Autoimmune link to schizophrenia

By Eleanor McDermid, Senior medwireNews Reporter

Patients with schizophrenia have an increased risk for developing autoimmune diseases, especially if they contract an infection, shows a registry-based study.

The Danish registry included 3.77 million people, 39,364 of whom were diagnosed with schizophrenia. Of these patients, 3.6% were later diagnosed with an autoimmune disease, giving them an incidence rate ratio that was elevated a significant 1.53-fold, with the risk highest in the first year after schizophrenia diagnosis.

But the researchers, led by Michael Benrós (Aarhus University, Denmark), found the risk differed according to whether patients had previous hospital contact for infection. Schizophrenia patients without infection had a 1.32-fold increased risk for developing autoimmune diseases. Although statistically significant, this was markedly lower than the 2.70-fold increase observed among those with infection. Infection in the absence of schizophrenia had a smaller effect, conferring a 1.75-fold increase.

“This suggests a biological interaction between the two exposures on the risk of developing autoimmune diseases,” say Benrós et al in The American Journal of Psychiatry.

“In our previous study of the reverse temporal associations, infections interacted in synergy with autoimmune diseases and additionally increased the risk of schizophrenia. Hence, infection could be a common risk factor for both autoimmune diseases and schizophrenia.”

Patients with schizophrenia had an increased risk for a range of autoimmune diseases, except for rheumatoid arthritis, and the increase was notably higher for diseases believed to involve brain-reactive antibodies, at a 1.91-fold increased risk, compared with a 1.21-fold increased risk for other autoimmune diseases.

“Infections may act as a common pathological mechanism for both psychosis and autoimmune diseases, or persons with psychosis may simply be more susceptible to acquiring infections than the general population, increasing the risk of autoimmune diseases,” suggest the researchers.

Patients with bipolar disorder also had an increased risk for autoimmune disease, with the interaction with infection also present. However, in the overall cohort, a family history of bipolar disorder did not predict autoimmune disease diagnosis. By contrast, a family history of schizophrenia was associated with a small but significant 4% increase after accounting for personal history of schizophrenia.

This suggests that common genes may contribute to the risk for both conditions, say the researchers, but the small size of the association means it is unlikely “to be the sole explanation of our findings.”

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