Cancer in Horses

Cancer. One very powerful word that can evoke feelings of fear, pain, triumph and loss. Nearly every person’s life has been in touched by cancer in one way or another and our horses are no different. The incidence of cancer in horses is less common than that in humans, but it is still a growing concern. Because horses have longer lifespans than dogs or cats, cancer often takes longer to develop. It is often more difficult to find because horses’ bodies are so large. The most obvious signs of cancer are scaly circular areas of hair loss on the skin, swollen lymph nodes and growing / changing lumps, but cancer can emerge in many forms. Other chronic warning signs that can be overlooked as your horse ‘just not feeling well’ include lethargy, weight loss, colic, difficulty breathing, lameness, swelling, difficulty urinating, oral odor or a wound that doesn’t heal. An enlarged abdomen could be a sign of a growing mass inside the horse’s body and unexplained bleeding from the mouth or nose could mean a tumor in the head. These symptoms do not necessarily mean your horse has cancer, but you should still mention them to your veterinarian to be sure.

Biology of Cancer

All tissues in the body are made of cells. Tissues are continuously broken down, repaired and rebuilt with new cells. The code for cell production is written into our DNA inside the nucleus of all cells. Sometimes errors can occur in the DNA that cue the body to produce too many cells. Usually these errors are minor and the excess cells produced are normal causing a benign tumor. Cancer is the uncontrolled growth of abnormal cells. It develops when DNA is damaged enough to reproduce incorrectly. It can be localized or it can spread (metastasize) to nearby tissue or even go on to grow throughout the body. Metastatic tumors require lots of blood vessels to fuel the rapidly growing and spreading cell growth.


Type                                        Description                             Appearance

Papilloma                                benign skin tumor                  cauliflower-like warts

Lipoma                                    benign fatty tumor                 soft & squishy to the touch

Sarcoids                                  skin cancer                             circular area of hair loss, grey scaly

Advanced Sarcoids                 skin / lymph node cancer      lumpy and ulcerated

Melanoma                               skin cancer                             firm, dome-shaped lumps

Squamous Cell Carcinoma    skin or organ lining cancer    open sore, scaly red patches, warts

Lymphoma                              tumor of the lymph nodes     solid growths, enlarged lymph nodes

What causes Cancer?


Horses are exposed to many of the same carcinogenic materials as us so they can develop cancer for many of the same reasons.   Environmental toxins, second-hand cigarette smoke, radiation, UV rays, pollutants, viral infections, trauma, stress, artificial chemicals found in food, exercise and even digestion can cause normal atoms or molecules to lose electrons. This change to the molecular structure creates a highly reactive molecule called a free radical. Free radicals within the body usually contain oxygen atoms so another name for free radicals is “reactive oxygen species”. Free radicals are looking to steal electrons from other molecules so they can easily create a domino effect of molecules stealing electrons creating more free radicals. Everyone has some free radicals in their bodies, but when toxins create too many free radicals they can damage cell membranes, cell proteins and even DNA. Once the DNA is damaged it can begin to replicate the error which could trigger abnormal cell growth – cancer.



Purebred horses tend to have a higher hereditary predisposition toward different types of cancers because they have been bred by people to promote certain physical traits, which limits genetic variability. Once you start selecting for a specific look, it becomes easy for genetic mutations to pop up within a bloodline. Cross-breeds have a bigger gene pool so they are less likely to have hereditary mutations, but they are still at risk from environmental causes. Surprisingly, color also plays a role. Up to 70% of grey and white horses develop melanomas. While malignant, melanomas grow slowly so early surgical intervention is recommended.

What can I do if I think my horse has Cancer?

Any time you have concerns about your animals’ health you should always see your vet because they know your horse’s individual history and have the tools to make a diagnosis & treatment plan. Your vet will want to perform a thorough physical exam, do blood tests and probably take radiographs (x-rays) or do an ultrasound. In order to confirm the diagnosis of cancer, a biopsy will need to be done. A biopsy is a minor surgical procedure where the vet removes a small tissue sample so it can be tested by the lab to see if cancer is present and, if so, determine the severity. Because there are so many different types of cancer, the treatments can vary too. Your vet will assess your horse and determine if surgery, chemotherapy, radiation or immunotherapy are right for them. Severe cases may be referred to a veterinary oncologist (a vet that specializes in treating cancer). When detected and treated early you have a much better chance of beating it! Today’s advances in both human and veterinary medicine can provide some form of benefit to nearly any patient and more than half can be cured completely.


Regular grooming helps horse owners closely observe their horse’s health so any unusual spots or lumps can be watched. Light colored horses are at a higher risk of developing squamous cell carcinomas so sun exposure should be limited or sunscreen can be used.

How do Cancer Nutraceuticals work?

Most cancer patients require a comprehensive treatment plan that includes pharmaceutical therapies and surgery, but more than half of horse owners are now asking their vet for complementary / natural medical options when faced with serious health problems.



Commonly referred to as “free radical scavengers”, antioxidants are natural chemicals that neutralize free radicals by providing extra electrons. The body makes many of its own antioxidants, but additional support comes from the diet. Studies on antioxidants and humans with cancer have yielded mixed results and there is not a strong body of evidence supporting their efficacy in treating cancer, but the trials tend to use purified substances as opposed to fresh fruits, vegetables and grains that are naturally rich in antioxidants. All antioxidants are different and work differently when they interact with free radicals so the best results come from giving your pet a variety of antioxidants. The National Institutes of Health created a unit of measuring antioxidant activity called the ORAC (Oxygen Radical Absorbance Capacity). This value is often listed on antioxidant supplement labels so you can see how potent they are.


Examples of Antioxidants:

  • Beta-carotene – Lycopene
  • Vitamin A – Grape Seed Extract
  • Vitamin C – Apricot Pits
  • Vitamin E – Dimethylglycine (DMG)
  • Alpha Lipoic Acid (dog only) – Resveratrol
  • Pycnogenol – Glutathione
  • Coenzyme Q10 – Flavonoids
  • Polyphenols – Astaxanthin
  • Super Oxide Dismutase (SOD) – Catalase (CAT)
  • Omega 3 Fatty Acids – Epigallocatechin-3-gallate (EGCG)


Fresh foods like blueberries, sweet potatoes, nuts, beans, dark green vegetables and many herbs are excellent sources of antioxidants.


Consult with your veterinarian before giving antioxidants to a horse that is already being treated for cancer because some supplements can interact with drugs, making them less effective.


Shark Cartilage

The skeletons of sharks are made up of cartilage, which is a flexible but strong connective tissue. Researchers have observed that sharks do not tend to have problems with cancer and believe the reason for that is the protein in their cartilage that inhibits the development of new blood vessels. Cancerous tumors require a big blood supply so slowing the body’s development of blood vessels to the tumor would restrict its nutrient supply. The study of “anti-angiogenesis” (the inhibition of blood vessel development) is getting more attention from cancer researchers, but the jury is still out on whether or not shark cartilage is truly effective.


Vitamin D

Vitamin D3 acts more like a hormone than a vitamin because it helps to activate immune system lymphocytes. It is essential to the endocrine system because it helps control cell growth, cell differentiation, synthesis of enzymes and repair of DNA. Studies show that individuals deficient in vitamin D3 have a higher risk for cancer. New evidence is showing that it may even help shrink tumors by killing cancer cells and repairing tissues.



A compound in the spice, turmeric, curcumin is thought to have multiple cancer-fighting abilities. It works by binding a protein that helps cancer grow and spread. Curcumin also interferes with inflammatory molecules that can cause cancer. Combining curcumin supplements with traditional chemotherapy can make cancerous cells more vulnerable to the drugs. Oral supplementation is not always well absorbed, but new treatment options are currently being studied.




Many mushrooms including shiitake, maitake and reishi have been found to be high in alpha and beta glucans, which help modulate the immune system. Many mushrooms have been found to slow growth and kill cancer cells. Reishi and maitake mushrooms in particular have been studied for their ability to protect the body from chemotherapy side effects including hair loss, pain and nausea. Mushrooms have a long history of use in Traditional Chinese Medicine, but were used by the ancient Egyptians even earlier. Laboratory studies are looking optimistic, but more research is needed in living people and animals confirm that mushrooms taken orally really kill cancer cells.


Transfer Factors

Nutrients isolated from dairy and egg yolks called transfer factors have been found to support the immune system through modulation (regulating the immune response up or down depending on the animal’s condition). Transfer factors help the immune cells recognize and respond to pathogens more efficiently by increasing communication between the cells.


While more research needs to be done to come to a stronger conclusion on these nutrients’ roles in cancer-fighting health, it is clear that there are many pathways that can benefit from good nutrition. Many of the actions these supplements theoretically take can be extrapolated into new treatments to help improve the lives of horses and the people who love them.



Animal models and therapeutic molecular targets of cancer: utility and limitations.

Application of the comet assay for investigation of oxidative DNA damage in equine peripheral blood mononuclear cells.

Constitutive activation of the ERK pathway in melanoma and skin melanocytes in grey horses.

Detection of papillomavirus in equine periocular and penile squamous cell carcinoma.

Clinical trials of immunogene therapy for spontaneous tumors in companion animals.

Neurotropic T-cell-rich B-cell lymphoma in a 14-year-old Morgan gelding.

Equine malignant lymphomas: morphologic and immunohistochemical classification.

B-cell multicentric lymphoma as a probable cause of abortion in a quarter horse broodmare.

Primary epitheliotropic intestinal T-cell lymphoma as a cause of diarrhea in a horse.