Human parainfluenza viruses (HPIVs) are a group of spherical, enveloped, negative-sense, single-stranded RNA viruses in the paramyxovirus family. There are four types of HPIV (Types 1 to 4) and two subtypes (4A and 4B). A fifth putative HPIV, PIV5, (formerly known as SV5) is commonly categorised as an 'unknown haemabsorbing agent' when clinical respiratory samples are propagated in laboratory cell culture. These viruses are unstable in the environment and are readily inactivated with soap and water.
HPIV1-4 infection is one of the common causes of upper and lower respiratory tract disease, especially in young children. Similar to respiratory syncytial virus (RSV), HPIVs 1-4 can cause repeated infections throughout life, and HPIV types 1-4 can cause a full spectrum of respiratory illness, including the common cold, croup, and severe lower respiratory tract illness, such as bronchitis, bronchiolitis and pneumonia. Disease association with HPIV5 is not well established, although it has been implicated in a range of chronic diseases outside the respiratory tract Among adults, most HPIV 1-4 infections cause mild disease showing as upper respiratory tract symptoms. However, HPIV infections may also cause more severe diseases especially among the elderly and among patients who are immunocompromised. HPIV infections are important causes of mortality among immunocompromised patients (1-3). The incubation period is from 1-7 days.
HPIVs are spread from respiratory secretions through close contact with infected persons or contact with contaminated surfaces or objects. HPA analysis of HPIV infections in England and Wales found that HPIV-3 was the major HPIV virus type causing most infections and outbreaks, followed by HPIV-1, HPIV-2 and HPIV-4 (4).
HPIV-3 infections had an annual epidemic cycle with a peak in late spring or summer, whereas peaks of HPIV-1 and HPIV-2 occurred at one or two year intervals in the late autumn or early winter. HPIV-4 has been infrequently detected and mainly appeared in the late autumn or early winter. Children under one year of age were the most commonly affected group, followed by those aged from 1-4 years . Other age groups were much less affected. Males were slightly more affected than females.
No specific antiviral is presently licensed for treatment of HPIV. There are currently no licensed vaccines available to protect against infection caused by any of the HPIVs (5,6,7). However, research is still on-going to try to develop vaccines against HPIV-1, HPIV-2 and HPIV-3 infections in laboratory and animal studies (6,8,9,10). A phase 2 study for the HPIV-3 vaccine found it safe and immunogenic in children 6-18 months old (10).
In outbreak and cluster settings, to prevent onward transmission, strict attention to infection control measures such as hand washing and wearing protective equipments should decrease or prevent spread of infection.
The Health Protection Agency monitors HPIV activity in the UK. Laboratory surveillance data (by specimen date) are shown in the following Figures 1-5 for England and Wales between 01/01/1975-26/08/2008. Regular reports are published in the HPA's weekly bulletin- the Health Protection Report (HPR) every four weeks throughout the year, e.g. http://www.hpa.org.uk/hpr/archives/2008/hpr2708.pdf.
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