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The human metapneumovirus was first isolated in 2001 from children with acute respiratory infections in the Netherlands. A relative of the turkey rhinotracheitis virus (avian pneumovirus), it is a cousin of the respiratory syncytial virus and a more distant cousin of the mumps, measles, and parainfluenza viruses. It has been related to acute respiratory tract infections in children, and some adults, in Canada, Australia, the UK, and the USA. Now a study in Nashville, Tennessee (John V Williams and colleagues.
see also perspective article, ibid: 431–3) has added to knowledge about the importance of this virus as a respiratory pathogen in children.
At a research primary care clinic over a period of 25 years (1976–2001) a total of 2009 children were followed from birth for an average of 2.5 years. A diagnosis of acute lower respiratory tract infection was made at 1127 patient visits and nasal-wash samples were obtained for viral culture at 687 of these visits. The viruses cultured were respiratory syncytial virus (103), parainfluenza virus (58), influenza virus (32), adenovirus (28), and other viruses (50). No virus was cultured from 408 samples from 321 children. Forty-nine saved, frozen samples from 248 of these 321 children were positive for human metapneumovirus on reverse transcription PCR. Twenty-two of these were confirmed by culture. Only one of 86 controls was positive. The virus caused cough and coryzal symptoms, each present in around 90%. Rhinitis was the most common finding (77%), followed by wheezing and abnormal ear drum (each 50%). The clinical diagnoses were bronchiolitis (59%), croup (18%), asthma exacerbation (14%), and pneumonia (8%). Chest X-rays were abnormal in seven of 14 children, mostly showing diffuse perihilar infiltrates.
The 49 human-metapneumovirus-positive children were 1.5 to 50 months old (mean 11.6 months, median 6.5 months). Three quarters (36) were infants and 38 of the 49 infections occurred between December and April. Although each of the five viruses (human metapneumovirus, respiratory syncytial virus, parainfluenza virus, influenza virus, and adenovirus) was associated with any of the clinical diagnoses, children infected with human metapneumovirus, respiratory syncytial virus, or adenovirus were most likely to have bronchiolitis and children infected with parainfluenza or influenza viruses were most likely to have croup. [It seems likely that some children with a diagnosis of bronchiolitis in the USA would in the UK be given a diagnosis of pre-school viral wheeze.] Human metapneumovirus was demonstrated in 21% of virus-positive cases of bronchiolitis, 12% of virus-positive croup, and 9% of virus-positive pneumonia. In any given year it accounted for up to 31% (0–31%) of otherwise virus-negative pneumonia. (The figures for human metapneumovirus and other viruses are not strictly comparable because of different methods.)
Human metapneumovirus also caused upper respiratory infection (15% of a random sample of 261 virus-culture negative children with URTI) but at a later age (mean 19.6 months) than lower respiratory tract infection.