Rhinovirus bronchiolitis: to be or not to be?

Bronchiolitis is a viral lower respiratory tract infection characterised by inflammation and obstruction of the small airways. Respiratory syncytial virus (RSV) is the most common causative agent, followed by parainfluenza viruses, adenoviruses and the recently discovered metapneumovirus. Birth cohort studies have reported that the incidence of bronchiolitis is 18–32% during the first year of life, and 1– 3% of infants need hospitalisation for bronchiolitis, with the majority being admitted before 6 months of age (1). In European countries, bronchiolitis is usually defined as the first virus-induced wheezing episode in infants under the age of 12 months, but in the USA, and in most previous studies, the upper age limit has been 24 months (2). The risk of severe bronchiolitis is highest in infants under the age of 3 months, who are less likely to exhibit wheezing than older infants. From a clinical point of view, the optimal upper age limit for bronchiolitis is likely to be 6 months (3). This age tends to separate a homogeneous group of young infants who often need to be hospitalised, due to a severe illness that puts them at risk of hypoxia and insufficient fluid intake, from the more heterogeneous group of wheezing older infants who are usually treated at home. Diagnostic tests for respiratory viral infections have markedly improved over the last 20 years. The development of polymerase chain reaction (PCR) technique has enabled us to identify previously unknown viruses such as the metapneumovirus and human bocavirus, and to re-evaluate the role of known viruses such as rhinoviruses. Rhinoviruses comprise more than 100 serologically differing subgroups with no major common antigens, which results in two important consequences: first, the development of workable serological tests has not been successful and second, children face many repeated rhinovirus infections during the first years of life (1). PCR-based methods have played a key role in improving rhinovirus diagnostics, which further has changed our concepts on the aetiology, predisposing factors and long-term outcome of early-life wheezing (1). Despite this progress, the question of whether rhinoviruses are real causative agents of bronchiolitis in young infants remains unresolved. In this issue of the journal, Midulla et al. (4) discuss their findings on a 3-year follow-up of infants hospitalised in Italy for bronchiolitis under the age of 12 months. As previously published, 195 viruses were detected by PCR in nasal wash samples from 174 (56%) of the 313 bronchiolitis patients (5). Rhinoviruses were identified in 24 cases (14% of viruspositive, 7.7% of all patients). The parents were contacted by phone on three occasions after their child’s admission for bronchiolitis and the response rates were 85% at 12 months, 77% at 24 months and 73% at 36 months. Wheezing during the first and second postbronchiolitis years was a significant predictor of wheezing during the third postbronchiolitis year (4). In adjusted analyses, blood eosinophils >0.4 9 10E9/L (odds ratio 7.7) and rhinovirus identification during bronchiolitis (odds ratio 3.1), were the only independently significant risk factors for postbronchiolitis wheezing during the third postbronchiolitis year (4). The prevalence of rhinovirus-associated wheezing increases steadily with age and the cut-off age of the dominance between RSV and rhinovirus has been about 12 months in hospital studies (1). Recent multicentre studies that focused on 1620 American (6) and 206 Finnish