Equine Cancer

Written by L.A. Pomeroy (Holistic Horse)

Breast, ovarian, prostate and testicular cancers are common in humans. Do they occur in horses, too?

October is breast cancer awareness month and, according to the Department of Health and Human Services’ Centers for Disease Control (CDC), it is the most commonly diagnosed cancer among US women. Behind skin cancer, it’s also the most fatal. The CDC also notes that, other than skin cancer, prostate cancer is the second leading cause of cancer deaths among US men and seventh in leading causes of death nationally, just behind the mortality rate (fifth) for ovarian cancer. The Cancer Treatment Centers of America say testicular cancer is not only the most common cancer of young men ages 15 to 35, but has more than doubled in incidence worldwide over the last 40 years.

What does any of this have to do with equine health? Horses, unlike humans, do not have advocates like CBS’ Katie Couric, actress Christina Applegate, or athletes Scott Hamilton and Lance Armstrong urging greater awareness of these very public diseases.

Horses do get breast, ovarian, prostate, and testicular cancer, and like these celebrities, star status does not assure immunity. On March 30, 2005, McQuay Stables in Tioga, Texas, put down possibly the greatest reining stallion ever, NRHA $4 Million Sire, Hollywood Dun It, when the beautiful, 22-year-old, buckskin American Quarter Horse lost his battle against testicular cancer. “I don’t know if we’ll ever be able to put another horse in that stall,” said a grief-stricken Colleen McQuay.

While such cancers are relatively uncommon in equines, for the horse owner facing a positive diagnosis, just one case is one too many. Oncologist Dr. John L. Robertson, with Biomed Sciences and Pathobiology at the College of Veterinary Medicine at Virginia Tech in Blacksburg, Virginia, suggests that the most fascinating aspect of equine cancer is not where it strikes, but how often it does NOT strike.


What is cancer? It is a state of cellular growth occurring when normal cells become abnormal, and continue to grow abnormally, characterized by the ability of those abnormal cells to subdivide and grow while limiting the ability of normal cells to do the same.

“Cancer,” says Bennie Jean Kuehnle, director of the Animal Institute of Holistic Health, and co-director of the International Alliance for Animal Therapy and Healing (IAATH), in Marietta, Oklahoma, “has been called by some researchers ‘a disease of civilization.’ It is rare in nature.”

Naturally speaking, given the horse’s status as a prey animal on the food chain, where signs of weakness or unsoundness attract a predator, cancer is also a discreet disease.

Kuehnle continues, “The horse’s survival instinct is a major roadblock to the healing process. As a prey animal, a horse will continue to perform while his innate sense of hiding weakness takes over. The very ability to mask symptoms and pain can often delay diagnosis of illness and treatment. There are documented cases of horses that showed no visible signs until the advanced stages. There are more than 70 types of animal cancers, and horses are susceptible to most, including the same types in humans involving the reproductive organs. The number of horses with cancer is growing and often remains undiagnosed.”

Kuehnle points to some half dozen theories currently seeking to explain the potential causes of cancer: from oxygen deficiency at the cellular level (Dr. Otto Warburg); or distortion of cell growth, at a microscopic level involving electron behavior (Dr. Albert Szent-Gyorgyi); to the negative affects of stress or a lack of natural sunlight. Dr. Hedwig Budwig, an expert on the therapeutic uses of oils, believes cancer is a fat-protein problem involving the underproduction of interferon by the body, while the late Professor Dr. Brauschle’s Hospital for Natural Healing sees it as a fungus whose growth can be halted or diminished, while strengthening the immune system, through herbal formulas.


Breast cancer, or more correctly, mammary neoplasia (tumors), in mares is uncommon in comparison to rates among women. Abbatoir studies (cited by Equine Disease Quarterly July 2008) report an incidence of barely 2%, and less than a dozen published case reports are known to exist. But of that compilation, all but one were malignant. And the results of necropsies, conducted between 1994 and 2008 by the Livestock Disease Diagnostic Center at the University of Kentucky on 11 mares diagnosed with malignant mammary neoplasia (cancer), have also concluded that equine mammary cancer is “much more likely” to be malignant than benign and carries a “poor prognosis” for long-term survival.

Albeit rare, but often fatal, conventional treatment of such cancer has a familiar ring for anyone whose wife, sister, mother or loved one has faced this disease: total removal of mammary tissue and any accessible lymph nodes. The equine ability to mask illness is the greatest challenge toward early diagnosis.

“Unfortunately, by the time a definitive diagnosis is made and surgery performed, metastasis to regional and distant lymph nodes and tissues has most likely occurred,” says Dr. Maria Shank with the Livestock Disease Diagnostic Center. Such diagnosis is possible through microscopic biopsy, depending on the tissue sample and its proximity to the neoplasm.

“Surgery is not recommended except in the most critical situations, as it does not prevent tumors from coming back,” says Kuehnle. In fact, surgery can accelerate growth of neoplastic cells, due to “incomplete excision of cancerous tissue, scarring, or a depletion of organ function.” Most mares seem to do well shortly after surgery to remove diseased mammary tissue, arguably buying a beloved friend more time, but inevitably suffer a recurrence at the excision site, or metastasis elsewhere, like the lymph nodes. Ensuing deterioration inevitably leads to euthanasia.

Mares also have a higher incidence of ovarian tumors (granulosa-theca cell, cystadenoma, dysgerminoma, teratoma) than other domestic animals. “While the percentage of ovarian tumors is reported to be 2.5% of all equine cancers, this particular location is significant,” says Kuehnle. “Mares with ovarian tumors, specifically granulose-theca cell (the most common type of equine ovarian cancer) tend to have reproductive dysfunction and be aggressive.”

Other veterinarians put ovarian tumors at a higher incidence percentage – as much as 6% – of all neoplastic (abnormal growth) conditions in horses. Granulosa-theca cell tumors occur most frequently in mares ages 5 to 9 years. Those exhibiting stallion-like behavior commonly show high levels of testosterone, because the tumor’s abnormal cell growth occurs in cells that naturally produce sex hormones and thus, can cause overproduction of estrogen, testosterone, etc.

You’d be cranky, if not downright aggressive, too, if suffering from the same painful conditions as this type of tumor can produce. Mares may show signs of lameness, or appear uncomfortable or irritable when performing, when in fact they are reacting to the pull of a tumor on their ovarian ligaments. As the tumor grows, it can place pressure on the broad ligament that suspends the ovary from the roof of the abdominal cavity. “Horses respond adversely to their tumor’s pull on this ligament,” says Kuehnle, “and may be reluctant to move forward. This causes gait problems and pain in the flank.” Poor performance could be more than just ‘mare-ishness.’ It could indicate a much more serious condition.

Other, more rare cancers include cystadenoma, a usually benign tumor that affects one ovary, producing multiple cyst-like structures within the ovary that do not spread, and can be corrected by surgical removal.

Teratomas are usually benign but uniquely unsettling. Tera, Greek for “a wonder or monster,” alludes to the strange character of these tumors, which contain “germ” cells – the genetic material for all components of the body. Teratomas can contain cartilage, bone, hair, and even teeth, but because they do not secrete sex hormones, have no negative impact on reproductive performance. Such odd tumors are often discovered incidentally, during a reproductive examination or rectal palpitation.

Equally rare, but less successfully treatable, are dysgerminomas, which can metastasize from one ovary into the abdominal and chest cavities with little to no outward signs of disease (other than symptoms akin to colic) until the cancer has irrevocably spread.


“Male horses have prostate glands and are susceptible to the same cancers, conditions, and issues as humans, with not-so-rare cases of death” says Kuehnle.  “It is becoming more of a problem as horses are exposed to abnormal internal, and external, factors. Despite the wonderful benefits to horses’ lives today, dependence on man has presented a Pandora’s box of issues that would seldom be encountered in their natural state.”

C. Edgar Sheaffer, VMC, a holistic consultant who operates Clark Veterinary Clinic in Palmyra, Pennsylvania, has observed the most cancer incidences not in the prostate, but in the sheath, penis, and glans, as small, cauliflower growths. “In a stallion, the primary growth is on the sheath, as a sarcoma, or occasionally, a sarcoid or melanoma.” Its presence can be insidious long before it is detected. “In breeding or semen-collecting situations, it may be detected earlier but most laypeople would not detect any changes until it was obvious. The next step, if it is a valuable animal or stud, would be unilateral castration, especially involving testicular tissue.” In such cases it is possible to collect semen in advance to store for future breeding seasons before performing the castration. His patients begin with surgery, at his eminent neighbor, the New Bolton Center of the University of Pennsylvania, followed by homeopathic prescriptions for what he describes as deep-acting, miasmatic disease (miasm meaning, “chronic by nature”).

“We see chronic conditions,” Sheaffer says, “become suppressed in the body elsewhere. Deep-seated disease then manifests as growths or tumors, on the sheath, scrotum, etc., so there is no one remedy or medicine. It takes a whole study of the animal to approach the condition.”

Squamous cell carcinoma, a common cancer in older horses, can also affect the perineal area beneath the tail, and the penis or prepuce of male horses. The most proactive steps against this cancer are to have your veterinarian conduct a thorough exam of the penis and sheath, once or twice annually while your aged gelding or stallion is sedated, to check for tumors.


Continued study is required to find new ways to combat equine cancer. Some of the most exciting research is coming out of Virginia Tech and its College of Veterinary Medicine. Three years ago, Dr. Robertson began conducting clinical trials of injectable, and topical, medical-grade frankincense oil into 42 horses diagnosed with equine malignant melanomas and squamous cell carcinomas. The trials concluded in 2007.

Frankincense, or Boswellia serrata, is an Indian herb credited with anti-inflammatory properties and its healing oil has been in use since 2300 BC in Egypt. The oil used in this study was “extremely high grade and concentrated,” and grown and distributed by a certified organic farmer in the Republic of Somaliland. “We have seen in some horses that application of frankincense will destroy portions of the tumor. Most of the tumors we see here are very large and advanced. We have not cured any horses, but can continue to study this as a potential therapy for tumors. Much more publishable research needs to be done, and we are doing it,” says Dr. Robertson.

“We intend to use frankincense as the only medicinal. There are over 200 organic compounds in this mixture, and our hope is to use it directly as a chemotherapeutic. The bottom line,” he says, “is there are lots of herbal old farmers’ tales out there about what helps horses. It’s our job to find out which ones work, and how, and why.”

“To reduce the impact of cancer,” says Kuehnle, “the horse has no greater ally than an educated owner. If your horse has been diagnosed with cancer of the reproductive system, or you suspect it, please seek to build a qualified team to help you and your horse with the healing process. Find someone to help you identify environmental factors and make adjustments. Find qualified individuals who can help with a nutritional plan. Since stress is a contributing factor, it is crucial not to panic and start adding everything you can find on the Internet or from well-meaning folks at the local vitamin shop. The horse’s body, under normal circumstances, will naturally eliminate what it doesn’t need. But a horse under stress can get overloaded and have negative reactions from detoxification. Healing cancer depends not only on the destruction of cancer cells, but in correcting the underlying causes.”

Equine cancer may not yet have celebrities promoting the need for more research and education, but with dedicated researchers and healers continuing to study and approach an old scourge in new ways, horse owners can draw hope.

L.A. Pomeroy of Northampton, Massachusetts, has been an equestrian photojournalist, award-winning publicist, and member of American Horse Publications since 1992, working with the U.S. Equestrian Team, Atlanta Committee for the Olympic Games, Equisearch.com, and Intercollegiate Horse Shows Association, as well as heading development and marketing for zoological institutions in New England. She enjoys trail riding in her native Catskill Mountains of upstate New York, collecting/researching equestrian art and collectibles, and making life better for the animals that share this planet with us.

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