Merck Manual

Please confirm that you are a health care professional

honeypot link

Parainfluenza Virus Infections


Brenda L. Tesini

, MD, University of Rochester School of Medicine and Dentistry

Reviewed/Revised Apr 2022

Parainfluenza viruses include several closely related viruses that cause many respiratory illnesses Overview of Viral Respiratory Infections Viral infections commonly affect the upper or lower respiratory tract. Although respiratory infections can be classified by the causative virus (eg, influenza), they are generally classified... read more 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 and classified as 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 is an uncommon cause of respiratory disease that requires medical attention.

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 year; 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 is a frequent cause of croup Croup Croup is acute inflammation of the upper and lower respiratory tracts most commonly caused by parainfluenza virus type 1 infection. It is characterized by a brassy, barking cough and inspiratory... read more Croup (laryngotracheobronchitis), primarily in infants aged 6 to 36 months. 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 type 2 can cause similar disease but is typically less severe.

Usually, a specific viral diagnosis is clinically unnecessary but may help distinguish a parainfluenza virus infection from a bacterial infection in patients with severe lower respiratory tract disease. The virus can be detected by polymerase chain reaction, usually using a multiplex panel of common respiratory pathogens.

Treatment of parainfluenza virus infection is symptomatic.

NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
quiz link

Test your knowledge

Take a Quiz!