Serious Bacterial Infections May Lead to Cognitive Decline
People who develop infections severe enough to require hospitalization face a higher risk of developing dementia later in life, according to a new Finnish study.
The research, published in PLOS Medicine, analyzed health records from more than 375,000 older adults. Certain severe infections remained associated with dementia even after researchers accounted for a wide range of other illnesses known to increase dementia risk.
The findings add to growing evidence that infections likely play a direct role in the biological processes that contribute to cognitive decline.
Studying Dementia Causes is Complicated
Dementia risk is multi-faceted, making the study of dementia risk complicated.
Older adults who experience serious infections are also more likely to have illnesses that are not the result of infection which are associated with dementia, such as diabetes, cardiovascular disease, depression, or neurological disorders. Nearly half of the dementia patients had experienced at least one of these conditions before their dementia diagnosis, raising the question of whether a serious infection is itself an independent risk factor.
Researchers from the University of Helsinki and collaborating institutions examined nationwide Finnish health registries. They identified 62,555 adults aged 65 and older who were diagnosed with dementia between 2017 and 2020. Each person was matched with dementia-free individuals of the same age and sex, creating a comparison group of more than 312,000 people.
The team then looked back as far as 21 years before diagnosis to determine which hospital-treated diseases had occurred before dementia developed.
Those who had been hospitalized with cystitis were about 19% more likely to develop dementia later on, even after accounting for the other conditions. Similar results were seen for other serious bacterial infections.
The findings were consistent across men and women and different educational backgrounds. The association was even stronger among people who developed early-onset dementia, before age 65.
How Infections Affect the Brain
Various theories exist on how infections might contribute to dementia.
One possibility is that severe infections trigger widespread inflammation throughout the body. This inflammatory response can affect the brain, potentially damaging neurons or accelerating processes already linked to neurodegenerative diseases like the build up of plaques.
Infections may disrupt the blood-brain barrier, the protective system that helps shield the brain from harmful substances circulating in the bloodstream. When that barrier becomes compromised, inflammatory molecules and other potentially damaging factors may gain greater access to brain tissue.
Another theory involves harm to blood vessels. Serious infections can increase clotting, damage blood vessel linings, and promote vascular problems that may contribute to cognitive decline over time.
Study Limitations
Although the study was large and drew on comprehensive national health records, it remains an observational study. That means it cannot prove that infections directly cause dementia.
Researchers were unable to measure many behavioral and biological factors that affect dementia risk. Other factors not captured in the health registries such as smoking, physical activity, diet, social engagement, or genetic risk could be influencing the results.
It is important to note that the study focused on infections severe enough to require hospital treatment. It does not consider whether milder infections carry similar risks.
What the Findings Mean
Dementia prevention has traditionally focused on factors such as blood pressure control, diabetes management, physical activity, hearing loss, and smoking cessation. This study suggests that preventing serious infections may deserve attention as well.
The results do not mean that every person hospitalized with an infection will develop dementia. Most will not. But they do strengthen the evidence that infections may be one piece of the dementia puzzle.
More studies are needed to determine if preventing or treating infections early reduces risk of dementia. Until then, the findings reinforce a broader message emerging from dementia research: protecting long-term brain health may require attention to many aspects of physical health, including the body's response to infection.
Related Articles on viral infections and dementia risk:
Current Shingles Vaccine Protects Against Dementia
Shingles Vaccine Reduces Risk of Dementia
