Respiratory Infections

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Respiratory Infections

Parainfluenza Virus

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Causative Agent

Human parainfluenza viruses (HPIV) are members of the genus Paramyxovirus and belong to the Paramyxoviridae family, which also includes mumps, measles and respiratory syncytial virus (RSV). HPIV is composed of a single RNA strand, surrounded by a lipid cell envelope of host cell origin. The five major serotypes of HPIV's can be grouped antigenically into two divisions: (1) HPIV-1 and HPIV-3 and (2) HPIV-2, HPIV-4 and mumps.

Clinical Description

Parainfluenza viruses are important causes of respiratory disease, especially in infants and young children, being responsible for 15% of childhood colds, croup, bronchitis and pneumonia. The average duration of illness is 7 - 10 days.


Transmission occurs invariably by inhalation of airborne virus and mucous membranes of the nose and throat are the initial site of para-influenza virus infection. Innoculation studies have demonstrated viral shedding to occur for up to 11 days, which occurs for longer in immunocompromised hosts.

Surveillance in Scotland

Like influenza, parainfluenza is not a notifiable disease. SCIEH information therefore depends wholly on laboratory reports, which for the most part come from hospitalised patients.

Trends in Scotland (1995-2000)

Numbers of reports of HPIV-3 have been increasing steadily since 1995 with the largest number of reports for any year recorded for 1999, which also saw an expanded season for the most frequently reported of these viruses. HPIV-2 has a more approximately biennial behaviour, although the 1998 outbreak was significantly less than that which occurred in 1996.

Incidence and Risk

Incidence and risk profiles differ between serotypes. HPIV-1 occurs biennially during the autumn in both hemispheres, with a peak incidence in the second and third year of life. HPIV-2 epidemiology is less well established. Outbreaks can occur simultaneously with HPIV-1 or alternately with a peak in autumn /early winter and peak incidence in the second year of life. HPIV-3 is unique among the parainfluenza viruses in its ability to infect young infants less than 6 months of age, with bronchiolitis and pneumonia being the most common clinical syndromes, and rates only second to RSV as a cause of LRI among neonates and young children. Infection with HPIV-4, based on sero-prevalence data is almost universal, but disease is either rare or difficult to detect.


Prevention measures not generally indicated or effective.

Graph of Parainfluenza type 3 reports 1995-2001: 4 weekly periods to midyear 2001