From The MS Information Sourcebook, produced by the National MS Society.
Epidemiology is the study of disease patterns, which takes into account variations in geography, demographics, socioeconomic status, genetics, and infectious causes. Epidemiologists contribute to knowledge about MS by studying the relationships between these factors, as well as patterns of migration, in an effort to understand who gets MS and why, and identify and explain areas with high or low rates of MS.
To date, epidemiological studies have helped to identify factors that may be related to the risk of developing MS, including geography, genetics, and infectious processes, but we still have few definitive answers. This is due, at least in part, to the challenges inherent in conducting these types of studies in a disease that can be difficult to diagnose. Since there is no single test for MS, the diagnosis can be missed, delayed, or even incorrect. While the availability of MRI technology is helping to address this problem, it remains difficult to determine the extent to which the combined data from earlier epidemiological studies are truly representative of the MS population, particularly since the investigators used different methods for identifying and counting people with MS, as well as different strategies for analyzing their data. Therefore, it is important to keep in mind that epidemiologic numbers are, by definition, estimates, with a certain margin of error associated with them.
Incidence and Prevalence of MS
People often want to know how many individuals there are with MS in various parts of the country or the world, and whether those numbers are increasing. While it might seem easy to simply count them, this task is hampered by the same diagnostic challenges already described. The incidence of a disease refers to the number of new cases occurring in a given period of time (usually a year) in a given population (100,000 is often the population size that is used). With the challenges inherent in promptly and correctly identifying people with MS, arriving at an accurate incidence figure has been virtually impossible. Most epidemiologists have chosen instead to focus on the prevalence of MS, which refers to the number of people with MS at a particular point in time, in a particular place. While prevalence is a bit easier to determine, since all persons with MS are included in the figure, regardless of how long they have had the disease, the diagnostic issues can distort these figures as well. Of the prevalence studies in MS that have been conducted in various parts of the world, the data from the northern hemisphere are the most reliable thus far.
From existing epidemiological studies, the following observations have been made:
- In the United States, there are estimated to be 400,000 people with MS. 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 it can occur in young children and significantly older adults.
- Worldwide, MS occurs with much greater frequency in higher latitudes (above 40° latitude) away from the equator, than in lower latitudes, closer to the equator. Even within one geographic area, however, where latitude and climate are fairly consistent, prevalence rates may differ significantly. These differences demonstrate that geographical factors are not the only ones involved.
- MS is more common among Caucasians (particularly those of northern European ancestry) than other ethnic groups, and is almost unheard of in some populations, such as Inuit, Yakutes, Hutterites, Hungarian Romani, Norwegian Lapps, Australian Aborigines, New Zealand Maoris. Thus, ethnicity and geography seem to interact in some complex way to impact prevalence figures in different parts of the world.
- Scientists have long been searching for an infectious agent that might trigger MS. While many different viruses have been suggested, including rabies, herpes simplex virus, measles, corona virus, canine distemper virus, HTLV-1, Epstein-Barr virus, among others, none has yet been confirmed. Chlamydia pneumoniae, a bacterial agent, has also been suggested but never proven. Although no trigger has yet been identified, most MS experts believe that some infectious agent is involved in initiating the disease process.
- Migration from one geographic area to another seems to alter a person's risk of developing MS. Studies indicate that immigrants and their descendents 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 before the age of 15 tend to take on the new risk themselves. For those who move after the age of 15, 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.
- MS is approximately 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.
- Genetic factors are thought to play a significant role in determining who develops MS. The average person in the United States has about one chance in 750 of developing MS. But close (first-degree) relatives of people with MS, such as children, siblings or non-identical twins, have a higher chance—ranging from one in 100 to one in 40. The identical twin of someone with MS, who shares all the same genes, has a one in four 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 1 in 4 demonstrates that other factors, including geography, ethnicity, and the elusive infectious trigger are likely involved as well.
- Certain outbreaks or "clusters" of MS have been identified, but the cause and significance of these outbreaks are not known.
Clearly, many unanswered questions remain. While it may be tempting to try and find short-cuts to the answers, or to tell ourselves that we know more than we actually do about who gets MS and why, we need to recognize the complexities involved and look to future epidemiological studies to help unravel the facts.
Kalb R. (ed.) Multiple Sclerosis: The Questions You Have; The Answers You Need (3rd ed.). New York: Demos Medical Publishing, 2004.
—Ch. 2 Neurology
Murray TJ. Multiple Sclerosis: The History of a Disease. New York: Demos Medical Publishing, 2005.