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 is 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 an astonishing 60-fold, and the feline immunodeficiency virus increases it 5- to 6-fold.
In addition, a cat living with a person who smokes is 2.4 times more likely to develop lymphoma than a cat living in a smoke-free home. Two smokers increase the risk to 4.1 times normal.
Above-average cigarette use and longer duration of exposure further increase risk. Secondhand smoke is particularly harmful to cats because they inhale it and ingest the toxins trapped in their fur when they groom themselves.
Fortunately, we now have chemotherapeutic medications that induce remission in most cats with lymphoma. Best wishes to you and Tao.
Q: Occasionally, Chico, our 2-year-old Chihuahua, skips or holds up one of his hind legs and runs on only three legs. Then, just as suddenly, he returns to running on all four legs. What's going on?
A: It sounds like one of Chico's kneecaps may be intermittently dislocating.
His knees are built like ours, complete with a kneecap, or patella, that rides in a V-shaped groove. In some dogs, the groove is shallow, allowing the patella to slip out and sit alongside it.
I suspect that's happening to Chico. When his patella dislocates, or luxates, the pain and instability cause him to skip or hold his leg up.
At some point while running on three legs, Chico extends his dislocated leg behind him, the patella pops back into its groove, and he runs on four legs again.
Patellar luxation, or PL, can occur in any dog, but it is most common in small breeds, usually becoming evident during young adulthood. Both kneecaps dislocate in about half the dogs with this problem.
In dogs with mild cases, the kneecap is stable during normal walking and running, and the condition is only discovered during a veterinary exam.
Chico appears to have moderate patellar luxation, with the kneecap dislocating and popping back into place on its own.
Dogs with more severe cases have chronically dislocated kneecaps that could develop arthritis and frequent or continuous lameness.
All dogs with this condition should be kept slim. Further treatment depends on the severity of the condition and ranges from anti-inflammatory pain medication to surgery.
Since it can be inherited, dogs like Chico should not be bred.
Only your veterinarian can determine a diagnosis, so make an appointment to have Chico examined. With your vet's help, I'm sure Chico will enjoy a long, comfortable, active life.
Lee Pickett, VMD, practices companion animal medicine in North Carolina. Contact her at