Comorbidity of schizophrenia and infection: a population-based cohort study.

PURPOSE: In this paper, we investigate the hypothesis that there is an overlap between infection and schizophrenia. Infections have been identified as a risk factor for schizophrenia, but the possible overlap between schizophrenia and infections remains unidentified so far. Here, we describe the use of the comorbidity index, a method for objectively integrating associations into a single measure estimating overlap. METHODS: Data were drawn from three population-based registers, the Civil Registration Register, the Danish Psychiatric Central Research Register, and the Danish National Hospital Register. We selected a cohort of 1,403,183 persons born in Denmark 1977-2002. RESULTS: Our results indicate that persons who have had a hospital contact with an infection (IRR 1.53, CI 1.46-1.61) are more likely to develop schizophrenia than persons who have not had such a contact. Persons who have had a diagnosis with schizophrenia are more likely to have had a hospital contact with an infection (IRR 1.73, 95 % CI 1.57-1.91) than persons who have had no schizophrenia diagnosis. A comorbidity index of 1.40 (95 % CI 1.34-1.46) was found, indicating an overlap between schizophrenia and infection. CONCLUSION: Our findings indicate that schizophrenia and infections overlap and that they share risk factors. The comorbidity index showed that the co-occurrence of schizophrenia and infection was 40 % higher than if the two disorders had occurred independently. Although the incidence of schizophrenia and infection was associated with each factor, the overlap could not be explained by urbanicity, parental history of psychiatric admission and infection.