[Consensus conference on acute bronchiolitis (II): epidemiology of acute bronchiolitis. Review of the scientific evidence].

A review of the evidence on epidemiology, risk factors, etiology and clinical-etiological profile of acute bronchiolitis is presented. The frequency estimates are very heterogeneous; in the population under two years the frequency of admission for bronchiolitis is between 1 and 3.5%, primary care consultations between 4 and 20% and emergency visits between 1 and 2%. The frequency of admissions for respiratory infection by respiratory syncytial virus in the risk population is: in premature infants < or =32 weeks of gestation between 4.4 and 18%, in patients with bronchopulmonary dysplasia between 7.3 and 42%, and in infants with congenital heart disease between 1.6 and 9.8%. The main risk factors are: prematurity, chronic lung disease or bronchopulmonary dysplasia, congenital heart disease and age less than 3-6 months at onset of the epidemic. Other factors are: older siblings or day care attendance, male gender, exposure to smoking, breastfeeding for less than 1-2 months and variables associated with lower socioeconomic status. Respiratory syncytial virus is the dominant etiological agent, constituting just over half the cases (median 56%; interval 27% to 73%). Other viruses implicated, in descending order of frequency, are rhinovirus, adenovirus, metapneumovirus, influenza viruses, parainfluenza, enterovirus and bocavirus. In studies with genomic detection techniques, between 20 and 25% of cases the virus involved is not identified and between 9% and 27% of cases have viral co-infection. Although respiratory syncytial virus bronchiolitis shows more wheezing and retractions, longer duration of respiratory symptoms and oxygen therapy and are associated with lower use of antibiotics. This pattern is associated with the younger age of the patients and does not help us to predict the etiology. In general, the etiological identification is not useful for the management of patients. However, in young infants (<3 months) with febrile bronchiolitis in the hospital environment, conservative management may help these patients and avoid diagnostic and therapeutic procedures.