Swamp Cancer (Pythiosis) in Horses: Symptoms, Diagnosis & Treatment

Pythiosis in horses is an uncommon but serious skin infection that can cause extensive tissue damage. Affected horses develop skin sores that enlarge rapidly, drain persistently, and cause intense itching.

Sometimes called “swamp cancer,” pythiosis is associated with exposure to contaminated water, particularly in warm, wet environments. Pythiosis occurs most frequently in tropical and subtropical regions, but equine cases have occurred in other climates.

Recognizing pythiosis early is important because the infection can spread to deeper tissues, making treatment increasingly difficult. The signs of pythiosis resemble other skin conditions, so veterinary testing is required for an accurate diagnosis.

Pythiosis in Horses

Pythiosis is a serious infection caused by Pythium insidiosum, an aquatic organism classified as an oomycete, or water mold. Although oomycetes resemble fungi, they are more closely related to algae and have a different cellular structure. ^[1][2][3][4]

In horses, pythiosis primarily affects the skin and the tissues beneath it, causing severe inflammation and progressive tissue damage. Less commonly, the infection can involve deeper structures or the gastrointestinal tract. ^{[1][2][3][4][5][6][7]}

Pythiosis is sometimes called “swamp cancer” because affected tissue sometimes resembles an aggressive skin tumor. Despite this name, pythiosis is an infectious disease, not a form of cancer. ^[1][2][3]

Table 1. Pythiosis in horses at-a-glance

Symptoms of Pythiosis in Horses

The characteristic symptom of pythiosis in horses is rapidly enlarging skin sores, often accompanied by intense itching and persistent drainage. ^[1][2][5][8]

Other symptoms of cutaneous, or skin, pythiosis include: ^{[1][2][3][5][8]}

• Thick, sticky, or blood-tinged discharge
• Openings in the sores that continuously drain fluid
• Bleeding and damaged surrounding tissue
• Swelling around affected areas

The sores commonly develop on the lower legs, abdomen, and underside of the chest, although the face, neck, and other areas can also be affected. Early sores may be small but can rapidly develop into large, circular, ulcerated masses. ^{[1][2][5][9][8]}

One particularly characteristic feature is the presence of kunkers, which are firm, yellow-white or gray masses embedded within the affected tissue. These irregular, sometimes branching structures contain: ^[1][3][8]

• Dead tissue
• Inflammatory cells and cytokines
• The infecting organism

Sometimes kunkers are visible within the sores. They may also be present in fluid discharged from the wounds. ^[1][3][8]

Pythiosis can spread beneath visible sores, affecting tissue that appears normal at the skin’s surface. The infection can produce channels through the affected tissue, with inflammatory material and kunkers accumulating within them. As a result, the area requiring surgical removal may be larger than the visible sores suggest. ^{[1][5][10][11]}

In more advanced cases, horses may develop: ^[1][3][5]

• Reduced appetite
• Weight loss
• Enlarged nearby lymph nodes

Infection involving deeper tissues, including bones or joints, is associated with swelling and severe lameness. ^{[1][3][5][7][8]}

Rarely, pythiosis affects the gastrointestinal tract rather than the skin, with abdominal pain, or colic, among the reported symptoms. ^[1][6]

When to Call the Veterinarian

Prompt veterinary attention is required for rapidly enlarging or draining skin sores, especially following exposure to standing water.

Pythiosis is difficult to distinguish from other skin conditions, making veterinary testing important for an accurate diagnosis. Delays in diagnosis and treatment may allow pythiosis to progress and involve deeper tissues. ^{[1][8][10][11]}

While awaiting veterinary assessment, the following precautions may help protect affected tissue: ^{[1][3][8][11]}

• Keeping the horse away from standing water
• Preventing rubbing or further damage to sores
• Leaving kunkers in place for possible diagnostic testing
• Avoiding reliance on routine wound treatments without veterinary guidance

Pythiosis is a potentially destructive infection that requires prompt veterinary attention. Although treatment can be challenging, horses have a better chance of recovery when the disease is identified early and infected tissue can be successfully removed.

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Causes & Risk Factors for Pythiosis in Horses

Pythiosis infection develops when Pythium insidiosum infects a horse’s tissues. The organism produces microscopic, free-swimming spores called zoospores, which move through water and attach to suitable growth media, including damaged animal tissue. ^[1][3][10]

In horses, infection occurs primarily when zoospores contact damaged skin, although the exact circumstances leading to infection are not always known. Exposure to the organism does not necessarily result in disease, and pythiosis can develop in otherwise healthy horses. ^[1][3]

Environmental conditions associated with equine pythiosis include: ^{[1][9][10][12]}

• Exposure to contaminated water
• Standing fresh water
• Warm, wet conditions
• Seasonal flooding

Existing skin damage provides an entry point for the organism when horses encounter contaminated water. ^[1][3]

Pythiosis occurs most frequently in tropical and subtropical regions but has also been documented in temperate climates. Most cases are associated with warm, wet southern regions, although infections have been reported elsewhere. ^{[1][9][10][12][13]}

Although pythiosis is not always preventable, limiting horses’ access to standing water and flooded pastures may help reduce exposure to Pythium insidiosum in high-risk regions. ^[1][3][12]

Diagnosing Pythiosis

Several conditions are associated with skin sores that resemble pythiosis, including: ^{[1][2][8][12]}

• Summer sores
• Certain fungal infections
• Excessive healing tissue (proud flesh)
• Skin tumors

An accurate diagnosis helps veterinarians distinguish pythiosis from these conditions and determine the appropriate treatment. ^{[1][2][8][12]}

Veterinarians may suspect pythiosis based on: ^[1][10][12]

• The appearance of the sores
• A history of exposure to standing water in at-risk environments
• The presence of characteristic kunkers

However, appearance alone does not distinguish pythiosis from other conditions, making laboratory testing important for confirmation. ^{[1][8][10][12]}

Veterinarians may collect a biopsy, or small tissue sample, from the affected area for laboratory examination. When present, samples of kunkers from the wound sites also provide useful material for testing because they contain the organism for identification.

Several laboratory methods are available to assess tissue samples for pythiosis, including: ^[3][10]

• Histopathology: Examines thin sections of tissue under a microscope to assess inflammation and look for the organism’s thread-like structures, called hyphae. Special stains help make these structures visible
• Culture: Attempts to grow the organism from tissue or kunkers so the laboratory can identify it
• PCR testing: Detects genetic material from Pythium insidiosum in the sample
• Immunohistochemistry: Uses antibodies to help identify Pythium insidiosum within tissue sections

Microscopic examination can reveal changes consistent with pythiosis, but identifying the organism is difficult using only microscopy. Definitive diagnosis usually requires a combination of histopathology with more specific testing, such as PCR or immunohistochemistry. ^[3][4][10]

Blood antibody testing can provide additional evidence by detecting the horse’s immune response to Pythium insidiosum. Veterinarians interpret these results alongside the appearance of the sores and findings from tissue testing. ^[3][9][14]

Veterinarians may use X-rays to assess whether the infection has spread to nearby bone, particularly when a horse develops lameness or swelling around a persisting sore. The images can reveal changes in the bone that help veterinarians determine the extent of disease and plan treatment. ^[7][11]

Treating Pythiosis

Treatment for pythiosis generally involves surgery, immunotherapy, or both. The choice depends on the size and location of the affected area and how deeply the infection extends into surrounding tissues. ^{[1][5][11][12][15]}

Surgical Removal

Surgery is generally the preferred treatment when all infected tissue can be safely removed. This includes infected tissue beneath the visible sore. ^[1][2][11]

Surgery is more difficult when the infection covers a large area or is close to important structures, such as joints and tendons. If infected tissue remains after surgery, the disease may persist or return. ^[1][2][11]

Immunotherapy

Immunotherapy is a treatment designed to help the horse’s immune system recognize and fight Pythium insidiosum. It involves injections containing proteins from the organism that stimulate an immune response. ^[1][14][15]

Veterinarians may use immunotherapy alongside surgery or when complete surgical removal is difficult. Some horses respond successfully, but results vary, and immunotherapy does not reliably eliminate the infection in every case. ^{[1][12][14][15]}

Medications & Wound Care

Standard antifungal medications are often less effective against pythiosis than against true fungal infections. This is partly because Pythium insidiosum lacks ergosterol, a substance targeted by several common antifungal medications. ^[1][3][11]

Veterinarians may use antifungal medications alongside other treatments in selected cases, although results are inconsistent. ^[1][3][11]

Horses may require additional wound care to manage drainage and protect healing tissue. It’s important to seek veterinary guidance in cases of pythiosis, as routine wound treatment is not sufficient to eliminate the infection. ^[1][5][11]

Recovery & Prognosis

Horses with pythiosis have a better chance of recovery when the infection is detected early and all affected tissue is removed. The prognosis is less favorable when the infection is extensive or involves deeper structures, such as joints or bones. ^{[1][2][7][11]}

Some horses experience persistent infection or recurrence after treatment, particularly when infected tissue remains. Recovery may require continued wound care and veterinary monitoring. ^{[1][5][11][12]}

Care following treatment focuses on monitoring wound healing and identifying any symptoms of persistent or returning infection. Veterinary assessments may include checking whether the wound is becoming smaller and whether drainage, itching, or kunkers remain or reappear. ^[5][11]

Signs that indicate the need for further veterinary investigation include: ^[5][11]

• Persistent drainage
• The reappearance of kunkers
• New draining openings

Some horses require additional surgical treatment when infection recurs after the initial procedure. ^[5][11]

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Prevention

There is no guaranteed way to prevent pythiosis, but reducing exposure to environments where Pythium insidiosum is most likely to occur lowers the risk. Owners should keep horses away from standing or stagnant water, flooded pastures, and persistently wet areas whenever possible. ^{[1][3][9][10][12]}

Because infection occurs when zoospores contact damaged skin, wounds and other breaks in the skin warrant particular care when horses are exposed to wet environments. Monitoring a horse’s wounds for unusual swelling, persistent drainage, rapid enlargement, or excessive itching may help reduce exposure and allow suspicious lesions to be identified sooner. ^[1][3][12]

Prevention depends on recognizing suspicious lesions promptly and seeking veterinary assessment rather than assuming a persistent wound is a routine skin injury. Early diagnosis does not prevent infection itself, but it can help limit progression into deeper tissues and improve the opportunity for successful treatment. ^[1][12][13]

Frequently Asked Questions

Here are some frequently asked questions about pythiosis in horses:

Summary

Pythiosis is a potentially destructive infection caused by Pythium insidiosum, an aquatic organism found primarily in warm, wet environments. Prompt diagnosis and treatment are essential because the infection can spread beneath visible sores and involve deeper tissues.

• Equine pythiosis commonly causes rapidly enlarging, draining skin sores with intense itching and progressive tissue damage
• Kunkers are firm yellow-white or gray masses within affected tissue that can provide useful material for diagnostic testing
• Diagnosis may involve histopathology, culture, PCR, blood antibody testing, and X-rays
• Treatment generally involves surgical removal of all infected tissue, immunotherapy, or a combination of both
• Recovery is more likely when pythiosis is identified early and the infection has not spread to deeper structures