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Abstract & Commentary
Exposure to Environment Micro-organisms and Childhood Asthma
By Hal B. Jenson, MD, FAAP, Professor of Pediatrics, Tufts University School of Medicine, and Chief Academic Officer, Baystate Medical Center, Springfield, MA, is Associate Editor for Infectious Disease Alert.
Dr. Jenson reports no financial relationship to this field of study.
Synopsis: Children living full-time on family-run farms were exposed to a wider spectrum of indoor microbes, which may account for the relationship between growing up on a farm and the reduced risk for developing childhood asthma.
Source: Ege MJ, et al. Exposure to environmental microorganisms and childhood asthma. N Engl J Med 2011;364:701-709.
Two separate, cross-sectional studies involving 6,843 and 9,668 children 6-13 years of age in South Germany randomly compared children living on farms with reference groups for the prevalence of asthma relative to the exposure to and diversity of indoor microorganisms. The first study screened 489 samples of mattress dust for bacterial DNA using single-strand conformation polymorphism (SSCP), which detects environmental bacteria that cannot be cultured. In this first study population, 52% of children lived on farms and had an 8% prevalence of asthma. The second study screened settled dust collected over 14 days by electrostatic dust collectors from 444 children's rooms for bacteria and fungi using quantitative cultures. In this second study population, 16% of children lived on farms with an 11% prevalence of asthma.
Children living full-time on family-run farms were classified as members of the farm group, and all other children were classified as members of the reference group. Asthma was defined as physician-diagnosed asthma on at least one occasion, or "wheezy bronchitis" on more than one occasion.
In both studies, children who lived on farms were exposed more frequently and to a greater variety of environmental microorganisms, and had lower prevalences of asthma and atopic disease. The diversity of microbial exposure was inversely related to the risk of asthma (odds ratio [OR] for the first study, 0.62; 95% confidence interval [CI], 0.44 0.89; OR for the second study, 0.86; 95% CI, 0.75-0.99), but not to the risk of atopy, independent of farming. Exposure to certain species was also inversely related to the risk of asthma, including exposure to species in the fungal taxon eurotium (adjusted loves ratio, 0.37; 95% CI, 0.18-0.76) and to a variety of bacterial species including Listeria monocytogenes, Bacillus, corynebacterium, and others (adjusted OR, 0.57; 95% CI, 0.38-0.86).
In this study, children living on farms had exposure to a greater diversity of environmental microorganisms than children who did not live on farms. A greater diversity of indoor environmental microbial exposure was inversely related to the risk of asthma, but not to atopy. It is possible that the indoor microbial diversity for children living on farms in these studies was a marker for exposure to even greater outdoor microbial diversity.
These results are consistent with the "hygiene hypothesis," which states that reduced childhood exposure to environmental microorganisms actually increases the risk of childhood asthma and atopic disease, presumably by affecting the natural development of certain aspects of immunity. Exposure to nonpathogenic microbes in early life might promote tolerance by triggering the innate immune system and activating signaling pathways that induce T cells. Activation of type I helper T cells may promote immunologic tolerance and counterbalance the predominance of type 2 helper T cells, which are more characteristic of asthma. Another hypothesis might be that low-level and continuing exposure to a broad range of microorganisms may prevent colonization with bacteria and fungi that increase the risk of asthma.
The studies used state-of-the-art methods for microbiological detection, though the methods are relatively crude and did not permit identification of specific protective pathogens, only broad families of species within microbial taxa. It is intriguing to consider that future studies using much more advanced technologies could identify specific microbes that confer protection to developing childhood asthma. The studies could also shed light on the mechanism of the protective benefit of exposure to microbial diversity.