Cats living in homes where people smoke cigarettes are more than twice as likely as other cats to acquire a deadly form of cancer known as feline lymphoma, according to a first-of-its kind study in cats conducted by scientists at Tufts University School of Veterinary Medicine and the University of Massachusetts.

The study, entitled "Environmental Tobacco Smoke and Risk of Malignant Lymphoma in Pet Cats," was published in the American Journal of Epidemiology. The authors conclude that these findings offer a compelling reason for further study of the relationship between passive smoke and non-Hodgkins lymphoma in humans, which is similar to lymphoma in cats.

"It has long been believed that the major cause of feline lymphoma was feline leukemia virus," explained Antony S. Moore, VMSc, a board-certified veterinary oncologist and director of Tufts' Harrington Oncology Program. "The results of our study clearly indicate that exposure to environmental factors such as second-hand tobacco smoke has devastating consequences for cats because it significantly increases their likelihood of contracting lymphoma."

Several recent studies in humans have suggested that people who smoke tobacco may have an increased risk of contracting non-Hodgkins lymphoma. In addition, other studies have suggested that children of parents who smoke may have an increased risk of developing lymphoma. The results of these studies, however, are often hard to prove due to the myriad of other risk factors that people face.

In sharing their living environments with humans, cats are exposed to many of the same environmental contaminants as their owners, including tobacco smoke. Exposure levels in cats continuously kept indoors may actually be higher than those of human household members, who often spend extended periods of time outside their homes. Cats may become exposed by inhaling the smoke or by ingesting it when they groom themselves and lick particulate matter off of their fur.

"We believe that feline exposure patterns to environmental tobacco smoke may mimic those of young children living in households where adults smoke and where the children inhale tobacco smoke or ingest particulate matter by mouthing contaminated objects," said Elizabeth R. Bertone, ScD, Department of Biostatistics and Epidemiology, University of Massachusetts at Amherst.

An epidemiologist and lead author of this study, Bertone added: "Our findings offer another reason for smokers living with pets and children to try to 'kick the habit.' Quitting smoking will not only reduce their risk of cancer, heart disease and diabetes, but may reduce the risk of cancer in their children and pets as well."

To evaluate whether exposure to household environmental tobacco smoke may increase the risk of feline lymphoma, the authors conducted a case-control study of this relationship in 180 cats who 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 100 were treated for renal failure.

After adjusting for age and other factors, the relative risk of lymphoma for cats exposed to any household environmental tobacco smoke was more than double (2.4) that of cats not exposed to tobacco smoke. The risk of cats acquiring cancer increased with both their duration and quantity of tobacco smoke exposure. Cats that were exposed for five or more years had a risk of more than triple (3.2) that of other cats.

Risk of lymphoma also appeared to be related to the number of smokers living in the home, with nearly a double relative risk (1.9) for cats living with one smoker, and a four-fold increase in risk (4.1) for cats living with two or more smokers. In addition, cats living in households where humans smoked a pack or more of cigarettes per day had a significant three-fold (3.3) increase in risk compared to cats living in homes where people did not smoke.

Previous studies have not addressed the association between environmental tobacco smoke and lymphoma in pets. While no clear mechanism has been proposed to explain an association between active or passive smoking and the development of non-Hodgkins lymphoma in humans, the new study indicates several components of tobacco smoke may be carcinogenic to lymphoid tissue and may cause mutation in certain tissues.

Laura A. Snyder, DVM, a recent graduate of Tufts Veterinary School, co-authored this study in collaboration with Drs. Bertone and Moore. The investigation was supported by the National Institutes for Health, the Geraldine R. Dodge Foundation, the Cape Cod Cat Club and the International Feline Foundation.

Facts about Feline Lymphoma

Feline lymphoma is the most common cancer in cats, and often involves their intestinal tracts. Cats that contract lymphoma are usually about 10-years-old. The typical treatment protocol involves chemotherapy and possibly radiation therapy for a course of about six months. The cost of treatment is $2,000 to $3,000. Approximately 65 percent of cats that receive treatment go into remission, and about 25 percent of them survive more for more than two years.

- Tufts University