Nothing to horse around with

By Kathy Smith

Be on the lookout for the three most common skin cancers afflicting horses and contact your veterinarian promptly if you find suspicious lumps or bumps.

Skin cancer is the most prevalent type of equine cancer, accounting for up to 80 percent of all cancers reported.

Sarcoids are the most frequently diagnosed type, with squamous-cell carcinoma the next most prevalent, followed by melanoma. 

Learning about these cancers and what to watch for will allow you to closely monitor your horse and seek veterinary advice promptly if you notice something that looks abnormal. While skin cancer poses a health risk for our beloved horses, many cancers are treatable, and early diagnosis will improve the horse’s quality of life and chance of survival. 

Sarcoid Tumours

The most common equine cancer, sarcoid tumours account for approximately 40 percent of all equine cancers. They can occur in any breed and at any age, typically developing in horses three to six years old. Sarcoids can be found anywhere on the body, often on the head, near the genitals, and on the underside of the abdomen. They may also form at sites of previous injury or scarring. Although non-malignant, these tumours can spread aggressively, occurring as single or multiple lesions ranging from small and wart-like to large, ulcerated, fibrous growths that can impact quality of life. Bovine papillomavirus is believed to be involved in the development of sarcoid tumours, although its precise role is not known.

Sarcoids are the most frequently recognized equine tumour. Photo: Photos.com/Melissa Jones

The Merck Veterinary Manual describes the six recognized clinical varieties:

• Occult — flat, gray, hairless, and persistent, often somewhat circular;
• 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.

Treatment options vary depending on the size, location, and type of sarcoid. New lesions should be treated promptly, as larger sarcoids are more difficult to treat. Treatment options may include surgery, chemotherapy (often topical), radiotherapy, laser surgery, or a combination of therapies. 

Due to the risk of recurrence, continue to monitor the horse diligently after treatment. Once a horse has developed a sarcoid it will be prone to developing more in the future.

A sarcoid tumour on the underside of the abdomen of a wild horse. Photo: Shutterstock/Asmodiel

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.

Related: Understanding Equine Vision and Eye Disease

Squamous cell carcinoma is the most common cancer of the equine eye and the second most common tumour in horses. Photo courtesy University of California, Davis

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.

Smooth, raised plaque on upper eyelid of a Paint horse. The horse developed a carcinoma secondary to sunburn. Photo: Wikimedia/Caroldermoid

Melanoma

A tumour of dark-pigmented skin, most melanomas in horses are found in those with gray coats where the hair turns gray or white with age. It is estimated that approximately 80 percent of grey horses over 15 years of age will have melanomas, which typically appear as rounded black nodules of varying size under the dock of the tail, in genital areas, and on the head around the mouth and eyelids. Melanomas are more common in Lipizzaners, Arabians, and Percherons, and in non-gray horses these tumours can be more dangerous. 

Related: A Genetic Connection to Equine Sarcoid Skin Tumours

Above: Small rounded melanoma nodules at the lip and corner of the mouth. Photo: Wikimedia/Caroldermoid

Perianal gray horse melanoma of a 23-year-old Andalusian mix mare. Photo: Shutterstock/Juerginho

Melanomas may begin small and slow-growing, but over time can increase in number and size. Most are benign but as time goes on they can become malignant and metastasize to other sites, resulting in systemic complications. 

These tumours are removed through surgery, with laser treatment or cryotherapy (freezing). Removal is much easier when melanomas are small. The outcome is excellent if tumours are benign. A canine melanoma vaccine is showing promise as treatment for equine melanoma.

Related: The Healthy Senior Horse

To read more by Kathy Smith on this site, click here.

Main Photo: Equine sarcoid. Credit: AdobeStock/Chelle129