Severe infections may raise the risk of dementia independently of other coexisting health conditions, according to a new study published on 24 March in the journal PLOS Medicine. The research was led by Pyry Sipilä at the University of Helsinki and adds important clarity to a long-standing question: whether infections themselves contribute to dementia risk, or whether the observed link is largely explained by other underlying illnesses.
Previous research has consistently suggested that people who experience serious infections face a higher likelihood of developing dementia later in life. However, the relationship has remained difficult to interpret, as many individuals with infections also live with other chronic conditions that could independently increase dementia risk. These overlapping health issues have made it challenging to determine whether infections play a direct role or reflect broader vulnerability in already at-risk populations.
To address this uncertainty, the researchers analysed nationwide Finnish health registry data, focusing on more than 62,000 individuals aged 65 and older who were diagnosed with late-onset dementia between 2017 and 2020. These cases were compared with over 312,000 matched individuals without dementia. Taking a comprehensive approach, the team examined hospital-treated conditions recorded over the previous two decades, identifying 29 diseases that showed a consistent association with increased dementia risk. Notably, nearly half—47 per cent—of those diagnosed with dementia had experienced at least one of these conditions before their diagnosis.
Among the 29 diseases identified, two were infections: cystitis, a common urinary tract infection, and bacterial infections of unspecified origin. The remaining conditions were non-infectious, with the strongest links observed for neurological and psychiatric disorders, including mental disorders due to brain damage or physical illness, Parkinson’s disease, and alcohol-related mental and behavioural disorders. These findings underscore the complex interplay between physical health, neurological conditions, and cognitive decline.
Crucially, when the researchers adjusted for the presence of the 27 non-infectious conditions, the association between infections and dementia remained largely unchanged. In fact, less than one-seventh of the increased dementia risk among individuals with hospital-treated infections could be explained by pre-existing illnesses. The connection appeared even stronger in cases of early-onset dementia, diagnosed before the age of 65, where multiple infections—including pneumonia and dental conditions such as caries—were linked to elevated risk. These patterns suggest that infections may contribute independently to processes that accelerate cognitive decline.
The authors caution that the study has important limitations, including the absence of baseline cognitive assessments and detailed clinical data prior to dementia diagnosis, as well as limited information on how infections were treated. As an observational study, it cannot establish a definitive cause-and-effect relationship. Nonetheless, the findings point to the possibility that severe infections may hasten underlying neurodegenerative processes, often occurring five to six years before a dementia diagnosis. The researchers emphasise the need for intervention studies to determine whether improved infection prevention or treatment could reduce dementia risk or delay its onset, highlighting a potentially important avenue for future public health strategies.
More information: Pyry Sipilä et al, The role of noninfectious comorbidities in the association between severe infections and risk of dementia in Finland: A nationwide registry study, PLOS Medicine. DOI: 10.1371/journal.pmed.1004688
Journal information: PLOS Medicine Provided by PLOS
