Lymphoma

Lymphoma is one of the most common cancers in equines, but diagnosing it is difficult. Vet and equine internal medicine specialist Michael Hewetson explains all to Catherine Welton

-  Catherine Welton

LYMPHOID tissue is found throughout the horse’s body and helps fight infection. Lymphoma occurs when the white blood cells (lymphocytes) in this tissue change and become malignant.

What happens next, and the prognosis, varies widely depending on where in the horse’s body the affected cells are.

“We tend to classify equine lymphoma based on the anatomical location of the disease,” explains Michael Hewetson, associate professor in equine internal medicine at the Royal Veterinary College. “Multicentric is the most well-known and involves two or more organs in the body, either the spleen and the liver or perhaps the spleen, liver and kidneys.”

The disease also occurs in the gut (alimentary), the chest (mediastinum) and the skin (cutaneous). Finally, solitary or extranodal lymphoma can develop anywhere in the body.

“I’ve seen them everywhere from the eyes, to the spinal column and the heart,” says Dr Hewetson.

While lymphoma is relatively rare in the general equine population, it accounts for 2–3% of all cancers and is the most common form of internal malignant cancer in horses.

But it’s not understood why some horses develop it while others don’t.

“There’s no good evidence of any clear underlying risk factors that will result in lymphoma developing. It tends to be a disease of younger horses – primarily four to 10 years of age.

“But, based on the current evidence available, there’s no suggestion of a genetic link and no breed predilection.”

WHAT ARE THE CLINICAL SIGNS?