doi:10.1136/thorax.55.suppl_1.S2 2000;55;2-10 Thorax DP Strachan of the 'hygiene hypothesis'Family size, infection and atopy: the first decade http://thorax.bmj.comUpdated information and services can be found at: These include: References http://thorax.bmj.com#otherarticles53 online articles that cite this article can be accessed at: serviceEmail alertingthe top right corner of the article Receive free email alerts when new articles cite this article - sign up in the box atTopic collections (1845 articles) Other Pediatrics (1200 articles) Asthma Articles on similar topics can be found in the following collections Notes http://www.bmjjournals.com/cgi/reprintformTo order reprints of this article go to: http://www.bmjjournals.com/subscriptions/ go to: ThoraxTo subscribe to on 10 April 2007 thorax.bmj.comDownloaded fromThorax 2000;55(Suppl 1):S2–S10S2Family size, infection and atopy:the first decade of the “hygienehypothesis”David P StrachanDepartment of Public Health Sciences, St George’s Hospital Medical School,London SW17 0RE, UKIntroductory articleThe magnitude of the effect of smaller family sizes on the increase in theprevalence of asthma and hay fever in the United Kingdom and New ZealandK Wickens, J Crane, N Pearce, R BeasleyBackground. Declining family size is one factor that has been proposed to contribute to increasingasthma and hay fever prevalence, but its relative importance has not been quantified. Objective. Ourpurpose was to determine the change in asthma and hay fever prevalence that would be expectedfrom the reduction in family size that has occurred in England/Wales and New Zealand over recentdecades. Methods. The relative change in family size between 1961 and 1991 in England/Wales andNew Zealand was determined from census data for these years. Summary weighted odds ratios werecalculated for the associations among birth order, family size, and asthma and hay fever prevalence.The expected increase in the prevalence of asthma and hay fever between 1961 and 1991 resultingfrom changes in family size was then calculated. Results. The expected relative increase in the prevalenceof asthma between 1961 and 1991 as a result of the smaller family size was 1% and 5% for England/Wales and New Zealand, respectively; smaller family size would be expected to increase the prevalenceof hay fever prevalence in England/Wales by 4%. Conclusions. Changes in family size over the last 30years do not appear to explain much of the reported increase in asthma or hay fever prevalence. Thecontribution that other risk factors have made to these increases could be assessed with use of a similarapproach. (J Allergy Clin Immunol 1999;104:554–8)In 1989 I proposed a novel but speculative explanation suppressing the Th2 immune responses involved in IgEmediated allergy.34Although the Th1/Th2 paradigmfor the principal epidemiological features of hay feverand the apparent rise in the prevalence of allergic dis- may not be as clear in humans as it first appeared inrodents,5the “hygiene hypothesis” has remained ofeases.1Colloquially named the “hygiene hypothesis”,this stated, in summary: interest to both immunologists and epidemiologiststhroughout the 1990s and has been the subject of a“These observations . . . could be explained if allergicdiseases were prevented by infection in early childhood, number of editorials and review articles.6–14This commentary summarises the epidemiologicaltransmitted by unhygienic contact with older siblings, oracquired prenatally . . . Over the past century declining family evidence which has emerged during the 1990s relatingfamily size, infections, and immunisations to atopy andsize, improved household amenities and higher standards ofpersonal cleanliness have reduced opportunities for cross- allergic diseases, and looks forward to the researchagenda for the next decade.infection in young families. This may have resulted in morewidespread clinical expression of atopic disease.”At first this hypothesis was received with scepticismbecause the prevailing immunological thinking con- Epidemiology of atopy and allergic diseases sidered infection as a potential trigger of allergic sens-itisation rather than as a protective influence.2However, The introductory article by Wickens et al15concentrateson one of the most striking epidemiological features ofduring the early 1990s a plausible mechanism arosefrom the distinction of Th1 and Th2 lymphocyte popu- allergy – the inverse association with family size, whichhas been consistently found in studies of hay fever, skinlations in laboratory animals and the recognition that“natural immunity” to bacterial and viral infections prick positivity, and specific IgE.16Many, but not all,studies find a stronger “protective” influence for olderinduces a Th1 pattern of cytokine release, potentially on 10 April 2007 thorax.bmj.comDownloaded fromFamily size, infection and atopy: the first decade of the “hygiene hypothesis” S3siblings than for younger siblings and, among the tendance in childhood was associated with a small, non-significant excess of atopy.28British and New Zealand studies summarised by Wick-ens et al,15this is the predominant pattern for hay fever, The balance of evidence does not therefore suggesta relationship between allergy and early child contactsalthough not for asthma – a point to which we returnshortly. outside the home, which is difficult to reconcile withthe “hygiene hypothesis”. In retrospect, it is interestingThe “hygiene hypothesis”1was an attempt to offer aparsimonious and coherent explanation for the current to note that Finnish investigators publishing in 198436had obviously set out to test the hypothesis that earlyobservations of differential hay fever risk within familiesand the historical evidence suggesting that hay fever infection increased,2rather than decreased the risk ofallergic symptoms. Finding no difference with respecthad emerged as a “post-industrial revolution epidemic”during the 19th century17and continued to increase in to day care attendance, they concluded that “atopyprobably cannot be prevented by protecting small chil-prevalence throughout the 20th century.18–20In 1989 the influence of family size and structure was dren from infection”. The direction of the causal hypo-thesis has certainly changed in the intervening years!apparent only
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