Environmental factors determine multiple sclerosis risk in migrants to Israel

Background: Multiple sclerosis (MS) is more common in developed than in underdeveloped regions. Several studies have shown that MS frequency may depend on age at immigration from low to high MS frequency zones. Israel's population includes immigrants from high and low MS frequency regions. MS is high in native-born Jewish Israelis, Israeli immigrants from Europe and in Israeli-born Jewish offspring of Afro-Asians (AA). Objective: To investigate whether MS risk depends on age at migration or length of exposure to lifestyle/geo-climatic factor(s). Methods: The Israeli National MS Registry (INMSR) listed 3935 MS patients with onset after 1950; 476 were AAs. We analyzed MS frequency by age at immigration and length of residence in Israel. We also compared the incidence rate of MS in 300 Israeli-born individuals whose parents were AAs and immigrated to Israel less than or more than five years before the birth of the Israeli born offspring with MS (assuming adaptation takes about 5 years). The Central Bureau of Statistics provided the total population at risk for MS. Results: Among AA immigrants, the incidence of MS decreased linearly with increasing age at immigration and increased with increased length of exposure to Israel's environment; there was no sharp change at any particular age. MS incidence in Israeli-born offspring was highest when both parents immigrated to Israel from AA more than 5 years before the child's birth in Israel and was lowest when both parents immigrated to Israel less than 5 years before birth of the child in Israel (annual IR of 2.3 versus 0.9/1 000 000). Adopting the Israeli lifestyle appeared to increase risk of MS in these offspring. Conclusions: Cumulative exposure to Israeli lifestyle/geo-climatic factors appeared to determine MS with no critical exposure age.