Discussion

In this study, the risks of the cumulative incidence rates of asthma and wheezing increased during 0-36 months old in children with a high level of dust mite allergen exposure in bed dust. Regarding the endotoxin exposure level, a significant positive OR was occasionally noted in the onset of asthma and wheezing in the high exposure group. In the simultaneous exposure to allergen, microorganisms, fungi, and air pollutants, these were found to interact with each other and promote or prevent allergic disease.27 As dust mite allergen and endotoxin were simultaneously investigated in only limited epidemiological studies, these are individually discussed below. In an integrated analysis of five geographically different European birth cohort studies, an association was noted between dust mite allergen exposure (2-6 months old) and sensitisation to house dust mite (≤6 years old), but no association was observed with an increase in the risk of asthma (≤6 years old), suggesting that house dust mite exposure alone is insufficient to increase the asthma risk and sensitisation with allergen bridges allergen exposure and the development of asthma.6,27 Similarly, in this study, the association of an increase in the risk of high levels of total serum IgE with dust mite allergen exposure was observed in the Q4 group and the OR was 1.58 (95% CI: 1.08-2.31). In addition, a trend was observed, suggesting an association between dust mite allergen exposure and sensitisation with allergen. This finding is consistent with the results of the five European birth cohort studies. On the contrary, no increase in the asthma risk was noted in the European cohorts, but a trend was observed between dust mite allergen exposure and the cumulative incidence rates of asthma and wheezing in the present study. This may have been due to differences in the dust mite allergen sampling point and the difference in the unit of Der 1 (μg/m2, μg/g). Regarding the sampling point, dust on the floor of the children’s bedroom was collected in the European cohort studies, whereas dust mite allergen in bed dust was evaluated in our study. The dust mite allergen concentration is higher in bed dust than that in dust on the bedroom floor and the dust mite allergen exposure level in children was also higher.26 Although it is necessary to pay attention to a simple comparison of the Der 1 concentration, the geometric mean was within a range of 0.06-0.97 μg/g in the European cohort studies, but the Der 1 concentration was higher at 3.88 μg/g in our study performed in Japan. Regarding the second point, the difference in the unit of Der 1 level, μg/g was used in the European cohort studies, whereas it was measured as μg/m2 in our study. The risk of mite sensitisation markedly increased when the Der 1 level exceeded 2 μg/g.28 In the present study, based on analysis of the amount per gram dust (unit: μg/g), no significant increase in total IgE levels was noted in the Q3 (3.17-8.37 μg/g) or Q4 (8.37 μg/g or higher) groups (Supp Info Figure 5), but in the evaluation of the amount per dust sampling area, a marked increase in total IgE levels was noted in the Q4 group (158.27 μg/m2 or larger) (Fig. 2), suggesting that the evaluation of the amount per dust sampling area is more important. In this study, a significant association with dust mite allergen was observed for the development of wheezing during 0-12 months old and the development of asthma during 0-24 months old in analysis by the onset age. The most important steps towards the development of mature systematic immune responses are taken in the first 1-2 years of life.5,29 Therefore, avoidance of early-life exposure to high concentrations of dust mite allergen may be important to prevent allergic diseases, such as asthma. Previous studies have lacked consistency in the association between endotoxin and asthma. For example, a cohort study was performed in an urban area of Boston involving children with parents having a medical history of atopy, in which simultaneous exposure to dust mite allergen (in the children’s bed) and endotoxin (in family room dust) was investigated until 2-3 months after birth, and the associations with asthma, wheezing, and atopy were surveyed until 7 years old. Endotoxin exposure was associated with an increase in the risk of asthma at 7 years old and wheezing at 1-7 years old (no change in the risk due to children’s age with wheezing).26 In a meta-analysis of 19 studies involving infants, the OR of wheezing in the high endotoxin exposure group (endotoxin concentration per gram of dust) compared to that in the low exposure group was 1.48 (95% CI: 1.10-1.98), demonstrating a positive correlation, whereas the OR of asthma after the participants reached school age was 0.82 (95% CI: 0.67-0.97), demonstrating an inverse correlation.12 Shamosollahi et al. systematically reviewed endotoxin exposure and health influence and discussed that “early life exposure to endotoxin at the environmental level induces respiratory symptoms, including wheezing, because of airway inflammation”. They reported that early-life environmental exposure to endotoxin may induce inflammatory reaction, but continuous exposure to endotoxin may activate immunity in healthy individuals and prevent the later onset of asthma.30In our study, significantly positive ORs of asthma and wheezing onsets were occasionally noted in the Q2-Q4 groups of the endotoxin exposure level in comparison with the Q1 group, supporting the possibility that environmental exposure to endotoxin in early life induces an inflammatory reaction. This finding is consistent with the results of the study in the urban area of Boston, the meta-analysis, and the systematic review. As the evaluation was performed for up to 3 years in our study, the risk of (or preventive effect on) endotoxin exposure-induced asthma at school age will be investigated through a follow-up survey in the future. The geometric mean endotoxin concentration was 15.5 EU/mg in a survey conducted in the US (n=6,953), and it was 2.5-76 EU/mg in other surveys performed in US inner cities,31 whereas that in dust in the present study was 16.0 EU/mg, which is within a range similar to that seen in US. The main strength of this study was that this is the first large-scale birth cohort study wherein an environmental measurement for both dust mite allergen and endotoxin and health influence were evaluated using standardised methods involving approximately 3,000 infants. The cumulative incidence rate of wheezing and asthma in early life could be evaluated by conducting annual questionnaire surveys. However, this study has the following limitations. First, dust mite allergen and endotoxin exposure were evaluated only at 18 months old. Exposure to high concentrations of allergens immediately after birth strongly influences the advancement of paediatric allergic disease.5,6 Samples were not collected immediately after birth in our study, but dust was collected at the age of 18 months old. The association with allergic disease over school age after growth will be investigated in a follow-up survey. Second, asthma symptoms were investigated using self-administered questionnaires filled out by mothers. However, a validation study of questionnaires that followed the diagnostic criteria of asthma used by allergists reported that the sensitivity and specificity of asthma symptoms were 0.91 and 0.64, respectively, in 0-24-month-old children.21 Third, total IgE levels at 24 months old alone were used as the cut-off for outcome evaluation of high serum IgE levels, and the specific IgE level could not be used. The associations with specific IgE and with changes accompanying growth remain to be investigated.