The human parainfluenza virus 1 (HPIV‑1) is an enveloped, single‑stranded, negative-sense RNA virus belonging to the genus Respirovirus in the family Paramyxoviridae. It is one of four antigenically distinct human parainfluenza viruses that infect the respiratory tract and is a leading cause of croup in young children.
Virology & Pathogenesis
HPIV‑1 has a nonsegmented RNA genome of roughly 15 kilobases that encodes six major structural proteins (nucleocapsid N, phosphoprotein P, matrix M, fusion F, hemagglutinin–neuraminidase HN and large polymerase L). As with other parainfluenza viruses, its genome is negative-sense and must be transcribed by the virion-associated polymerase before translation. The HN glycoprotein binds sialic‑acid–containing receptors on the surface of respiratory epithelial cells, and the F protein promotes fusion of the viral envelope with the host cell membrane. After entry the virus replicates in the cytoplasm following the “rule of six” common to respiroviruses, assembling progeny virions that bud from the cell surface. HPIV‑1 lacks the segmented genome of influenza viruses and instead produces a single pleomorphic particle. It belongs to the Respirovirus genus together with HPIV‑3, whereas HPIV‑2 and HPIV‑4 are placed in the Rubulavirus genus. Replication is restricted to the respiratory mucosa and results in necrosis and sloughing of ciliated cells and submucosal edema, changes that narrow the airway and contribute to the pathogenesis of croup.
Seasonal outbreaks and clinical impact
HPIV‑1 circulates worldwide and causes biennial outbreaks in the autumn. Infection is spread by respiratory droplets and direct contact with contaminated surfaces. In young children, HPIV‑1, HPIV‑2 and HPIV‑3 account for about 75 % of croup cases and constitute a large fraction of pediatric acute respiratory infections. The illness often begins with symptoms of a mild upper respiratory infection—nasal congestion, fever and cough—but can progress to laryngotracheitis characterised by a barky cough, hoarseness and inspiratory stridor. In some patients, particularly infants and those with underlying lung disease, the virus can spread to the lower airways causing bronchiolitis or pneumonia. There is no licensed vaccine or specific antiviral therapy for HPIV‑1; management is supportive, including humidified air, corticosteroids or nebulised epinephrine for croup, and infection control measures such as hand hygiene and isolation help limit transmission. HPIV‑1 remains an important respiratory pathogen in early childhood and continues to cause periodic outbreaks of croup and other respiratory illnesses despite advances in medical care. Ongoing research into the molecular biology of respiroviruses may aid future vaccine development and antiviral strategies. Related Terms: Paramyxoviridae, Respirovirus, Croup, Hemagglutinin–neuraminidase, Fusion protein