Adult major affective disorder after prenatal exposure to an influenza epidemic

BACKGROUND

We have previously reported an increase in schizophrenia diagnoses in a population exposed during the second trimester to the 1957 influenza epidemic. These basic findings together with a fair number of replications have been interpreted as supporting a neurodevelopmental contribution to the origins of schizophrenia. Recent neuroimaging findings suggest that affective illness may also have a neurodevelopmental origin. We examined the hypothesis that exposure to an influenza epidemic during the second trimester would increase the risk for adult major affective disorder.

METHODS

The subjects had been exposed as fetuses to the type A2/Singapore influenza epidemic in greater Helsinki, Finland. Control subjects were born in the 6 years before the epidemic.

RESULTS

We found a significant (P < .001) increase in the proportion of hospital diagnoses for major affective disorder for individuals exposed to the influenza epidemic during their second trimester of fetal development compared with control subjects (13% vs 2%). This second-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 estimated population-based rates (2.1 vs 0.6 per 1000) (P < .05) of major affective disorder. Additional analyses revealed that the increase of major affective disorder among subjects in the index group who were exposed during the second trimester was due to a significant (P < .002) elevation 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 depressive disorder. These encouraging findings, if replicated, may suggest that some mental disorders may stem, in part, from a disturbance in the development of the fetal brain during the second trimester.

Prenatal influenza infections and adult schizophrenia

We reported previously that residents of Greater Helsinki, Finland, whose mothers were exposed to the 1957 influenza epidemic during their second trimester of gestation had a significantly elevated risk of developing adult schizophrenia. The majority of the replication studies to date have not determined whether the mothers actually contracted an infection or the stage of gestation based on mother's last menstruation. We read prenatal clinic records of the mothers of the Helsinki-born schizophrenia subjects to determine timing of infection, as noted by the prenatal clinic obstetric nurse at a time close to the actual infection. Schizophrenia subjects who were exposed in the second trimester had a significantly higher rate of definite influenza infection (86.7%) in that period compared to those who were exposed during the first and third trimesters (20.0%). These results are interpreted with caution because of the small number of cases.

Hospitalization patterns in schizophrenia. A 13-year follow-up

All first admitted patients in 1972 from a catchment area of 582,000 inhabitants aged 15 years or more who were diagnosed as schizophrenic at least once from 1972 until September 1983 (n = 53) were followed-up on average 13 years after first admission. About 20% of the cohort was hospitalized on any given day throughout the length of the follow-up period. The duration of hospitalization decreased from a mean of 8.2 months for the first admission to 1.7 months for the tenth or later admission. The readmission risk increased as a function of the number of previous admissions. Patients with income from occupation or from grants for education had shorter duration of first in-patient period. If the patients were diagnosed as schizophrenics already during the first hospitalization the risk for prolonged duration of the first in-patient period was increased but the readmission risk diminished. Furthermore, readmission risk after the first discharge was diminished by own income and by out-patient treatment and increased by low social status. High proportion of follow-up time in hospital (greater than or equal to 30%) was correlated to affective flattening present at first admission. Of the cohorts' total number of admissions (n = 493) 12% were involuntary. Involuntary admissions were more frequent in the first half of the follow-up period and were correlated to a previous involuntary admission.

Seasonality, birth complications and schizophrenia in a high risk sample

It has been suggested that viral infections, more prevalent in the winter months, might increase pregnancy and birth complications, which might in turn be responsible for the excess of winter-born patients with schizophrenia. In this study it was found that the rate of schizophrenia among high risk winter-urban births who had suffered a perinatal complication was significantly greater than that for those who had not, thus supporting the hypothesis.

The interaction of seasonality, place of birth, genetic risk and subsequent schizophrenia in a high risk sample

Birth occurring in winter months, which are high viral infection months, have been repeatedly shown to produce a slight excess of later-diagnosed schizophrenics. As a result, some researchers have speculated on the possible aetiological effect of viral infections on some forms of schizophrenia. The implications of the viral hypothesis were indirectly tested in the context of an ongoing prospective study of Danish children at high-risk (HR) for schizophrenia. A third-order analysis of variance interaction was hypothesized. Genetically vulnerable individuals, born in winter, in an urban environment (which increases the likelihood of the presence and transmission of viruses) would be more likely, as foetuses or neonates, to have suffered some CNS damage due to the infection; thus they would show higher rates of schizophrenia in the HR-urban-winter birth condition reached 23.3 per cent, considerably above population base rates (1 per cent) or rates for the HR subjects (8.9 per cent). Alternative explanations for the results were explored.