Lymphoma is a general term for the malignant transformation of the solid tissues of the lymphatic system and is synonymous with lymphosarcoma. Lymphoma though one of the most common internal cancers of horses only has an incidence rate of less than five per 10,000 horses. There do not appear to be any breed or sex predilections but is most common in horse four to 10 years of age. This article discusses symptoms, diagnosis, prognosis and treatment.

Because lymphoma can arise in any organ, the clinical signs are frequently referable to a dysfunction of the organ that is involved. In one study of 20 cases, the clinical findings in order of frequency:

• weight loss
• fever,
• peripheral lymphadenopathy
• abdominal mass
• respiratory signs
• ocular signs
• colic
• diarrhea

There are four forms of the disease based on the anatomic location of the tumors:

• Multicentric or generalized 50%
  • Represents multiple metastasis via the blood stream.
  • Once discovered the course of the disease is rapid.
  • May have circulating cancer cells
• Alimentary 19%
  • May present as a very acute onset of one or more of the following: weight loss, fever, dependent edema, colic and/or diarrhea.
  • Signs and lab findings of malabsorption are the rule.
• Mediastinal 6%
  • Signs of compression of the organs of the chest is most common and include: pleural effusion, tachypnea dyspnea, and dependent edema. Less commonly: a cough, tachycardia, and hypercalcemia.
  • Neoplastic cells may be found in the pleural fluid.
• Extra nodal 25%
  • The skin and upper respiratory system are most commonly affected.
  • Tumors are nonpainful masses that are firm and well circumscribed.
  • There may be a solitary or multiple masses.
  • Tumors may develop rapidly or slowly and spontaneously regress and reappear.

In 50% of the cases more than one form is present.