Researchers have traced a connection between some infections and mental illnesses like schizophrenia, depression and bipolar disorder. New research from Denmark bolsters that connection. The study, published on Thursday in JAMA Psychiatry, shows that a wide variety of infections, even common ones like bronchitis, are linked to a higher risk of many mental illnesses in children and adolescents.
The findings support the idea that infections affect mental health, possibly by influencing the immune system, report agencies.
“This idea that activation of the body’s immune inflammatory system as a causative factor in … select mental illnesses is one that has really caught on,” says Dr. Roger McIntyre, a professor of psychology and pharmacology at the University of Toronto, who wasn’t involved in the study. “This study adds to that generally, but builds the case further in a compelling way.”
In the new study, the researchers gathered data on hospitalizations and prescription medications for the 1.1 million children born in Denmark between Jan. 1, 1995, and June 30, 2012.
“We could follow individuals from birth, so there was no missing information during the study period,” says Dr. Ole Köhler-Forsberg of Aarhus University Hospital, a neuroscientist and one of the authors of the study.
Köhler-Forsberg and his colleagues used two national registries — one to get data on hospitalizations because of severe infections like pneumonia and another for data on antimicrobial or antiparasitic medications prescribed to children for less severe infections. “Most of them are those infections that you and I and all others have experienced,” says Köhler-Forsberg.
While the majority of kids in the sample had some of kind of infections, only a small portion of them were diagnosed with any kind of mental illness. About 4 percent were diagnosed with conditions like schizophrenia, attention deficit hyperactivity disorder, anxiety and personality disorders. And a total of 5.2 percent were prescribed medications for mental disorders.
For all mental illnesses — excluding depression and bipolar disorder — the team found that being hospitalized for an infection was associated with a 84 percent higher risk of subsequently being diagnosed with a mental health disorder at a hospital and a 42 percent increased risk of being prescribed a medication for mental illness.
Less serious infections, treated in an outpatient setting with an antibiotic, antiviral, antifungal or antiparasitic medications were associated with a 40 percent greater risk of getting a mental illness diagnosis in a hospital and a 22 percent higher risk of getting a prescription.
However, the increased risk for mental illness following infections was less dramatic when the authors did an additional analysis to look into the influence of genetics and the home environment. The authors compared outcomes for more than 800,000 siblings in this population — siblings that did have infections with ones who did not. In this analysis, they found that the increased risk for mental illness following hospitalization fell to 21 percent (from 84 percent). Similarly, the risk of being prescribed a psychotropic medication following hospitalization fell to 17 percent.
But the risk didn’t drop to zero, notes Köhler-Forsberg. “That’s also another finding that made us more confident that there is some link between infections, or the immune system and mental disorders,” he says.
The authors also found that the risk for getting a mental disorder was the highest within the first three months following an infection.
“And also we found that the more the infections, and the more severe the infections, then the risk increased as well,” says Köhler-Forsberg. “So there is this load of infection that seems to impact the brain and mental disorders.”
Previous studies have also illustrated a connection between infections and mental illness, says McIntyre. For example, a 2013 study by a couple of members from the current research team showed that hospitalizations due to autoimmune disorders and infections were associated with an increased risk of mood disorders like depression and bipolar in adults. Similarly, influenza in pregnant women has been linked with an increased risk of schizophrenia in their children. Other studies have found that other infections are also associated with schizophrenia. But the mechanism underlying the link is still not fully understood. One theory that’s supported by various studies is that infections contribute to mental illness by activating the body’s own inflammatory response.