The associations between childhood acute lymphoblastic leukemia (ALL) and several factors related to early stimulation of the immune system, that is, farm residence and regular contacts with farm animals (livestock, poultry) or pets in early childhood, were investigated using data from 13 case–control studies participating in the Childhood Leukemia International Consortium. The sample included 7847 ALL cases and 11,667 controls aged 1–14 years. In all studies, the data were obtained from case and control parents using standardized questionnaires. Pooled odds ratios (ORs) and 95% confidence intervals (CIs) were estimated by unconditional logistic regression adjusted for age, sex, study, maternal education, and maternal age. Contact with livestock in the first year of life was inversely associated with ALL (OR = 0.65, 95% CI: 0.50, 0.85). Inverse associations were also observed for contact with dogs (OR = 0.92, 95% CI: 0.86, 0.99) and cats (OR = 0.87, 95% CI: 0.80, 0.94) in the first year of life. There was no evidence of a significant association with farm residence in the first year of life. The findings of these large pooled and meta-analyses add additional evidence to the hypothesis that regular contact with animals in early childhood is inversely associated with childhood ALL occurrence which is consistent with Greaves’ delayed infection hypothesis.
Bibliographical noteFunding Information:
1INSERM U1153, Epidemiology and Biostatistics Sorbonne Paris Cité Center (CRESS), Epidemiology of Childhood and Adolescent Cancers Team (EPICEA), Paris-Descartes University, Villejuif, France 2Dipartimento di Medicina Traslazionale, Università del Piemonte Orientale, AOUMaggiore della Carità & CPO, Piemonte, Novara, Italy 3Department of Hygiene, Epidemiology, and Medical Statistics, School of Medicine, University of Athens, Athens, Greece 4Department of Medicine, Clinical Epidemiology Unit, Karolinska Institute, Stockholm, Sweden 5Department of Preventive and Social Medicine, Dunedin School of Medicine, University of Otago, Dunedin, New Zealand 6School of Public Health, University of California, Berkeley, California 7Telethon Kids Institute, University of Western Australia, Perth, Western Australia, Australia 8Unit of Occupational Medicine, Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden 9Department of Epidemiology, Biostatistics, and Occupational Health, Faculty of Medicine, McGill University, Montréal, Quebec, Canada 10School of Public Health, University of Saõ Paulo, Saõ Paulo, Brazil 11Central American Institute for Studies on Toxic Substances (IRET), Universidad Nacional, Heredia, Costa Rica 12Division of Epidemiology and Clinical Research, Department of Pediatrics and Masonic Cancer Center, University of Minnesota, Minneapolis, Minnesota
We would like to thank our dear colleague and friend, Patricia Buffler, who passed away after the initiation of the present pooled analysis. She was a founding member and Chair of CLIC as well as the driving force behind the NCCLS. The French authors are grateful to Maëlle Meurant, Sandrine Pinto, Boris Sidje, Sofiene Ben Salha for technical assistance and Jérémie Rudant who contributed to this work. Additional acknowledgements by study are provided in Supporting Information.
- childhood leukemia
- farm residence