Marked differences in prevalence of multiple sclerosis between ethnic groups in Oslo, Norway

Objectives Oslo, the capital of Norway, has a high prevalence of multiple sclerosis (MS). In recent decades there has been substantial immigration to Oslo from Asia, the Middle East and Africa. The aim of the study was to estimate the prevalence of MS among non-Western immigrants living in Oslo, adjusted for duration of residence. Methods Data were obtained from the MS registry at Ullev (a) over circlel University Hospital. The prevalence of MS was adjusted for ethnicity, age and duration of residence in Norway. Results A total of 786 definite MS patients were alive and resident in Oslo on 31 December 2005, yielding a crude prevalence of 148/10(5). Twentyseven patients were of non-Western origin: Middle East 14, Asia 9, Africa 4. The non-Western patients' mean age at migration was 20. The crude prevalence (95 % CI) of MS patients was 170/10(5) (159-182) for the Norwegian/Western, 85/10(5) (50-143) for the Middle East, 21/10(5) (11-41) for the Asian, and 20/10(5) (7-53) for the African cohorts. The high MS prevalence in the Middle East cohort and the low prevalence among Asian/African immigrants were also pronounced after adjustment for age and duration of residence. Conclusions The Middle East immigrants had a markedly higher prevalence of MS despite a shorter duration of residence than other non-Western patients. These findings suggest that people from the Middle East have a greater genetic disposition for MS. Furthermore, the high age at migration among the non-Western immigrants indicates that possible environmental factors affecting MS risk may also act on adults.