A groundbreaking study published in Nature’s Scientific Reports has revealed a significant geographic association between amyotrophic lateral sclerosis (ALS) and multiple sclerosis (MS), suggesting a potential shared environmental cause. This correlation persists even after accounting for variables such as race, gender, wealth, latitude, and access to neurological healthcare.
The findings, led by NYU Stern Professor Melissa Schilling, challenge previous assumptions that no mechanistic or genetic link exists between ALS and MS. “The results of the study are surprising because previous studies have typically concluded there was no evidence for a mechanistic or genetic link between the two diseases,” Schilling explained. Her research involved analyzing large-scale datasets with econometric methods.
Unveiling Geographic Patterns
The study’s heat maps illustrate the geographic distribution of ALS and MS across the United States, revealing a strong positive correlation in their distribution. This relationship, however, has been obscured in the past due to a statistical phenomenon known as “Simpson’s Paradox.” This occurs when a trend appears in separate groups but disappears or reverses when these groups are combined.
In this case, the paradox arises from gender differences: while both men and women show a strong geographic correlation between ALS and MS, pooling the data across genders obscures this relationship. On average, ALS is more prevalent in men, whereas MS is more common in women.
Rethinking Latitude and Disease
For decades, researchers have observed a north-south gradient in MS distribution, speculating that factors such as UV light or vitamin D might influence the disease. However, studies supplementing MS patients with these elements have yielded inconsistent results. The new study suggests a stronger geographic link between MS and ALS than with latitude, indicating a shared environmental factor that varies imperfectly with latitude.
“I started gathering and analyzing every dataset I could find relevant to ALS about nine years ago when a friend with ALS asked me if I would take a look at the data,” Schilling noted. “I was very surprised to find such a strong geographic pattern as most of the research on ALS does not emphasize the role of geography. I was even more surprised to find that ALS has a very strong association with the geography of MS.”
Potential Environmental Culprits
The study posits that environmental factors, both natural and human-made, might be at play. These include viruses, parasites, algae, molds, as well as practices like the use of heating oil, agricultural methods, industrial activities, mining, and chemical contamination of fisheries.
“The list of suspects is long, but comparing across geographies and, in particular, across outlier locations, such as the Faroe Islands, where MS increased strikingly after military troops arrived there in the 1940s, could significantly narrow the hunt,” Schilling observed.
Data and Methodology
The research utilized mortality and demographic data from the US Centers for Disease Control and Prevention WONDER database, which mandates standardized mortality data collection. This was combined with latitude, economic, and healthcare access data. The primary analysis focused on US state-level crude mortality rates and was replicated globally using World Health Organization data, yielding similar results.
Reference: Schilling MA. The geographic association of multiple sclerosis and amyotrophic lateral sclerosis. Sci Rep. 2025;15(1):34665. doi: 10.1038/s41598-025-18755-8
The study’s implications are profound, suggesting that identifying and understanding these environmental factors could lead to new strategies for preventing and treating both ALS and MS. As research continues, the hope is that these insights will pave the way for breakthroughs in combating these debilitating diseases.
