Common Equine Skin Cancers
- Verrucose — well-defined or large, ill-defined areas with a gray, scabby, or warty appearance that may contain small, solid nodules, and possible surface ulcerations;
- Nodular — multiple, discrete, solid nodules of variable size that may ulcerate and bleed;
- Fibroblastic — fleshy masses with either a thin pedicle (stalk) or wide, flat base that commonly bleeds easily and has a wet, bloody surface;
- Mixed — variable mixtures of two or more types;
- Malevolent — an extremely rare but rapidly growing tumor that spreads extensively through the skin with infiltrating extension to underlying tissue.
Squamous Cell Carcinoma
Squamous cell carcinoma is the most common malignant skin tumour in horses, typically affecting nonpigmented, poorly-haired areas near mucous membranes such as around the eyes, lips, nose, anus, and external genitalia, especially the penis sheath. Usually appearing as raised, irregular masses, this is an invasive, often slow-growing cancer that can later spread internally. Squamous cell carcinoma is most commonly diagnosed in horses 12 years of age and older. A genetic risk factor for ocular squamous cell carcinoma has been identified in Haflinger, Belgian, and potentially Rocky Mountain Horse breeds. The penile form is most often found in older geldings; hence, regular sheath inspection is important to identify and treat lesions.
Risk of squamous cell carcinoma can be reduced by simply providing shade as this cancer is frequently related to sun exposure, especially in horses with white or partially white hair coats.
Treatment may include surgery, chemotherapy, or radiation therapy, or a combination of therapies. Survival is dependent upon size and malignancy of the tumour before treatment. Recurrence rate of squamous cell carcinoma is high, and these horses should be monitored vigilantly after treatment.
