Exposure to Environmental Microorganisms and Childhood Asthma

The New England Journal of Medicine February 24, 2011; Vol. 364, No. 8, pp. 701-9 Markus J. Ege, M.D., Melanie Mayer, Ph.D., Anne-Cécile Normand, Ph.D., JonGenuneit, M.D., William O.C.M. Cookson, M.D., D.Phil., Charlotte Braun-Fahrländer,M.D., Dick Heederik, Ph.D., Renaud Piarroux, M.D., Ph.D., Erika von Mutius, M.D. FROM ABSTRACT: Children who grow up in environments that afford them a wide range of microbialexposures, such as traditional farms, are protected from childhood asthma andatopy. In previous studies, markers of microbial exposure have been inverselyrelated to these conditions. Results In both studies, children who lived on farms had lower prevalences of asthma andatopy and were exposed to a greater variety of environmental microorganisms thanthe children in the reference group.Increased microbial exposure was inversely related to the risk of asthma:Reduced by 38% in one study and 14% in the other study, for an average reductionby 24%. Conclusions Children living on farms were exposed to a wider range of microbes than werechildren in the reference group, and this exposure explains a substantial fraction ofthe inverse relation between asthma and growing up on a farm. KEY POINTS FROM THIS STUDY: 1) This study looked at two groups of children whose exposure to microorganisms was evaluated differently. Each group involved children who grewup on rural farms and a group of controls who were not farm raised children.Combined, the study group consisted of 16,511 children between ages 6-13 years. 2) One study analyzed childrens mattress dust collected with a vacuum cleaner.One study analyzed childrens room dust collected with electrostatic dust collectors. 3) After collection, the dust was incubated for 7 days. The colonies weremicroscopically counted and identified and treated with Grams stain. 4) Environmental exposure to microorganisms has repeatedly been found to beinversely related to the manifestation of atopic diseases such as asthma and hayfever. 5) In the population with higher bacterial exposures, the prevalence of asthmaand atopy was substantially lower. 6) Children living on farms had a lower prevalence of asthma than children in thereference groups in both studies by a combined 25%. 7) Indoor microbial exposure is much more common and diverse in the farmingenvironment than in the nonfarming environment. 8) The risk of asthma decreased significantly with the increase in the number ofdetectable bacteria and fungi. 9) Children growing up on farms were protected from asthma and atopy. Thesechildren were exposed to a greater variety of environmental fungi and bacteria ascompared with children in the reference group who lived in the same regions. 10) These data support the idea that the greater diversity of microbial exposureamong children who live on farms is associated with protection from thedevelopment of asthma. 11) The transport of environmental microorganisms from animal sheds and barnsto the indoor environment has been documented. Even when indoors, childrenliving on farms were exposed to a greater variety of microbes than children who didnot live on farms. 12) Microorganisms may be protective against asthma by triggering the innateimmune system for a prominent Type 1 helper T cell response; predominance oftype 2 helper T cells is characteristic of asthma. [Innate Immune Response] 13) The results of both studied groups showed that children living on farms had awider range of microbial exposures than children in the reference groups, whichlargely explained the protective effect of the farming environment on thedevelopment of asthma in children. A COMMENT FROM DAN MURPHY Recall Article Review 9-12 indicated that early life exposure to antibiotics (which killthese microbes) was associated with a 52% increase in asthma.The book by immunologist Mary Ruebush, PhD, Why Dirt Is Good further detailswhy innately children are meant to come into contact with microbes; when theydont or when the microbes are killed with antibiotics, the immune system does not develop properly, there is a switch to a predominant Th2 response, and atopicdisorders (asthma, hay fever, eczema, etc.) develop.