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Parainfluenza Virus Infections

by Craig R. Pringle, BSc, PhD

Parainfluenza viruses include several closely related viruses that cause many respiratory illnesses varying from the common cold to an influenza-like syndrome or pneumonia; croup is the most common severe manifestation. Diagnosis is usually clinical. Treatment is supportive.

The parainfluenza viruses are paramyxoviruses types 1, 2, 3, and 4. They share antigenic cross-reactivity but tend to cause diseases of different severity. Type 4 has antigenic cross-reactivity with the mumps virus and appears to be an uncommon cause of respiratory disease.

Childhood outbreaks of parainfluenza virus infections can occur in nurseries, pediatric wards, and schools. Types 1 and 2 tend to cause epidemics in the autumn, with each serotype occurring in alternate years. Type 3 disease is endemic and infects most children < 1 yr; incidence is increased in the spring.

Parainfluenza viruses can cause repeated infections, but reinfection generally causes milder illness. Thus, in immunocompetent adults, most infections are asymptomatic or mild.

The most common illness in children is an upper respiratory illness with no or low-grade fever.

Parainfluenza type 1 probably causes croup (laryngotracheobronchitis—see Croup), primarily in infants aged 6 to 36 mo. Croup begins with common cold symptoms. Later, fever, a barking cough, hoarseness, and stridor develop. Respiratory failure due to upper airway obstruction is a rare but potentially fatal complication.

Parainfluenza virus type 3 may cause pneumonia and bronchiolitis in young infants (see Bronchiolitis). These illnesses are generally indistinguishable from disease caused by respiratory syncytial virus (see below) but are often less severe.

A specific viral diagnosis is unnecessary. Treatment is symptomatic.

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