Diseases and conditions image
Diseases and conditions image EquiMed

Also Known As

Lump, Tumor - malignant


Cancers appears in horses when normal mechanisms of cell growth change and cells proliferate in a disorganized and unchecked way, forming masses of cells that disrupt the normal development and functioning of the body. If this growth of cells happens deep within the body, the lumps can press against or spread into vital organs, causing them to malfunction and shut down.

Cancers growing in the head region of the horse may cause distortion of the face due to local invasion and may affect hearing, eyesight, and the ability to eat and breathe properly.

As the cancer cells spread to other areas of the body, these "metastases" grow and become additional cancerous lumps. Cancers of the intestine and abdominal organs may become apparent only when the disease is in an advanced state.

When cancerous growths obstruct intestines or when they occupy a large portion of another organ, the horse will begin to show signs of the disease.

Some cancer cells produce inappropriate hormones that disturb the normal hormonal balance. For example, there may be excessive production of testosterone in a mare that has cancer of the ovary known as equine graulosa cell tumor. This leads to stallion-like behavior and aggression towards other horses.

Gray horses often have nodular masses or "melanomas." These are frequently found under the tail and around the dock, but can occur behind the jaw and in the eye. Sometimes they are found within the abdomen and in the throat area. Many melanomas remain small and harmless to the horse, however, on rare occasions, they spread throughout the body and transform into invasive tumors.


  • Swollen lymph nodes
  • Enlarging or changing lumps
  • Abdominal distension
  • Chronic weight loss
  • Chronic diarrhea
  • Unexplained bleeding
  • A dry, non-productive cough
  • Lameness
  • Straining to urinate
  • Oral odor


Cancer develops when cells start to proliferate in a disorganized way. This leads to lumps of cells that can press on or invade vital organs, preventing them from working properly, or, if near the surface of the body, are visible and, at times, disfiguring.

The cause of any particular cancer is difficult to determine. In the case of melanomas, it is likely that a number of key mutations linked to coat color may be the basis.

Most cancers are due to mutations and much research is currently under way to determine what triggers the mutations. Once the causes of various mutations are known, researchers can begin to determine how to control and eventually eliminate them.


More research is needed to determine how best to prevent cancer in horses. So far, catching cancers in their earliest stages and treating them appropriately is the best course of action that horse owners and veterinarians can take.


Many of the treatments used for human cancers work well for cancers in horses. In the case of swollen lymph nodes or enlarging/changing lumps, a biopsy or cytology of the nodes or lumps may be useful in determining if a malignant tumor exists.

When the abdomen is distended, when diarrhea is ongoing, or weight loss occurs, radiographs, ultrasounds, or endoscopy may help determine where and how serious any tumors are. Radiographs are helpful with dry coughs, lameness, straining to urinate, or when unusual oral odor indicates a serious internal problem.

In all cases, the help of an experienced veterinarian will be needed to diagnose, formulate surgery or treatment plans, and make sure that the horse is kept comfortable throughout the ordeal.

Diagnosis is accomplished by taking a piece of any existing mass and examining it under a microscope. If the lump is small and not causing any problems, it is often best left alone.

Some equine surgeons argue that operating on some tumors, including melanomas, "activates" the cells and increases the chances of tumor growth or of cells escaping through the blood system to other areas of the body.

Other treatments include the use of drugs and vaccines. Researchers have developed a tissue-based vaccine made from a horse's own tumor cells. Good responses have been obtained for melanomas using this vaccine, and, in equine cases, melanomas do not seem to be as destructive as they are in humans.

Many cancers are amenable to treatment through surgery, chemotherapy, and radiotherapy, and these treatments have been applied to equine cancer with considerable success.

Laser surgery is very helpful in the treatment of equine cancer where it can be used to cut out tumors and seal blood vessels to promote healing. If a cancer is confined to the skin, creams can be applied to keep cancerous cells in check.

Additional anti-cancer drugs and treatments such as radiation are currently being developed and gene therapy shows great promise in allowing the manipulation of genetic signals to prevent cells from becoming cancerous or to turn off cancer-like functions once they begin.

Dig Deeper

This section contains articles specially selected by EquiMed staff for visitors wanting more information about this disease or condition. These articles are copyrighted by their respective owners and are available to you courtesy of EquiMed

About the Author

EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.