This review article looks at the female-to-male ratio of MS, which is similar to other immune-mediated diseases, with the susceptibility higher in females than males.
A meta-analysis carried out by Koch-Henriksen et al, showed that the world-wide sex ratio of MS has been increasing substantially over the last century. In a study from Canada, the sex ratio of MS patients born in the 1930s was lower than 2 and then increased to more than 3 females for each male patient in the latest birth cohort studies in the 70s. The increasing frequency of MS among females is a key driver for the worldwide increasing prevalence of MS.
In relation to why this is happening, the author suggests that environmental factors are at work in a sex-specific manner and this change occurring within a century is too short a time for a genetic cause. In relation to environmental agent/s influencing the sex ratio of MS, possibilities include pregnancy, older age at birth, reduced offspring number and fertility rates. However, reproductive history may well have an influence on female susceptibility to MS; this single factor is unlikely to entirely explain temporal trends in the MS sex ratio.
Additional factors include vitamin D deficiency, Epstein-Barr virus infection and smoking history. The sex ratio of MS remains a puzzle and further research is needed to understand the environmental factors involved.
Authors: Disanto G, Ramagopalan SV.
Source: Mult Scler. 2013 Jan;19(1):3-4.
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