By: Julia Calderone
Published: May 22, 2015 11:16am ET on LiveScience.
People who have had an infection that made them so sick they had to be hospitalized may have IQs that are slightly lower than average, a new study suggests.
Researchers from the University of Copenhagen and Aarhus University in Denmark examined the hospital records of 190,000 Danish men born between 1974 and 1994. All the men took IQ tests at age 19, as part of the process of signing up for Denmark's mandatory draft. The tests were designed to assess their logical, verbal, numerical and spatial reasoning.
After adjusting for factors known to track with people's IQ scores, such as social conditions and the education levels of their parents, the researchers found that the average IQ score of the men who had been hospitalized for an infection before they took the IQ test — about 35 percent of the study cohort — were 1.76 points below the average of the men in the study who had not been hospitalized for an infection.
"Infections in the brain affected the cognitive ability the most, but many other types of infections severe enough to require hospitalization can also impair a patient's cognitive ability," study author Dr. Michael Eriksen Benrós, a researcher at the National Centre for Register-Based Research, said in a statement.
Moreover, the more times a person was hospitalized, the lower his IQ, researchers found. Those with five or more hospitalizations for infection had an average IQ that was 9.44 points below the average of those who were not hospitalized. [10 Ways to Keep Your Mind Sharp]
Hospitalization for bacterial infections tended to lower cognitive scores by about 1.55 points, and hospitalizations for viral infections lowered them by about 1.34 points, the researchers said.
The study shows that there is a strong relationship between the number and severity of infections a person has and that person's cognitive ability, according to the authors. The infections seen in the study included those of the stomach, urinary tract and skin, as well as some sexually transmitted infections such as herpes.
Infections have previously been linked to increased risks of depression and schizophrenia, and may even worsen the cognitive declines associated with dementia, according to the study. But this is the first study to suggest that infection may harm the brain and cognition of healthy people.
Although it's not clear exactly how infections may affect a person's IQ, the study authors said it's possible that the immune system, and not the infection itself, affects the brain. When the body launches an attack against a foreign invader, it activates an immune response that can lead to inflammation. The brain is generally protected from this attack, but perhaps sometimes the brain can be affected.
"It seems that the immune system itself can affect the brain to such an extent that the person's cognitive ability measured by an IQ test will also be impaired many years after the infection has been cured," said Benrós in a statement.
It could also be that inflammation elsewhere in the body negatively affects the brain, the researchers suggest. Animal experiments and some recent, small studies on people have indicated that the immune system may contribute to cognitive decline. However, more research is needed to determine whether genetic or environmental factors may play a role, the researchers said.
The study authors said they hope that their results will spark more research into the immune system's possible role in the development of psychiatric disorders. It is unclear whether infection-related inflammation may actually cause mental disorders to develop, or whether other factors may be involved, such as a genetic predisposition toward both infection and a lowered cognitive ability, the researchers said.
The study was published May 13 in the journal PLOS ONE.