Association of bacterial load in drinking water and allergic diseases in childhood

Background Treatment of drinking water may decrease microbial exposure. Objective To investigate whether bacterial load in drinking water is associated with altered risk of allergic diseases. Methods We recruited 1,110 schoolchildren aged 6-16 years between 2011 and 2013 in Pozega-Slavonia County in Croatia, where we capitalized on a natural experiment whereby individuals receive drinking water through public mains supply or individual wells. We obtained data on microbial content of drinking water for all participants; 585 children were randomly selected for more detailed assessments, including skin prick testing. Since water supply was highly correlated with rural residence, we compared clinical outcomes across four groups (Rural/Individual, Rural/Public, Urban/Individual and Urban/Public). For each child, we derived quantitative index of microbial exposure (bacterial load in the drinking water measured during the child's first year of life). Results Cumulative bacterial load in drinking water was higher (median [IQR]: 6390 [4190-9550] vs 0 [0-0]; P < .0001), and lifetime prevalence of allergic diseases was significantly lower among children with individual supply (5.5% vs 2.3%, P = .01; 14.4% vs 6.7%, P < .001; 25.2% vs 15.1%, P < .001; asthma, atopic dermatitis [AD] and rhinitis, respectively). Compared with the reference group (Urban/Public), there was a significant reduction in the risk of ever asthma, AD and rhinitis amongst rural children with individual supply: OR [95% CI]: 0.14 [0.03,0.67], P = .013; 0.20 [0.09,0.43], P < .001; 0.17 [0.10,0.32], P < .001. Protection was also observed in the Rural/Public group, but the effect was consistently highest among Rural/Individual children. In the quantitative analysis, the risk of allergic diseases decreased significantly with increasing bacterial load in drinking water in the first year of life (0.79 [0.70,0.88], P < .001; 0.90 [0.83,0.99], P = .025; 0.80 [0.74,0.86], P < .001; current wheeze, AD and rhinitis). Conclusions and Clinical Relevance High commensal bacterial content in drinking water may protect against allergic diseases.