Cancer in Horses

Updated December 15, 2023 | By: Jamie Whittenburg, DVM

Understanding the Big C

Like humans, horses can be affected by cancer. Cancer is a disease state in which cells in the body that make up tissues and organs grow uncontrollably. This uncontrolled growth typically results in tumors and the cells may also spread to distant sites in the body.

Cancers may result from genetics, exposure to certain substances, and age. Cancer affects all animals, though there is more current research available for cancers affecting dogs and cats.

Cancer Terms and Definitions

• Tumor – An enlargement of a part of the body that is caused by an uncontrolled or abnormal growth of cells. These masses may be benign or malignant.
• Benign - Benign tumors are masses of cells that are proliferating abnormally but are not cancerous. These tumors do not spread to other areas of the body.
• Malignant - Malignant tumors are also masses of cells that are proliferating abnormally, but these tumors are cancerous and can often spread to other sites and organs in the body.
• Metastasis – The process by which cancer cells spread to other sites and organs in the body, distant from the original tumor.

Common Types of Cancer and Treatment Options

According to the Morris Animal Foundation, “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.”

The necessary treatment for equine cancer will be dependent upon the type, location, stage, and health of the horse.

Squamous Cell Carcinoma (SCC)

Squamous cells are the flat skin cells that make up the outer layer of skin. SCC is an abnormal overgrowth of some of these cells. The most common areas for SCC to occur in horses are the eyes and penis, though they may occur anywhere there are squamous cells present. SCC often affects the third eyelid of horses which can lead to serious visual impairment.

Treatment of these tumors is largely dependent on their location. For some tumors, radiation is an excellent choice as SCC is typically very sensitive to radiation. However, due to the location of the tumor or lack of nearby radiation oncology services, other treatments such as surgical removal, cryotherapy, and intralesional injections may be utilized.

Melanoma

Melanomas are very common in grey horses. In fact, by the age of 15, approximately 80% of greys will have a melanoma. Though they may be found in many areas on the horse’s body, melanomas are especially common under the tail and near the sheath or genitals.

Often, if the melanoma is small and not causing issues for the horse, they will not require any treatment. Owners should keep a vigilant eye on the area and call their veterinarian if the mass is rapidly growing or impeding the horse’s function. In these cases, surgical removal is likely the first line of therapy, and could be followed by chemotherapy.

Sarcoids

Sarcoids are the most commonly diagnosed equine tumor and account for 36% of skin tumors and 20% of all tumors in horses. It is theorized that these tumors may be caused by a papilloma virus, and there is no known sex or age predisposition. These tumors may occur as single or multiple masses and typically first appear like small, wart-like lesions. Left untreated, sarcoids can become large and ulcerated, causing pain, and hindering the horse’s performance.

Depending on the size and location of the tumor, equine sarcoids are typically treated with surgical removal, cryotherapy, laser, or chemotherapy. Small sarcoids, if caught early, may respond to topical treatment prescribed by a veterinarian that the owner can carefully apply. In the future, there may be a possibility of a vaccine-type treatment that could enable the body to destroy the tumor itself.

Ask the Vet Video on Sarcoids in Horses

Lymphoma

Lymphoma, though not a skin cancer, is also somewhat common in horses. Lymphoma (medically referred to as lymphosarcoma) is a malignant cancer of the lymphocytes, which are a type of white blood cell involved in the immune response. Lymphoma in horses is often found in the stomach and gastrointestinal tract but commonly spreads to other areas of the body, including the lymph nodes. There is no age, sex, or breed disposition for equine lymphoma.

Lymphoma in horses is a serious disease and typically carries a guarded to poor prognosis, even with treatment. Treatment attempts consist of chemotherapy and corticosteroids.

Signs of Cancer in Horses

There are some signs that horse owners should watch for to help them to catch any potential cancers early. These include:

• New masses or lumps and bumps on the skin
• Lack of appetite
• Weight loss
• Swollen lymph nodes
• Decreased activity or being lethargic

If any of the above signs are seen, owners should contact their veterinarian as soon as possible.

Potential Causes of Cancer

Currently, it is unknown exactly what causes cancer in horses, though researchers and veterinarians theorize that both genetics and environment are involved. Prolonged sun exposure, especially to areas of skin that are lightly pigmented, plays a role in the development of squamous cell carcinoma, especially of the eye.

Diagnosing Cancer in Horses

If your horse is exhibiting the signs mentioned above, it is time to call your veterinarian. The first step of the vet’s diagnostic process will be a complete physical examination and a thorough history. Depending on the horse’s age, history, signs, and physical exam findings, blood work, internal exams, x-rays, ultrasound, or biopsies may be required.

There is currently research underway to be able to identify cancer biomarkers in the blood of horses. This would make diagnosis of cancers such as lymphoma much easier and more accurate and may drastically improve the prognosis for the affected horses.

Prognosis for Horses with Cancer

Skin tumors, such as sarcoids, are typically the easiest to treat, whereas tumors in the eye or internal organs may pose a bigger challenge and have a poorer prognosis.

In general, although horses can have cancer, it is not one of the leading causes of death for them. Many cancers, if caught early and treated appropriately, can be successfully managed, allowing a normal life for the horse.