Skin cancer in horses
Skin cancer, or neoplasia, is the most common type of cancer diagnosed in horses, accounting for 45 to 80% of all cancers diagnosed. Sarcoids are the most common type of skin neoplasm and are the most common type of cancer overall in horses. Squamous-cell carcinoma is the second-most prevalent skin cancer, followed by melanoma. Squamous-cell carcinoma and melanoma usually occur in horses greater than 9-years-old, while sarcoids commonly affect horses 3 to 6 years old. Surgical biopsy is the method of choice for diagnosis of most equine skin cancers, but is contraindicated for cases of sarcoids. Prognosis and treatment effectiveness varies based on type of cancer, degree of local tissue destruction, evidence of spread to other organs and location of the tumor. Not all cancers metastasize and some can be cured or mitigated by surgical removal of the cancerous tissue or through use of chemotherapeutic drugs.
Sarcoids
Sarcoids account for 39.9% of all equine cancers and are the most common cancer diagnosed in horses. There is no breed predilection for developing sarcoids and they can occur at any age, with horses three to six years old being the most common age group and males being slightly more prone to developing the disease. Sarcoids are also more prevalent in certain familial lines, suggesting that there may be a heritable component. Several studies have found an association between the presence of Bovine papillomavirus-1 and 2 and associated viral growth proteins in skin cells with sarcoid formation, but the exact mechanism that controls or induces epidermal proliferation remains unknown. However, high viral loads within cells are strongly correlated with more severe clinical signs and aggressive lesions.Clinical signs
The appearance and number of sarcoids can vary, with some horses having single or multiple lesions, usually on the head, legs, ventrum and genitalia or around a wound. The distribution pattern suggests that flies are an important factor in the formation of sarcoids. Sarcoids may resemble warts, small nodules, oval hairless or scaly plaques or very rarely, large ulcerated masses. The occult form usually presents on skin around the mouth, eyes or neck, while nodular and verrucous sarcoids are common on the groin, penile sheath or face. Fibroblastic sarcoids have a predilection for the legs, groin, eyelid and sites of previous injury. Multiple forms may also be present on an individual horse. Histologically, sarcoids are composed of fibroblasts that invade and proliferate within the dermis and sometimes the subcutaneous tissue but do not readily metastasize to other organs. Surgical biopsy can definitively diagnose sarcoids, but there is a significant risk of making sarcoids worse. Therefore, diagnosis based solely on clinical signs, fine-needle aspiration or complete excisional biopsy are safer choices.Treatment
While sarcoids may spontaneously regress regardless of treatment in some instances, course and duration of disease is highly unpredictable and should be considered on a case-by-case basis taking into account cost of the treatment and severity of clinical signs. Surgical removal alone is not effective, with recurrence occurring in 50 to 64% of cases, but removal is often done in conjunction with other treatments. Topical treatment with products containing bloodroot extract for 7 to 10 days has been reported to be effective in removing small sarcoids, but the salve's caustic nature may cause pain and the sarcoid must be in an area where a bandage can be applied. Freezing sarcoids with liquid nitrogen is another affordable method, but may result in scarring or depigmentation. Topical application of the anti-metabolite 5-fluorouracil has also obtained favorable results, but it usually takes 30 to 90 days of repeated application before any effect can be realized. Injection of small sarcoids with the chemotherapeutic agent cisplatin and the immunomodulator BCG have also achieved some success. In one trial, BCG was 69% effective in treating nodular and small fibroblastic sarcoids around the eye when repeatedly injected into the lesion and injection with cisplatin was 33% effective overall. However, BCG treatment carries a risk of allergic reaction in some horses and cisplatin has a tendency to leak out of sarcoids during repeated dosing. External beam radiation can also be used on small sarcoids, but is often impractical. Cisplatin electrochemotherapy, when used with or without prior surgery to remove the sarcoid, had a non-recurrence rate after four years of 97.9% in one retrospective study. There is a chance of sarcoid recurrence for all modalities even after apparently successful treatment. While sarcoids are not fatal, large aggressive tumors that destroy surrounding tissue can cause discomfort and loss of function and be resistant to treatment, making euthanasia justifiable in some instances. Sarcoids may be the most common skin-related reason for euthanasia.Squamous-cell carcinoma
Squamous-cell carcinoma is the most common cancer of the eye, periorbital area and penis, and it is the second most common cancer overall in horses, accounting for 12 to 20% of all cancers diagnosed. While SCC has been reported in horses aged 1 to 29-years, most cases occur in 8 to 15-year-old horses, making it the most common neoplasm reported in older horses. Carcinomas are tumors derived from epithelial cells and SCC results from transformation and proliferation of squames, epidermal skin cells that become keratinized. Squamous-cell carcinomas are often solitary, slow-growing tumors that cause extensive local tissue destruction. They can metastasize to other organs, with reported rates as high as 18.6%, primarily to the lymph nodes and lung.Clinical signs and predisposing factors
Tumors related to squamous-cell carcinoma can appear anywhere on the body, but they are most often located in non-pigmented skin near mucocutaneous junctions such as on the eyelids, around the nostrils, lips, vulva, prepuce, penis or anus. The tumors are raised, fleshy, often ulcerated or infected and may have an irregular surface. Rarely, primary SCC develops in the esophagus, stomach, nasal passages and sinuses, the hard palate, gums, guttural pouches and lung. The eyelid is the most common site, accounting for 40-50% of cases, followed by male and female genitalia. Horses with lightly pigmented skin, such as those with a gray hair coat or white faces, are especially prone to developing SCC, and some breeds, such as Clydesdales, may have a genetic predisposition. Exposure of light-colored skin to UV light has often been cited as a predisposing factor, but lesions can occur in dark skin and in areas that are not usually exposed to sunlight, such as around the anus. Buildup of smegma on the penis is also linked to SCC and is thought to be a carcinogen through penile irritation. Pony geldings and work horses are more prone to developing SCC on the penis, due to less frequent penile washing when compared to stallions. Equine papillomavirus-2 has also been found within penile SCCs, but has not been determined to cause SCC.Treatment and prevention of SCC
Before treatment of squamous-cell carcinoma is initiated, evidence of metastasis must be determined either by palpation and aspiration of lymph nodes around the mass or, in smaller horses, radiographs of the thorax. Small tumors found early in the disease process can be treated with cisplatin or radiation with favorable results. For more advanced cases, surgical removal of eye, mass or penile amputation can be curative provided all cancerous cells are removed and there is no metastasis. However, young horses that have a hard or "wooden" texture to SCCs on the glans penis have a very poor prognosis for treatment and recovery.Regular washing of the penis and prepuce in males as well as cleaning the clitoral fossa in mares is recommended to remove smegma buildup, which also gives the opportunity for inspection for suspicious growths on the penis or on the vulva.