Human parainfluenza viruses

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

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Human parainfluenza viruses (HPIVs) are a group of four distinct serotypes of single-stranded RNA viruses belonging to the paramyxovirus family. They are the second most common cause of lower respiratory tract infection in younger children.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

HPIVs are negative-sense, single-stranded RNA viruses that possess fusion and hemagglutinin-neuraminidase glycoprotein “spikes” on their surface. There are four serotypes types of HPIV (1 through 4) and two subtypes (4a and 4b). The virion varies in size (average diameter between 150 and 300 nm) and shape, is unstable in the environment (surviving a few hours on environmental surfaces), and is readily inactivated with soap and water.

The four serotypes include:

• Human parainfluenza virus type 1: HPIV-1 (most common cause of croup; also other upper and lower respiratory tract illnesses typical)
• Human parainfluenza virus type 2: HPIV-2 (causes croup and other upper and lower respiratory tract illnesses)
• Human parainfluenza virus type 3: HPIV-3 (associated with bronchiolitis and pneumonia)
• Human parainfluenza virus type 4: HPIV-4 (includes subtypes 4a and 4b)

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

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Repeated infection throughout the life of the host is not uncommon. Symptoms of later breakouts include upper respiratory tract illness as in a cold and sore throat. The incubation period of all four serotypes is 1 to 7 days. Parainfluenza viruses can be detected via cell culture, immunofluorescent microscopy, and PCR. Though no vaccines currently exist, research into vaccines for HPIV-1, -2, and -3 is underway. Parainfluenza viruses last only a few hours in the environment and are inactivated by soap and water.

• American Academy of Pediatrics. Parainfluenza Viral Infections. In: Peter G, ed. 1997 Red Book: Report of the Committee on Infectious Diseases. 24th ed. Elk Grove Village, IL: American Academy of Pediatrics; 1997: 379.
• Collins PL, Chanock RM, McIntosh K. Parainfluenza viruses. In: Fields BN, Knipe DM, Howley PM, eds. Fields Virology. 3rd ed. Philadelphia: Lippincott-Raven; 1995: 1205-41.
• Glezen WP, Denny FW. Parainfluenza Viruses In: Evans A, Kaslow R, eds. Viral Infections in Humans: epidemiology and control. 4th ed. New York: Plenum; 1997:551-67.

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  This transmission electron micrograph (TEM) depicts parainfluenza virions, and free filamentous nucleocapsid material. From Public Health Image Library (PHIL). ^[1]
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  Transmission electron micrograph (TEM) of parainfluenza virus. From Public Health Image Library (PHIL). ^[1]
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  This transmission electron micrograph (TEM) revealed the presence of the human parainfluenza type 4A virus (HPIV-4A), which like the mumps virus, is also a Paramyxoviridae family member, and a member of the genus, Rubulavirus. From Public Health Image Library (PHIL). ^[1]
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  This transmission electron micrograph (TEM) revealed the presence of the human parainfluenza type 4A virus (HPIV-4A), which like the mumps virus, is also a Paramyxoviridae family member, and a member of the genus, Rubulavirus. From Public Health Image Library (PHIL). ^[1]
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  This transmission electron micrograph (TEM) revealed some of the nucleocapsid morphologic features displayed by the human parainfluenza virus Type-4a (HPIV-4), a member of the Paramyxoviridae family. From Public Health Image Library (PHIL). ^[1]

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Each of the four HPIVs has different clinical and epidemiologic features. The most distinctive clinical feature of HPIV-1 and HPIV-2 is croup (i.e., laryngotracheobronchitis); HPIV-1 is the leading cause of croup in children, whereas HPIV-2 is less frequently detected. Both HPIV-1 and -2 can cause other upper and lower respiratory tract illnesses. HPIV-3 is more often associated with bronchiolitis and pneumonia. HPIV-4 is infrequently detected, possibly because it is less likely to cause severe disease. The incubation period for HPIVs is generally from 1 to 7 days.

HPIVs are spread from respiratory secretions through close contact with infected persons or contact with contaminated surfaces or objects. Infection can occur when infectious material contacts mucous membranes of the eyes, mouth, or nose, and possibly through the inhalation of droplets generated by a sneeze or cough. HPIVs can remain infectious in aerosols for over an hour. HPIVs are ubiquitous and infect most people during childhood. The highest rates of serious HPIV illnesses occur among young children. Serologic surveys have shown that 90% to 100% of children aged 5 years and older have antibodies to HPIV- 3, and about 75% have antibodies to HPIV-1 and -2. The different HPIV serotypes differ in their clinical features and seasonality. HPIV-1 causes biennial outbreaks of croup in the fall (presently in the United States during odd numbered years). HPIV-2 causes annual or biennial fall outbreaks. HPIV-3 peak activity occurs during the spring and early summer months each year, but the virus can be isolated throughout the year.

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