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.
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. It is challenging to conduct 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 MRI technology is helping to address this problem, it remains difficult to determine how accurately data from earlier epidemiological studies truly represent 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 all epidemiological numbers are estimates.
Incidence and Prevalence of MS
People often want to know how many people have MS in various parts of the country or the world, and whether those numbers are increasing. Simply counting them is easier said than done due to the diagnostic challenges described above.
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). 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 — the number of people with MS at a particular point in time, in a particular place.
While prevalence is a bit easier to determine than incidence, the diagnostic issues can distort these figures as well, since all persons with MS are included in prevalence figures, regardless of how long they have had the disease. Of the MS prevalence studies that have been conducted worldwide, the data from the northern hemisphere are the most reliable thus far.
The following observations have been drawn from existing epidemiological studies:
- 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 significantly older adults.
- Worldwide, MS occurs with much greater frequency above 40° latitude than closer to the equator. However, prevalence rates may differ significantly even within a geographic area, where latitude and climate are fairly consistent. 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, but people of African, Asian, and Hispanic ancestry also develop the disease. In spite of the latitude at which they live, MS is almost unheard of in some populations, such as Inuit, Yakutes, Hutterites, Hungarian Romani, Norwegian Lapps, Australian Aborigines and New Zealand Maoris — indicating that ethnicity and geography interact in some complex way to impact prevalence figures in different parts of the world.
- Scientists have long searched 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 and others — none has been confirmed. To date, the most promising candidate appears to be the Epstein-Barr virus. Chlamydia pneumoniae, a bacterial agent, has also been suggested but never proven. Although no trigger has yet been confirmed, 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 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.
- 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. And some recent 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 chance in 750 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 one in 40 — 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 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 one in four 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 questions remain. While it may be tempting to try and find shortcuts 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.