parainfluenza
Summary
Human parainfluenza virus (HPIV) is a paramyxovirus (enveloped, negative-sense, single-stranded RNA) that is the leading cause of croup (laryngotracheobronchitis) in young children, and can also cause bronchiolitis, pneumonia, and common cold-like illness. It typically presents with a barking (seal-like) cough, inspiratory stridor, and hoarseness, often with a characteristic 'steeple sign' on neck X-ray.
Detail
Human parainfluenza virus (HPIV) belongs to the Paramyxoviridae family, which includes enveloped, helical, negative-sense single-stranded RNA viruses. There are four serotypes (HPIV-1 through 4), with HPIV-1 and 2 most classically associated with croup, and HPIV-3 more associated with bronchiolitis and pneumonia in infants. The virus has hemagglutinin-neuraminidase (HN) and fusion (F) glycoproteins on its envelope, which mediate attachment/entry and cell fusion, respectively (leading to characteristic multinucleated giant cell formation in respiratory epithelium). Transmission is via respiratory droplets and fomites, with seasonal peaks in fall. Croup (laryngotracheobronchitis) classically affects children 6 months to 3 years old, presenting with a barking cough, inspiratory stridor, hoarseness, and low-grade fever, often worse at night. Chest/neck X-ray may show subglottic narrowing, the 'steeple sign,' reflecting airway edema. Diagnosis is usually clinical, though PCR or antigen testing can confirm. Management is supportive: humidified air, corticosteroids (dexamethasone) to reduce airway inflammation, and nebulized racemic epinephrine for moderate-severe cases to reduce edema via vasoconstriction. Unlike influenza, no specific antiviral is routinely used, and no vaccine is currently available. It's a classic USMLE association: 'parainfluenza = croup,' paired with RSV (bronchiolitis) and influenza as major pediatric respiratory pathogens.
Sources
- First Aid for the USMLE Step 1
- Sherris Medical Microbiology
- UpToDate: Croup: Clinical features, evaluation, and diagnosis
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