Seasonal occurrence of human metapneumovirus infections in Croatia.

Human metapneumovirus (HMPV) is one of the commonest causes of viral acute respiratory infections (ARI) especially among pediatric patients. Its incidence varies from year to year and in countries belonging to moderate climate zone shows periodic exchange. The aim of this study was to investigate epidemiological characteristics of HMPV infections in Croatia. During a 4-year period (January 1st, 2009 to December 31st, 2012) nasopharyngeal aspirates from 2610 children under the age of 10 admitted to hospitals with ARI were collected. Direct immunofluorescence assay (DFA) (Argene, France) was used to detect the virus from clinical samples. Demographics and clinical data were also analyzed. HMPV was detected in 8.4% of patients. While the majority 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 children aged 2-5 (11.4% of all children in this age group). HMPV caused 7.1% of upper respiratory tract infections (URTI) and 11.7% of lower respiratory tract infections (LRTI). 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. This study indicates that HMPV infections in Croatia show biennial outbreak pattern characterized by alternation of winter and spring activity. HMPV outbreaks are also in time connection and alternate with RSV outbreaks.