Introduction

The prevalence of paediatric allergic diseases has recently increased in developed countries. Adoption of Western lifestyle was noted as a contributing factor, but it has not been sufficiently verified.1,2 Western lifestyle is characterised by an increase in the time spent indoors, highly insulated airtight houses, and a more hygienic environment; in addition, it may also be characterised by changes in the pattern of exposure to house dust mite allergen and microorganisms that influence the prevalence of paediatric allergic diseases.2There is sufficient scientific evidence for dust mite allergen as a factor aggravating asthma symptoms, but there is no evidence that it is a risk factor for asthma incidence.3,4 A European large-scale birth cohort study showed that exposure to a high concentration of allergens immediately after birth influences the development of allergic sensitisation.5,6 Moreover, exposure to dust mite allergens is associated with dust mite sensitisation.7 However, intervention studies reported no associations between dust mite exposure in infancy and asthma onset at 3 8 or 8 years of age.9Reports suggest no consistency in the association between endotoxin, a cell wall component of gram-negative rods and microorganism exposure marker, and asthma development. Braun-Fahrländer et al. reported that in farming areas in Germany, Austria, and Switzerland, the endotoxin concentration in bed dust was higher in children of farmers than in that of other children, and significantly fewer children of farmers had pollinosis, asthma, and atopic sensitisation.10,11Mendy et al. reported a positive correlation between the endotoxin concentration in dust (endotoxin concentration per gram of dust) and wheezing in infants and toddlers (0-3 years old) but an inverse correlation with asthma after they reached school age.12Furthermore, recent studies in adult Americans revealed that the influence on allergic disease differs due to climate and lifestyle because of differences in exposure to endotoxin and indoor allergens such as mites and moulds.13,14 Therefore, it is necessary to clarify the association of exposure to dust mite allergen and endotoxin with asthma development in Asian regions under different climatic regions and lifestyles in comparison with that in Western countries. Although large-scale birth cohort studies have been conducted in Western countries, there are few reports from Asia, including Japan. We conducted a large-scale birth cohort study on the effects of early-life exposure to dust mite allergen and endotoxin in mattresses on asthma and wheezing development by 0-36 months old in Japan.