Epidemiology of Multiple Sclerosis: Environmental Factors

This chapter discusses the environmental factors associated to epidemiology of multiple sclerosis. The epidemiologic evidence points to three environ­mental risk factors—infection with the Epstein-Barr virus (EBV), low levels of vitamin D, and cigarette smoking—whose association with multiple sclerosis (MS) seems to satisfy in varying degrees most of the criteria that support causality, including temporality, strength, consis­tency, biologic gradient, and plausibility. None of these associations, however, has been tested experimentally in humans and only one––vitamin D deficiency is presently amenable to experimental interventions. The evidence, albeit more sparse and inconsistent, linking other environmental factors to MS risk are summarized. Epidemiologic clues to the hypothetical role of infection in MS are com­plex and often seem to point in opposite directions. The ecological studies, database/linkage analyses, and longitudinal studies of sunlight exposure and vitamin D are reviewed. Biologic mechanisms for smoking and increased risk of MS could be neuro­toxic, immunomodulatory, vascular, or they could involve increased frequency and duration of respiratory infections. Some other possible risk factors include––diet and hepatitis B vaccine.