Lymphoma risk in cats more than doubles if owners are smokers

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Cats living in homes where people smoke cigarettes are more than twice as likely as other cats to acquire a deadly cancer, lymphoma, according to a study published in the August issue of the American Journal of Epidemiology.

Researchers have shown an increase of certain types of cancer in dogs if their owners smoke, but the new study is the first to provide evidence of this effect in cats.

"Smoke has devastating consequences for cats," said Dr. Antony S. Moore, who was involved in the study and is director of Tufts University's Harrington Oncology Program. Board-certified by the Specialty of Oncology, American College of Veterinary Internal Medicine, he says that cats may inhale secondhand smoke or ingest particulate matter from their fur when they groom themselves.

Lymphoma is the most common cancer in cats. Only feline leukemia virus and feline immunodeficiency virus are known to play a role in causing the disease.

"The reason we looked at lymphoma was that it is so common, and our ability to treat it is just not very good," Dr. Moore said. "Twenty-five percent of cats [that have it] live no more than a year with chemotherapy."

The researchers tested a number of possible cancer risks, including diet, spay and neuter status, age, sex, breed, grooming, home characteristics, and the use of flea control products, shampoos, and oral medications. Only one factor, smoking, was associated with the cancer, in a significant manner.

The 194 cats in the study were treated at Tufts veterinary school's Foster Hospital for Small Animals between 1993 and 2000. Eighty of the cats were treated for lymphoma and 114 for renal disease. The scientists used cats with renal disease as controls because it is a nonmalignant, serious condition that is not known to be associated with passive smoking in cats or humans.

After adjusting for age and other factors, the relative risk of lymphoma for cats exposed to any household environmental tobacco smoke was roughly two-and-a-half times that of cats not exposed.

Risk increased with both the duration and quantity of tobacco smoke exposure. Exposure for five years or more tripled the risk of acquiring the malignancy. Exposure to two or more smokers in the house quadrupled the risk. And cats living in households where humans smoked a pack or more of cigarettes per day had more than a threefold increase in risk, compared with cats living in smoke-free homes.

The results, says Elizabeth Bertone, ScD, an assistant professor of epidemiology at the University of Massachusetts, who was involved in the study, also provide compelling reasons for studying the relationship between secondhand smoke and non-Hodgkin's lymphoma in humans. This human cancer is similar to lymphoma in cats.

Right now, however, the implications for cats are clear. Dr. Moore says veterinarians should think about educating clients about the risks of smoking on their cats and dogs.

"It's not something we normally bring up or talk about with pet owners," Dr. Moore said. "But I think, just like in human medicine now, we need to say [to our clients] that there has started to be a building body of evidence that there are health effects on cats and dogs from living with smokers."