The aim of this study was to perform detailed investigations of a presumed high-risk area, namely the county of Värmland, to see if previous results from our group indicating a high frequency of multiple sclerosis (MS) could be confirmed. We soon identified an aggregation of MS cases in the parish of Lysvik located in the north of Värmland and initiated epidemiological and genetical analyses of the population. We also extended our genetic research to include studies of a possible association between MS immunopathic trait and the MS susceptibility gene HLA-DR(2)15 type, but in another geographic area.

The onset-adjusted prevalence of MS in Värmland was 170/105 (95% CI: 154-185) in December 2002, which is higher than prevalence previously reported from other Swedish areas. There was a great variation in MS frequency between communities in Värmland. We found a persistently high occurrence of MS in Torsby and Sunne communities. In the community of Årjäng MS frequency had increased substantially since the previous study performed by our group.

Epidemiological analysis of a cluster of MS cases in Lysvik revealed 27 MS patients, of whom 23 were the descendants of a Finnish family originated from a common ancestor born in Savolaks in Finland in the 16th century and 18 had relatives with MS. Since this cluster was most likely to have a genetic basis (located in an area with a high inbreeding rate) the mode of MS inheritance was investigated. The linkage study using the genome-wide transmission disequilibrium test (TDT) provided several regions of interest, especially on chromosome 14q (14q24-31). The linkage peak on chromosome 17q was also confirmed by this study.

The frequency of the HLA-DR(2)15 allele was higher in healthy siblings of MS patients without MS immunopathic trait (MSIT) than in siblings with the trait, which provides further support for the hypothesis that MSIT and MS are two independent, albeit, synergistic conditions.

The prevalence study supports that Värmland County is a high-risk area. Furthermore, the aggregation of MS cases in Lysvik indicates a concentrated risk zone, possibly due to a combination of genetic, environmental and social risk factors. A widely and evenly spread environmental (i.e., infectious) agent together with cultural changes and industrialisation could possibly induce disease in subgroups of genetically more susceptible individuals. The evidence of linkage to chromosome 14 found in this study indicates that further genetic research is required.