MS and spru are both genetic issues IIRC. So to me it seems pretty good odds the latitude variations in incidence are due to a nordic gene and not the actual geography itself.
He's the only one in his family to have the problem ~ but because antibiotics alleviate the symptoms the theory in Pakistan/India (a major tropical spru area) is that there's an unknown bacteria or virus that triggers it in susceptible individuals.
The fact D3 has effects in mega doses ~ (on other parts of the intestines) ~ is important.
Lack of sunlight at the northern latitudes has been mentioned as well.