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Cancer Med. 2018 Jun;7(6):2665-2681. doi: 10.1002/cam4.1466. Epub 2018 Apr 16.

Living on a farm, contact with farm animals and pets, and childhood acute lymphoblastic leukemia: pooled and meta-analyses from the Childhood Leukemia International Consortium.

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INSERM U1153, Epidemiology and Biostatistics Sorbonne Paris Cité Center (CRESS), Epidemiology of Childhood and Adolescent Cancers Team (EPICEA), Paris-Descartes University, Villejuif, France.
Dipartimento di Medicina Traslazionale, Università del Piemonte Orientale, AOUMaggiore della Carità & CPO, Piemonte, Novara, Italy.
Department of Hygiene, Epidemiology, and Medical Statistics, School of Medicine, University of Athens, Athens, Greece.
Department of Medicine, Clinical Epidemiology Unit, Karolinska Institute, Stockholm, Sweden.
Department of Preventive and Social Medicine, Dunedin School of Medicine, University of Otago, Dunedin, New Zealand.
School of Public Health, University of California, Berkeley, California.
Telethon Kids Institute, University of Western Australia, Perth, Western Australia, Australia.
Unit of Occupational Medicine, Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden.
Department of Epidemiology, Biostatistics, and Occupational Health, Faculty of Medicine, McGill University, Montréal, Quebec, Canada.
School of Public Health, University of Saõ Paulo, Saõ Paulo, Brazil.
Central American Institute for Studies on Toxic Substances (IRET), Universidad Nacional, Heredia, Costa Rica.
Division of Epidemiology and Clinical Research, Department of Pediatrics and Masonic Cancer Center, University of Minnesota, Minneapolis, Minnesota.


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.


Animals; childhood leukemia; contact; farm residence

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