Am J Epidemiol 2003; 158:1227.
Copyright © 2003 by the Johns
Hopkins Bloomberg School of Public Health
LETTERS TO THE EDITOR |
RE: "ENVIRONMENTAL TOBACCO SMOKE AND RISK OF MALIGNANT LYMPHOMA IN PET CATS"
Thame Thrombosis and Haemostasis Research Foundation, Thame, Oxon OX9 3NY, United Kingdom
The paper by Elizabeth Bertone et al. (1) on environmental tobacco smoke exposure and the increased risk of malignant lymphoma in pet cats is unconvincing.
For the association of active smoking in humans and risk of non-Hodgkin’s lymphoma, most large studies have shown no increased risk. A recent review of all cohort and case-control studies concluded that there were no grounds for rejecting the null hypothesis (2). The relative risk of 3.8 quoted by Bertone et al. was for the smallest cohort study that represented a sample size of 2 percent of all cohort studies (3). A similar small study gave the opposite result with a relative risk of 0.58 (2). Coupled with this is the inescapable observation that the incidence of non-Hodgkin’s lymphoma has increased year by year since 1975 despite increasing smoking cessation during the last four decades (4). Yet this study on environmental tobacco smoke exposure in cats appears to give greater risks for malignant lymphoma than for the risks of non-Hodgkin’s lymphoma with active smoking in humans.
Bertone et al. refer to the association between environmental tobacco smoke exposure prior to a child’s birth and risk of lymphoma but, as with many other diseases in children attributed to environmental tobacco smoke exposure (5), the correlation between environmental tobacco smoke exposure and childhood cancer is extremely tenuous. In one of the references cited by Bertone et al. (6), there was no significant trend for risk of childhood cancer with maternal smoking before pregnancy when using general population controls, and there was a negative trend for maternal smoking when comparison was made against hospital controls (p = 0.001).
The dosimetry of environmental tobacco smoke exposure in humans must also be considered. Using the most sensitive methods for the determination of serum cotinine at the National Center for Environmental Health, Pirkle et al. (7) showed that the mean level in environmental tobacco smoke-exposed nonsmokers was 0.6 ng/ml compared with 300 ng/ml in active smokers, or 1/500th of the level. Even though cats have a raised respiration rate compared with humans, their cotinine levels by environmental tobacco smoke inhalation must be infinitesimal compared with those of actively smoking humans. Particulate matter ingested by grooming seems to have little effect because this would be greater in long-haired cats, but they had a nonsignificant inverse risk for malignant lymphoma. Bertone et al. made no attempt at an objective measurement of environmental tobacco smoke exposure by comparing cotinine levels in exposed and nonexposed cats, although it appeared from their Materials and Methods that they were able to do so.
Finally, it is true that the proportion of malignant lymphoma cases with concurrent feline leukemia virus infection has decreased. However, it is important to know how many cats tested feline leukemia virus positive and how many had been vaccinated against this virus in the exposed and nonexposed groups. Cigarette smoking is becoming rare in higher socioeconomic groups but common in lower socioeconomic groups. It is the latter group that would not be able to afford to have their animals vaccinated and thus would place them at a higher risk for malignant lymphoma. Considering all the evidence, this is a more likely explanation for the higher risk in exposed cats.
REFERENCES
REFERENCES
- Bertone ER, Snyder LA, Moore AS. Environmental tobacco smoke and risk of malignant lymphoma in pet cats. Am J Epidemiol 2002;156:268–73.
[Abstract/Free Full Text] - Peach HG, Barnett NE. Critical review of epidemiological studies of the association between smoking and non-Hodgkin’s lymphoma. Hematol Oncol 2001;19:67–80.[CrossRef][Web of Science][Medline]
- Linet MS, McLaughlin JK, Hsing AW, et al. Is cigarette smoking a risk factor for non-Hodgkin’s lymphoma or multiple myeloma? Results from the Lutheran Brotherhood Cohort Study. Leuk Res 1992;16:621–4.[CrossRef][Web of Science][Medline]
- Dinse GE, Umbach DM, Sasco AJ, et al. Unexplained increases in cancer incidence in the United States from 1975 to 1994: possible sentinel health indicators. Annu Rev Public Health 1999;20:173–209.[CrossRef][Web of Science][Medline]
- Denson KWE. Passive smoking in infants, children and adolescents. The effects of diet and socioeconomic factors. Int Arch Occup Environ Health 2001;74:525–32.[Web of Science][Medline]
- Sorahan T, McKinney PA, Mann JR, et al. Childhood cancer and parental use of tobacco: findings from the inter-regional epidemiological study of childhood cancer (IRESCC). Br J Cancer 2001;84:141–6.[CrossRef][Web of Science][Medline]
- Pirkle JL, Flegal KM, Bernert JT, et al. Exposure of the US population to environmental tobacco smoke: the Third National Health and Nutrition Examination Survey, 1988 to 1991. JAMA 1996;275:1233–40.
[Abstract/Free Full Text]
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