Geographic similarities between varicella and multiple sclerosis: an hypothesis on the environmental factor of multiple sclerosis.

The environmental factor causing multiple sclerosis (MS) is unknown. Kurtzke et al. (Neurology 1979; 29: 1228-1235) depicted a north to south diminishing gradient in the case/control ratios for MS among American veterans in the United States. A similar, but less precise, gradient emerged when the incidence rates of varicella from 37 states during 1978-91 were compared. A loose correlation appears to exist between the mean incidence of varicella and the MS risk ratio (n = 0.344 Spearman rank correlation coefficient, p = 0.037). Further, the data on the fat of migrants moving from a high risk MS country to a low risk country and the reverse, plus the great importance of the age at migration, raise the question of a possible connection between the two diseases. Because of these epidemiological and other similarities between the two diseases a further comparative study was suggested.