Multiple sclerosis in the Faroe Islands and the lack of protection by exposure in infancy.

Using data from 32 patients with symptom onset between 1943 and 1973, we described the occurrence of clinical neurologic multiple sclerosis (CNMS) in the Faroe Islands as then constituting three epidemics. We concluded that CNMS is the rare late result of infection with the primary MS affection (PMSA), a state requiring some 2 years of exposure for acquisition by Faroese. Our theses are that PMSA was first transmitted during World War II by affected by asymptomatic British troops to Faroese aged 11-45; that this (F1) cohort of affected asymptomatic Faroese under age 27 in 1945 transmitted PMSA to the next (F2) cohort of Faroese comprising those attaining age 11 each year from 1945 until F1 input ceased; that the F2 cohort similarly transmitted PMSA to the third (F3) cohort of Faroese. Cases of CNMS defining epidemics I-III were members of the respective F1-F3 cohorts. Within the F4 cohort of Faroese there is now a fourth epidemic of CNMS, with 7 patients with symptom onset between 1984 and 1989. Intermittency of the year of birth for CNMS cases is thus a reflection of membership in these separate population cohorts, and does not indicate 'protection' in infancy or childhood. There is no evidence for an extra-Faroese source of MS after the first epidemic. No model of acute infection with short transmissibility fits the data.