Biodiversity and childhood asthma: A nationwide retrospective birth cohort study in Japan.

Background: Empirical evidence to support an association between biodiversity and childhood asthma remains limited, and the contributions of specific taxonomic groups have been poorly understood. This nationwide, retrospective cohort study aimed to evaluate the associations between a wide range of biodiversity indices during pre-, peri-, and post-natal periods and childhood persistent asthma. Methods: We included child-mother dyads between January 2005 and January 2017 from the Japan Medical Data Center database. Species richness, net relatedness index (genetic diversity), and land-use types were defined based on the municipalities in which the children resided during each exposure period. The primary outcome was persistent asthma in children aged 4-5 years. Modified Poisson regression models were used to evaluate the association between the biodiversity indices and childhood persistent asthma. Results were adjusted for confounders and predictors of asthma, including infant sex, cesarean section delivery, parental history of asthma, particulate matter ≤2.5 µm in aerodynamic diameter, nitrogen dioxide, population density, metropolitan area status, municipality-level area deprivation index, and prefectural-level smoking prevalence. Results: A total of 109,233 child-mother dyads were included; 34.7% of the children had persistent asthma at 4-5 years of age. Greater species and genetic diversity of freshwater fish, along with stagnant water areas, were consistently associated with a 2-5% lower risk of childhood asthma. Conclusions: Biodiversity during early life was heterogeneously associated with childhood asthma. These findings suggest that conserving freshwater fish species and their habitats may contribute to asthma prevention in children.