The degenerative neurological condition is now so common in the Orkney Isles that one in every 170 women is a sufferer – and some villages are hotspots where the levels are even higher than this.
Authors of the research, which is the first study of its kind for 40 years, say they cannot determine why Orcadians and those who move to the islands, which have a population of about 20,000, run such a high risk of having the disease.
However, genetics expert Dr Jim Wilson, who led the investigation, believes it may be at least partly explained by an inherited genetic weakness yet to be discovered by scientists.
He said: "With this clustering, some people would try to say it is due to the soil or something in the water. But, at least in the past, people married in their own community very often. At some level with their genetic background people in a parish are part of the same extended family. Even if we have not been able to find a genetic factor [to explain the dense levels of MS] it does not mean that it is not out there."
The findings have been released little more than a week after figures showed a seven-fold increase in the number of Scots under the age of 30 developing MS in some health board areas.
It has long been known that Scotland, and the Orkney islands in particular, have among the world's highest rates of MS along with parts of Canada and Scandinavia.
However, Mr Wilson said the last comprehensive study was in 1974. It found the number of people in Orkney diagnosed with probable or definite MS was 309 per 100,000. The new research, published in the Journal of Neurology, Neurosurgery & Psychiatry, found this has now increased to 402 per 100,000, which would equate to roughly 80 people on the islands.
This compares to 295 per 100,000 in Shetland and 229 per 100,000 in Aberdeen. The most recently reported rates for Alberta and Nova Scotia in Canada were about 350 per 100,000.
Dr Elizabeth Visser, who worked on the research, visited patients and trawled medical notes in Orkney, Shetland and Aberdeen to ensure those included as sufferers met strict diagnostic criteria.
Mr Wilson, of Edinburgh University's Centre for Population Health Sciences, said better diagnostic tests, along with better survival chances, may help explain why the number of sufferers had increased. However, he added: "It could also be a real underlying trend – that there is a rise in the number of people developing the disease."
There were "in-comers" with MS as well as lifelong residents, but no evidence that lots of people chose to live in Orkney because they had MS.
MS has long been linked to the so-called sunshine-drug Vitamin D, but Mr Wilson said that as the Shetland Islands, further north, appeared to have less of a problem than Orkney this was unlikely to fully explain the Orkney situation.
A study measuring vitamin D levels in thousands of Orcadians is currently taking place.
Mr Wilson said: "We saw within Orkney and Shetland there were hotspots and cold spots. Some isles and parishes and villages had a much increased rate and in other parts there were hardly any residents who had it."
MS causes myelin – a layer that insulates nerve cells in the brain – to break down. This weakens and slows the messages sent through nerves cells from the brain to other parts of the body. The symptoms include numbness, loss of eyesight, fatigue, dizziness and muscle weakness that can accumulate, leading to disability.
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