Skin cancer in horses

[4] Prognosis and treatment effectiveness varies based on type of cancer, degree of local tissue destruction, evidence of spread to other organs (metastasis) and location of the tumor.

[9] 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 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.

[6] Histologically, sarcoids are composed of fibroblasts (collagen producing cells) that invade and proliferate within the dermis and sometimes the subcutaneous tissue but do not readily metastasize to other organs.

[14] However, BCG treatment carries a risk of allergic reaction in some horses[10] and cisplatin has a tendency to leak out of sarcoids during repeated dosing.

[5] 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.

[5] Rarely, primary SCC develops in the esophagus, stomach (non-glandular portion), nasal passages and sinuses, the hard palate, gums, guttural pouches and lung.

[20] Horses with lightly pigmented skin, such as those with a gray hair coat or white faces, are especially prone to developing SCC,[19] and some breeds, such as Clydesdales, may have a genetic predisposition.

Pony geldings and work horses are more prone to developing SCC on the penis, due to less frequent penile washing when compared to stallions.

[22] Before treatment of squamous-cell carcinoma (SCC) 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.

[23] However, young horses (usually geldings less than 8-years-old) that have a hard or "wooden" texture to SCCs on the glans penis have a very poor prognosis for treatment and recovery.

[28] Common sites for metastasis include lymph nodes, the liver, spleen, lung, skeletal muscle, blood vessels and parotid salivary gland.

[24] Tumors will initially begin as single, small raised areas that may multiply or coalesce into multi-lobed masses (a process called melanomatosis) over time.

However, gene therapy injections utilizing interleukin-12 and 18-encoding DNA plasmids have shown promise in slowing the progression of tumors in patients with metastatic melanoma.