Human parainfluenza virus 3 (HPIV‑3) is an enveloped, single‑stranded, negative-sense RNA virus in the genus Respirovirus of the family Paramyxoviridae. It is one of four antigenically distinct human parainfluenza viruses and a common cause of lower respiratory tract infections in infants and young children.
Virology & Pathogenesis
HPIV‑3 possesses a nonsegmented negative‑sense RNA genome approximately 15 kb long that encodes six structural proteins (N, P, M, F, HN and L) and several nonstructural proteins. As a respirovirus, it shares a replication strategy with HPIV‑1 but differs antigenically. The hemagglutinin–neuraminidase glycoprotein binds to sialylated receptors on respiratory epithelial cells, and the fusion protein mediates membrane fusion and syncytium formation. Once inside the cytoplasm the virion-associated polymerase transcribes and replicates the genome in the “rule of six” pattern. HPIV‑3 replicates efficiently in the lower respiratory tract, infecting bronchiolar and alveolar epithelium as well as upper airways. Viral replication triggers the release of pro‑inflammatory mediators and causes sloughing of ciliated cells, mucosal edema and increased secretions. HPIV‑3 is classified in the genus Respirovirus together with HPIV‑1, whereas HPIV‑2 and HPIV‑4 are rubulaviruses.
Clinical significance
HPIV‑3 circulates year‑round but tends to peak in spring and early summer. It is a major cause of bronchiolitis and pneumonia in infants, immunocompromised patients and older adults. Together, HPIV‑1, HPIV‑2 and HPIV‑3 cause roughly three‑quarters of croup cases and a large share of acute respiratory infections in children. HPIV‑3 infection often begins as an upper respiratory illness with rhinorrhea, cough and low‑grade fever, but it more commonly progresses to lower respiratory disease than HPIV‑1 or HPIV‑2. Clinical manifestations range from bronchiolitis with wheezing to frank pneumonia characterised by dyspnea, hypoxia and diffuse infiltrates on imaging. Management is supportive, including supplemental oxygen, hydration and bronchodilators. No licensed vaccine or specific antiviral agent is available, though passive immunoprophylaxis and candidate vaccines are under investigation. Prevention focuses on hand hygiene, respiratory etiquette and isolation of symptomatic individuals. HPIV‑3 remains an important cause of serious respiratory illness in early childhood and immunocompromised hosts. Continued research into respirovirus biology may facilitate development of vaccines and targeted therapies. Related Terms: Paramyxoviridae, Respirovirus, Bronchiolitis, Hemagglutinin–neuraminidase, Fusion protein