Case description: A 17-year-old Friesian gelding was examined at a referral hospital because of a 1-month history of mild exercise intolerance and marked lymphocytosis.
Clinical findings: Physical examination revealed no peripheral lymphadenopathy or other abnormalities. Results of an abdominal palpation examination per rectum and thoracic and abdominal ultrasonographic examinations were unremarkable. B-cell chronic lymphocytic leukemia (CLL) was diagnosed on the basis of severe lymphocytosis and positive expression of the B-cell marker CD20 by lymphocytes in the bone marrow and peripheral blood.
Treatment and outcome: Treatment with prednisolone (2 mg/kg [0.9 mg/lb], PO, every other day) and chlorambucil (20 mg/m2, PO, every 3 weeks for 2 doses, then every 2 weeks) was initially associated with improvement in clinical signs and a decrease in the lymphocyte count. However, 3 weeks after administration of the first dose of chlorambucil, the lymphocyte count began to increase. One week later, the horse developed episodes of recurrent fever and the lymphocyte count continued to increase. Despite continued administration of the prednisolone-chlorambucil protocol, the horse's clinical condition deteriorated rapidly, and it was euthanized 6 weeks after initial examination at the referral hospital because of a poor prognosis. A necropsy was not performed.
Clinical relevance: B-cell CLL has been infrequently described in horses. This report was the first to describe the use of chemotherapy, albeit unsuccessful, for the treatment of B-cell CLL in a horse. This information should be useful for guiding expectations for prognosis and management of other horses affected with the disease.