, not a standard preventive vaccine. Today’s Veterinary Nurse specifically describes the immunotherapy product as treatment rather than prevention. (Today's Veterinary Nurse)

Will My Horse Recover?

Recovery depends on how early the disease is identified and whether the lesion can be controlled completely.

The outlook is better when:

• The lesion is small

• The lesion is recent

• Critical structures are not involved

• Complete surgical removal is possible

• Immunotherapy is started early when appropriate

• The horse is not severely debilitated

• Treatment is not delayed by weeks of trial-and-error wound care

The outlook becomes more guarded when lesions are large, chronic, recurrent, deep, close to joints or tendons, or difficult to remove surgically. MSD Veterinary Manual notes that size, site and duration of infection influence prognosis, and that timely recognition and treatment are essential. (MSD Veterinary Manual)

FAQs

Is pythiosis a fungus?

No. Pythiosis is caused by Pythium insidiosum, an oomycete. It is fungus-like in some ways, but it is not a true fungus, which is one reason standard antifungal treatment may be unreliable. (Pavlab)

What are kunkers in horse wounds?

Kunkers are firm, yellow-gray, coral-like necrotic masses found inside many equine pythiosis lesions. Their presence in a fast-growing draining wound is a major clue that pythiosis should be investigated. (MSD Veterinary Manual)

Can pythiosis spread from horse to horse?

Documented horse-to-horse spread is not expected. Horses usually acquire infection from contaminated wet environments, standing water or moist vegetation through small wounds. (Pavlab)

Can pythiosis be cured?

Some horses can recover, especially when lesions are diagnosed early and treated aggressively with surgery, immunotherapy or a combination plan. Large, chronic or unresectable lesions are much harder to cure and may carry a guarded prognosis. (MSD Veterinary Manual)

When should I call a vet?

Call your vet promptly if a wound is rapidly growing, draining blood-tinged fluid, very itchy, contains firm gritty material, smells bad, follows exposure to stagnant water, or fails to respond to ordinary wound care within a few days.

Final Thoughts

Pythiosis is rare compared with everyday wounds, rain scald, scratches or proud flesh, but it is serious enough that owners in wet-risk areas should know the pattern.

The warning signs are a fast-growing, itchy, ulcerated, draining wound, especially on the lower limbs or abdomen, after exposure to stagnant water, flooded pasture, marshy ground or wet vegetation. Kunkers make the concern even stronger.

The safest approach is not to guess. Get the wound assessed, sample it properly and act early. With pythiosis, time is tissue.

If you are unsure whether your horse’s wound is pythiosis, proud flesh, summer sores, infection, a tumor or another skin disease, ASK A VET™ can help you work through the signs and decide how urgently your horse needs veterinary care.