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	Human Parainfluenza
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Human Parainfluenza Viruses

Clinical Features of human parainfluenza viruses (HPIVs)

Human parainfluenza viruses (HPIVs) are common causes of respiratory tract disease in infants and young children. 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 or swelling around the vocal chords and other parts of the upper and middle airway); HPIV-1 is the leading cause of croup in children, whereas HPIV-2 is less frequently detected. HPIV-3 is more often associated with bronchiolitis (swelling of the small airways leading to the lungs) and pneumonia. HPIV-4 is detected infrequently, and is less likely to cause severe disease; but it may be more common than once thought.

HPIVs can cause repeated infections with all serotypes throughout life. Reinfections usually manifested by an upper respiratory tract illness (e.g., a cold, sore throat). HPIVs can also cause serious lower respiratory tract disease with repeat infection (e.g., pneumonia, bronchitis, and bronchiolitis), especially among older adults and patients with compromised immune systems. The incubation period (time from exposure to the virus to onset of symptoms) for HPIVs generally ranges from 2 to 7 days.

The Viruses

HPIVs belong to the Paramyxoviridae family and are negative-sense, single-stranded RNA viruses that possess fusion and hemagglutinin-neuraminidase glycoprotein "spikes" on their surface. There are four serotypes (1 through 4) and two subtypes (4a and 4b). The virion varies in size (average diameter between 150 and 200 nm) and shape, is unstable in the environment (surviving a few hours on environmental surfaces), and is readily inactivated with soap and water.

Epidemiologic Features

HPIVs are spread person to person by direct contact with infected secretions through respiratory droplets or 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 airborne droplets 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, large peaks occur during odd-numbered years, with smaller peaks in even-numbered years. HPIV-2 causes smaller annual or biennial fall outbreaks. HPIV-3 peak activity occurs during the spring and early summer months each year in temperate climates, but the virus can be isolated throughout the year, particularly during seasons when HPIV-1 and HPIV-2 are absent. The patterns of infection with HPIV-4 are still unknown.

Diagnosis

Infection with HPIVs can be confirmed in various ways:

1. by isolation and identification of the virus in cell culture,
2. by direct detection of viral antigens in respiratory secretions by use of immunofluorescence, enzyme immunoassay, or fluoroimmunoassays,
3. by polymerase chain reaction assay, or
4. by demonstration of a significant rise in specific IgG antibodies between appropriately collected paired serum specimens, although infection may not always elicit a significant antibody response.

Prevention

No vaccine is currently available to protect against infection caused by any of the HPIVs; however, researchers are developing vaccines. Passively acquired maternal antibodies may play a role in protection from HPIV types 1 and 2 in the first few months of life, highlighting the importance of breast-feeding. Strict attention to infection-control practices, including standard and contact precautions, should decrease or prevent spread of infection in healthcare settings. In the community, frequent hand washing and not sharing items such as cups, glasses, and utensils with an infected person should decrease the spread of virus to others.

References

• American Academy of Pediatrics. Parainfluenza Viral Infections. In: Pickering LK, Baker CJ, Kimberlin DW, Long SS, eds. Red Book: 2009 Report of the Committee on Infectious Diseases. 28th ed. Elk Grove Village, IL: American Academy of Pediatrics; 2009: 485-487.
• Karron RA, Collins PL. Parainfluenza viruses. In: Fields BN, Knipe DM, Howley PM, eds. Fields Virology. 5th ed. Philadelphia: Lippincott-Williams & Wilkins Publishers; 2007: 1497-1526.
• 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 page was last reviewed on March 25, 2011