The Other Cancers

Is there an uglier word in medicine than "cancer?" This is true in human and veterinary medicine. Many of us view cancer as invasive, debilitating, and fatal. But those who know horses know that while cancer occurs in equids, most of these cancers are sarcoids and melanomas--cancers that are generally benign, but should receive attention from a veterinarian to ensure they aren't a danger to the horse.

Still, there are other forms of cancers that can strike horses. Although uncommon, sometimes even rare, these cancers range from the local and treatable to the aggressive and deadly. They include lymphoma, squamous cell carcinoma, granulosa cell tumor, and cancers of the pituitary gland, thyroid, gastrointestinal (GI) tract, and bone.

With our aging horse population, we seem to be seeing more cancers. And while chances are you'll never see one of these cancers in your horse, knowing about the disease, prognosis, and treatment options can help you identify cancer early and make informed decisions should it strike.


This is a malignant tumor of the lymphoid tissue, which can include the lymph nodes, spleen, and bone marrow. Very rarely, horses have involvement of the lymphocytes (a type of white blood cell partly responsible for controlling immune response) in circulation; when present, this is referred to as lymphoid leukemia.

Although rare, affecting less than 1% of the horse population, lymphoma is the most common metastasizing tumor (one that spreads to other parts of the body) that affects the horse. It is seen primarily in older horses and occasionally in younger horses, and it can occur in the thorax (the chest cavity), gastrointestinal tract, skin, and other organ systems. As it spreads, it can cause severe disease elsewhere.

Clinical signs of lymphoma are sometimes vague and include weight loss, poor body condition, poor performance, and sometimes fever or diarrhea.

Diagnosis of lymphoma is usually via cytology (microscopic examination of cells) and histopathology (microscopic examination of diseased tissue). However, Elizabeth Davis, DVM, Dipl. ACVIM, assistant professor at Kansas State University, also has been using a flow cytometer to investigate the potential for a more rapid diagnosis (within a few hours rather than a few days).

"The flow cytometer measures what the DNA content is of that cell population," she explained. In comparing cells from healthy horses to horses with infectious diseases and to horses with cancer, the cell populations from cancerous horses have an abnormal concentration of DNA. "It is not the concentration that we would expect for normal replicating or dividing cells; they are very proliferative. There's a lot of active division due to the neoplastic (abnormally growing) cells overwhelming the normal population of cells."

Davis hopes that flow cytometry will correlate the characteristics of the tumor and the response to treatment, particularly if early diagnosis is made.

"In human medicine, findings from the flow cytometer can be an indicator of how well the patient responds to treatment," says Davis. But a study in dogs did not yield a good correlation, nor will horses with lymphoma likely be easy study candidates. "Often, the prognosis is very poor and the horses don't have much time left. If they are in very poor condition, we put them down," she says.

"At this point, the information that we gain from the cell cycle analysis is supportive to the diagnosis of lymphoma," she adds. "It is important to realize that this information is used in conjunction with our more conventional diagnostic methods, such as histopathology."

Few treatment options exist for most forms of equine lymphoma; treatment might be initiated in a breeding animal such as a late-term broodmare or a stallion during the breeding season in order to save the horse for a few months. This therapy will, in some cases, allow the veterinarian to prolong life through the pregnancy or breeding season.

A diagnosis of lymphoma that involves only the skin (cutaneous lymphoma) carries the best prognosis for long-term survival, and some cases might respond to systemic corticosteroid therapy. Davis says, "Treatment for cutaneous lymphoma will potentially prolong the life of the horse and therefore improve the quality of life. Some horses with this form of disease will even go into remission. This is in contrast to the other forms of lymphoma that horses get (i.e., thoracic, gastrointestinal, or multicentric)."

Squamous Cell Carcinoma

This is a malignant skin tumor of the outermost layer of the skin. These are somewhat common tumors found primarily in the genital area, light-colored skin, eyelids, and occasionally the mucous membranes.

Davis says that squamous cell carcinoma is usually locally very aggressive, but is late or slower to metastasize. She noted that light-skinned horses and horses with white skin around their eyes are more disposed to this type of cancer.

Clinical signs can vary depending on the location of the tumor, although the mass is usually visible. Although the diagnosis might be suspected on visual evidence, definitive diagnosis is made following biopsy and histopathology.

Traditionally, treatment consists of removing or debulking the tumor unless the lesion is deeply invasive or in a complicated area. Other medical therapies include chemotherapeutic drugs, cryosurgery (destruction of unwanted tissue by freezing), anti-inflammatory therapy, radiation therapy, or penile amputation if the penis is affected.

Radiation therapy was recently used at Colorado State University (CSU) to treat an aggressive, inoperable squamous cell carcinoma located in the jaw of an older horse. CSU assistant professor of surgery Dean A. Hendrickson, DVM, Dipl. ACVS, says radiation therapy in horses is very rare because of limitations in accommodating large animals. Consequently, CSU veterinarians did not know how the horse would respond to this kind of treatment.

"We don't have a lot of experience in that arena," Hendrickson says, "and we were pushing the envelope. The carcinoma was growing quickly and was pretty invasive, so the prognosis with not doing anything was poor." The radiation treatment was intended to slow down cancer progression and extend quality and quantity of life.

The horse received one round of radiation treatment. Afterward, the mass began shrinking and the horse resumed eating. No follow-up treatment was scheduled other than placing the horse on pelleted rations and moist hay (to make chewing easier) and (phenylbutazone) Bute for pain relief. Regrettably, although the horse was progressing, he colicked; rather than put the animal through colic surgery, the owners had the horse euthanized.

Nevertheless, this experience indicated that radiation therapy might be a viable option for other horses.

"This is an area we need to keep open and to offer as an alternative," states Hendrickson. "Some of the common skin cancers could well be treated with radiation therapy and perhaps give us a greater success rate. The challenge is there aren't very many radiation therapy units available in veterinary medicine, particularly for large animals."

Prognosis for squamous cell carcinoma varies widely; aggressiveness of the tumor, size, and location are influential factors in success of the treatment.

Granulosa-Theca Cell Tumor

This tumor occurs in the ovaries of mares. Usually, only one ovary is affected and the tumor is benign, although malignant and metastasizing tumors have been reported. The affected ovary becomes very enlarged, interferes with hormone production, and causes infertility, disruption of the estrous cycle, and atrophy of the other ovary.

Clinical signs of granulosa-theca cell tumor include failure to conceive and behavior changes. Says Davis, "The mare may become very aggressive to riders or herd mates, act bossy, behave like a stallion, and develop a thick, cresty neck."

Diagnosis is based on rectal palpation, ultrasound, a hormone panel to measure hormone levels, and histopathology following treatment, which involves surgical removal of the affected ovary.

"Prognosis," Hendrickson says, "is very good if we can remove the ovary before it becomes very large or before it ruptures and disseminates cancer cells throughout the abdomen." After the affected ovary is removed, the remaining ovary usually returns to normal size and regains fertility, and hormone levels return to normal. Recovery of these functions takes anywhere from two to six months.

Gastrointestinal Tumors

Most tumors of the equine stomach are carcinomas (malignant new growth that arises from the epithelium found in skin or the lining of body organs); lymphoma is sometimes involved with the stomach and is usually involved in intestinal tract tumors. The most common presenting signs are abdominal pain, diarrhea, malabsorption of nutrients, weight loss, inappetence, fever, and/or anemia.

Preliminary diagnosis of a GI tumor is via endoscopy, microscopic examination of abdominal fluid that might reveal tumor cells, and palpation of larger masses. The definitive diagnosis of cancer is via surgery.

"Surgery is where we diagnose the tumor as malignant," Hendrickson explains. "We suspect that we'll find an obstruction, but we don't know what that obstruction will be."

Treatment involves removal or reduction of the tumor. Prognosis depends on how quickly diagnosis is made and how advanced the tumor is, Hendrickson says. "Most of the time surgery is curative because they are isolated masses. But in some cases, they will disseminate throughout the abdomen, and those are very, very difficult to treat as are cases where much intestine is diseased."

Tumors of the Pituitary Gland

The pituitary gland is located at the base of the brain. Tumors here are usually benign and associated with hyperpituitarism or Cushing's disease, a common disease in aged horses. The tumor causes hyperactivity of the adrenal cortex, resulting in the overproduction of several adrenal hormones.

"(Affected) horses present with varied clinical signs, including poor body condition that may include weight loss, laminitis (founder), redistribution of fat (fat pads), increased water intake and urine output, and a thickened hair coat that doesn't shed out," Davis says. Diagnosis is via history, clinical signs, and in some cases hormone measurements and hormone suppression tests.

There are no surgical or medical options for removing or treating a tumor in this location, but medical therapy can reverse some or all of the clinical signs, depending upon how advanced the disease is when it is diagnosed. Current drugs of choice, Davis notes, are dopamine agonists and serotonin antagonists, which can help normalize hormone levels. Good routine health management is very important for warding off secondary problems associated with Cushing's disease.

Although the tumor will continue to grow, prognosis for extending the quality and quantity of life for many years with treatment and good management is generally favorable for horses with mild signs, particularly if laminitis is not too severe.

Tumors of the Thyroid Gland

These are mostly benign, are common in the older horse, and often found only incidentally. "Benign thyroid tumors generally don't cause the horse any problems or produce any clinical signs, so no treatment is necessary," says Davis. "For malignant tumors, removal of the thyroid gland is recommended."

Prognosis for a horse with a malignant thyroid gland depends on whether the cancer has already spread to other sites, in which case treatment could be difficult.


Malignant bone cancer occurs rarely (although benign bone tumors of the skull, ribs, cartilage, long bones, or flat bones are occasionally seen). Osteosarcoma--which is rarely seen in horses--presents with a painful, hard, enlarged mass. Radiographs can also pick up the abnormality. "Most of the time, bone cancer is pretty distinctive in what it looks like radiographically," says Hendrickson. "Once we see that and are suspicious, we do a bone biopsy to try to determine definitively what's going on."

There is no treatment for osteosarcoma, and prognosis is poor.

Minimize Risk

Rapidly progressive cancer in horses is rare. But because it does occur occasionally, it pays to be vigilant to changes that occur in your horse. Check periodically for bumps or swellings in areas susceptible to squamous cell carcinoma--under the tail and around the vulva and penile areas. Don't ignore strange behavioral changes in mares or attribute unthriftiness, appetite changes, or vague personality or physical changes to "old age" in the senior horse.

Warns Hendrickson, "We still don't know what causes the vast majorities of cancer, so the best we can do is to diagnose them early and treat these tumors while they are small, occupying very small spaces, and are more easily removed."


Briggs, K. Sarcoids and Melanomas. The Horse, June 1999, 41-52.

Cable, C. Ovarian Tumors. The Horse, March 2001, 55-62.

See the Cancer category under Ailments/ Syndromes at


By far, the most frequent tumors seen in horses are sarcoids and melanomas. Sarcoids are generally benign skin cancers found most often on the head, lower abdomen, or limbs. The lesions might remain small, static, or disappear, or they could suddenly proliferate. They often appear in clusters. Sarcoids are strictly skin tumors that are usually locally confined to that area. Diagnosis is based on visual exam combined with histopathology to establish the definitive diagnosis, as sarcoids are easily recognizable by a veterinarian.

Although the sarcoid might not cause any problems for the horse, there is the possibility the tumor could become locally invasive and spread to other sites, so precautionary treatment is often recommended. Says Elizabeth Davis, DVM, Dipl. ACVIM, assistant professor at Kansas State University, "If the tumor is large, we'll surgically remove it and apply a local treatment--an immunomodulator or a cytotoxic drug (cell-killing)." Other treatments include cryosurgery (freezing) or laser surgery.

Melanomas are skin tumors occurring most commonly in gray horses; more than 80% of gray horses over the age of 15 will develop at least one melanoma during their lifetimes. "They are black in color and occur under the tail and sometimes in the head or throat latch area," Davis says. "They are often benign, but occasionally can be malignant and life-threatening." Melanomas can be diagnosed on inspection, with definitive diagnosis provided via testing of cells obtained with fine needle aspiration.

Because a melanoma can appear small on the outside but be relatively internally invasive, Davis prefers to remove them if possible or to have the client carefully monitor the tumor for growth. "Melanomas are potentially going to get bigger and spread," Davis explains. "Therefore, if it is possible to surgically remove the tumor, this is sometimes the treatment of choice."

A form of medical therapy involves the administration of cimetidine. The use of this drug in treating equine melanoma is expensive, controversial, and not always effective.

Davis says, however, that, "this drug, which is a histamine-type 2 blocker, is well recognized as a treatment for gastric ulcers. Interestingly, this drug has also been proposed to allow restoration of normal tumor surveillance mechanisms (within the horse's body) to reduce or control the spread of melanoma. Therefore, we commonly prescribe this therapy for older gray horses suffering from the development of multiple melanomas, or those that are rapidly progressive."

Additionally, the veterinarian might treat the site with anti-cancer drugs or cryosurgery. Barring complications, the prognosis for treatment of sarcoids and melanomas is favorable in some cases.--Marcia King

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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