Migration and multiple sclerosis in immigrants from United Kingdom and Ireland to Australia: a reassessment. III: risk of multiple sclerosis in UKI immigrants and Australian-born in Hobart, Tasmania

Our previous work suggested that migrants from the United Kingdom and Ireland (UKI) to Australia who left their home country at a young age had a longer interval between immigration and onset and likely acquired MS in Australia. In the present study, we reassessed Australian-born cases of MS identified in Hobart, Tasmania, a relatively high-risk zone, in our 1981 survey and compared these with cases of MS in UKI immigrants incident in Australia. The incidence of MS in Australian-born residents rose from 1.63 per 100,000 in 1941-1965 to 3.48 per 100,000 in 1966-1981. The bulk of UKI immigrants who developed MS in Australia migrated after the age of 15 years, and likely acquired their disease in the UKI. The mean interval from immigration to onset differed significantly (p < 0.01) between those migrating before (22 years) versus after (6 years) the age of 15, suggesting acquisition of MS in Australia in the former group. Identified environmental risk factors such as smoking, sunlight and exposure to Epstein-Barr virus do not fully account for the epidemiology of multiple sclerosis. The apparent introduction of MS into Hobart from the mid-1940s on could provide circumstantial support for the theory that MS is a transmissible disease.