Solid Tumors in Horses: Characteristics and Treatments

While tumors are a relatively common occurrence in humans, horses aren't as likely to develop these growths on their bodies. However of those that develop, skin and subcutaneous (under the skin) tumors in horses comprise 50-80% of all equine neoplasia (tumors). At the 2011 American College of Veterinary Internal Medicine Forum, held June 15-18 in Denver, Colo., Jeff Phillips, DVM, Dipl. ACVIM, assistant professor of oncology and genetics at the University of Tennessee College of Veterinary Medicine, discussed characteristics and treatment of equine sarcoids, squamous cell carcinomas (SCC), and melanomas in horses.


Phillips began discussing sarcoids and their different forms: nodular, fibroblastic (cauliflowerlike appearance), verrucose (flat), and sarcoma (loss of epithelium [outer layer] overlying the tumor). He noted that equine sarcoids are strongly associated with bovine papilloma virus, and the amount of viral DNA found in each tumor correlates with disease severity. While its mode of transmission is unknown, this virus can be detected in flies and in the environment, and researchers believe it could be transmitted via fomites (grooming tools, tack, etc.) rather than direct horse-to-horse contact. The most common sites of sarcoid formation are areas that have experienced some trauma in the past, such as the girth area.

"Sarcoids are not thought to be heritable," noted Phillips.

Sarcoids are best treated early and aggressively. Up to 80-90% of tumors are currently controlled by surgery or cryotherapy (freezing the sarcoid off). Phillips explained that veterinarians might treat more advanced lesions using intralesional chemotherapy with cisplatin oil suspension or beads. Very thin lesions might also respond well to topical 5-fluorouracil (5-FU, a chemotherapeutic cream) application.

Other immunotherapeutic options he noted include topical imiquimod (an immune-boosting drug), BCG (Bacillus Calmette-Guerin) injections, and autogenous treatment achieved by grinding a piece of tumor and injecting it along the crest of the horse's neck. Various sarcoid vaccines are currently in development but not yet commercially available, he noted.

Squamous Cell Carcinomas

Phillips continued the discussion with SCC, a skin disorder commonly induced by sun exposure, particularly in nonpigmented tissue (e.g., around the eyes or rectum) of at-risk breeds such as Paint horses, Appaloosas, and Draft breeds. He also mentioned a form found on male horses' penises that might be associated with a papilloma virus.

Typically, SCC develops in areas that have previously experienced trauma or chronic inflammation. Penile SCC tends to occur in older geldings but with a low (10-15%) metastatic rate (ability to spread to other areas of the body) into regional lymph nodes; less than 5% of affected horses develop metastases in areas not immediately surrounding the SCC. Prognosis depends on the lesion's physical appearance and grade of severity.

There are numerous treatment options for SCC:

  • Surgical excision;
  • Cryotherapy
  • Radiation therapy (although this is sometimes cost prohibitive);
  • Chemotherapy with 5-FU for early lesions;
  • Mitomycin C eye drops (for SCC around the eye); and
  • Cisplatin (Phillips noted this treatment option isn't as effective against SCC as it is for sarcoids).

Phillips uses piroxicam (a type of non-steroidal anti-inflammatory drug) as an adjuvant treatment particularly in high risk horses with multiple lesions, with which he sees 23% resolution.


Melanoma is the third main skin cancer of horses, typically developing as nodular lesions. Phillips discussed specific gene mutations--specifically the gray coat color genes--that might predispose a horse to developing melanoma. A familial history of melanoma also is associated with disease risk, meaning melanoma development could have a hereditary or genetic aspect. There is no preventive treatment for at-risk horses.

Phillips remarked, "Melanoma treatment rarely achieves a cure, but the objective is to control tumor growth in a way that a horse can live with it." He added that veterinarians often find it difficult to treat large or aggressive melanoma tumors effectively.

A promising novel therapy for melanoma employs immune system-based treatment using the canine melanoma vaccine (xenogenic plasmid DNA) that targets and attempts to eliminate tyrosinase, an enzyme critical in the production of melanin (pigment).

Phillips also described another novel treatment option that could be used in the future to manage various solid tumors in horses: thermochemotherapy, which uses heat to work synergistically with other chemotherapies, in particular injection with carboplatinin (a compound similar to cisplatin). Phillips noted that when applied, this treatment method specifically targets the tumor tissue and does no damage to the surrounding healthy tissue. So far, Phillips has seen an encouraging 81% favorable response rate when treating solid tumors with thermochemotherapy.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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