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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,Myoplasma 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 Myoplasma 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. |
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