ADULT MAJOR AFFECTIVE-DISORDER AFTER PRENATAL EXPOSURE TO AN INFLUENZA EPIDEMIC

Background: We have previously reported an increase in schizophrenia d iagnoses in a population exposed during the second trimester to the 19 57 influenza epidemic. These basic findings together with a fair numbe r of replications have been interpreted as supporting a neurodevelopme ntal contribution to the origins of schizophrenia. Recent neuroimaging findings suggest that affective illness may also have a neurodevelopm ental origin. We examined the hypothesis that exposure to an influenza epidemic during the second trimester would increase the risk for adul t major affective disorder. Methods: The subjects had been exposed as fetuses to the type A(2)/Singapore influenza epidemic in greater Helsi nki, Finland. Control subjects were born in the 6 years before the epi demic. Results: We found a significant (P<.001) increase in the propor tion of hospital diagnoses for major affective disorder for individual s exposed to the influenza epidemic during their second trimester of f etal development compared with control subjects (13% vs 2%). This seco nd-trimester effect seems somewhat stronger in men (16% vs 2%) (P<.001 ), although the rates of major affective disorder in women (8% vs 3%) (P>.05) were similar. The second-trimester effect remained when we est imated population-based rates (2.1 vs 0.6 per 1000) (P<.05) of major a ffective disorder. Additional analyses revealed that the increase of m ajor affective disorder among subjects in the index group who were exp osed during the second trimester was due: to a significant (P<.002) el evation of unipolar forms, although a similar though not significant ( P>.05) elevation was observed for the bipolar forms of major affective disorder. Conclusions: These data are consistent with the hypothesis concerning the possible neurodevelopmental contribution to the origins of some forms of major affective disorder, especially unipolar depres sive disorder. These encouraging findings, if replicated may suggest t hat some mental disorders may stem, in part, from a disturbance in the development of the fetal brain during the second trimester.