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Human metapneumovirus (hMPV) was recently discovered in children with acute respiratory tract infection. We have studied the occurrence of hMPV and report clinical findings of 50 hMPV-infected children who were hospitalized during an outbreak in Norway.During 5 months from November 15, 2002 to April 14, 2003 we collected nasopharyngeal aspirate specimens from 236 children admitted because of respiratory tract infection (RTI). Samples were analyzed for influenza virus A/B, parainfluenza viruses 1, 2 and 3 and respiratory syncytial virus by direct immunofluorescence assays and cell culture. Rhinovirus, adenovirus and hMPV were identified by polymerase chain reaction.Human metapneumovirus was identified in 50 of 236 children (21%). Most (41 of 50) hMPV-infected children were hospitalized between November 15 and January 15, and during these 2 months hMPV was the most common isolate (41 of 72 isolates; 57%). Respiratory syncytial virus was identified in 36 children (15%), among whom 34 were admitted after the hMPV outbreak.The median age of hMPV-infected children was 12 months (range, 1 to 115 months), and one-half of the children had an underlying chronic disease. The most common symptoms were fever (86%), cough (90%), dyspnea (80%), wheezing (56%), rhinorrhea (44%), anorexia (48%) and vomiting (36%). Eight (16%) had an upper respiratory tract infection (rhinopharyngitis, n = 6; laryngitis, n = 2), 24 (48%) had bronchiolitis and 17 (34%) had pneumonia. Two-thirds with a lower RTI also had signs of upper RTI. Fourteen (28%) children needed supplemental oxygen, 1 was treated with continuous positive airway pressure and 2 were ventilated mechanically.Human metapneumovirus was the most common virus isolate during the winter season 2002 to 2003 in children hospitalized for respiratory tract infection. Upper respiratory tract infections and mild to severe bronchiolitis were most common, but a relatively high proportion of hospitalized children developed severe pneumonia.