Q: Tao, our 11-year-old part-Siamese cat, was diagnosed with lymphoma. He has always been an active, healthy indoor cat, so I don’t understand how this could have happened. What causes lymphoma in cats? A: I’m sorry to hear about Tao’s lymphoma, also called lymphosarcoma. It’s the most common cancer in cats—and the incidence is higher in cats than any other species. Lymphoma develops when lymphocytes, a type of white blood cell, proliferate until they are out of control. As with most cancers, the cause in a given patient is usually unknown. However, veterinarians do recognize a few risk factors. Siamese, Oriental shorthairs, Javanese, and related breeds are predisposed to lymphoma, so genes probably play a role. Cats infected with two particular viruses that suppress the immune system are more likely to develop lymphoma than cats that test negative for these viruses. The feline leukemia virus increases the risk of lymphoma …