Multiple sclerosis among first- and second-generation immigrants in Denmark: a population-based cohort study.

Area of Science:

• Neurology
• Epidemiology
• Environmental Health

Background:

• Multiple sclerosis (MS) incidence varies globally, and its exact causes remain unclear.
• Understanding environmental risk factors is crucial for elucidating MS aetiology.

Purpose of the Study:

• To investigate the impact of immigration and timing of environmental exposures on multiple sclerosis risk.
• To evaluate the role of early-life environmental factors in MS aetiology using a large cohort study in Denmark.

Main Methods:

• Utilized the Danish Civil Registration System and Danish Multiple Sclerosis Registry (1968-2015).
• Analyzed incidence rates of MS among first and second-generation immigrants versus ethnic Danes.
• Employed log-linear Poisson regression to estimate relative risks (RR) based on immigration status and age at arrival.

Main Results:

• First-generation immigrants from low, intermediate, and high MS risk areas showed significantly reduced risk compared to ethnic Danes (RR=0.21, 0.43, 0.75 respectively).
• Immigrants arriving before age 15 had higher MS risk than their country of birth but lower than Danes (RR=0.69).
• Those arriving later had MS risk closer to their country of birth (RR=0.45).

Conclusions:

• Environmental factors influencing MS risk appear to exert their effects during childhood or adolescence.
• Early-life environmental exposures are critical determinants in the aetiology of multiple sclerosis.