Danish researchers have uncovered an association between infection and decreased cognitive ability. The finding was published last Wednesday to PLoS ONE.

A wide range of bacterial and viral infections, from influenza and pneumonia to hepatitis and encephalitis, can seriously stress the immune system. Oftentimes, the battle inflicts collateral damage on systems across the body. Some of these systems may not even be directly involved. For example, scientists are increasingly noting that infections and associated immune responses like inflammation can affect the brain.

This peaked the curiosity of researchers at the National Centre for Register-based Research at Aarhus University in Denmark. So, with a nationwide register of 161,696 young Danish men at their disposal, they decided to investigate whether or not infections were in any way linked to cognitive ability.

The Danish Conscription Registry tracked almost all males born between 1976 and 1994, collecting a variety of data, including the time, number, and detail of all infections requiring hospitalization. At age 19, all of the men in the register took a 3-hour test designed to measure cognitive ability. In hunting for an association between infection and cognitive ability, the researchers controlled for parental educational level, year of testing, birth order, multiple birth status, birth weight, gestational age, a parental history of infections, parental and individual history of psychiatric disorders, and substance abuse.

Analyzing the data, the researchers discovered that the more infections a subject endured throughout his life, the lower he scored on the test of cognitive ability. Subjects who were hospitalized once for an infection scored 0.83 points lower on a 100-point scale, while subjects who fell ill to 5 or 6 different infections scored 5.29 points lower. Unsurprisingly, infections of the central nervous system were associated with larger detriments, but respiratory, gastrointestinal, and skin infections also registered significant decreases in cognitive ability. (Below: Graph shows the unadjusted decreases in cognitive ability.)

The researchers also found that time factored into cognitive ability scores. Subjects with more recent infections faired worse than those with more distant infections.

What could account for the link between infection and a decrease in cognitive ability?

"The observed associations might be due to a biologically-mediated effect of the infection or associated immune responses causing an acute and possibly transient effect on general cognitive ability," the researchers say. "Inflammation and immune components can directly affect the glutamate, serotonin and dopamine systems that are considered central in cognition."

So does this mean that you get a little duller every time you succumb to an infection? Perhaps. But maybe not. Reverse causality can't be ruled out.

"Lower cognitive ability may be a risk factor for acquiring infections," the researchers admit. "Studies have indicated that immune related genes might be implicated in cognition, and individuals with genetic liability towards a lower general cognitive ability might also be more genetically vulnerable towards infections."

The study's size and extensive controls were key strengths, however, the study could not account for the effects of less severe infections that didn't require hospitalization and did not include women.

Source: Benros ME, Sørensen HJ, Nielsen PR, Nordentoft M, Mortensen PB, Petersen L (2015) The Association between Infections and General Cognitive Ability in Young Men – A Nationwide Study. PLoS ONE 10(5): e0124005. doi:10.1371/journal.pone.0124005

(Image: AP)