Multiple sclerosis (MS) is a chronic autoimmune disease that affects the central nervous system, specifically the brain and spinal cord. In individuals with MS, the immune system mistakenly attacks myelin, the protective sheath covering nerve fibers, disrupting communication between the brain and the rest of the body. Epidemiology is the scientific study of how health-related states, such as diseases, are distributed across populations and the factors that influence these patterns. This field aims to understand who gets a disease, where it is most common, and how frequently it occurs, providing insights into its causes and potential prevention strategies.
Global Prevalence and Incidence
Prevalence refers to the total number of existing cases of a disease in a population at a specific time, while incidence represents the number of new cases diagnosed over a defined period, typically a year. Globally, an estimated 2.9 million people are living with MS. This figure reflects a rise in prevalence across every world region since 2013.
The worldwide incidence of MS stands at approximately 2.1 new cases per 100,000 people per year. Europe has the highest prevalence rate among all World Health Organization (WHO) regions, with about 133 cases per 100,000 people. High-income North America, including the United States, also reports elevated prevalence, with nearly 1 million people diagnosed with MS in the U.S. alone.
The Geographic Pattern of MS
A notable characteristic of MS distribution is the “latitude gradient,” where the prevalence of the disease increases in populations living farther from the equator. For instance, countries like Canada, Scotland, and Scandinavian nations exhibit high MS prevalence, while equatorial Africa and Asia report fewer cases.
The strong geographic association suggests an environmental influence on MS risk. Migration studies provide evidence for this environmental role. Individuals who move from a low-risk region to a high-risk area before adolescence tend to acquire the risk profile of their new location. Conversely, those who migrate from a high-risk to a low-risk area often retain the lower risk of their childhood environment. This indicates that early life environmental exposures play a substantial part in determining an individual’s long-term MS risk.
Demographic Characteristics of MS
Multiple sclerosis manifests in young adults, with symptom onset and diagnosis most common between 20 and 40 years old. While MS can appear at any age, including in children and individuals over 50, the peak incidence occurs in these younger adult years.
There is a sex disparity in MS, with the disease more common in women than in men. The female-to-male ratio is approximately 3:1 for the most frequent form of MS, relapsing-remitting MS. This gap has been observed to widen over time in some regions.
MS affects individuals across all racial and ethnic groups, though prevalence varies considerably. Historically, MS was considered primarily a disease of people of Northern European descent, who still exhibit the highest risk. However, recent studies indicate that MS prevalence in Black Americans is similar to that in White Americans. Hispanic and Asian populations show lower rates of MS compared to White and Black populations in the United States.
Identified Risk Factors
Multiple sclerosis development arises from a complex interplay of genetic predisposition, environmental influences, and lifestyle choices. Understanding these interacting factors helps to explain the observed epidemiological patterns.
Genetic susceptibility plays a role, though MS is not directly inherited in a simple Mendelian fashion. Having a first-degree relative with MS, such as a parent or sibling, increases an individual’s risk compared to the general population. The human leukocyte antigen (HLA) gene complex, particularly the HLA-DRB11501 allele, is the strongest genetic factor associated with MS risk across various populations. This gene is involved in immune system function and how the body recognizes foreign substances.
Environmental factors are implicated in MS, helping to explain the geographic patterns observed. Low levels of vitamin D, often linked to reduced sun exposure in higher latitudes, are consistently associated with an increased risk of MS. The Epstein-Barr virus (EBV), which causes infectious mononucleosis, is considered an environmental trigger; nearly all individuals with MS show evidence of prior EBV infection, and a history of mononucleosis further increases risk.
Lifestyle factors also contribute to MS risk. Smoking is a risk factor, with studies estimating it contributes to MS cases. Obesity, particularly during childhood and adolescence, has also emerged as an independent risk factor for developing MS. The combination of these genetic, environmental, and lifestyle factors is thought to drive the development of multiple sclerosis.