Seasonal Occurrence of Human Metapneumovirus Infections in Croatia

Background: Human metapneumovirus (HMPV) is 1 of the commonest causes of viral ARI especially among pediatric patients. Its incidence varies from year to year in countries belonging to moderate climate zone. The aim of this study was to investigate epidemiologic characteristics of HMPV infections in Croatia. Methods: During a 4-year period (January 1, 2009, through December 31, 2012), nasopharyngeal aspirates were collected from 2610 children <10 years who were admitted to hospitals with acute respiratory infections. Direct immunofluorescence assay was used to detect the virus from clinical samples. Demographics and clinical data were also analyzed. Results: HMPV was detected in 8.4% of patients. While many of HMPV-infected children were 13-24 months of age (30.9% of all proven HMPV infections), the highest incidence of HMPV infection was recorded in 2- to 5-year-old children (11.4% of all children in this age group). HMPV caused 7.1% of upper respiratory tract infections and 11.7% of lower respiratory tract infections. Annual prevalence rates of HMPV infection varied significantly from year to year (P < 0.001). Peak incidence was detected in spring or winter months, depending on the year. Conclusions: This study indicates that HMPV infections in Croatia show a biennial outbreak pattern characterized by alternation of winter and spring activity. HMPV outbreaks alternate with respiratory syncytial virus outbreaks.