Although horses do not often get cancer other than melanoma, researchers now believe that lymphoma is more prevalent than previously thought. Unfortunately, most lymphomas in horses don’t enlarge the lymph nodes, but instead involve internal organs inaccessible for biopsy making the cancer difficult to diagnose.
Lymphoma in the horse generates a considerable inflammatory response characterized by influx into the tumor of cells, many of which are reactive lymphocytes. These inflammatory cells may contribute substantially to the size of the tumor masses.
The cause of lymphoma in horses remains unknown; no viral cause has been found, as it has in other animals. The most common signs are often mistaken for colic or other internal disorders. As the cancerous masses in the spleen and other abdominal areas grow in size, the horse becomes very uncomfortable and attempts to relieve the pressure by actions that mimic colic.
One of the main symptoms associated with lymphoma is weight loss, usually associated with intestinal involvement or the effects of cancer on the body's metabolism.
Lymphoma can involve a variety of areas of the body, both in the abdomen and in the chest cavities, as well as nodules in the skin or around the eyelids."
A review of scientific literature sheds little light on this unusual form of equine cancer. Based on studies of how lymphoma spreads within the horse's body, many researchers believe that it may be more prevalent than reported because many horses that die or are euthanized are presumed to have common disorders, such as colic, and owners often do not want to necropsy them.
A classic case of lymphoma was seen at the University of California, Davis veterinary school when Lost in the Fog, a robust four-year-old colt sprinter was brought to the hospital in the belief that the horse was suffering from colic based on the symptoms.
At the University of California at Davis veterinary school, clinicians theorized that the two football-size tumors they found in Lost in the Fog's spleen and near his spine had been developing over the past four months, but perhaps as long as a year. A third tumor the size of an egg was found in the ligament supporting his spleen. The verdict was the worst possible: nothing could be done but to make the colt as comfortable as possible until his condition deteriorated to the point where euthanasia would be the humane route.
Dr. Alain Theon, chief of the Medical and Radiation Oncology Services at the Davis, and one of the few equine oncology specialists in the world, has performed a substantial amount of pioneering research on equine tumors and has made tremendous progress in establishing safe protocols for high dose systemic chemotherapy in horses. He has now developed a standard protocol of combination chemotherapy that includes doxorubicin and has used this protocol to treat a substantial number of horses with tumors
Also, by suppressing the inflammatory reaction, corticosteroids often shrink lymphomas considerably, making them more amenable to surgical removal or chemotherapy, as well as reducing their impact on the function of the horse’s organs, including the gastrointestinal tract.
Researchers and veterinarians are in agreement that more research into the causes of lymphoma is needed and better methods of diagnosis will help save the lives of many horses.