Is multiple sclerosis an age-dependent host response to measles?

Several lines of evidence support the possibility that multiple sclerosis (M.S.) may be an age-dependent host response to measles. In animals, measles evokes different responses depending upon age at inoculation. In man, measles is already known to produce at least two age-dependent responses: risk of subacute sclerosing panencephalitis is increased among those who have had measles before 2 years of age and risk of measles encephalitis increases with age, at least during adolescence. Studies of immigrant populations indicate that an event at or before adolescence but not infancy affects risk of M.S. In the tropics where M.S. is rare, measles tends to be acquired very early in life, usually before the age of 3, whereas in temperate areas, measles tends to be acquired later, after the age of 5. A retrospective study has shown that M.S. patients tend to have had measles later than controls. Mechanisms which might underlie an age-dependent host response to measles include maturation of an immune system (k.g., number of available B cells) or change in the metabolic state of a target cell (e.g., oligocytes which change from laying down myelin to maintaining it). If the hypothesis that M.S. is a host response to later measles infection is valid, then mass measles vaccination programmes should produce a decline in the rate of M.S., but the effect may not be discernible before 1980.