Multiple Sclerosis (MS) is a chronic, potentially disabling disease affecting the central nervous system, including the brain and spinal cord. The immune system mistakenly attacks the protective myelin sheath surrounding nerve fibers, disrupting communication between the brain and the rest of the body. While the exact cause remains unknown, research suggests MS results from a complex interaction between genetic predisposition and environmental factors. This interplay is central to understanding the disease’s distribution, which shows notable variations across populations and geographic locations.
Measuring Geographical Differences in MS
Researchers use specific metrics to understand the distribution of multiple sclerosis across the United States. Prevalence measures the total number of people living with MS at a specific point in time, while incidence tracks the number of new cases diagnosed over a specific period. Prevalence is often the metric cited in geographic studies, as it provides a clearer picture of the disease burden in a given area.
Accurately measuring these rates across different states presents significant challenges. The United States lacks a national registry for MS, meaning researchers must rely on estimates derived from large administrative health claims data sets. Variability in diagnostic criteria and differences in healthcare access between states can also affect the reported numbers. Furthermore, population migration introduces complexity, as a person’s risk is largely established during early life, meaning the disease risk may have been acquired in a different state than where the diagnosis occurred.
Despite these difficulties, a clear global pattern has been observed: MS rates generally increase with distance from the equator, a phenomenon known as the latitude gradient. Areas closer to the poles report higher prevalence rates than those closer to the tropics. This pattern suggests that an environmental factor linked to latitude plays a substantial role in determining MS risk. Within the US, this trend is reflected in the higher rates found in northern regions compared to the southern states.
The Region with the Highest Reported Rates
The geographic concentration of MS in the northern United States is sometimes referred to as the “MS Belt,” referencing the global trend of higher prevalence at higher latitudes. Recent, large-scale studies using comprehensive health claims data confirm this northern clustering. A 2023 analysis found that the highest overall prevalence of multiple sclerosis in the United States was reported in Montana.
Montana reported an overall prevalence estimate of approximately 186.2 per 100,000 people. The same analysis showed the highest prevalence for White individuals was also in Montana, estimated at 193.3 per 100,000. Other states in the upper Midwest and Northeast regions also rank highly, including Michigan, Nebraska, Wisconsin, and Ohio. These states consistently demonstrate a higher burden of MS than their southern counterparts. For instance, the Northeast region has an estimated prevalence of 377 cases per 100,000 people, compared to the South’s estimated 277 cases per 100,000.
While Montana showed the highest overall rate, the burden shifts when examining specific racial and ethnic groups. Michigan had the highest prevalence for Black or African American individuals (233.2 per 100,000) and the highest for Hispanic/Latino individuals (73.3 per 100,000). This finding underscores that genetic ancestry and environmental factors interact in complex ways not solely explained by latitude. The consistent trend, however, is that northern states experience the greatest overall disease burden.
Leading Theories for Geographic Variation
The persistent geographic pattern of MS has led researchers to focus on environmental elements that vary with latitude. The most prominent hypothesis revolves around the link between sunlight exposure and Vitamin D levels. People living farther from the equator receive less intense ultraviolet B (UVB) radiation, especially during winter, which is necessary for the skin to synthesize Vitamin D.
Lower levels of Vitamin D are thought to impair the immune system’s regulatory functions, potentially contributing to the autoimmune response seen in MS. Studies suggest that low sun exposure may increase MS risk both directly and indirectly through its effect on Vitamin D status. The protective effect of sunlight appears to involve both Vitamin D-dependent and non-Vitamin D-dependent pathways.
Beyond the Vitamin D hypothesis, other environmental factors are being investigated to explain the geographic clustering. The “migration effect” suggests that individuals who move from a high-risk region to a low-risk region before adolescence acquire the lower risk of their new home. This indicates that exposure to an environmental trigger, likely an infectious agent, during early development is a determinant of later risk.
Infectious agents, such as the Epstein-Barr virus (EBV), are strongly implicated in the development of MS. Exposure to EBV is common, but the timing of this exposure, which varies geographically, could influence the risk of an abnormal immune response. Other lifestyle factors that show geographic variation, including smoking and adolescent obesity, also contribute to the overall environmental risk profile.