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Updated June 23, 2022 – While cancer is not as common in horses as in dogs, cats and people, it still poses a health risk for our equine companions. Many cancers affecting horses are treatable, so monitoring your horse for cancer and seeking prompt veterinary care for any suspicious lumps or bumps can lead to many more healthy years together.

Approximately 80% of reported cancers in horses are associated with the skin or the tissue layer beneath the skin. The three most commonly reported cancers in horses are squamous cell carcinoma, melanoma and sarcoid tumors.

“Knowing a little about these cancers and what to look for can make all the difference in the long-term prognosis for your horse,” said Dr. Janet Patterson-Kane, Morris Animal Foundation’s Chief Scientific Officer and co-author of Clinical Equine Oncology. “As with any species, the earlier the cancer is caught, the better the chance at improving your animal’s survival odds and quality of life.”

Squamous Cell Carcinoma

  • Most common type of tumor affecting the eye and eye structure of the horse
  • Usually locally invasive and slow to spread
  • Often related to sun exposure and more prevalent in regions where solar radiation (ultraviolet rays) is higher
  • Risk can be lowered simply by providing shade for your animals
  • Most often diagnosed in horses 12 years and older
  • High recurrence rate following treatment requires vigilant monitoring

“There is a fine line between benign and malignant tumors,” said Dr. Patterson-Kane. “Consult your veterinarian if you notice any swelling or redness. If caught early, squamous cell carcinoma can be a treatable cancer with surgical excision and topical therapy.”


  • Appears as growth in or under the skin
  • Affects about 80% of older gray horses
  • In non-gray horses, melanoma tends to be more dangerous
  • Arabians and Lipizzaner horses seem to have a higher disease prevalence
  • Typical tumor sites include the underside of tail, genital areas and head
  • Canine melanoma vaccine shows promise as treatment for equine melanoma

“Although melanomas might begin as small, slow-growing tumors, over variable periods of time these tumors can become malignant and metastasize to distant sites,” said Dr. Patterson-Kane. “The spread of tumor cells to other places in the body can result in systemic complications and decreased longevity. The best defense is, if you see something, get rid of it – when this can be done without complications. These tumors are much easier to remove when small.”

Sarcoid Tumors

  • Most common tumor in horses
  • Found anywhere on the body
  • Often challenging to treat
  • Seldom fatal, but can prevent riding if tumors develop in saddle or bridle area
  • Several sarcoid tumor subtypes with some horses having a mixed combination
  • A possible link between the development of tumors and infection with bovine papillomavirus

“These tumors come in all shapes and sizes,” says Dr. Patterson-Kane. “They can look like circular hairless areas of skin, or round lumps or warts. One more aggressive form, the fibroblastic sarcoid, can have a stalk or be flatter and more obviously invasive; these masses are often ulcerated. Alert your veterinarian if you suspect sarcoids of any type or if a previously treated horse develops sarcoids again. This is especially of concern if the tumors begin showing up on other parts of the body. Any new lesions should be treated promptly.”

How Morris Animal Foundation Is Helping
Morris Animal Foundation has been funding cancer research in horses for almost two decades, including squamous cell carcinoma, sarcoid tumors and melanoma. With time and funding, we are making strides in finding ways to tackle these top equine cancers.

Our funded researchers developed a screening test for ocular squamous cell carcinoma in Haflinger horses, a high-risk breed for the disease. This team now is looking to develop tests for related breeds.

In another squamous cell carcinoma study, researchers focused on cancer-causing genes associated with the disease. Researchers hope their findings will inform the development of novel therapeutic targets or a way to improve early cancer detection.

For melanoma, we now know a commercially available canine melanoma vaccine also can help treat this cancer in horses. Horses in the study showed dramatic shrinkage of their tumors.

In sarcoid research, investigators found evidence supporting a genetic component to sarcoid development. In another study, researchers are exploring the feasibility of a novel oncolytic therapy, using a genetically modified virus that infects and breaks down cancer cells, but not normal cells.

Together, these studies will inform the next generation of prevention measures and treatments to give affected horses more healthy years, possibly free of recurrent lesions.

“Always let your veterinarian know of any unusual lumps or bumps on your horse. What you find may save or greatly extend the life of a horse with cancer,” says Dr. Patterson-Kane. “Many cancers that were rapidly fatal even a decade or two ago are treatable today, thanks to ongoing research.”

How You Can Help

Please consider making a gift today. The more research we can fund, the faster we will be able to find effective treatments, preventives and maybe even cures for the cancers that affect the animals we love.


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