Summary: A large nationwide study from Finland has found that severe, hospital-treated infections are linked to a higher risk of dementia. Examining health registry records for more than 374,000 people, researchers identified 29 diseases associated with increased dementia risk—27 of them non-infectious conditions such as Parkinson’s disease and alcohol-related disorders, and two infectious conditions. The association between serious infections (notably hospital-treated urinary tract infections and unspecified bacterial infections) and later dementia remained robust even after accounting for other comorbid illnesses.
The study’s authors emphasize that their results do not prove infections directly cause dementia, but they suggest that severe infections may accelerate underlying cognitive decline in the years before a clinical diagnosis.
Key Facts
- Independent risk factor: Fewer than one-seventh of the excess dementia cases linked to severe infections could be explained by other pre-existing noninfectious illnesses, indicating infections act as a largely independent risk marker.
- Timing: On average, hospital-treated infections associated with later dementia occurred about five to six years before the dementia diagnosis.
- Stronger in early-onset cases: Associations were stronger for individuals diagnosed with dementia before age 65; five specific infections, including pneumonia and dental infections, were tied to higher risk in this group.
- Most robust infection associations: Hospital-treated cystitis (urinary tract infection) and bacterial infection of an unspecified site showed the clearest links to later dementia.
Source: PLOS Medicine
Severe infections treated in hospital are associated with an increased risk of dementia, independent of many common noninfectious comorbidities, according to a nationwide Finnish registry study published March 24 in PLOS Medicine.
Researchers used comprehensive Finnish health registries to compare 62,555 people aged 65 or older who received a late-onset dementia diagnosis between 2017 and 2020 with 312,772 matched dementia-free controls. The analysis covered hospital-treated diagnoses recorded up to 21 years before dementia, with adjustments for education, marital status, employment, and area of residence.
Applying strict selection criteria, the team identified 29 hospital-treated diseases that were consistently associated with higher dementia risk and had occurred at least 1–21 years before diagnosis with a minimum prevalence threshold. Of those 29, twenty-seven were noninfectious conditions spanning mental, neurological, cardiometabolic, endocrine, digestive and sensory disorders, plus injuries. Two were infections: cystitis (a urinary tract infection) and bacterial infection of an unspecified site. Nearly half (47%) of dementia cases had at least one of these 29 diseases recorded prior to diagnosis.
When researchers adjusted the models to account for the 27 noninfectious dementia-related diseases, the infection–dementia associations remained essentially unchanged. For hospital-treated cystitis the adjusted rate ratio was 1.22 before and 1.19 after comorbidity adjustment; for unspecified bacterial infections the ratios were 1.21 and 1.19, respectively. These results indicate that pre-existing noninfectious comorbidities explain only a small portion of the excess dementia risk seen after severe infections.
The associations held across subgroups defined by sex and education level and were particularly pronounced for early-onset dementia (diagnosed before age 65), where five types of infection—including pneumonia and dental caries—showed elevated risk. The authors note that because dementia develops over many years, a serious infection may accelerate an already ongoing cognitive decline.
Study limitations include the absence of baseline cognitive testing and detailed clinical examinations before dementia diagnosis, and a lack of information on infection treatments and some psychosocial or biological factors that registries do not capture. Because the research is observational, unmeasured confounding cannot be fully excluded, and causality cannot be established.
“Overall, our findings are consistent with the possibility that severe infections contribute to increased dementia risk,” the authors write, adding that randomized intervention studies are needed to determine whether better infection prevention or more effective treatment can reduce dementia incidence or delay its onset.
Key Questions Answered
A: This study focused on severe, hospital-treated infections. Everyday mild infections were not the subject of the analysis. The evidence points to infections severe enough to require hospital care as being linked to later cognitive decline.
A: Severe infections can trigger systemic inflammation and immune responses that may affect the brain. This inflammatory activity could damage neurons or accelerate pathological processes already present during the silent phase of dementia.
A: The study suggests this is plausible, but it does not provide direct evidence. The authors recommend clinical trials to test whether improved infection prevention or more aggressive treatment strategies can delay or prevent dementia in people at higher risk.
Editorial Notes
- This article was edited by a Neuroscience News editor.
- The journal paper was reviewed in full.
- Additional context was added by editorial staff.
About this neurology research news
Author: Claire Turner (PLOS)
Source: PLOS Medicine
Contact: Claire Turner – PLOS
Image credit: Neuroscience News
Original Research (open access): “The role of noninfectious comorbidities in the association between severe infections and risk of dementia in Finland: A nationwide registry study” by Adriana Michalak et al., PLOS Medicine. DOI: 10.1371/journal.pmed.1004688
Abstract
Title: The role of noninfectious comorbidities in the association between severe infections and risk of dementia in Finland: A nationwide registry study
Background
Severe infections have previously been associated with increased dementia risk, but both infections and dementia frequently coexist with other illnesses that may confound this relationship. This study used nationwide Finnish registry data to evaluate whether noninfectious mental and physical illnesses explain the observed link between severe infections and dementia.
Methods and findings
The register-based study included 62,555 people aged 65 or older diagnosed with late-onset dementia between 2017 and 2020 and 312,772 matched dementia-free controls. Analyses adjusted for demographic and socioeconomic factors. Applying a one-year lag, researchers identified 29 hospital-treated diseases that occurred 1–21 years before dementia diagnosis, had at least 1% prevalence in the population studied, and were robustly associated with increased dementia risk. Two of these were infectious (cystitis and unspecified bacterial infections); the others covered a wide range of mental and physical conditions.
About 47% of dementia cases had at least one of the identified diseases prior to diagnosis. The associations between the two infectious diseases and dementia risk were largely independent of the 27 noninfectious comorbid diseases diagnosed before infections. Findings were consistent across sex and education subgroups and were stronger for early-onset dementia. The study could not assess certain psychosocial, behavioural, or biological confounders not captured in registries.
Conclusions
This nationwide Finnish registry study found multiple mental and physical diseases associated with increased dementia risk and demonstrated that the higher dementia incidence following severe infections is not explained by the identified noninfectious comorbidities. These results support the view that severe, hospital-treated infections are independent risk markers for later dementia and highlight the need for intervention studies to test whether improved infection prevention or treatment can reduce dementia occurrence or delay its onset.