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

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


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.

Keywords: Animals; childhood leukemia; contact; farm residence.

Publication types

  • Meta-Analysis
  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Adolescent
  • Age Factors
  • Animals
  • Animals, Domestic*
  • Case-Control Studies
  • Child
  • Child, Preschool
  • Environmental Exposure / adverse effects*
  • Farms*
  • Female
  • Humans
  • Infant
  • Male
  • Odds Ratio
  • Precursor Cell Lymphoblastic Leukemia-Lymphoma / epidemiology*
  • Precursor Cell Lymphoblastic Leukemia-Lymphoma / etiology*
  • Public Health Surveillance
  • Risk Assessment
  • Risk Factors
  • Socioeconomic Factors