Purpose of review
Reduced exposure to childhood infections may explain the increased prevalence of allergic diseases in industrialized countries (the hygiene hypothesis). This review will examine recent epidemiologic studies of the hygiene hypothesis and asthma.
Recent studies have confirmed previous findings of an inverse association between increased exposure to other children during childhood and either allergen sensitization or hay fever. However, there is conflicting evidence regarding the relation between exposure to other children and asthma. Although it has been hypothesized that vaccinations may influence the development of asthma, recent findings do not support this association. Serologic evidence of exposure to certain gastrointestinal pathogens (eg, hepatitis A virus) has been inversely associated with either allergen sensitization or asthma in some, but not all, recent studies. Although heavy infestation with certain parasites (eg, helminths) is protective against allergen sensitization, there is conflicting evidence regarding the relation between parasitic infection and asthma. The results of recent studies suggest that the relation between endotoxin exposure and asthma is complex and likely influenced by factors related to the exposure itself, the host, and other covariates. Although it has been postulated that antibiotic use in early life is a risk factor for asthma, this hypothesis is not supported by recent findings.
For every exposure studied with regard to the hygiene hypothesis, there are inconsistent findings in relation to asthma. The hygiene hypothesis is not likely to be the sole explanation for the ongoing asthma epidemic in industrialized nations.