Figure 7: Border between Finland and Russian Karelia, with a 6-fold difference in the incidence of TID, from “Karelia today” (77)
The countries share a common border and ancestry and thus have similar geography, climate, vitamin D levels, and prevalence of HLA risk haplotypes (78). However, Finland has 6-fold higher incidence of TID. This markedly higher rate of TID is accompanied by a much lesser rate of infectious disease. In Finland as compared to Karelia 2% vs 24% had hepatitis A; 5% vs 24% had toxoplasma gondii; and 5% vs. 73% for helicobacter pylori (79). There is an ongoing study aiming to better understand the mechanisms that may underlie these differences.