According to Danish researchers, the study — the largest of its kind to date — shows a clear correlation between infection levels and impaired cognition.
"Our research shows a correlation between hospitalization due to infection and impaired cognition corresponding to an IQ score of 1.76 lower than the average," said Dr. Michael Eriksen Benrós, who is affiliated with the National Centre for Register-Based Research at Aarhus University School of Business and Social Sciences and the Mental Health Centre Copenhagen at the University of Copenhagen.
"People with five or more hospital contacts with infections had an IQ score of 9.44 lower than the average," he continued.
The study also found that an infection's effect on cognitive ability "increased with the temporal proximity of the last infection and with the severity of the infection," he said.
"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," he said.
"Moreover, 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."
The nationwide study tracked 190,000 Danes born between 1974 and 1994, who had their IQ assessed between 2006 and 2012. According to the researcher, 35 percent of these individuals had a hospital contact with infections before the IQ testing was conducted.
"Infections can affect the brain directly, but also through peripheral inflammation, which affects the brain and our mental capacity," explained Benrós.
"Infections have previously been associated with both depression and schizophrenia, and it has also been proven to affect the cognitive ability of patients suffering from dementia. This is the first major study to suggest that infections can also affect the brain and the cognitive ability in healthy individuals."
He noted that the brain is affected by all types of infections.
"Therefore, it is important that more research is conducted into the mechanisms which lie behind the connection between a person's immune system and mental health," he said.
Experiments on animals have shown that the immune system can affect cognitive capabilities, and more recent studies in humans have also pointed in that direction, according to the researcher.
Normally, the brain is protected from the immune system, but with infections and inflammation the brain may be affected. Benrós says his research suggests that it may be the immune system that causes the cognitive impairment, not just the infection, because many different types of infections were associated with a decrease in cognitive abilities.
The researchers said they hope the study's findings will lead to further research on the possible involvement of the immune system in the development of psychiatric disorders.
The study was published in PLOS ONE.
Source: Aarhus University
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