It is a very interesting area, and I'm about to sound like a broken record too. I believe Vitamin D has something to do with it, as issues with Vitamin D levels, sunlight exposure, etc. show some correlations. Increasing incidence in areas closer to the equator may, as gwa suggested, be due to increasing quality of health care in those areas, with increasing diagnosis. Or could other changes in lifestyles be playing a role?
One complicating factor on just the vitamin D level angle is dramatically changing lifestyles that result in more and more people, worldwide, spending more time indoors. My mother grew up on a farm, worked in the fields, and walked long distances to the bus stop to go to school and come home, etc - the difference between her childhood and mine was stark. As a pre-teen, I at least played outdoors on a fairly frequent basis. I look around my neighborhood now and I hardly ever see kids playing outdoors for any length of time. The difference between my childhood and kids today is almost as great as the difference between mine and my mother's. In other parts of the world, including the tropics, people have moved from an agrarian existence where they worked outside, to a more urban one, where their work is more likely to be indoors.
I have seen several articles on Vitamin D levels measured in boys and girls in Iran, and how high percentages, particularly girls, are deficient in Vitamin D. Many Iranians live in a sunny climate, but the clothing and lifestyle precludes getting sunlight exposure. Were people's dress and lifestyle different back in the 60s and 70s in Iran? I think so. The young adults in Iran now have were all born after the Iranian revolution in 1979. Below is an recent abstract examining the prevalence of MS in Isfahan, Iran. Isfahan is south of Tehran. The authors say their area is a medium to high risk of MS in relation to the rest of the world's prevalence, which argues against the gradient hypothesis. They have a higher ratio of female: male cases at 3.6 - I don't think it is nearly that high in the U.S. and northern Europe. But if they were to consider the degree of sunlight exposure and vitamin D levels their patients get, how would they compare to people to the north of them? What if they compared Vitamin D levels of Iranian women to men, would that explain why their female:male ratio is so high? Also, if I read the abstract correctly, the prevalence rates (I'm assuming prevalence, not incidence) drop off with increasing age over 40, in the people who reached teens/early adulthood before 1979. Does that have any relationship to the revolution, and the marked changes in lifestyle that may have come with that?
I'm just speculating, and don't want to read too much into this abstract, especially when I don't have the whole paper to look at. The Iranian study may disprove the latitude gradient. But it doesn't disprove the sunlight exposure/Vitamin D level relationship; it may actually support it.
Neuroepidemiology. 2006 Jun 27;27(1):39-44 [Epub ahead of print] Related Articles, Links
Prevalence of Multiple Sclerosis in Isfahan, Iran.
Etemadifar M, Janghorbani M, Shaygannejad V, Ashtari F.
Departments of Epidemiology and Neurology, Isfahan University of Medical Sciences and Health Services, Isfahan, Iran.
Background: The prevalence of multiple sclerosis (MS) shows considerable variability all over the world. According to Kurtzke, Iran is considered to have a low prevalence. Objective: To estimate the period prevalence and risk factors of MS in Isfahan, central part of Iran. Methods: A cross-sectional case register study conducted between 2004 and 2005. In the province of Isfahan, Iran, all patients known to have definite MS during 2004 and 2005, being alive and resident within Isfahan as well as being a member of the Isfahan MS Association were included in the study. Demographic and case-related information was recorded. 1,391 definite MS patients (308 men and 1,083 women) from the Isfahan MS Association, Iran, have been identified. The disease was confirmed using clinical information and MRI findings by a neurologist and radiologist. The patients were evaluated by interview and a questionnaire. Population data were obtained from the year 1999 Iran Census. The mean (SD) age of the participants was 32.5 (9.3) years with a mean (SD) duration of the disease of 6.4 (5.1) years for men and 6.9 (5.3) years for women. Results: The period prevalence of MS was 35.5 per 100,000 [95% confidence interval (CI) 33.6-37.3] in a population of 3,923,255, with a higher rate in women than men [54.5 (95% CI: 51.1-57.
for women and 14.9 (95% CI: 13.3-16.6) for men]. The female/male ratio was 3.6 (95% CI: 3.2-4.1). The direct age-adjusted period prevalence was 59.5 per 100,000 (95% CI: 44.8-75.2) for women and 17.0 per 100,000 (95% CI: 8.9-25.1) for men. MS rates were highest among 30- to 39-year-olds and decreased with increasing age. Sensory and visual disturbances were the most common initial presentations with a prevalence of 51.1% (95% CI: 48.4-53.7) and 47.0% (95% CI: 44.4-49.7), respectively. Conclusion: Isfahan could be considered as an area with a medium to high risk of MS. This is in sharp contrast with the gradient hypothesis. Copyright (c) 2006 S. Karger AG, Basel.
PMID: 16804333 [PubMed - as supplied by publisher]