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We read with interest the results of Rothwell and Charlton regarding the incidence and prevalence of multiple sclerosis in south east Scotland.1 They have identified standardised multiple sclerosis prevalence rates for the Lothian and Border Regions of 203 and 219 per 100 000 respectively, the results challenging the theory that the high prevalence rates previously reported in Scotland are peculiar to the north east and its offshore islands. The authors postulate that the apparent step in prevalence rates between England and Scotland may be due to the distinctive Celtic ancestry of the Scottish population as can be crudely measured by surnames prefixed with Mc or Mac.
In Northern Ireland we have also identified a much higher prevalence rate for the disease than exists in England and Wales and have speculated that the similar rate to that in Scotland is at leastly partly a function of the common ethnic origins of the two populations.2 The contiguous region of Coleraine, Moyle, Ballymena, and Ballymoney lies less than 20 miles from Scotland at its closest point and has a standardised prevalence rate for all multiple sclerosis, based on the 1961 census population for Northern Ireland,3 of 230 (95% confidence interval (95% Cl) 207–256) per 100 000. Using a similar method to Rothwell and Charlton (British Telecom phone book of the area), 17% of the study population had a surname prefixed with Mc or Mac and it is of note that 22.9% of prevalent cases had such a surname prefix (odds ratio = 1.46, 95% Cl 1.09–1.93, χ2=6.82, p=0.009).
Our results support the conclusion of Rothwell and Charlton that Celtic ancestry is a risk factor for multiple sclerosis and confirm the existence of a step in multiple sclerosis prevalence in the British Isles between England/Wales and Scotland/ Northern Ireland.
The author’s reply:
The study of the prevalence of multiple sclerosis in Northern Ireland by McDonnell and Hawkins is interesting.1-1 The findings are similar to those of the recent study in south east Scotland.1-2 Both studies suggest that there is an increased prevalence of multiple sclerosis in the north of the British Isles compared with the south. It seems likely, as McDonnell and Hawkins suggest, that this at least partly reflects differences in the genetic susceptibility of the respective populations.
The south east Scotland study did, as Shepherd suggests, attempt to link the high prevalence of multiple sclerosis to Scottish ancestry. However, the study used a standard text of several hundred surnames which are considered to have originated in Scotland,1-3rather than just those prefixed with Mc or Mac. This is obviously still a very crude approach to the problem, but any bias is likely to have weakened rather than strengthened the association. The proportion of cases in the telephone book with a surname pre-fixed with Mc or Mac was simply used as a crude illustration of the fact that the differences in apparent ancestry between the Scotland and England are still considerable. This is supported by major differences in the HLA types of the two populations.1-4 Contrary to Shepherd’s assertion, the Highlands and Islands telephone book does include Orkney. However, he is correct to point out that the prevalence of surnames pre-fixed with Mc or Mac is indeed lower on Orkney than in the region as a whole.
Further insights into the high prevalence of multiple sclerosis in the north of the British Isles might come from a prevalence study which is currently being planned on the Isle of Skye.