Severe Bacterial Infections Linked to Higher Dementia Risk

People are hospitalized with severe infections — such as COVID-19, pneumonia, or other diseases — are more likely to develop dementia years later. Scientists have long speculated that the infections might not be to blame, but underlying factors, like diabetes or other chronic diseases, might predispose people to both infections and dementia.  

But now, new research provides growing proof that it’s the severe bacterial infections that drive dementia risk. 

A study published in PLOS Medicine of over 370,000 older adults in Finland compared the health histories of individuals who developed dementia to those who remained healthy to see how severe infections and other chronic, non-infectious diseases played a role in driving up dementia risk. 

“It was unclear whether infections indeed would contribute to dementia risk if the influence of other diseases was taken into account,” study author Pyry N. Sipilä, from the University of Helsinki, told Being Patient. °But even when they accounted for the effects of other underlying health conditions, it was still clear that the severe infections were driving the risk.”

Why severe infections might be to blame

Sipilä’s team compared over 62,000 older adults diagnosed with dementia between 2017 and 2020 and compared them to a set of 310,000 adults who remained healthy. They combed through their health records and noted any diseases they developed that required hospital treatment anytime in the 21 years. 

After accounting for age, sex, education levels, and other demographic factors, they found 29 different diseases linked to a higher risk of developing dementia. Mental health disorders and Parkinson’s more than tripled the risk. 

Among them, bladder infections — also called urinary tract infections (UTIs) — and bacterial infections where the region of the body infected wasn’t recorded increased the risk by about 19 percent. In other words, if the chances of getting diagnosed with dementia at age 65 are 5 percent, one of these infections could up the risk to 6 percent. 

Sipilä and her team looked at the relationships between different diseases to make sure other conditions that increase dementia risk couldn’t explain away the impact of infections. For example, if someone with Parkinson’s develops an infection and later dementia, they would try to disentangle the relationship to see if the infection or the Parkinson’s were driving the risk. 

On average, they found people developed dementia about five to six years after the infection. Sipilä said that previous research found that severe infections are tied to an increased risk of vascular dementia. While she cautioned that the research doesn’t prove cause and effect, this new study provides another clue that strong inflammation related to these infections affects the vascular system in the brain. 

Ryan Demmer, a professor of epidemiology at the Mayo Clinic, who wasn’t involved in the study, told Being Patient that when someone is hospitalized, it increases the risk of them developing other chronic diseases. Some people are inactive and lose muscle mass during a hospital stay, others gain weight, or become physically deconditioned. “A lot of people have to stop medications when they get hospitalized that are important for preventing a lot of cardiovascular diseases, which are linked to dementia,” Demmer also explained.

He said the methods these researchers used controlled for hospitalization, providing more evidence that infections themselves might be contributing factors. Dr. Thad Abrams at the University of Iowa who has conducted similar research, but wasn’t involved in the study, told Being Patient that Sipilä’s study supports the notion that the inflammatory response to the infection raises dementia risk. 

Preventing and treating infections for dementia prevention

Though many studies have come to similar conclusions, it isn’t clear why infections are linked to higher dementia risk. Demmer thinks observational studies are important because electronic health records aren’t completely accurate. Researchers can also test whether preventing severe infections could reduce the occurrence of dementia risk factors, like cardiovascular disease, and dementia itself, he added.

In the meantime, both Sipilä and Demmer recommend vaccinations against infectious disease, as many including those for the flu, COVID-19, RSV, and shingles are linked to a reduced dementia risk.   

Studies of antivirals targeting one such chronic infection — herpes — failed to slow cognitive decline in mild cognitive impairment and Alzheimer’s. Other studies have linked various vaccines to lower dementia risk, an unexpected side effect for protecting against severe infection.

Despite the emerging evidence, infections have yet to be included in the list of modifiable risk factors for dementia. “I don’t think doctors blow them off, but I don’t think that it’s in the culture yet to think of these as dementia [risk factors],” Dr. Robert Wallace, a clinical and emeritus professor of epidemiology at the University of Iowa, told Being Patient.

For Abrams, the data is enough to change his practice. 

“If you told me, as a primary care provider, that there’s a risk for dementia with this particular kind of infection, that would probably change my practice, and I would be more aggressive in managing it,” said Abrams. That might mean, he said, keeping a closer eye on patients in their 40s and 50s with a history of these infections and screening for neurological changes sooner.