Since we're talking about MS, of course nothing will be clear. I found a couple of Pubmed abstracts that seem relevant. Both studies, for different reasons, seem to suggest that the latitude connection with MS may not be quite as clear as is commonly thought.
Geographic variation of MS incidence in two prospective studies of US women.
Neurology. 1999 Nov 10;53 ( 8 ):1711-8
Hernán MA, Olek MJ, Ascherio A.
Department of Epidemiology, Harvard School of Public Health, Boston, MA 02115, USA.
OBJECTIVE: To estimate the incidence of MS and its relation to latitude in two ongoing prospective studies of US women.
BACKGROUND: A higher incidence of MS has been found in northern areas compared with southern areas of the United States and other countries, but the attenuation of this gradient in Europe in the last few decades and the consideration of ethnic factors have led some authors to question the existence of a strong association between MS and latitude.
METHODS: The authors identified new cases of MS among participants in the Nurses' Health Study (NHS), which took place between 1976 and 1994, and in the Nurses' Health Study II (NHS II), which took place between 1989 and 1995. The NHS included women born between 1920 and 1946, and the NHS II included women born between 1947 and 1964.
RESULTS: The incidence of MS among NHS participants (181 definite/probable patients) increased significantly with latitude (p = 0.03, trend). Adjusted rate ratios were 3.5 (95% CI, 1.1, 11.3) for the north and 2.7 (95% CI, 0.8, 8.9) for the middle tiers relative to the southern tier. Among NHS II women (131 definite/probable patients), no association between latitude and MS was found (p = 0.89, trend). Adjusted rate ratios were 0.8 (95% CI, 0.4, 1.6) for the northern areas and 0.9 (95%, 0.4, 1.8 ) for the middle areas, relative to the southern areas.
CONCLUSION: The association between latitude and risk of MS in the United States was corroborated, but there was an attenuation of the north-south gradient over time. If confirmed, this finding could provide new clues to identifying environmental causes of the disease.
The use of standardized incidence and prevalence rates in epidemiological studies on multiple sclerosis. A meta-analysis study.
Neuroepidemiology. 2003 Jan-Feb;22(1):65-74.
Zivadinov R, Iona L, Monti-Bragadin L, Bosco A, Jurjevic A, Taus C, Cazzato G, Zorzon M.
Department of Clinical Medicine and Neurology, University of Trieste, Italy. firstname.lastname@example.org
OBJECTIVE: To demonstrate whether or not the age and sex adjustment of incidence and prevalence rates in multiple sclerosis (MS) could allow more reliable comparison between epidemiological studies performed in different areas of the world and to establish if the latitude gradient theory could be confirmed after the standardization for age and sex distribution.
METHODS: A meta-analysis of population-based incidence and prevalence studies on MS from 1980 through 1998 using the terms 'multiple sclerosis', 'prevalence' and 'incidence' in the bibliographic databases MEDLINE and EMBASE was performed. We included studies that reported the diagnostic criteria, number of cases and the population studied, the date of the study, the latitude, and the age- and sex-specific crude incidence and prevalence rates. According to the inclusion criteria, 69 of 127 papers on prevalence and 22 of 70 papers on incidence were considered for age adjustment and 27 prevalence and 8 incidence studies for sex adjustment. The mean incidence and prevalencerates and the 95% confidence intervals age- and sex-adjusted to the World and the European standard populations were calculated.
RESULTS: The Spearman rank correlation and the multiple regression analyses indicated that age adjustment to standard populations could overcome the limitations in comparing the crude prevalence and incidence rates of different epidemiological studies on MS. When the mean crude and age- and sex-adjusted prevalence and age-adjusted incidence rates were stratified by latitude (from south to north), the latitudinal gradient, which was highly significant for the crude rates, became less remarkable for the age- and sex-adjusted prevalence rates and not significant for the age-adjusted incidence rates.
CONCLUSIONS: The crude incidence and prevalence rates in epidemiological studies on MS should be age- and sex-adjusted to a common standard population to permit a more reliable comparison among studies performed in different countries. Our findings support the opinion that the latitude does not play a key role in determining the onset of MS. Whenever possible, the crude incidence and prevalence rates should be adjusted to the ethnic origin and migration characteristics.