Declining incidence of multiple sclerosis in the Orkney Islands.

The incidence of MS in the Orkney Islands has been updated from 1941 to September 21, 1983. Since 1965, MS incidence rates have fallen significantly when compared with those for 1941 to 1964. Alterations in age-specific prevalence, mean duration of illness, and mean age of the MS population are consistent with the decline in incidence of MS in recent years. Although the reason for the decreasing incidence is uncertain, it is consistent with the hypothesis that MS may be caused by canine distemper virus.