Multiple sclerosis is believed to affect nearly 1 million adults over the age of 18 in the US. While the disease is not contagious or directly inherited, epidemiologists — scientists who study patterns of disease — have identified factors in the distribution of MS around the world that may eventually help determine what causes the disease. These factors include gender, genetics, age, geography and ethnic background.
Epidemiology is the branch of medicine that deals with the incidence, prevalence, distribution, and possible control of diseases and other factors relating to health. Epidemiological studies in MS are challenging for several reasons:
Therefore, all epidemiological numbers, such as prevalence, are estimates.
The National MS Society funded an important prevalence study
- The incidence of a disease is the number of new cases occurring in a given period of time (usually a year) in a given population (usually 100,000).
- The prevalence of MS is the total number of people with MS at a particular point in time, in a particular place. Most epidemiological studies in MS focus on prevalence.
that provided a scientifically sound estimate of the number of people over the age of 18 living with MS in the United States. The Centers for Disease Control (CDC) received federal funding to implement a neurological disease surveillance system and they are actively working on the development of that system. The National MS Society is working with the CDC to help with the development of the surveillance system. The scientific community will be able to learn more about MS with such as system in place.
Although more people are being diagnosed with MS today than in the past, the reasons for this are not clear. Likely contributors, however, include greater awareness of the disease, better access to medical care and improved diagnostic capabilities. There is no definitive evidence that the rate of MS is generally on the increase.
Most people are diagnosed between the ages of 20 and 50, although MS can occur in young children
and older adults.
In general, MS is more common in areas farthest from the equator. However, prevalence rates may differ significantly among groups living in the same geographic area regardless of distance from the equator. For example, in spite of the latitude at which they live, MS is almost unheard of in some populations, including the Inuit, Yakutes, Hutterites, Hungarian Romani, Norwegian Lapps, Australian Aborigines and New Zealanders — indicating that ethnicity and geography interact in some complex way to impact prevalence figures in different parts of the world.
- Migration from one geographic area to another seems to alter a person’s risk of developing MS. Studies indicate that immigrants and their descendants tend to take on the risk level — either higher or lower — of the area to which they move. The change in risk, however, may not appear immediately. Those who move in early childhood tend to take on the new risk themselves. For those who move later in life, the change in risk level may not appear until the next generation. While underlining the complex relationship between environmental and genetic factors in determining who develops MS, these studies have also provided support for the opinion that MS is caused by early exposure to some environmental trigger in genetically susceptible individuals.
- Read about MS clusters, the perception that very high numbers of cases of MS have occurred in a specific time period or location. Clusters may provide clues to environmental or genetic risk for the disease.
MS is at least two to three times more common in women than in men, suggesting that hormones may also play a significant role in determining susceptibility to MS. Studies have suggested that the female to male ratio may be as high as three or four to one.
Genetic factors are thought to play a significant role in determining who develops MS.
- The average person in the United States has about one in 750 (.1%) chance of developing MS.
- For first-degree relatives of a person with MS, such as children, siblings or non-identical twins, the risk rises to approximately 2.5-5% — with the risk being potentially higher in families that have several family members with the disease.
- The identical twin of someone with MS (who shares all the same genes) has a 25% chance of developing the disease. If genes were solely responsible for determining who gets MS, an identical twin of someone with MS would have a 100% chance of developing the disease; the fact that the risk is only one in four demonstrates that other factors, including geography, ethnicity and the elusive infectious trigger, are likely involved as well.
Research has demonstrated that MS occurs in most ethnic groups, including African-Americans, Asians and Hispanics/Latinos, but is most common amongst Caucasians of northern European ancestry. Susceptibility rates vary among these groups, with recent findings suggesting that African-American women have a higher than previously reported risk of developing MS.