Cognitive Labs

The latest research shows that viral and bacterial infections could have lasting legacies on cognition.

In a recent study, researchers found that people with higher levels of common infectious agents in their blood — including Chlamydia pneumoniae, Helicobacter pylori, cytomegalovirus and herpes simplex type 1 that causes cold sores — are more likely to have memory and cognitive problems than people with lower levels.

The researchers are not the first to make the connection. Other studies have linked infections and their effects on the body, such as inflammation, disruption of lipid production, and amyloid plaque build-up, with circulatory disorders such as heart disease and stroke. And these in turn have been linked to memory disorders like Alzheimer’s and dementia. Some studies even suggested that memory loss may be a result of the brain reacting to invading infections.

In the latest study, researchers analyzed the memory in 1,625 people around age 69, living in New York City. They compared the scores on these cognitive tests against blood samples from the participants that detected five common infections: Herpes simplex type 1 (oral) and type 2 (genital), cytomegalovirus, the bacteria responsible for causing pneumonia, and H. pylori, which is found in the digestive tract and has been linked to inflammation in gum disease and heart disease.

Participants who had higher levels of infections were at 25% higher risk of scoring low on a cognition test called the Mini-Mental State Examination, which tested for recall. The association was strongest among women, participants with lower education levels, those on Medicaid or without health insurance, and people who did not exercise regularly. “This observation provides some indirect evidence that the negative effects of chronic infection might be mitigated by beneficial behaviors such as physical activity, and evidence is accumulating that exercise has anti-inflammatory effects,” the authors write.

The researchers continued to test the memory of the participants every year for an average of eight years, but the infections were not linked to changes in memory and thinking skills as time went on. That could mean that the the infections had already altered the memory and cognition of the participants. “The damage [was] already done at the time of enrollment,” said study author Dr. Mira Katan of the Northern Manhattan Study at Columbia University Medical Center in New York and a member of the American Academy of Neurology in an email response to questions about the results. “Another explanation would be that our duration of follow-up may have been insufficient to detect a change.”

How could infections that the body essentially fights off leave such a lasting legacy on cognition? Although the scientists aren’t sure, they have some theories. It’s possible that chronic, or persistent infections from the pathogens are leading to an overall higher level of inflammation in the body, and over time, that damage can compromise the blood vessels in the brain that contribute to making memories. Or, the infectious agents could be directly causing cognitive decline.

If the link between viral and bacterial infections and memory loss disorders is confirmed with further research, it could reveal new ways of thinking about, and treating memory loss. A viral or bacterial agent that’s driving memory problems could lead to a bigger push for controlling such infections from a public health perspective, and focusing on new, antimicrobial approaches to treating disorders such as dementia or Alzheimer’s. In a corresponding editorial, Dr. Timo Strandberg of the department of medicine at the University of Helsinki, Finland and Dr. Allison Aiello, an associate professor of epidemiology at the University of Michigan School of Public Health write: “Undoubtedly, demonstrating that old-age cognitive disorders, including [Alzheimer's disease], are slowly progressing diseases of viral etiology would revolutionize the dementia research field (and be Nobel Prize worthy). However, great challenges remain.”

One of those is the fact that no study has established conclusively that pathogens can cause cognitive decline. But the latest findings suggest that infections, and the levels of inflammation, could potentially be used to better identify at-risk populations.

“While this association needs to be further studied, the results could lead to ways to identify people at risk of cognitive impairment and eventually lower that risk. For example, exercise and childhood vaccinations against viruses could decrease the risk for memory problems later in life,” said Katan in a statement.