By Siobhan McAuliffe, MVB, DACVIM, Foal Medicine Specialist

Other than melanomas, cancer in horses is rare. Amongst those that can be seen are lymphomas and many researchers now believe that lymphoma is more prevalent than previously thought. Lymphoma in horses is broadly categorized into:

Cutaneous: Single or multiple lumps (enlarged lymph nodes) are seen on the body. This type responds well to treatment with corticosteroids.

Intestinal: Single or multiple solid tumors or as a diffuse infiltrate of neoplastic cells. The small intestine and intestinal lymph nodes are most commonly affected. This form has a poor prognosis in general but some cases of singular masses have responded well to tumor removal and chemotherapy.

Generalized: In this form lesions are commonly seen in the intestine but also in the thorax. This form has the poorest prognosis, as many lesions are frequently present at the time of diagnosis.

The cause of lymphoma in horses remains unknown; no viral cause has been found, as it has in other animals.

7-year-old mare with a history of poor performance and poor weight gain despite a distended abdomen. This mare had the intestinal form of lymphosarcoma with lesions in all abdominal organs.

Clinical signs

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. Weight loss is the most common clinical sign. Other signs that may occur alone or together are; fever, dependent edema, diarrhea or increased water content of feces and recurrent colic. Diagnosis in many cases is difficult particularly in the early stages of disease as many other disorders may result in the same clinical signs.

It is important to try to differentiate the intestinal lymphoma from those conditions with similar general clinical signs, but which are resolvable, such as heavy parasitic infestation, dental disease and diet-related digestive disturbances.

Extensive lesions visible in the liver of the mare pictured above, as seen at necropsy.