Severe infections independently amplify the risk of dementia later in life

People who experience severe, hospital-treated infections face an elevated risk of developing dementia later in life, and this connection operates independently of other underlying medical conditions. Researchers reached this conclusion by analyzing the vast electronic health records of the Finnish population. The findings were published in the journal PLOS Medicine.

Medical professionals have observed a link between infectious diseases and cognitive decline for some time. Proposed explanations revolve around how the immune system interacts with the central nervous system. A severe infection causes widespread inflammation throughout the body. This persistent inflammation can affect the blood-brain barrier, which is a tight layer of cells that normally protects the brain from toxins and pathogens circulating in the bloodstream.

When the blood-brain barrier becomes compromised, harmful proteins and inflammatory cells can enter the brain tissue. This infiltration can promote neuroinflammation, a state of chronic immune activation inside the brain. Such an environment plays a role in the destruction of brain cells, a hallmark of dementia cases. Infections also trigger vascular issues across the body, which can involve changes in blood clotting and potential damage to the delicate blood vessels supplying the brain with oxygen and nutrients.

The biological timeline of cognitive decline complicates this picture. The condition typically arises late in life, generally after the age of 80. By this time, most patients already suffer from a variety of other physical and mental ailments. Many of these age-related diseases, such as diabetes and cardiovascular conditions, are known risk factors for both dementia and severe infections.

Because of these overlapping risks, researchers wanted to test whether an infection acts independently to elevate dementia risk. It remained entirely possible that a patient who develops dementia after pneumonia simply had pre-existing heart disease that actually caused both problems. To isolate these variables, an epidemiological research team initiated a massive data investigation. Pyry N. Sipilä, a researcher at the University of Helsinki, led the study.

Sipilä and his colleagues accessed Finland’s nationwide health registry data. The dataset included 62,555 individuals aged 65 or older who received a late-onset dementia diagnosis between 2017 and 2020. They compared this group to 312,772 control individuals without dementia. The researchers matched each patient with dementia to five control subjects of the same sex, birth year, and specific clinical timeline.

This epidemiological approach ensures that generic variables like age and the natural passage of time do not artificially inflate the results. The matching comparison allows researchers to pinpoint variations restricted to specific health events. To conduct the analysis, the researchers reviewed up to 21 years of prior medical records for each participant. They intentionally excluded the single year immediately preceding the dementia diagnosis to ensure that the cognitive decline itself had not caused the other medical events.

The researchers then cataloged every disease or condition that had sent these individuals to the hospital. Out of 170 common conditions, the team identified 29 specific diseases that reliably preceded a dementia diagnosis. This list included 27 non-infectious conditions. Examples ranged from cardiovascular events like cerebral infarction to metabolic issues like type 2 diabetes, along with mental health conditions like severe depression and physical traumas like head injuries.

The final two items on the list of 29 conditions were infectious diseases. Specifically, these were cystitis, an infection of the urinary system, and general bacterial infections without a specifically identified site. Almost half of all patients with dementia had experienced at least one of the 29 conditions in the two decades before their cognitive decline. Many patients experienced a sequence of these conditions over the years.

The researchers mapped how these illnesses connected to one another. They found a web of interrelated conditions, where an initial diagnosis of a stroke often led to a subsequent diagnosis of a urinary tract infection. The majority of the 27 non-infectious diseases also elevated a patient’s likelihood of eventually contracting one of the severe hospital-treated infections. To answer their core question, the researchers needed to isolate the statistical effect of the infections alone.

They adjusted their mathematical models to account for all 27 non-infectious conditions. Even after this adjustment, the association between the two types of infections and later dementia remained robust. An individual hospitalized for cystitis faced roughly a 19 percent relative increase in the rate of eventually developing dementia compared to someone who avoided such an infection. Unspecified bacterial infections showed an identical rate increase.

Only roughly ten to fourteen percent of the excess dementia risk for these patients could be explained by their other physical and mental health issues. The statistical modeling demonstrates that infections act as distinct risk factors entirely on their own. The team repeated their analysis for early-onset dementia, a variation that strikes people before the age of 65. The sample size for this secondary inquiry included 2,639 cases.

The researchers identified a wider variety of infections linked to an increased risk of early cognitive decline in this younger cohort. This list included severe gastrointestinal conditions, bacterial pneumonia, and severe dental caries. Just as with the older cohort, the associations between infections and early-onset dementia held steady after researchers accounted for all other co-occurring medical conditions.

The exact biological reasons for the differences between early-onset and late-onset dementia remain a subject of active research. The two forms of the condition rely on distinct genetic and physiological underpinnings. The overarching study design provides high confidence in the final reported metrics. Finland’s healthcare system maintains nearly complete electronic health records for its citizens, eliminating the typical biases found in studies that rely on self-reporting or memory.

Still, as an observational study, the research does not definitively establish that infections directly cause dementia. The findings carry an additional limitation regarding non-severe events. The registry only documents infections serious enough to require hospital care. Minor respiratory infections or simple illnesses managed with prescribed oral antibiotics at home were not included in the primary dataset.

The researchers suspect that in older individuals, severe infections may accelerate cognitive decline rather than initiate the disease entirely from scratch. A large inflammatory event might simply speed up the deterioration in a brain already poised to develop dementia. Future scientific endeavors must verify if treating or preventing infectious diseases actively alters the trajectory of cognitive decline. Clinical intervention trials, such as studies analyzing the long-term cognitive impacts of large-scale vaccination programs, will offer related guidance in the coming years.

The study, “The role of noninfectious comorbidities in the association between severe infections and risk of dementia in Finland: A nationwide registry study,” was authored by Pyry N. Sipilä, Kaarina Korhonen, Joni V. Lindbohm, Mika Kivimäki, and Pekka Martikainen.