Causes of Bronchiolitis
Bronchiolitis is primarily caused by viral infection of the respiratory epithelial cells, with respiratory syncytial virus (RSV) being the predominant pathogen, followed by rhinovirus, human metapneumovirus, influenza, parainfluenza, adenovirus, and coronavirus. 1, 2, 3
Primary Viral Pathogens
Respiratory Syncytial Virus (RSV)
- RSV is the most common cause of bronchiolitis, accounting for more than 80% of cases during epidemic periods 4, 5, 6
- RSV causes approximately 90,000 hospitalizations annually in the United States and is the leading cause of hospitalization in infants under 1 year of age 2
- Approximately 90% of children are infected with RSV within the first 2 years of life, with up to 40% experiencing lower respiratory tract infection during their initial infection 2
- RSV transmission occurs through contact with respiratory droplets either directly from an infected person or through self-inoculation from contaminated surfaces 5
Other Viral Causes
- Rhinovirus is recognized as a primary cause of bronchiolitis by infecting respiratory epithelial cells 1, 3
- Human metapneumovirus can cause bronchiolitis with clinical presentations similar to RSV 2, 3
- Influenza virus is an established cause of bronchiolitis in young children 1, 2, 3
- Parainfluenza viruses are recognized viral causes of bronchiolitis 1, 2, 3
- Adenovirus can cause bronchiolitis, sometimes with more severe presentations 1, 2, 3
- Coronavirus is identified as a viral cause of bronchiolitis 1, 3
Pathophysiologic Mechanism
The American Academy of Pediatrics describes the underlying mechanism as:
- Acute inflammation, edema, and necrosis of epithelial cells lining the small airways 7
- Increased mucus production contributing to airway obstruction 7
- Bronchospasm, though this plays a relatively minor role compared to inflammatory obstruction (evidenced by poor response to bronchodilators) 7
- Bronchiolar obstruction results from a combination of epithelial destruction, inflammatory cell infiltration, and debris accumulation 7
Clinical Context
Age Distribution
- Bronchiolitis is defined as occurring in children aged less than 2 years following an upper respiratory illness 1
- The majority of RSV bronchiolitis cases (81.9%) occur in infants in the first six months of life 8
Seasonal Pattern
- Peak incidence occurs between December and March in North America, though regional variations exist 2
Important Clinical Pitfall
A common pitfall is assuming all bronchiolitis requires extensive diagnostic testing to identify the specific viral cause. Bronchiolitis remains a clinical diagnosis, and diagnostic testing is not routinely recommended 5, 6. The specific viral etiology does not typically change management, as treatment is primarily supportive regardless of the causative virus.