US study suggests passive smoking may increase the risk of lymphoma in cats

In the past a number of studies have looked at the potential association between smoking and an increased risk of lymphoma in humans. These studies have not all agreed, but a number of them have suggested that smoking can increase the risk of developing this type of tumour.

Lymphoma is the single most common tumour seen in domestic cats, and in the past, the common occurrence of lymphoma has been largely linked to relatively widespread infection with feline leukaemia virus (FeLV) which is a known cause of lymphoma. However, several studies over recent years have suggested that relatively few cases of feline lymphoma seen now are directly associated with FeLV infection (probably largely due to greater control of this virus through both testing and vaccination of cats).

The study published recently by Elizabeth Bertone and colleagues (Bertone ER, Snyder LA, Moore AS (2002) Environmental Tobacco Smoke and the Risk of Malignant Lymphoma in Pet Cats. American Journal of Epidemiology 156:268-273) looking at the possible association between environmental tobacco smoke (ETS) exposure and the development of lymphoma in cats was therefore extremely interesting. We often think of the phrase 'passive smoking' when we consider exposure to tobacco smoke in the environment, but for cats in particular, exposure to environmental smoke can result in exposure to harmful toxins not only by inhalation (breathing) but also through grooming - tobacco smoke can become deposited on the haircoat and through grooming this could perhaps result in a significant alternative source of potential harmful chemicals.

In their paper, the authors of this study found that any exposure to ETS more than doubled the risk of a cat developing lymphoma (compared to cats not exposed to ETS). They further showed that this risk of developing lymphoma increased with a longer duration of exposure to ETS (more than five years), with an increased number of cigarettes being smoked in the household on a daily basis, and also where there was more than one smoker living in the same house as the cat.

Although based on relatively small numbers of cats (80 cats that developed lymphoma, and 114 'control' cats (ie, cats without lymphoma), this study has nevertheless given some extremely valuable insights. More than 40 chemicals have been identified in cigarette smoke that may potentially be toxic to cells in a way that could result in tumour formation, and this study provides strong evidence that exposure to ETS can be harmful to cats and may be a trigger for development of lymphoma.

It is clear that just as FeLV does not cause all cases of feline lymphoma, neither does exposure to tobacco smoke. Nevertheless, the clear message from this study is that exposure to tobacco smoke can increase the risk of a cat developing lymphoma, and the more smoke a cat is exposed to, the greater that risk is. This study does not provide direct evidence for a cause and effect association, but certainly raises the possibility of this being the case. Further, although this study was conducted in the USA , due to the nature of the study, there is no reason why the results would not be equally applicable to cats elsewhere.

This is an extremely important study - it raises the serious possibility that exposure to ETS is quantifiably detrimental to the health of cats. This is undoubtedly an area that will attract further research in the future, including the potential effect of ETS exposure on the development of other tumours and other diseases.