Infections during childhood are linked to an elevated risks for mental disorders, according to research published in JAMA Psychiatry.

“The etiology of mental disorders is rather unknown, and several studies have indicated that the immune system may play a role in at least a subgroup of individuals with mental disorders,” said study author Ole Köhler-Forsberg from Aarhus University.

The researchers used Danish nationwide registries to investigate the relationship between infections treated since birth and subsequent risk of mental disorders in childhood and adolescence (but not adulthood). The study included 1,098,930 individuals born in Denmark between 1995 and 2012.

The study showed that children who had been hospitalized with an infection had an 84% increased risk of suffering a mental disorder. Children who were not hospitalized — but whose infections had been treated with medication — had an increased risk of 40%.

Schizophrenia spectrum disorders, obsessive-compulsive disorder, personality and behavior disorders, mental retardation, autism spectrum disorder, attention-deficit/hyperactivity disorder, oppositional defiant disorder and conduct disorder, and tic disorders were associated with the highest risks after infections.

“The temporal correlations between the infection and the mental diagnoses were particularly notable, as we observed that the risk of a newly occurring mental disorder was increased by 5.66 times in the first three months after contact with a hospital due to an infection and were also increased more than twofold within the first year,” said research director Dr. Michael Eriksen Benrós from the Psychiatric Centre Copenhagen at Copenhagen University Hospital.

“This is another piece of evidence showing that the brain and the body are tightly connected. Furthermore, our findings indicate that infections and the immune system may play a role in the development of mental disorders among some individuals. Whether and how specific infections may lead to a mental disorder needs to be investigated in clinical and preclinical studies,” Köhler-Forsberg told PsyPost.

“We hope that our findings will contribute and help lead to a better understanding of the complex interplay between the peripheral immune system and the central nervous system (CNS). Clearly, the CNS is linked to the rest of the body in an intimate connection.”

“Furthermore, we hope that our findings will encourage a greater focus on a broad and detailed somatic evaluation of children and adolescents with mental disorders. Some mental health problems may be caused by infectious agents, but in addition it is important to treat somatic diseases among patients with mental disorder, as sufficient somatic treatment may have a positive impact on the mental disorder as well,” Köhler-Forsberg added.

But the researchers noted that several confounding factors need to be considered when interpreting the study. Though infections may directly or indirectly contribute to mental disorders, it’s also possible that infections are related to particular genetic or socioeconomic factors, which are in turn associated with mental disorders.

The study, “A Nationwide Study in Denmark of the Association Between Treated Infections and the Subsequent Risk of Treated Mental Disorders in Children and Adolescents“, was authored by Ole Köhler-Forsberg, Liselotte Petersen,Christiane Gasse, Preben B. Mortensen, Soren Dalsgaard, Robert H. Yolken, Ole Mors, and Michael E. Benros.