Cancer is much less prevalent in horses than humans, dogs, cats, and other species. It’s uncommon, but not impossible, for a horse to have cancer that affects their internal organs. Horses are much more likely to develop cancer of the skin or tissue close to the surface of their body. By getting to know the most common cancers in horses, you may be able to recognize a potentially cancerous growth while it’s still at an early stage of development.

Melanoma
Around 80% of grey or pale-colored horses will develop melanomas. It’s usually seen in horses over 7-8 years old, but it can happen to younger horses. Most melanomas start off benign and become malignant, but you cannot tell by looking at one whether it’s dangerous or not. Some become malignant quickly, others can take years, and some never metastasize.
Melanomas can appear alone or in clusters. They start as a hard lump under the skin and eventually appear black or grey. They often appear around the eyes, genitals, or mouth, but can also affect the guttural pouch, lymph nodes, heart, lungs, and other internal organs.
While melanomas are generally distinct in appearance, they can be mistaken with other skin conditions. It’s important to have your veterinarian examine any potential melanoma and confirm the diagnosis with a biopsy. If the melanoma is found to be benign, your vet may decide to monitor it. It it does require treatment, the most effective options are removal, freezing, and chemotherapy. There is also a vaccine called ONCEPT, developed for dogs, that can help the horse’s immune system attack the cancer cells.

Squamous Cell Carcinoma
Squamous cell carcinoma (SCC) is the second most common type of cancer that affects horses. It’s most often seen on or around the eye, though it can also affect the mouth or genitals.
SCC starts out as a small lesion or sore that may be weepy or crusty. When it affects the eyes, it can cause excessive eye discharge in the early stages. As it advances, it takes on a clustered, cauliflower-like appearance. It does not tend to metastasize throughout the body, but it does harm nearby tissues, like the eye itself, and the lymph nodes under the jaw. It can be surgically removed or treated with chemotherapy. SCC has been linked to sun exposure. When it affects the genitals, it’s thought to be related to poor hygiene.

Lymphosarcoma
Lymphoma or lymphosarcoma is the most common blood cancer in horses, though it’s rare overall, affecting less than 0.5% of all horses globally. It does not favor any particular breed but seems to affect young adult horses between 4 and 10 years old.
In other animals, lymphoma causes noticeable swelling of the lymph nodes, but this is often not the case with horses, making it more difficult to diagnose. Cutaneous lymphoma causes visible lumps just under the skin, but this form is uncommon. Other forms of lymphoma affect the internal organs, causing a wide array of symptoms from anemia, difficulty breathing, coughing, weight loss, lethargy, diarrhea, decreased appetite, and abdominal pain.
Lymphoma may be more common in horses than previously believed, as the symptoms are nonspecific. A horse that seems to have colic may actually be suffering from stomach pain and pressure from masses on their internal organs.
Treatments like surgery and chemotherapy can treat lymphoma, but for the most part, prognosis is poor.

Preventing Common Cancers In Horses
Naturally, horses spend their days outside in the sun, and all of that UV exposure puts them at greater risk for skin cancers, especially for horses with light-colored hair and skin. For at-risk horses, you can limit their sun exposure in the middle of the day, when the sun’s rays are strongest. You can protect delicate tissue around the eyes with a UV-blocking fly mask.
Always take lumps, bumps and lesions seriously. Talk to your veterinarian if you notice any unexplained symptoms in your horse. It’s unlikely that your horse will develop cancer, but if they do, it’s important it’s diagnosed as soon as possible. Caught early, many cancers in horses are treatable and even curable.