Child infections can increase the risk of developing certain mental illnesses in childhood and adolescents. Rebecca Nelson / Getty Images…
Rebecca Nelson / Getty Images
Researchers have traced the connection between certain infections and mental illnesses such as schizophrenia, depression and bipolar disorder. New research from Denmark supports that connection. The study, published Thursday in JAMA Psychiatry, shows that a variety of infections, also common as bronchitis, are associated with a higher risk of many mental illnesses in children and adolescents.
The findings support the idea that infections affect mental health, possibly by affecting the immune system.
“This idea that activation of the body’s immune-inflammatory system as a causal link in … choose mental illnesses is one that has really hit,” says Dr. Roger McIntyre, Professor of Psychology and Pharmacology at the University of Toronto, who was not involved in the study. “This study adds it generally, but builds on convincingly.”
In the new study, researchers gathered data on hospitalizations and prescription drugs for 1.1 million children born in Denmark between 1 January 1995 and 30 June 2012.
“We could follow individuals from birth, so there was no missing information during study time, “says Dr. Ole Köhler-Forsberg at Aarhus University Hospital, a neuroscience and one of the authors of the study.
Köhler-Forsberg and his colleagues used two national registries – one for receiving hospitalization information due to severe infections such as pneumonia and another for data on antimicrobial or antiparasitic drugs prescribed for children for less serious infections. “Most of them are the infections you and I and everyone else have experienced,” said Köhler-Forsberg.
While the majority of the children in the sample had some infections, only a small proportion of them were diagnosed with some form of mental illness. About 4 percent were diagnosed with conditions such as schizophrenia, attention deficit hyperactivity disorder, anxiety and personality disorders. And a total of 5.2 percent prescribed medications for mental disorders.
For all mental illnesses – excluding depression and bipolar disorder – the team found hospitals for infection was associated with a 84 percent higher risk of later diagnosis with a mental illness in a hospital and a 42 percent increased risk of prescribing a medicine for mental illness.
Less Serious Infections Treated in the outpatient environment with antibiotics, antiviral, antifungal or antiparasitic drugs Noble was associated with a 40 percent greater risk of diagnosing mental illness in a hospital and a 22 percent higher risk of recipe.
However, the increased risk of mental illness after infections was less dramatic when the authors made an additional analysis to look at the impact of genetics and home environment. The authors compared the results of more than 800,000 siblings in this population – siblings who had infections with those who did not. In this analysis, they found that the increased risk of mental illness after hospitalization fell to 21 percent (from 84 percent). Likewise, the risk of pre-registered psychotropic drugs after hospitalization fell to 17 percent.
But the risk did not fall to zero, notes Köhler-Forsberg. “It is also another foundational that made us more convinced that there is a connection between infections or the immune system and mental disorders,” he says.
The authors also found that the risk of mental disorder was highest within the first three months after an infection.
“And we also found that the more infections, and the more difficult the infections are, the risk also increased,” says Köhler-Forsberg. “So there is this infection burden that seems to affect the brain and mental disorders.”
Previous studies have also illustrated the connection between infections and mental illness, says McIntyre. For example, a 2013 study by a couple of members from the current research group showed that hospital stays due to autoimmune disorders and infections were associated with increased risk of mood disorders such as depression and bipolar in adults.
Similarly, the flu in pregnant women has been associated with an increased risk of schizophrenia in their children. Other studies have shown that other infections are also associated with schizophrenia.
However, the mechanism underlying the link is still not fully understood. A theory supported by various studies is that infections contribute to mental illness by activating the body’s own inflammatory response.
A subset of people with different mental illnesses, from schizophrenia and depression to autism and bipolar disorder, has what is called “pro-inflammatory balance,” says McIntyre. In other words, their bodies are in a state of inflammation.
“So the naturally occurring proinflammatory proteins are higher and naturally occurring anti-inflammatory proteins that we produce are relatively lower,” he says. “So seesaw is balanced against an inflammatory condition.”
“What we have been fighting for a long time is … is this [inflammation] causing mental illness, or is it a consequence of mental illness?” says McIntyre. The new study suggests that it is the former, as researchers could show that the diagnosis of mental illness occurred shortly after the infections.
“At least from a time perspective, activation of the inflammatory system first occurred, followed by mental illness,” he says. “Therefore, it makes more sense of the idea that the immune system is causative, rather than being a consequence of mental illness.”
However, inflammation is not the only way for infections that affect mental health. In some cases pathogens can be the sins, says Köhler-Forsberg.
“Some infections begin as a peripheral infection in the body and can cross the blood-brain barrier and get into the brain and thereby harm and increase the risk of mental disorders,” he says.
A well-known example is a parasite called Toxoplasma gondii which spreads through contact with contaminated cat stools.
This pathogen was unlikely to be involved in the cases in this new study, “said Dr. Lena Brundin, associate professor at the Van Andel Research Institute in Grand Rapids, Michigan, who wrote an accompanying editorial to the new study. But she says, “We know that the parasite can go to the brain and be associated with some mental symptoms later in life.”
“We have seen that patients with Toxoplasma gondii may be more suicidal,” she adds.
Parasite affects brain function, including production of dopamine, says Brundin. “It can also cause localized inflammation in the brain.”
The herpes virus is also known to cross the blood-brain barrier and remain dormant in the nervous system and become reactivated years later.
Another way in which infections or treatment for an infection can affect mental health can be by altering the intestinal microbial, write the authors of the new study. “It’s very hard to say how or why, because we do not understand how microbiomics work,” says Köhler-Forsberg.
Some antibiotics are also known to enter the brain, notes McIntyre, so they can also have a direct impact as well. The new study found that all medicines given to the children for their infections, antibiotics were associated with the highest risk of a subsequent diagnosis of mental illness. More “acceptable use of antibiotics” can reduce some of that risk, he says.
Parents, McIntyre, Brundin and Köhler-Forsberg, warn that the results do not mean that every child who develops an infection becomes mentally ill.
“You should be sure that most children will not continue and develop a mental illness,” said McIntyre. After all, infections are necessary for normal development of the immune system.
As said, parents should be looking for signs of anxiety, behavioral problems or other potential symptoms of mental illness following an infection, says Köhler-Forsberg. If they notice something in their child, they should immediately seek help, he adds.
“If a child should be so unlucky to develop some mental problems, then we know it [in most cases] when it is discovered early and treated well, then the child will get better again,” he says.