A Hiroshima University study has found that higher levels of the periodontal bacterium Fusobacterium nucleatum in the mouth were associated with greater disability in people living with multiple sclerosis (MS), though the researchers caution the findings do not establish cause and effect.
The cross-sectional observational study, published in Scientific Reports, analyzed tongue-coating samples collected from patients treated at Hiroshima University Hospital between May and November 2023.
Of 112 eligible patients, 98 were included after exclusions for non-consent and missing data. The cohort included 56 patients with MS, 31 with neuromyelitis optica spectrum disorder (NMOSD) and 11 with myelin oligodendrocyte glycoprotein antibody-associated disease (MOGAD). All participants were aged 15 years or older.
Researchers collected baseline clinical characteristics, oral-care routines and neurological disability scores using the Kurtzke Expanded Disability Status Scale (EDSS).
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Among patients with MS, those with a high relative abundance of F. nucleatum were significantly more likely to have moderate disability — defined as an EDSS score of 4 or higher — compared with those with lower bacterial levels (61.5 per cent versus 18.6 per cent). The association remained statistically significant after adjustment for multiple comparisons.
“This study demonstrated that the relative abundance of F. nucleatum in the oral cavity was strongly associated with the EDSS score in patients with MS,” the authors wrote. They added that future research should explore immunological pathways — including cytokine-related mechanisms — and assess whether interventions such as improved oral care could influence disease outcomes.
MS prevalence rising worldwide
Global estimates of MS prevalence vary depending on data sources and methods. The Atlas of MS — a joint initiative of the Multiple Sclerosis International Federation and the World Health Organization (WHO)— reports that the number of people living with MS worldwide increased from 2.3 million in 2013 to 2.8 million in 2020 and approximately 2.9 million in 2023.
By contrast, the WHO’s most recent MS fact sheet, updated in 2023, estimates that more than 1.8 million people worldwide are living with the disease, reflecting differences in reporting systems and country-level data availability.
Canada continues to have one of the highest rates of MS globally. MS Canada estimates that more than 90,000 Canadians live with the disease — roughly one in every 400 people — and reports that MS prevalence has increased in every global region since 2013. The organization attributes the rise to improved diagnosis, better case tracking, longer life expectancy and population growth, while noting that an increase in underlying risk cannot be ruled out.
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Rising MS prevalence in Japan
Historically, MS prevalence in Japan has been lower than in Europe and North America, but longitudinal data indicate a steady increase since the 1980s.
The Hiroshima University researchers note that MS prevalence in Japan has risen over recent decades, a trend thought to be influenced by environmental changes. Prior studies have pointed in particular to alterations in the gut microbiome.
“In patients with MS, a reduction in the abundance of gut bacteria that produce short-chain fatty acids has been reported, suggesting a potential association with MS pathology,” the authors wrote. Other studies have found increased oxidative stress in the gut among individuals with secondary progressive MS, which may contribute to disease progression.
The WHO notes that MS has no cure, symptoms can fluctuate or worsen over time, and both genetic susceptibility and environmental factors are believed to play a role in disease development.
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Building on prior oral–systemic research
The study builds on earlier work by the same research group examining links between oral health and systemic disease. In collaboration with dental professionals, the authors have previously reported that poor oral health and elevated serum periodontal antibodies are associated with worse outcomes in stroke patients. Associations between periodontal disease and autoimmune conditions such as rheumatoid arthritis also are well established.
The authors also cite emerging research suggesting overlap between oral and gut microbial changes in MS. One recent study examining both saliva and fecal samples found increased levels of inflammation-associated bacteria — including Fusobacterium and Leptotrichia in saliva, and Enterobacteriaceae and Actinomyces in feces — among patients with MS.
“The abundance of F. nucleatum was also increased in the gut, indicating dysbiosis in both the oral and gut environments and suggesting a possible oral–gut microbiome axis,” the researchers wrote.