Squamous Cell Carcinoma in Horses

What is Squamous Cell Carcinoma?

Squamous Cell Carcinoma (SCC) is the most common malignant skin cancer in horses. It is usually seen in adult or older horses, but some cases have been reported in horses less than a year of age. SCC is typically found on lightly pigmented areas, so gray, palomino, and paint-colored horses are most common candidates. The most affected locations include the eyes, genital areas, and mouth.

Types of Squamous Cell Carcinoma

Different types of SCC are often categorized by the area involved, including:

• Ocular (involving the eye): This is the most common SCC. Horses with white or light pigmentation around the eye are most susceptible. It typically involves the outside corneal tissue, conjunctiva, third eyelid, or the skin directly next to the eye. These tumors may grow slowly, or rapidly, but typically affect only one eye at a time.

• Cutaneous (skin): One of the most common locations of cutaneous locations for SCC is the eyelid. Other cutaneous locations include the ear, which often stem from aural plaques, and the nose.

• Genital or mucosal: Ulcerative SCC is common around the vulva, under the tail, the perineal area, and on the sheath and penis. Sunlight exposure, skin pigmentation, and smegma (oily secretion in folds of skin) are common predisposing factors for these carcinomas.

While less common, internal SCC can occur; the most common locations for these carcinomas include the gastrointestinal system (primarily oral or involving the stomach), nasal cavities, guttural pouches, and bladder. These are often associated with a poor prognosis, as diagnosis (if made) is usually late in the disease process. Chances of metastasis (cancer spreading) are higher, and treatment can only be aimed at end-of-life care.

Symptoms of Squamous Cell Carcinoma

SCC is typically locally invasive and slow to metastasize but can get into local lymphoid tissue and spread throughout the body.

SCC may be proliferative or ulcerative. Proliferative lesions are more common around or on the eye; these lesions grow upward and outward, are typically red/white in color, and sometimes appear like cauliflower lesions. Ulcerative SCC appears like red irritated skin with possible discharge. Common symptoms of SCC in horses include:

• Ocular lesions can become uncomfortable on the eye, so horses may squint, tear excessively, or be jumpy/more reactive on that side if their vision becomes impaired.

• Raised red bumps

• Ulceration of affected tissue, with occasional mild discharge or odor

• Sensitivity to touch. Depending on location, this can make a horse head shy, resistant to wearing tack, and discomfort or abnormal posture when urinating.

• Gastric SCC may present very vaguely with clinical signs such as weight loss and decreased appetite.

Causes of Squamous Cell Carcinoma in Horses

While the exact cause of SCC is unknown, there are some factors that increase the likelihood of developing these lesions, including:

• Light or white skin pigmentation

• Prolonged UV exposure over time

• Chronic skin irritation such as aural plaques, local bacterial or viral infections like warts, or chronic abrasions/wounds from tack or other factors. The exact process of how these develop into cancerous lesions is still not fully understood.

• Genetic involvement for SCC is currently unknown, although draft horses, Haflingers, Appaloosas, and Paint horses are commonly affected breeds. There is ongoing research to determine genetic predisposition for SCC in these breeds.

• Excess smegma (a combination of shed skin cells, skin oils, and moisture) in the sheath is also a predisposing factor for genital SCC.

How Veterinarians Diagnose Squamous Cell Carcinoma

Squamous cell carcinoma can only be definitively diagnosed by histopathology, which involves evaluating a piece of skin tissue at the microscopic level. A presumptive diagnosis is often made based on a horse’s history and the clinical presentation of skin lesions; your veterinarian may then take a small sample of the lesion to send off for confirmation. Your veterinarian will also examine local lymph nodes; if they feel large or abnormal, a fine needle aspirate (FNA) will be performed to collect a sample to check for possible spreading of the cancer.

Suspect stomach SCC may be first noted during a gastroscopy, in which a small camera is passed through the nose, down the esophagus, and into the stomach. Gastric SCC often presents with vague clinical symptoms, or similarly to stomach ulcers. If abnormal tissue is noted, a biopsy can be taken for definitive diagnosis of the cause.

Treatment of Squamous Cell Carcinoma

There are various treatment options your veterinarian may discuss with you. Depending on the location and involvement of the carcinoma, the prognosis for complete recovery may be poor. If you notice any abnormal lesions on your horse’s skin, have them evaluated by your veterinarian immediately, as early intervention often leads to more favorable results.

Potential therapies for treatment of SCC include:

• Surgical removal involves cutting out as much affected tissue as possible. This may be used for small lesions early on, or in some larger masses if the horse is a suitable candidate. If other treatment methods are unsuccessful and the carcinoma is invading local tissue at a fast rate or the horse becomes uncomfortable, enucleation (removal of the affected eye) is often curative for ocular cases. Surgical intervention without additional treatment methods often leads to repeat cancerous lesions.

• Cryotherapy involves using liquid nitrogen to freeze the lesions without disturbing surrounding healthy tissue. This is typically done in a few freeze/thaw cycle sessions over several weeks' time. The spots typically scab over after they’re frozen, then eventually fall off. This method has the highest success with early SCC or after surgical removal.

• Hyperthermia or CO2 laser methods may be recommended for small areas or for lesions previously made smaller through surgery.

• Mitomycin-C is an antimicrobial that has some anti-tumor activity. This may be used in conjunction immediately following CO2 laser sessions.

• Chemotherapy may be used topically or injected into a mass.

  • Cisplatin is typically injected into lesions as a solution or using biodegradable beads spaced over larger tumors.

  • Five-fluorouracil (5-FU) is available as a topical cream, commonly used in small ocular or genital lesions, or as an injectable.

• Radiation is not a commonly used approach anymore, due to limited success, availability, and safety for involved personnel, but may be recommended for specific cases.

• Oral piroxicam (an anti-inflammatory medication) is sometimes used for ocular SCC or in combination with other treatments as it may inhibit tumor cell growth.

• Photodynamic therapy is a newer approach to SCC that is still being studied. Surgical debulking (making the tumors smaller) is performed, if possible, then a photosensitive agent is injected into the lesion. A small laser, with specified wavelength for that agent, is passed over the area. This is aimed at the rapidly reproducing tumor cells and is safe for local healthy tissue not containing the injected agent.

Recovery and Management of Squamous Cell Carcinoma in Horses

Depending on the location of the tumor, involvement of surrounding tissues, size, and treatment method, therapy may or may not be successful. Internal and metastatic (cancer that has spread) SCC both carry poor prognosis.

Genital and cutaneous carcinomas tend to recur once treated. For ocular SCC, if the bony orbit is not involved and other therapies haven’t been successful, eye removal is often curative with no long-term complications. Because this cancer often recurs, it can be difficult to treat completely, although research is ongoing to improve the odds of curative therapies.

Prevention of SCC in Horses

Prevention is important for light colored horses. UV fly masks are beneficial for those with light skin around the eyes, and partial stall boarding may appropriate to help prevent prolonged exposure of mucosal, genital, and cutaneous regions.

For stallions and geldings, regular sheath cleanings will prevent irritating smegma build-up.

If your horse develops aural plaques, early veterinary evaluation is recommended to attempt treatment and prevent evolution to SCC.