Dr. Suvisaari and Colleagues Reply

To the Editor: Drs. Fritzsche and Schmidli misunderstand our article in that we did not suggest that procreation in the summertime represents a constant hazard for schizophrenia. We observed that both patients with schizophrenia and their unaffected siblings are more often born during the winter and spring months, suggesting that among parents of patients with schizophrenia, there is a tendency to have children more often during the winter and spring. This tendency may not be related to the offspring’s risk of developing schizophrenia at all. It is also difficult to believe that there would be a link between a tendency of parents to have children during winter and spring and tick-borne infections.

However, we also observed that between 1955 and 1959 there was a pronounced seasonal variation in births of schizophrenia patients but not their normal siblings. On the basis of these findings, we suggested that the seasonal variation in schizophrenia births may consist of two components: a constant component caused by parental procreational habits and an irregular component caused by environmental risk factors that, when they occur, considerably increase the magnitude of the seasonal variation in births among patients but not their siblings and are genuine risk factors for schizophrenia. Infections caused by B. burgdorferi are one possible explanation for this irregular component. However, in Finland, Lyme borreliosis is endemic in southwestern coastal areas, in which the incidence of schizophrenia is lowest. In many northeastern areas, in which the incidence of schizophrenia is exceptionally high, ticks no longer survive (1–3).

Finally, we wish to point out that we do not share the belief that the incidence of schizophrenia is constant worldwide. In our other studies, we have shown that there is considerable temporal (4) and regional (3) variation in the incidence of schizophrenia in Finland, and we have also investigated time trends in the seasonal variation in births of individuals with schizophrenia (5). We have no reason to believe that this temporal and regional variation in the incidence of schizophrenia would be limited to Finland.