The incidence and epidemiology of encephalitis in children have been analyzed in a 20 year survey at the Department of Virology and the Children’s Hospital, University of Helsinki, Finland. There were 405 patients with an overall male to female ratio of 1.4:1. Patients were grouped according to age: One to 11 months (33), one to four years (139), five to nine years (159), and ten to 16 years (74). The etiology was determined in 69%; it was unknown among one-half of the youngest children and in only one-fourth of those at least ten years of age. The incidence was highest in children younger than two years (16.7/100,000 child years) and was very low in those 14 to 16 years of age (1/100,000 child years). After the introduction of vaccination against mumps, measles and rubella in 1983, the major encephalitides have been varicella-zoster, Mycoplasma pneumoniae, and respiratory and enteroviruses. Enteroviruses and Herpes simplex were most common in the infants younger than one year of age whereas respiratory viruses and Mycoplasma pneumoniae as well as varicella-zoster dominated in older children. Mumps and varicella were more common in boys, and adenovirus and Mycoplasma pneumoniae were more common in girls. Seasonal accumulation appeared in February and March and the incidence was lower in July and August. 
COMMENT. This study is important in emphasizing a greater interest in childhood encephalitis and the need for early diagnosis and treatment especially in young infants. The Mayo Clinic study in 1984 had shown that encephalitis in young children is often diagnosed late. The authors suggest that convulsions of unknown origin may be due to ill-defined encephalitis caused by respiratory viruses and adenovirus in winter months and enteroviruses in autumn. The etiology is often difficult to establish but antigen tests for respiratory viruses, adenoviruses, Herpes virus, and Mycobacterium pneumoniae are available and will provide the diagnosis within a few hours.