People in the study who'd suffered more viral and bacterial infections in the past, including infection with herpes simplex type 1 virus (HSV1), which causes cold sores, were at increased risk of scoring below average on a test of cognitive abilities, the researchers said.
The results held even after the researchers took into account factors that could affect people's test scores or brain function, such as age, education level, high blood pressure, diabetes and alcohol consumption.
The findings add to a growing body of evidence linking infections with cognitive function. For instance, previous studies have linked herpes infection with an increased risk of Alzheimer's disease in people with certain genetic mutations.
However, the new study found only an association, and cannot prove these infections were cause of cognitive impairment. It's possible other factors not accounted for by the study could explain the link, said study researcher Dr. Mira Katan of Columbia University Medical Center in New York City. Larger studies in different populations are needed to confirm the results, Katan, a neurologist, said.
Infections and cognitive impairment
Katan and colleagues previously found that past infections were linked with a higher risk of stroke.
The new study involved 1,625 adults around age 70 who lived in northern Manhattan.
Researchers tested participants' blood for evidence of previous infection withHSV1, herpes simplex type 2 (which causes genital herpes infections), cytomegalovirus (a common herpes virus), chlamydia pneumoniae (a respiratory infection that can cause pneumonia) and Helicobacter pylori (a bacteria found in the stomach). These particular infections have been linked with a higher risk of stroke. Those with a higher number of these infections were said to have a higher "infection burden."
They also took a test that measured their cognitive abilities, including attention, memory and language skills.
Participants with a higher infection burden were 25 percent more likely to score below average on the cognitive test.
The link was strongest among women, those with a low socioeconomic status, and those who with low levels of physical activity, the researchers said.
However, infection burden wasn't related to participants' risk of a decline in cognitive scores over time, the researchers said.
Inflammation risk
Infections increase levels of inflammation in the body, which contribute to cognitive impairment as well as stroke, Katan said.
Because exercise can reduce inflammation, it may counteract this risk, Katan said.
Dr. Nunzio Pomara, director of the geriatric psychiatry division at the Nathan S. Kline Institute for Psychiatric Research, in Orangeburg, N.Y., who was not involved in the study said he was "quite happy to see that, at least in some instances, there could be a potentially treatable [cause of] cognitive dysfunction," referring to the fact that infections can be treated or prevented.
However, Pomara said he hoped further studies would explore the link between infections and Alzheimer's disease. The current study did not find a link between infections and the risk of cognitive decline over time, which would be expected to occur in Alzheimer's disease patients, but it could be that participants were not followed for long enough to see a change in their cognition, Pomara said.
Given the growing evidence for the link between infections and cognitive impairment, studies that attempt to prove the link are worthwhile, Dr. Timo Strandberg of the University of Helsinki in Finland, and Allison Aiello of the University of Michigan, wrote in an editorial accompanying the new study. A first step would be a study that randomly assigns people with Alzheimer's disease to receive an antiviral medication or not, and examines the effect on disease outcome, they said.
The study and editorial are published in tomorrow's (March 26) issue of the journal Neurology.
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