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Schizophrenia Not Linked to Prenatal Exposure to Influenza

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Nov. 29, 1999 (New York) -- Nobody is quite sure where schizophrenia -- a mental illness characterized by hallucinations, delusional, and/or disordered thinking -- comes from. Most researchers agree there is a genetic or hereditary component involved, but clearly many people who have no family history develop the disease. One theory is that exposure in the womb or in childhood to infectious agents, like the influenza virus, can put a person at risk. But studies looking at these theories have yielded mixed results.

In one of the largest studies to date, Danish investigators report they were unable to find an association between exposure to influenza before birth and subsequent development of schizophrenia. But the study authors did find that children from large families, where there is potentially a higher risk of childhood infections, had a slightly increased risk for schizophrenia, especially when the births of the children were closely spaced together, according to a report in the November issue of the Archives of General Psychiatry.

"Our findings do not support the hypothesis that schizophrenia is associated with prenatal exposure to common infections or influenza. However, they are compatible with the hypothesis that environmental exposure, perhaps to common infections in childhood, may be a risk factor", write Tine Westergaard, MD, and colleagues. Westergaard is with the Statens Serum Institut in Copenhagen, Denmark.

Using data from a national registration system, the investigators utilized information from a database that included all Danish-born women born since 1935 and all their offspring who were alive on April 1, 1968 or born in the twenty years since. Of the nearly two million people who were followed, schizophrenia was identified in over 2,600 people using information from the Danish Psychiatry Case Register.

Monthly reported cases of influenza in Denmark were obtained from 1950 through 1988. The authors focused on the prevalence of influenza three to five months prior to the births of the study patients when investigating the association between schizophrenia and number of reported influenza cases in the population.

The number of children in a family (sibship size) correlated to the risk of developing schizophrenia, with the highest risk in those families with four or five children. No association was found between birth order and schizophrenia risk. Short intervals (less than two years) between the birth of the person with schizophrenia and the birth of the nearest oldest or youngest sibling also correlated to an increased risk of schizophrenia. More than 10% of the cases of schizophrenia came from large families or those with close spacing between siblings. "The association between sibship size and schizophrenia risk could be indicative of a possible association between schizophrenia and exposure to infections in childhood," say the authors.

No association was found between schizophrenia risk and prevalence of influenza three, four, or five months prior to birth, during any month prior to birth, or during the month of birth. "[O]ur study does not support the hypothesis that in utero [in the womb] exposure to influenza or other common infections may increase the risk of developing schizophrenia", writes Westergaard.

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