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A cluster of MS is the perception that a very high number of cases of MS have occurred over a specific time period and/or in a certain area. Such clusters of MS — or of other diseases where clusters are occasionally reported — are of interest because they may provide clues to environmental or genetic risk factors that might cause or trigger the disease. So far, cluster studies have not produced clear evidence for the existence of any causative or triggering factor or factors in MS.

Clusters are difficult to investigate

The problems associated with investigating clusters of any disease, and especially MS, are enormous. First, it is difficult to determine what constitutes an “excess” of cases of MS. To do this, one needs to calculate the expected incidence of MS (that is, the number of new cases that would be expected to occur in a given area over a given time period, based on the total population at risk in the area). The expected incidence can then be compared with the reported incidence. However, documented incidence rates may not exist for an area where a cluster has been reported because MS is not infectious — and is therefore not “reportable” according to federal  standards — and because the National Neurological Conditions Surveillance System at the Centers for Disease Control and Prevention (CDC) could not begin its work until fiscal year 2019. The challenge then becomes finding a suitable comparison population where the incidence of MS is known. This figure can help to determine the expected incidence of MS in the area of the reported cluster.

Calculating an expected incidence rate gets even more complicated. MS rates are known to vary by latitude. Furthermore, MS occurs more often in women than men, and more often in individuals of northern European ancestry than in others. Therefore, the expected incidence in an area must take into consideration not only its geographical location, but also the age, gender distribution and ethnic makeup of the people living there.

In addition, MS is more common in families where the disease already exists — an indication of genetic susceptibility. Therefore, family relationships within a reported cluster must also be considered.

Some of the other reasons that MS clusters are difficult to investigate include:

  • Uncertainty of the diagnosis of MS: When lay people identify an MS cluster, they may actually be including different diseases with similarities to MS. The difficulty of diagnosing MS accurately can make this problem worse.

  • Lag time between clinical onset and diagnosis: Since the first symptoms of MS often occur years before the disease is diagnosed, a person diagnosed with MS in an area where a cluster has been reported may actually have had clinical onset of MS somewhere else.

  • Coincidence: It is possible for clusters to happen by chance, with no common factor(s) causing the MS.

Clusters of MS

A confirmed cluster of MS means that there is a significantly higher incidence of definite MS in an area than expected. Surprising as it sometimes seems, however, an apparently extraordinary number of MS cases in a neighborhood or county may turn out to be the “expected” number.

The major resource for individuals with concerns or questions about an MS cluster in their community is the local public health department. Public health officials are qualified to investigate suspected clusters. If they lack adequate staffing and funding to analyze situations that they consider to be of concern, they may refer cases to the federal Agency for Toxic Substances and Disease Registry at 888-422-8737 or


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