Epidemiology of encephalitis in children: A 20‐Year survey

Koskiniemi, Marjaleena; Rautonen, Jukka; Lehtokoski‐Lehtiniemi, Eeva; Vaheri, Antti

Four hundred five children from the Helsinki area who were 1 month to 16 years old were treated for acute encephalitis at the Children's Hospital, University of Helsinki, from January 1968 through December 1987. Encephalitis occurred most commonly in children 1 to 1.9 years of age, among whom the incidence was 16.7 per 100,000 child‐years. The incidence remained quite high until the age of 10 years, and then gradually declined to 1.0 per 100,000 child‐years at the age of 15 years. Since 1983, when mumps, measles, and rubella vaccination eradicated the encephalitides associated with these microbes, the major associated agents have been varicella‐zoster, Mycoplasma pneumoniae, and respiratory and enteroviruses. In infants younger than 1 year of age, the major agents were enteroviruses, herpes simplex virus, and the group of “others,” whereas in older children, respiratory viruses and Mycoplasma Pneumoniae as well as varicella‐zoster virus, dominated. In children aged 1 to 11 months, the causal agent could not be identified in one‐half of all cases, whereas in children who were at least 10 years old, the etiology remained unknown in only one‐fourth of cases. Male dominance was most evident in the 4‐ to 9‐year age group. The difference in etiology between males and females was significant (p = 0.02); mumps and varicella were more common in boys, and adenovirus and Mycoplasma pneumoniae were more common in girls. The overall male‐to‐female ratio was 1.4:1. Characteristic seasonal variation occurred in encephalitides associated with mumps, measles, and entero‐ and respiratory viruses. In the whole series, some accumulation appeared in February and March. Less than one‐half of this number appeared in July and August.