Swamp Cancer in Horses

with Dr Paula Williams

BSc (Hons) BVSc MRCVS MANZCVS of WestVETS Animal Hospital

With the ongoing wet weather and humidity in many parts of Australia, there has been an increase in the number of cases of swamp cancer attended by equine veterinarians. Here, we are going to take a look at what swamp cancer is, the presenting signs and how it is treated.

What is Swamp Cancer?

Despite its misleading name, swamp cancer is not actually a cancer or tumour but a term used to describe granulomatous growths on horses caused by an aquatic pathogen similar to a fungus. It has many names: Pythiosis, Bursattii, Oomycosis, Phycomycosis and Leeches. The name swamp cancer has likely developed as a result of the fast-growing lesions that occur when horses are in swampy areas. Pythiosis is one of the more common terms used in Australia.

What causes it?

The organism causing swamp cancer is a pathogen in the class Oomycetes, which is often described as a fungus, but it is not a true fungus; it is more like algae. The most common organism causing it in Australia is PYTHIUM INSIDIOSUM. There is another similar organism called Basidiobolus which is seen in Australia, but less commonly, these lesions tend to be drier and slower growing.

Pythium is reported to cause lesions in horses, dogs and cats, cases have also been reported in people.

Where does it occur?

Pythium typically occurs in wetland conditions in contaminated still fresh water such as dams, lakes, swamps, bogs, muddy and flooded areas. Areas are often known for the occurrence, but heavy rains and flood can spread the organism to new areas.

Pythiosis tends to occur in tropical and subtropical areas of the world. It was first described by British vets in India in 1884. It has been reported in SE Asia, South America, the Gulf Coast of America and the east coast of Australia, particularly in Queensland and northern New South Wales.

How does infection occur?

Horses that are standing in contaminated water are prone to lesions. Often, there is a small irritation, insect bite, or wound that allows the organism to enter via an entry point. The organism is well adapted to the body temperature of the host. Zoospores are motile spores of the organism that use a specially developed flagella (tail) to move; they then encyst on the surface of tissues. They secrete a sticky substance that allows them to stick; they then develop hyphae that extend into the host tissues and infiltrate blood vessels. This unique behaviour allows infection to develop rapidly.

Where do lesions commonly present?

The distribution of lesions is attributable to where body parts are exposed to the organism in water –
• Legs (especially the lower limbs)
• Abdomen
• Chest
• Muzzle and lips
• Genitalia

Lesions are most common in the skin, but on rare occasions, they can invade bone and intestine.

What do the lesions look like?

The lesions are classic in their appearance:
• Often circular
• Granulomatous (like proud flesh)
• Ulcerated
• Fistulated
• Nodular
• Blood/pus discharge
• Itchy or irritating – horses often traumatise them, resulting in secondary infection.
• Foetid smell
• Grey or yellow necrotic cores - KUNKERS
• Rapid growing.

KUNKERS
• Specific to equine pythiosis
• Firm yellow–grey lumps
• Coral-like appearance
• Necrotic tissue clumps
• Variation in size from a grain of rice to a few cm.

How is it diagnosed?

Swamp cancer is diagnosed by the following:
• Location and history of access to possible contaminated water
• Clinical appearance with kunkers
• Biopsy
- Histopathology
- Presence of pythium
• PCR and antibody tests are available elsewhere

Are there other conditions that look similar?

The lesions that occur with swamp cancer can look like other conditions, so on occasion, a biopsy is necessary to differentiate to ensure that the therapy is appropriate. These conditions are:
• Granulation tissue / proud flesh
• Bone infection with non-healing wound – osteomyelitis/sequestrum
• Sarcoid
• Squamous cell carcinoma
• Habronema
• Botryomycosis

What is the treatment?

Treatment for pythium can be difficult, and there can be a frustratingly poor response to therapy. Topical treatments (creams, ointments and sprays) on their own often have a very poor effect. In order to successfully treat pythium, a MULTIMODAL APPROACH has the best result using multiple treatment options. These include:

Surgery
• Often, radical surgical excision is required
• Usually performed under a general anaesthetic
• Electrocautery and surgical lasers are often very helpful
• Repeat excision is often required

Antimycotic agents
• Although pythium isn’t a true fungus, there are reports of success with some of the antifungal agents in combination with the other modalities. Examples of these agents are:
• Systemic antimycotic agents
- Ketoconazole
- Sodium iodide
- Potassium iodide

Topical agents
Many different ones are used with veterinarians having their own recipes and favourite combinations that seem to work when used with surgical excision

Known as PHYCOBUSTERS or PHYCOFIXERS

Various combinations including the following agents:
• Ketoconazole, fluconazole, itraconazole, voriconazole
• Amphotericin B
• Rifampicin
• Metronidazole
• DMSO
• Turmeric

Immunotherapy
Pythium vaccines have been used for many years with good success. The vaccines are formulated from antigens secreted by pythium in cultures. The course is usually four doses a week apart. Unfortunately, whereas this has been available before, there is limited availability currently in Australia.

Metalaxyl
There has been recent interest in using Metalaxyl for the treatment of swamp cancer. It is a fungicide used in plants and is being used off-label in animals. A small animal veterinarian in Sydney has had extensive experience of its use in cases of pythiosis in dogs and in the last year several equine veterinarians have used it with success. It has been administered both topically and orally but MUST only be used under the guidance of a veterinarian.

Should I be worried that I can get pythiosis?

Cases with a high mortality rate have been reported in people across the world. Humans acquiring infection from animals has not been reported. It is however advisable that people handling cases wear appropriate personal protective clothing and cover wounds.

Prevention

If your horse is kept in an area where swamp cancer has been reported, then it is recommended to reduce their exposure to possible contaminated wet areas. If a horse has an abrasion or wounds, it is preferable that they don’t go swimming or stand in water.

Summary

In the recent wet and humid conditions, there has been an increase in the number of cases of swamp cancer/pythiosis. This is a non-transmissible disease that is seen in the tropical and subtropical regions. It is a tropical fungal-like organism that is found in freshwater, such as dams, lakes, flooded areas and swamps. The organism infects the horse by entering the skin via wounds or abrasions. The lesions are often quite classic in appearance, with fast-growing granulomatous masses with kunkers. Treatment requires a multimodal approach with surgery and the use of systemic and topical agents. If you are concerned that your horse has swamp cancer, then seek prompt attention from your veterinarian.

Paula qualified in the UK in 1993 and has been an equine veterinarian since then. She completed an internship at Rossdale and Partners in Newmarket, UK and has subsequently worked in equine hospitals both in the UK and Australia. Paula is currently an equine practitioner at WestVETS Animal Hospitals in Queensland. Her clinical interests include diagnostic imaging, the investigation and management of musculoskeletal issues in the equine athlete, the equine foot, neonatology and internal medicine.