When people in a community who live close to one another develop MS, it’s natural to wonder whether their disease resulted from common exposure to something in the local environment. Studies have been conducted to examine suspected MS clusters, defined as a high number of cases that happen in a certain geographical area and/or over a specific period of time.
While cluster studies have been conducted in places including the Faroe Islands; Galion, Ohio; DePue, Illinois; Rochester, New York; El Paso, Texas, no true MS cluster has ever been confirmed and no environmental exposure, such as a hazardous substance or toxin, has to date been linked to MS. Despite the fact that no MS cluster has been confirmed, such studies continue to be of interest because they have the potential to give us clues concerning environmental and genetic factors that may play a role in causing or triggering MS.
Among the trigger factors that have been considered in MS cluster studies are infectious agents, environmental and industrial toxins, diet, trace metal exposures, and climatic elements.
Studying clusters presents several difficulties
There are considerable difficulties involved in studying clusters of any disease and MS poses special challenges. The first challenge is to confirm that an excess of MS cases has indeed occurred. This requires determining how many new cases of MS might normally be expected during a given period of time for the area being considered. However, information about new cases of MS in an area may not be readily available because, unlike with certain infectious diseases, cases of MS are not required to be reported and there is currently no nationwide MS registry.
Determining expected rates of incidence of MS in an area may also be very difficult because prevalence of MS varies by geographical location, age, gender distribution, and ethnic composition. Since MS occurs more frequently in families with a history of the disease, this must also be taken into consideration when examining a potential cluster.
Additional difficulties involved in examining a potential cluster include uncertainty surrounding diagnosis, the lag-time between the appearance of initial MS symptoms and diagnosis, and the possibility that clusters of MS cases may have occurred simply by chance.