No Horsing Around with Cancer

Updated May 11, 2026 — 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. And thanks to newly launched research, scientists are closer than ever to better ways to detect and treat one of the most common, and most challenging, equine cancers.

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

Squamous Cell Carcinoma  
It's not often we get to talk about curing cancer. Still, this possibility came closer to fruition thanks to the fantastic work of Morris Animal Foundation-funded researcher Dr. Rebecca Bellone and her team at the University of California, Davis.

The group identified a genetic mutation for eye-related squamous cell carcinoma in Haflinger horses, a breed with a higher reported incidence of SCC. Findings helped inform the development of genetic screening tests for Haflinger and other breeds, including Belgian, Rocky Mountain Horse, Connemara Pony, Holsteiner and Belgian Warmblood horses.

"The risk factor we have identified for Haflinger horses is in a gene known to repair UV-caused damage to DNA," said Dr. Bellone. "We recommend using a UV-protecting fly mask on horses with two copies of the mutation. This is just one example of how understanding the genes involved can be used to reduce the risk of cancer."

In another ongoing study, researchers from Colorado State University are tackling SCC in horses from a different angle – investigating a novel treatment using a drug successfully used to treat a similar disease in people. In this small-scale study, the team hopes to show that this drug can also improve outcomes in affected horses and, if successful, will use findings to further study this novel SCC treatment.

Quick facts about SCC in horses:

• The most common type of tumor affecting the eye and eye structure of the horse.
• The second most common cancer found in horses.
• Usually locally invasive and slow to spread but can result in blindness
• Often related to sun exposure and more prevalent in regions where solar radiation (ultraviolet rays) is higher
• Providing shade for your animals can lower the risk
• Most often diagnosed in horses 12 years and older
• A high recurrence rate following treatment requires vigilant monitoring

Melanoma  
Melanoma is one of the most common types of skin tumors found on horses. This form of cancer is especially prevalent in gray horses, reaching an incidence rate of up to 80%. The tumors, which can be benign or malignant, often are advanced by the time they are diagnosed and can result in prolonged and significant discomfort for affected horses.

Unfortunately, veterinarians often instruct many horse owners to adopt a "wait and see" approach when it comes to cancer. One Morris Animal Foundation-funded researcher decided to challenge this approach.

Dr. Jeffrey Phillips, an Assistant Professor of Oncology and Medical Genetics at Lincoln Memorial University in Tennessee, set out to find practical ways to treat horses with melanoma. He had an interesting proposal – use a recently developed canine melanoma vaccine to treat horses with the disease. Phillips devised a treatment protocol for horses and received a grant from the Foundation for a clinical trial.

Most horses in the study (the initial study enrolled 15 horses) experienced dramatic tumor shrinkage, while the remaining patients had their tumor growth slowed. More research is needed in this area, but some veterinarians are using this tool as another way to tackle the disease.

The work doesn't stop there. A newly funded Morris Animal Foundation study at the University of Zurich is working on early detection and targeted therapy for equine melanoma, analyzing tumor tissue at the molecular level to identify new treatment targets, and developing a liquid biopsy that could catch the disease before it becomes advanced. Results are forthcoming.

Quick facts about melanoma in horses:

• 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 reportedly exhibit a higher prevalence of the disease.
• Typical tumor sites include the underside of the tail, genital areas and head.
• Canine melanoma vaccine shows promise as treatment for equine melanoma

Sarcoid Tumors  
Sarcoid tumors are the most common skin cancer diagnosed in horses and ponies. The good news is these tumors are nonmalignant (they don't spread to other parts of the body) and seldom fatal. However, they can grow quite large and spread and multiply locally, impacting the quality of life for affected animals.

Sarcoid tumors come in all shapes and sizes. They can look like circular hairless areas of skin, round lumps, warts or combinations. They can occur on any part of the horse's body but are more common near the genitals, head or underside of the abdomen. Tumors also can pop up at sites of previous injuries or scarring.

A recently completed Foundation-funded study is taking an unusual approach to treating sarcoids by using a modified virus usually found in chickens to attack the sarcoid cells. The team showed that the modified virus could infect the cancer cells and kill them. The team hopes to use their findings to develop a new immunotherapy.

Quick facts about sarcoids in horses:

• Most common tumor in horses
• Found anywhere on the body
• Often challenging to treat
• Seldom fatal, but can prevent riding if tumors develop in the 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

What Are Scientists Doing About Horse Cancer? 
Equine cancer research has advanced significantly in recent years, but important questions remain, especially around melanoma, where detection often comes too late and treatment options are limited. Here's a look at what researchers are working on right now.

Q: Why is melanoma so hard to detect early in horses?

A: Melanoma in horses — especially gray horses — typically develops slowly over many years. Because the tumors often start small and firm, owners and veterinarians may adopt a watchful waiting approach rather than intervening early. By the time the cancer is clearly progressing, it can already be advanced, making treatment significantly harder. There is currently no simple blood test to catch melanoma in its early stages, which is one reason the new University of Zurich liquid biopsy study is so significant.

Q: What is a liquid biopsy, and why does it matter for horses?

A: A liquid biopsy is a non-invasive test that checks the blood for signs of cancer — no tissue removal required. If researchers can develop a reliable liquid biopsy for equine melanoma, veterinarians would have a way to detect the disease earlier, before it becomes advanced, and to monitor whether a treatment is working over time. An aim of the Morris Animal Foundation-funded study led by Dr. Piotrowski at the University of Zurich is to develop exactly this kind of test.

Q: Surgery is the standard treatment for equine melanoma — why doesn't it always work?

A: Surgery can be effective, but melanoma recurs in roughly half of cases after surgical removal. Part of the reason is that it is difficult to distinguish melanoma tissue from healthy tissue at the margins, meaning cancer cells can be left behind. Developing a better molecular understanding of equine melanoma — another goal of the University of Zurich study — could point toward therapies that target cancer cells more precisely and reduce recurrence.

Q: What treatments are available for horse cancer today?

A: It depends on the cancer type. For squamous cell carcinoma, genetic screening tests allow owners of at-risk breeds to identify horses before disease develops. For melanoma, the canine vaccine (used off-label in horses) has shown some effectiveness in reducing or slowing tumors. Surgery remains the primary option, though recurrence is common. For sarcoid tumors, multiple treatment approaches exist, and novel immunotherapy options are in development. In all cases, early detection significantly improves outcomes, which is why research into better diagnostic tools remains a priority.

Q: Is Morris Animal Foundation currently funding horse cancer research?

A: Yes. The Foundation has funded equine cancer research for decades, and that work is active today. The newest grant, awarded to the University of Zurich in early 2026, is investigating both early detection and new treatment targets for equine melanoma. While results take time, the Foundation's commitment to finding better answers for horses with cancer is ongoing.

Science to Save Animals Starts with YOU!  
Cancer studies like these take vision as well as financial investment. That's why Morris Animal Foundation, with the generous support of our donors, continues to fund this critical work to change the odds for horses suffering from these devastating cancers. 

DONATE TODAY and fund lifesaving science for horses and all animals.

Additional Resources:  

Squamous Cell Carcinoma in Horses  

Taking on Equine Melanoma and Winning  

Watching and Waiting on Equine Melanoma? There's a Better Way  
Sarcoid Tumors in Horses  

Fresh Scoop Podcast Episode 4: Equine Eye Diseases and Genetic Testing  

Horse Health Studies