The incidence of febrile seizures (FS) in a cohort of children, ages 3 months to 5 years, living in a Netherlands province was compared with the incidence of common viral infections reported to a national registry and the results reported from the Department of Medical Microbiology, Public Health Laboratory Friesland, Leeuwarden, The Netherlands. In a 4-year period, April 1998 to April 2002, 267 of 303 (88%) general practitioners in the province registered 654 cases of FS (388 first FS and 266 recurrences) in a group of 429 children, an estimated incidence of 2.4 in 1000 patient-years. The ratio of simple versus complex FS was 7.3. A seasonal variation was observed, with peaks in the winter, nadirs during the summer, and small increases at the end of summer. Statistical comparison of the seasonal variation of FS with viral incidence figures showed a significant correlation between FS and influenza A, especially with FS recurrences, and to a lesser degree of significance, with complex FS. Poisson regression analysis revealed no association between FS and RSV infections, despite high rate of infection and clear seasonal variation. Influenza B virus, parainfluenza viruses, enteroviruses, and adenoviruses showed no correlation with FS also, but the incidence of these infections was lower and a seasonal trend could not be demonstrated. Influenza A has a significant role in the recurrence of FS in the Netherlands. Vaccination against influenza should be considered as a preventive therapy for FS recurrence after a first FS. 
COMMENT. The role of infections in neurologic disease is constantly changing, and epidemiological studies show the emergence of new or the re-emergence of old maladies . The association of influenza A with febrile convulsions (FC) has shown changes over time and geographically. In recent correspondence regarding influenza virus and the frequency and mechanism of FC , influenza A virus is currently a common cause of FC in Japan and in other Asian countries but is less frequently associated in the US and Europe. In the first half of the 20th century, apart from roseola infantum, viral infections as a cause of FC were rarely reported, and influenza A infection with FC was not recorded in the literature. Currently, in the US, human herpesvirus (HHV)-6 is a more frequent cause of FC than is infection with influenza A virus . During the 2003 outbreak of influenza A in Houston, Texas, children admitted to hospital with neurologic complications had seizures that were classified as encephalopathic, and none was typical of a FC (Ped Neur Briefs Nov 2004;18:83) . The distinction between seizures with encephalopathy and complex febrile seizures is often difficult. The threshold to FC is determined by the height of the fever, but other factors involved in susceptibility to FC include genetics, an increased cytokine and systemic immune response to infection and, especially with complex FC, a possible unrecognized viral encephalitis or toxic encephalopathy.
Circadian and seasonal variation of first febrile seizures has been studied at the University of Ferrara, Italy . The frequency of FC increased significantly in the evening hours, with a peak between 5 and 8 pm, the time of an expected circadian increase in body temperature. The peak seasonal incidence was in January and winter months, the time of greatest frequency of viral infections and respiratory illnesses responsible for fever. Parental alertness to the risks of FC recurrence and the need for prophylactic therapy should be heightened at these times.