The age incidence of multiple sclerosis: a decision theory model.

It is proposed that multiple sclerosis (MS) arises as an auto-immune response to antigens shared by common commensal bacteria and brain tissues. In particular it is suggested that the causative bacteria are normally spread by the faecal-oral route but first exposure can occur in the nasopharynx, particularly following a viral respiratory infection, and this increases the risk of MS. The interaction between bacterial colonisation and the immune response is analysed in terms of an information model derived from statistical decision theory. The model predicts a finite chance of autoimmune disease on first exposure which rises with age at exposure. The predicted age incidence of MS, which is the resultant of the rising error function and the age incidence of first exposure to common bacteria, rises to a peak in the third decade and matches published age incidence data. Furthermore the model predicts that subsets of the population, such as women, in whom the risk of MS is increased will have an earlier mean age of onset. This accords with observation which is hitherto unexplained. The model also explains the decreased incidence of MS in equatorial regions, data on migration studies, and is consistent with the observation that the mean age of onset is not consistently lower in low incidence regions. It also offers an explanation for conflicting data on the effect of social class, economic conditions and birth order. The hypothesis is amenable to laboratory investigation and should be pursued.