Schizophrenia and season of birth in a tropical region: relationship to rainfall.

Winter birth has been shown to increase the risk of schizophrenia in adult life. It has been hypothesized that this effect is due to seasonal variation in infectious diseases, including influenza, as exposure to influenza during mid gestation also increases the risk of schizophrenia. However, in many areas there is little variation in temperature during the year, although rainfall may vary greatly. We tested the hypothesis that, in a tropical region with wet and dry seasons, schizophrenia births would be related to rainfall. The data came from the city of Mossoro in north-eastern Brazil. In this area there is no meaningful variation in temperature, but there is a rainy season with little precipitation during the rest of the year. In this region, the prevalence of influenza parallels that of rainfall. There was a significant relationship between rainfall and the number of schizophrenia births three months later. In contrast, there was no significant relationship between rainfall and general population births three months later. The relationship of birth to rainfall, rather than winter birth, may be associated with risk of schizophrenia in tropical regions; exposure to influenza during gestation may be the basis for such a relationship.