Multiple sclerosis clusters might sound like a dodgy breakfast cereal (“we’ll stick to toast, thanks”) but they’re actually a pretty interesting phenomenon. So what exactly are they? Well, a cluster is when an unexpectedly high number of cases of a disease occur within a specific time period and/or in a certain area. Scientists get very excited whenever clusters of MS are reported because it gives them the opportunity to investigate the possible factors that might be at play – which could shed some light on the way the disease is caused or progresses.
Let’s say a surprising number of people developed MS on a tiny island off the Spanish coast. Scientists would want to question those people about their diet and lifestyle, and look at any common exposures such as chemicals or infections. If the people were all related it might suggest a genetic link. If not, then there may be something about their environment that could be causing a peak in incidence in the disease. If the scientists could identify what all the people have in common, it might point to an obvious trigger. Or not.
Unfortunately, clusters aren’t always so straightforward. It takes rigorous mathematical analysis to confirm that they even exist. Epidemiologists have to calculate a country’s incidence rate so they can work out if the number of cases that occur in a certain place is expected or not. Because sometimes, what looks like a cluster is in fact a simple coincidence. If 90 in 1000 people are expected to have the disease in a certain country, for example, it could simply be that 67 of these happen to live in the same small community. Strange, but possible.
Once clusters have been confirmed, the causes aren’t always obvious – largely because there’s typically a lag between possible triggers and the onset of the disease. This means that by the time the cluster has been detected, the agent that caused it may well have disappeared.
The Faroe Islands, located between Norway and the UK, is a classic example. A sudden occurrence of MS cases after 1943 appeared to tally with the British occupation of the island around this time, suggesting British soldiers may have brought over something that triggered the disease. Interestingly, many of the soldiers were from the Scottish Highlands, where the MS prevalence is fairly high. Despite years of investigation, however, no factor has yet been identified that can definitively account for the curious cluster.
Other clusters appear to be more clear-cut. In El Paso, Texas, USA, in 1994, the community was concerned that a concentration of cases of MS might be linked to toxic waste from a nearby metal processing plant. When investigators from the Texas Department of Health looked into the case they found that adults who had attended a school near the plant did indeed have twice the expected rate of MS for the area. What’s more, analysis of hair samples showed a high likelihood that these adults had been exposed to heavy metals in childhood.
Then a similar case occurred in in De Pue, Illinois, USA, in the late 1990s: not only was this cluster found to be statistically significant, there was also evidence that those affected had been exposed to heavy metals from a nearby zinc plant. While both these cases seem like no-brainers, they’ve yet to be solved completely. Experts have pointed out that there’s still no direct evidence that heavy metals can cause MS, so more research is needed before we can come to any conclusions.
Gathering the evidence in this way takes time, of course. For the scientists involved, sorting through the data for important clues can be a painstaking process – but one that is extremely worthwhile. By identifying the factors that can trigger MS, the hope is that one day we’ll be able to prevent the disease altogether. In other words, clusters may one day be a thing of the past rather than the source of mystery they currently are.