Cancer is the abnormal growth of cells that develop into masses known as tumors. Tumors often induce a state of wasting or a "hollowed-out" look (tumor cachexia). Tumors cause this appearance by releasing chemicals into the body that cause muscle wasting, weight loss, anemia (low blood cell count), loss of appetite and depression. When certain cell types (malignant tumors) become cancerous, they can spread through the blood or lymph system to any region of the body and begin tumor growth there.

Many equine tumors are benign, meaning that they do not spread beyond a local area. Lymphosarcoma, however, is a malignant tumor in the horse.

Lymphosarcoma is cancer of the lymphocyte inflammatory blood cell line. Lymphocytes are a type of white blood cell that are vital for immunity and are the primary cell of lymph nodes and other lymph tissue. The lymphatic system is a network of immune defensive centers spread around the body.

Lymph nodes (lymph glands) are clusters of cells (mostly lymphocytes) that are the control bases within this network, joined by lymph vessels. Lymph nodes fight infections in a body region, and notify the rest of the body of infection in the area by chemical signaling. Along with the lymph nodes under the skin, there are clusters of lymph tissue throughout the body, especially around the areas in contact with the outside world - the intestine and the lungs.

Any one of these clusters of lymph cells can become cancerous. Like other types of cancer, the signs of disease depend on where tumors are located and the organs affected. Lymphosarcoma is divided by pathologists into 4 types, based on location. The four main types are generalized (a/k/a multi-centric), cutaneous, mediastinal, and intestinal.

Intestinal lymphosarcoma is abnormal invasion by lymphocytes of the wall of the intestine. The center for this invasion is the lymph tissue within the intestine. The damage to the intestinal wall leads to loss of function of the intestine, which causes poor digestion and absorption of nutrients. This in turn appears as the observations of weight loss and/or diarrhea. These horses also often have swelling of the lower belly and legs from loss of certain body proteins in the intestine.

Diagnosis can require biopsy of the intestine, and sometimes rectal biopsy to reach this diagnosis.