Multiple sclerosis in the Faroe Islands. 5. The occurrence of the fourth epidemic as validation of transmission.

Based on 32 cases with clinical onset 1943-73, we previously described the occurrence of clinical neurologic multiple sclerosis (CNMS) on the Faroe Islands as constituting three consecutive epidemics, with 20, 9, and 3 cases respectively. As of 1991 there were seven additional cases of CNMS with clinical onset 1984-1989 constituting the fourth epidemic, as well as three more members of epidemic III. We have proposed that CNMS is the rare late result of infection with PMSA (the primary multiple sclerosis affection), a state requiring some two years of exposure from age 11+ for acquisition by Faroese, and that PMSA was first transmitted during World War II by affected but asymptomatic British troops to Faroese residents; part of this (F1) cohort of affected asymptomatic Faroese transmitted PMSA to the next (F2) cohort comprising Faroese reaching age 11 in the interval when that F1 subset was present, and the F2 cohort similarly transmitted PMSA to the third (F3) cohort. Cases of CNMS defining epidemic I-III were members of the respective F1-F3 cohorts. The existence of epidemic IV within the F4 cohort of Faroese may be taken as validation of our transmission models and of our theses as to the nature of multiple sclerosis.