A Systematic Review on the Development of Asthma and Allergic Diseases in Relation to International Immigration: The Leading Role of the Environment Confirmed

<div><p>Background</p><p>The prevalence of asthma and allergic diseases is rising worldwide. Evidence on potential causal pathways of asthma and allergies is growing, but findings have been contradictory, particularly on the interplay between allergic diseases and understudied social determinants of health like migration status. This review aimed at providing evidence for the association between migration status and asthma and allergies, and to explore the mechanisms between migration status and the development of asthma and allergies.</p><p>Methods and Findings</p><p>Systematic review on asthma and allergies and immigration status in accordance with the guidelines set by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement. The pooled odds ratio (OR) of the prevalence of asthma in immigrants compared to the host population was 0.60 (95% CI 0.45–0.84), and the pooled OR for allergies was 1.01 (95% CI 0.62–1.69). The pooled OR for the prevalence of asthma in first generation versus second generation immigrants was 0.37 (95% CI 0.25–0.58). Comparisons between populations in their countries of origin and those that emigrated vary depending on their level of development; more developed countries show higher rates of asthma and allergies.</p><p>Conclusions</p><p>Our findings suggest a strong influence of the environment on the development of asthma and allergic diseases throughout the life course. The prevalence of asthma is generally higher in second generation than first generation immigrants. With length of residence in the host country the prevalence of asthma and allergic diseases increases steadily. These findings are consistent across study populations, host countries, and children as well as adults. Differences have been found to be significant when tested in a linear model, as well as when comparing between early and later age of migration, and between shorter and longer time of residence.</p></div

The PRISMA Flowchart.

<p>The PRISMA Flowchart.</p

Search terms and equations used for this review in the PubMed database in March 2013.

<p>Search terms and equations used for this review in the PubMed database in March 2013.</p

Map of countries in which included studies on asthma or allergies and migration were conducted.

<p>Map of countries in which included studies on asthma or allergies and migration were conducted.</p

Forest plot estimating the difference in prevalence of asthma and allergic diseases between immigrants and the host population.

<p>Forest plot estimating the difference in prevalence of asthma and allergic diseases between immigrants and the host population.</p

Forest plot estimating the difference in prevalence of asthma and allergic diseases between first generation immigrants and those born to foreign parents (second generation immigrants).

<p>Forest plot estimating the difference in prevalence of asthma and allergic diseases between first generation immigrants and those born to foreign parents (second generation immigrants).</p

Differences in prevalence of asthma and allergies between migrants and the host population.

<p>* Odds Ratios for first-generation immigrants compared to the host population unless stated otherwise.</p><p>** Mean number of new asthma consultations/1000 patient years.</p><p>*** Prevalence and risk estimates only reported for groups with ≥50 immigrants.</p

Map of countries of origin of immigrants, based on available data from included studies on the relationship between asthma or allergies and migration.

<p>Map of countries of origin of immigrants, based on available data from included studies on the relationship between asthma or allergies and migration.</p

Studies comparing prevalences of asthma and allergies between immigrants with different duration of residence in host country.

<p>* Only subsamples of first-generation immigrants reported in this table.</p><p>** Significance levels reported for linear trends unless stated otherwise.</p

Levels of lung IL-17, IL-1β and TNFα.

<p>C57/Bl6 and IL-17R^−/− mice were exposed to air or to ozone. Data shown as mean ± SEM for n = 5 in each group; *p<0.05 compared to air in the same species; ^#p<0.05 compared to C57/Bl6 mice with corresponding exposure.</p