(HealthDay News) – Autoimmune diseases and infections are independent and synergistic risk factors for mood disorders, according to a study published online June 12 in JAMA Psychiatry.

Michael E. Benros, MD, from Aarhus University in Denmark, and colleagues examined the effect of autoimmune diseases and infections on the risk of developing mood disorders using data from Danish longitudinal registers. Participants included 3.56 million people born from 1945–1996 who were followed up through 2010, 91,637 of whom had hospital contacts for mood disorders.

The researchers found that the risk of a subsequent mood disorder was significantly increased for those with a prior hospital contact because of autoimmune disease (incidence rate ratio [IRR], 1.45). The risk of later mood disorders was increased with any history of hospitalization for infection (IRR, 1.62). There was a synergistic relationship between these two risk factors, with the risk of mood disorders further increased with both risk factors (IRR, 2.35). A dose-response association was noted, with the number of infections and autoimmune diseases increasing the risk of mood disorders. Of the participants diagnosed as having a mood disorder, 32% and 5%, respectively, had a previous hospital contact for an infection and autoimmune disease.

“In conclusion, autoimmune diseases and the number of severe infections are independent and synergistic risk factors for mood disorders, with hospital-treated infections being the most common risk factor,” the authors write. “These associations are compatible with the hypothesis of a general immunologic response affecting the brain in subgroups of patients with mood disorders.”

One author disclosed financial ties to the pharmaceutical industry.

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