Cancers in Horses Back »

Written by Haley Anderson under the direction and review of Sara Mastellar.


When you hear the word cancer, most people automatically think of a friend or family member that has either battled cancer or passed away from it. Many animals such as dogs, cats, and even horses can also develop cancers that can affect them severely and even cause death. In this article, we will discuss three of the more common forms of equine cancer.

Cancer appears in horses when normal moderators of cell growth fail and cells proliferate in disorganized and unchecked ways. These masses of cells, or tumors, can disrupt normal body functioning. Cancer that is located on the head or face of a horse can affect eyesight, hearing, as well as the ability to eat and breathe properly. The cause of any cancer is difficult to determine, but some horses are more predisposed to cancer than others. For example, melanoma is more likely to affect gray horses.

One way to improve a horse’s prognosis is to identify the cancer early and begin treatment. Surprisingly, many human cancer treatments work well for horses. Some cancers require the removal of the cancer with surgery or lasers. Some tumors are small and, if they are not causing problems, will be left alone. In skin cancers, creams may be applied in some cases to keep the cells in check. The use of drugs and vaccinations that target cancerous cells have had good responses (Loving, 2011).

Melanoma


Melanoma tumor on a gray horse. The placement on the neck is unusual. Photo by Sara Mastellar.

Melanoma, a skin cancer, develops in the melanin-producing cells. Melanin cells contain dark brown to black pigment. They are found in the hair, skin, and iris of the eye in people and animals. Melanomas usually appear as round, black nodules that are often found around the eyes, neck, vulva or rectum, base of the ears, and under the tail. The nodules that are found on the horse are usually smooth and are not painful. When the tissue from the nodules is observed under a microscope dark, black granules are visible. The exact cause of melanoma is not known, but may be related to a number of mutations linked to coat color. Gray horses are the most susceptible. Melanoma affects eighty percent of gray horses over the age of fifteen years old (Burks, 2012). Darker colored horses can develop melanomas, as well as horses that are only four or five years old. A tumor biopsy should be examined by a specialist that has experience with melanoma.

Accurately characterizing the malignancy of a melanoma is important because actively malignant melanoma has a high chance of killing a horse in a relatively short time due to faster cell division and spread. However, not all melanomas are malignant. There are some equine surgeons that do not like operating on horses that have melanoma because they believe that performing traditional surgery will activate the cells and increase the chance of the tumor growing or metastasizing. A metastasized tumor is when an existing tumor is broken apart and spreads cancerous cells throughout the body to seed new tumors. Instead, they recommend performing laser surgery where it is necessary, to remove large masses that are in the way of tack or interfere with the well-being of the horse. A tissue-based vaccine made from horse tumor cells has been showing good responses (Tannler, 2013). Some veterinarians have also recommended using radiation and other cancer drugs and treatments to help treat melanoma (Loving, 2011).

Lymphoma


Cutaneous lymphoma. Photo courtesy of Taintor & Schleis (2011).

Lymphoma is the most common internal cancer in equines. Lymphoma is a disease of the lymphoid system where some of the lymphoid cells become tumorous and grow uncontrollably. They are usually grape-sized and grow into a large mass. Unfortunately, there is no way to positively identify the disease before problems begin to occur (Bain, 2002). Some of the signs of lymphoma are fever, weight loss, masses, depression, and diarrhea (Posnikoff, 2013). Lymphoma usually occurs in horses that are four to ten years old.

Sarcoids

Sarcoids are skin tumors that make up 35-90% of all equine tumors. Sarcoids are tumors made up of connective tissue and are the most common external tumor. They appear more frequently on geldings and all equid species are susceptible, even donkeys and zebras. Usually, they are not life threatening because they tend to stay relatively small. They can affect a horse’s welfare or quality of life depending on the size and location of the tumor.

(Above) Fibroblastic sarcoid. Photo courtesy of John Ismay.

The tumors usually are located on the head and lower legs. Forty different sarcoid treatments have been described, but few have produced consistent results (University of Edinburgh, 2016). The injection of intratumoral drugs, such as cisplatin, may help the tumor decrease in size and has been known to help treat horses with sarcoid cancer. More information on sarcoid tumors in horses can be found in the iGrow article Equine Tumors – Sarcoids.

Take-Home Message

Cancer can appear in horses and can, sadly, be deadly if not caught and treated right away. If you suspect your horse has cancer, have your horse evaluated by a local veterinarian right away. Always check your horses for any changes in their attitude and on their bodies. Cancer can come in many different forms and some forms can spread quickly through the body.


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