Multiple sclerosis in the Faroe Islands: transmission across four epidemics.

As of 1991 we had ascertained 42 native resident Faroese plus 12 non-resident "migrant" Faroese with clinical onset of MS in this century. The resident series comprised four successive epidemics beginning in 1943 and then at 13-year intervals thereafter, a separation of very high statistical significance (p < 0.00001). We concluded that the first epidemic in the Faroes resulted from the introduction of a specific but unknown infection which we call the "primary MS affection" (PMSA) by occupying British troops during World War II. Clinical neurologic MS (CNMS) is then the rare late sequel of infection with PMSA. The first epidemic defined age of susceptibility to PMSA as age 11 to 45 at onset of exposure. Models of transmission used for the first three epidemics included the need for two years of exposure before PMSA acquisition and limitation of transmissibility of PMSA to age 13 to 26. With these conditions successive cohorts of susceptible Faroese were defined to account for the second and third epidemics, and they also predicted the occurrence of the fourth epidemic. Further consideration of these models suggests transmissibility is even more limited, perhaps to age 20 to 26 or so.