The hygiene hypothesis and atopic disease

The hygiene hypothesis revisited: does exposure to infectious agents protect us from allergy?

PURPOSE OF REVIEW

The increase in incidence and prevalence of allergic disease remains a mystery and cannot be explained solely by genetic factors. The hygiene hypothesis provides the strongest epidemiological explanation for the rise in allergic disease. This review evaluates the recent epidemiological and mechanistic research in the role of infectious agents in the pathogenesis of or protection from allergic disease.

RECENT FINDINGS

Recent literature has extended the epidemiological findings of the protective effect of being born and reared in a farm environment and associates an increased diversity of organisms in house-dust samples with protection from allergic disease. Furthermore, human and animal studies provide increasing evidence for the role of both the innate and adaptive immune systems, including regulatory cells, as mediators of this protective effect.

SUMMARY

There is evidence that exposure to some infectious organisms can protect from atopy, whereas other infections appear to promote allergic diseases. The timing of exposure to infection and the properties of the infectious agent, in addition to the genetic susceptibility of the host, play an important role in the future development of allergic disease.

Prevalence of allergic rhinitis in 3-6-year-old children in Wuhan of China

BACKGROUND

Only a few prevalence studies of allergic rhinitis (AR) have been reported in China. This study aimed to evaluate the prevalence of AR in a population of 3-6-year-old children in Wuhan, China.

METHODS

Sixteen kindergartens in Wuhan City were randomly selected; for each kindergarten, there were three classes from three grades (top, middle and bottom grade, 3-6 years old, respectively). Questionnaires generated by the authors were distributed and filled out by parents of the selected children, with a telephone interview subsequently. Skin prick test (SPT) was carried out on the children after a written consent was signed by the parents.

RESULTS

A total of 1211 (89.5%) valid questionnaires were returned for evaluation. The adjusted current prevalence of AR in 3-6-year-old children was 10.8% with the diagnostic criterion of nasal symptoms(+) and SPTs(+). In the SPTs(+) children, the most common inhalant allergen was house dust mites (94.7%), followed by moulds (28.4%). The prevalence of AR was higher in males than that in females (13.0% vs. 7.7%, P < 0.05). 15.8% and 23.2% of AR children were sensitive to egg and milk, respectively. The percentage of children sensitive to both inhalant and food allergens was 27.4%.

CONCLUSIONS

We found an unexpectedly high prevalence of diagnosed AR in 3-6-year-old children within the investigated population. Dust mite was the most important allergen source for 3-6-year-old children in Wuhan.

Allergic rhinoconjunctivitis: epidemiology

Allergic rhinoconjunctivitis has been studied much less frequently than asthma using epidemiologic approaches. Population-based studies are difficult to conduct because of misclassification arising from the reliance on self-reported questionnaires that use terms such as allergic rhinitis or hay fever to establish the diagnosis. In addition, many epidemiologic studies focus on diagnostic skin testing or allergen-specific IgE antibodies (RASTs) as an objective outcome to assess for hay fever. These techniques are helpful but not perfect measures for predicting hay fever outcomes in epidemiologic studies. It is generally accepted, however, that allergic rhinoconjunctivitis is one of the most common of chronic diseases and is the most common atopic disorder. This article reviews the definition of allergic rhinoconjunctivitis, the epidemiology of this disorder from infancy into adulthood, and environmental risk factors for development of sensitization to certain allergens.

Infections and atopy: an exploratory study for a meta-analysis of the "hygiene hypothesis"

BACKGROUND

According to the "hygiene hypothesis" selected allergic diseases could be prevented by exposure to infectious agents during early childhood.

METHODS

This study was performed to assess the feasibility of a future meta-analysis on the "hygiene hypothesis" and atopic diseases. Differences concerning the potential association with a history of infectious events, in terms of magnitude and homogeneity of global risk estimates between the three major atopic diseases (i.e. atopic dermatitis, asthma and allergic rhinitis) were examined. We conducted a preliminary analysis on a sample of articles published on this topic and cited in a recent and authoritative review.

RESULTS

The ranges of relative risks estimates (between 0.6 and 0.8) were similar for atopic dermatitis, allergic rhinitis and asthma. Compared with asthma and allergic rhinitis, reported global risk estimates were more stable for atopic dermatitis (lowest heterogeneity). Our analysis suggests that three main categories of indirect markers of exposure to infection can be identified: 1) geographical gradient, 2) indices of potential contact with infectious agents (such as number of siblings) and 3) history of infectious events.

CONCLUSIONS

In this exploratory study, we chose articles cited in a single review and obtained a preliminary quantification of the association between infections and atopic diseases. The association with indirect markers of infection corresponded to 20% protection for atopic dermatitis, 30% for allergic rhinitis and 40% for asthma. In a subsequent meta-analysis, diseases should be considered separately and differences between types of exposures should be taken into account as one of the major end-points, with attention to time since exposure and disease onset.