There has also been a well documented doubling in the prevalence of allergy and asthma in the last 25 years in industrialized countries. A number of theories have been proposed to account for this increase including poorer indoor air quality since houses were insulated to a greater degree for energy conservation. Another theory that has been proposed is referred to as the "hygiene" theory and suggests that we have steralized the environment surrounding infants to the point that their immune system is deflected towards the allergic immune mechanism. Partial support for this theory has come from epidemiologic studies performed in several countries that have found a lower prevalence of allergy in children living on farms compared to children living in cities.

Introduction:

Atopy has been shown in a number of studies to be a disease that occurs more commonly in western or industrialized societies compared to non-industrialized countries (1 2 3) and the prevalence of atopy has almost doubled in the last 25 years (4). It was hypothesized that this increased atopy might be related to the pollution associated with industrialization. However studies, which compared the prevalence of atopy in highly polluted cities in communist states with matched cities in western countries clearly showed that the increased atopy was related to western society and not the pollution (5 6 7 8)  There have been a number of hypotheses put forward to account for the increase level of atopy and it has been suggested that in westernized societies, infants are not challenged by contact with organisms found in more rural societies. As a result there immune systems may not move towards TH1 responses that are thought to turn off the TH2 responses that govern allergy.

In fact even in industrialized western societies it has been found that children in rural farm families have less atopy than their city dwelling cousins.

ABSTRACTS:

Ernst P, Cormier Y. Relative Scarcity of Asthma and Atopy among Rural Adolescents Raised on a Farm.(Canada)
Am J Respir Crit Care Med 2000;161:1563-1566.Go to Abstract

We determined the prevalence of markers of atopy and asthma among 1,199 rural secondary school students ages 12 to 19 yr. Subjects identified as having been raised on a farm and half as many subjects without regular exposure to a farming environment from the same school class completed a respiratory symptom questionnaire and underwent allergy skin tests and a methacholine bronchoprovocation test. Current wheeze, airways hyperresponsiveness (AHR), and skin test positivity to inhaled allergens were all significantly less common among adolescents raised on the farm and these differences were especially pronounced in girls. After adjusting for gender and current smoking, the odds ratios for being raised on a farm were: 0.70 (95% CI 0.52 to 0.95) for current wheeze; 0.59 (95% CI 0.37 to 0.95) for asthma, defined as the concomitant occurrence of wheeze and AHR; and 0.58 (95% CI 0.46 to 0.75) for atopy defined as a positive reaction to any one of 24 common inhaled allergens. These associations were also not significantly altered by adjusting for the difference in the number of siblings.

Kilpelainen M, Terho EO, Helenius H, Koskenvuo M. Farm environment in childhood prevents the development of allergies.(Finland)
Clin Exp Allergy 2000;30:201-8.Go to Abstract

BACKGROUND: A protective effect of infections in early life might explain the firmly reported finding of an inverse association between atopic disorders and large sibships.
OBJECTIVE: To study the effect of childhood farm, rural non-farm and urban environment, as well as family size and other factors on the occurrence of asthma, wheezing and atopic disorders up to young adulthood.
METHODS: Data on lifetime prevalence of physician-diagnosed asthma, allergic rhinitis and/or allergic conjunctivitis, atopic dermatitis, as well as self-reported episodic wheezing from 10 667 Finnish first-year university students aged 18-24 years were collected by a postal questionnaire. Associations of lifetime prevalence of the diseases with living on a farm, in a rural non-farm and urban environment during childhood were estimated by logistic regression analysis. Adjustment was made for potential confounding by gender, parental atopy, parental education, number of older siblings, day care outside the home and passive smoking.
RESULTS: The childhood farm environment independently reduced the risk for physician-diagnosed allergic rhinitis and/or allergic conjunctivitis (adjusted odds ratio 0.63, 95% CI 0.50-0.79, P <0.001), and for diagnosed asthma and episodic wheezing analysed together (OR 0.71, 95% CI 0.54-0.93, P < 0.05), but not for atopic dermatitis during lifetime. Urban childhood environment did not show independent increased risk when compared with rural non-farm residence. The inverse association of sibship size with the occurrence of allergic rhinitis and/or allergic conjunctivitis was found among subjects with one (OR 0.86, 95% CI 0.77-0.96, P < 0.01) or at least four older siblings (OR 0.47, 95% CI 0.26-0.84, P < 0.05). CONCLUSION: Childhood farm environment seems to have a protective effect against allergic rhinitis and/or conjunctivitis, and more weakly against asthma and wheezing irrespective of family size. Environmental exposure to immune modulating agents, such as environmental mycobacteria and actinomycetes, favouring manifestation of a nonatopic phenotype could explain the finding.

Von Ehrenstein OS, Von Mutius E, Illi S, Baumann L, Bohm O, von Kries R.
Reduced risk of hay fever and asthma among children of farmers. (Germany)
Clin Exp Allergy 2000;30:187-93.Go to Abstract

BACKGROUND: The prevalence of atopic diseases is on the rise. Traditional lifestyles may be associated with a reduced risk of atopy.
OBJECTIVES: To test the hypothesis that children living on a farm have lower prevalences of atopic diseases. To identify differences in living conditions between farmers and other families which are associated with the development of atopic conditions.
DESIGN: Cross-sectional survey among children entering school (aged 5-7 years). A written questionnaire including the ISAAC core questions and asking for exposures on a farm and elsewhere was administered to the parents.
Setting: School health entry examination in two Bavarian districts with extensive farming activity.
Subjects: 10,163 children.
MAIN OUTCOME MEASURES: The prevalence of doctor's diagnoses and symptoms of hay fever, asthma and eczema as assessed by parental report.
RESULTS: Farmers' children had lower prevalences of hay fever (adjusted odds ratio = 0. 52, 95% CI 0.28-0.99), asthma (0.65, 0.39-1.09), and wheeze (0.55, 0. 36-0.86) than their peers not living in an agricultural environment. The reduction in risk was stronger for children whose families were running the farm on a full-time basis as compared with families with part-time farming activity. Among farmers' children increasing exposure to livestock was related to a decreasing prevalence of atopic diseases (aOR = 0.41, 95% CI 0.23-0.74).
CONCLUSIONS: Factors related to environmental influences on a farm such as increased exposure to bacterial compounds in stables where livestock is kept prevent the development of allergic disorders in children.

Since the publication of the first three studies, the observation has been confirmed in other western countries.

Portengen L, Sigsgaard T, Omland O, Hjort C, Heederik D, Doekes G. Low prevalence of atopy in young Danish farmers and farming students born and raised on a farm. Clin Exp Allergy 2002 Feb;32(2):247-53. Go to Abstract

Downs SH, Marks GB, Mitakakis TZ, Leuppi JD, Car NG, Peat JK. Having lived on a farm and protection against allergic diseases in Australia. Clin Exp Allergy 2001 Apr;31(4):570-5 Go to Abstract

Horak F, Studnicka M, Gartner C, Veiter A, Tauber E, Urbanek R, Frischer T. Parental farming protects children against atopy: longitudinal evidence involving skin prick tests. Clin Exp Allergy 2002 Aug;32(8):1155-9 Go to Abstract

Kauffmann F, Oryszczyn MP, Maccario J. The protective role of country living on skin prick tests, immunoglobulin E and asthma in adults from the Epidemiological study on the Genetics and Environment of Asthma, bronchial hyper-responsiveness and atopy. Clin Exp Allergy 2002 Mar;32(3):379-86 Go to Abstract

Klintberg B, Berglund N, Lilja G, Wickman M, van Hage-Hamsten M. Fewer allergic respiratory disorders among farmers' children in a closed birth cohort from Sweden. Eur Respir J 2001 Jun;17(6):1151-7. Go to Abstract

Riedler J, Eder W, Oberfeld G, Schreuer M. Austrian children living on a farm have less hay fever, asthma and allergic sensitization. Clin Exp Allergy 2000 Feb;30(2):194-200 Go to Abstract

COMMENTS:

There is striking agreement between these studies. Thus even in western countries there are pockets of lower prevalence of atopy that approximate the differences seen between the countries in Eastern Europe and those in the west. Given that there has been a virtual doubling in the prevalence of atopy in western countries in the last 25 years, it is apparent that that observation may not apply to farming areas within the western countries. If it could be determined that this doubling has not occurred in farming areas i.e that the prevalence of atopy has remained constant and if the reason for this could be determined, we might have the beginning of an understanding of the regulation of the expression of the putative gene(s) for atopy. This would open the possibility to reversing the trend to increasing atopy. The hypothesis that exposure to bacterial compound on farms by stimulating a Th1 response that turns off the Th2 or allergic response is intriguing. If such bacterial compounds could be isolated then the possibility of a vaccine for preventing atopy might be in reach.