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Abstract:
We found 41 Faroese with clinical onset of MS 1900-1983, including 9 "migrants" who had lived in (high MS risk) Denmark for 3+ years before onset. Their major foreign residence was during the 10 years or so before clinical onset, began at age 11-29 and was of at least 2 years duration, following which clinical onset occurred an average of 6 years later. Among the 32 resident Faroese, clinical MS began between 1943 and 1973 and comprised three epidemics, each one significantly later in time and lower in incidence than the preceding. The risk of MS for Faroese of epidemic I, who acquired the disease from asymptomatic British troops in the World War II occupation, was 18 per 10,000. Risk for those of epidemics II and III, who acquired the disease from asymptomatic but affected Faroese, was not significantly different. We conclude that the primary MS affection (PMSA) is a single, widespread infectious disease whose acquisition in virgin populations follows two years of exposure starting between age 11 and 45, which then produces clinical neurologic MS (CNMS) in only a small proportion of the affected after an incubation period of 6 (virgin populace) or 12 (endemic areas) years, and which is transmissible only during the systemic PMSA phase which ends by age 27 or younger.