In a prospective birth cohort study we showed associations between ethnicity and the risk of lower respiratory symptoms. Compared to Dutch infants, Antillean infants had an increased risk of lower respiratory symptoms at 24 months, which was mediated by postnatal exposures. Infants of Turkish ethnicity reported more often infections, upper respiratory symptoms and eczema than Dutch in the first 2 years of life, which partly explained their increased risk of respiratory symptoms both at 12 and 24 months. The risk of lower respiratory symptoms in the first 2 years of life was not different in Cape Verdean, Moroccan or Surinamese infants, as compared to Dutch infants.
Our study focused on differences in respiratory morbidity between ethnic groups, coming from a multicultural urban society. The results are consistent with earlier findings of the PIAMA birth cohort, showing an increased risk of respiratory symptoms before the age of 2 years in Turkish infants living in the Netherlands [15
]. However, the relatively small size of the non-Dutch groups in that study did not allow for a comparison between ethnic minorities and the differences observed disappeared after adjustment for socioeconomic indices [15
]. In our study, ethnic groups had different prevalences of socioeconomic indicators of deprivation. Overall, ethnic minorities were more deprived than the indigenous Dutch population but in different ways, suggesting that different lifestyles and the behavioral adaptations to the environment of ethnic minorities may mask risk factors for respiratory symptoms during infancy.
It has been shown that daycare attendance is associated with frequent wheezing in the preschool years, but not afterwards [25
]. In our study, daycare attendance suppressed part of the association between Turkish ethnicity and LRS at 12 and at 24 months, suggesting that if non-Dutch children had used more daycare, their risk of LRS would be greater than observed. However, the effect of daycare attendance on LRS is mediated by respiratory infections [26
]; therefore, it is difficult to disentangle the different role played by daycare attendance and by infections. We found that both respiratory and non-respiratory infections were associated with increased risk of LRS in the first 2 years of life and, as patterns of respiratory diseases will become more clear, we will be able to assess whether the overall burden of infections may predispose towards a certain asthma phenotype.
Some possible limitations to our study have to be considered. We defined the ethnic background of infants according to the Dutch standard classification [18
]. This classification is objective, reproducible and can be easily applied in epidemiological studies, allowing comparison with future studies. However, some misclassification might have occurred as third generation migrants were labelled Dutch and were hence not distinguished. This would reduce the contrast between Dutch and other ethnic groups, and hence the effect size of Turkish ethnicity on LRS.
In the current study we used standard respiratory questionnaires for schoolchildren, which have shown satisfactory repeatability but may not be optimal for infants and preschool children [27
]. A recent study by Strippoli et al. [28
], showed poorer repeatability in infants for questions regarding cough and upper respiratory symptoms compared to wheeze and shortness of breath. However, our study did not focus on wheezing alone, but the outcomes included the combined variables LRS at 0–12 and LRS at 12–24 months, which were considered positive if at least one of the investigated symptoms occurred. Also, a study by Michel et al. [29
] conducted in the UK, showed that parents’ understanding of the term ‘wheeze’ was different between English speaking and non-English speaking parents. Yet, non-native Dutch in our study were approached in their own language and we did not found a systematic difference in reported respiratory symptoms between Dutch and non-Dutch parents, as shown by the higher risk of LRS in Turkish and the lower risk in Moroccans, as compared to Dutch. Therefore, we consider it unlikely that misclassification of the outcome variables occurred. We cannot exclude that the differences between the ethnic groups might be partly explained by cultural differences, different attitudes towards the use of the medical system and different cultural reporting of the symptoms.
A possible limitation of the current study is represented by the assessment of postnatal exposure to environmental tobacco smoke, which was only evaluated at 24 months and might have been susceptible of recall bias with regard to the analysis at 12 months. Yet, this might partly explain why a significant association between LRS and smoke exposure was found only at 12–24 months.
Missing data on the outcome variables were not completely at random, therefore a complete-case analysis was likely to introduce biased results [23
] and would lead to loss of a large number of study subjects in the multivariate analyses. In order to overcome this issue, we imputed the outcome variables using the predictors under study, thus minimizing the possible bias [30
Do our findings have practical implications? It would seem that reduction of the prevalence of infections and symptoms of the upper airways early in life could lead to a parallel reduction of the burden of lower respiratory symptoms in non-Dutch infants up to the levels of their Dutch peers. Environmental exposures could not entirely explain the association between ethnic background and respiratory symptoms, and it could be argued that ethnicity-specific genetic factors and gene-by-environment interactions among ethnic groups might predispose infants to the development of respiratory symptoms [31
]. A longer follow up of our cohort will reveal whether the increased prevalence of respiratory symptoms in certain ethnic groups represents a temporary association with respiratory infections in early childhood, or predicts progression to chronic persistent symptoms, including asthma. Appropriate focus on prevention of environmental factors as identified might favorably influence such progression.
In conclusion, we found associations between ethnic background and respiratory symptoms in the first 2 years of life that could be largely explained by environmental exposures.