Exposure to farming environments in childhood and asthma and wheeze in rural populations: a systematic review with meta-analysis.

Asthma is a chronic respiratory disorder that has become substantially more common over the past decades. One environmental factor for which particularly strong associations with asthma and allergic diseases have been described is exposure to farming environments in childhood. The aim of this systematic review was to update and extend existing narrative reviews, test for heterogeneity of effect across studies, and conduct a meta-analysis to report a summary effect measure. Published relevant literature was searched through PubMed including all articles added to PubMed before September 1, 2011. Articles were included if they reported an epidemiological study on the exposure to a farming environment in childhood and subsequent wheeze or asthma. Heterogeneity of effect measures across studies was evaluated using Cochran's Q and I(2). Random-effects meta-analysis was performed to summarize effect measures for various outcome definitions. In total, 357 retrieved abstracts revealed 52 original articles from 39 studies with data considered for the meta-analysis. Most studies were conducted among children or on childhood onset of disease. Most data were published on doctor-diagnosed asthma or current wheeze. The meta-analysis showed substantial heterogeneity across studies with similar outcome definitions. Nonetheless, the combined effects were statistically significant and showed an approximate 25% lower asthma prevalence among exposed subjects compared with unexposed subjects. The protective 'farm-effect' on asthma was reported in numerous studies. Its underlying factors ought to be studied, and promising efforts have been already made. However, the heterogeneity of the effect across studies should also be investigated because whatever causes it is a potential threat to valid synthesis of evidence and to the detection of specific protective factors.