Human Parainfluenza Viruses - Clinical Significance

Clinical Significance

In the USA it is estimated that there are 5 million children with lower respiratory infections (LRI) each year. Estimates have shown that hPIV-1, hPIV-2 and hPIV-3 have been linked with up to a third of these infections. Upper respiratory infections (URI) are also important in the context of hPIV, however are caused to a lesser extent by the virus. The highest rates of serious HPIV illnesses occur among young children and surveys have shown that about 75% of children aged 5 or older have antibodies to HPIV-1.

For infants and young children it has been estimated that ~25% will develop ‘clinically significant disease.'

Repeated infection throughout the life of the host is not uncommon and symptoms of later breakouts include upper respiratory tract illness, such as cold and a sore throat. The incubation period for all four serotypes is 1 to 7 days. In immunosuppressed people, parainfluenza virus infections can cause severe pneumonia which can be fatal.

hPIV-1 and hPIV-2 have been demonstrated to be the principal causative agent behind croup (laryngotracheobronchitis) which is a viral disease of the upper airway and is mainly problematic in children aged 6–48 months of age. Biennial epidemics starting in Autumn are associated with both hPIV-1 and 2 however, hPIV-2 can also have yearly outbreaks. Additionally, HPIV-1 tends to cause biennial outbreaks of croup in the fall. In the United States, large peaks have presently been occurring during odd-numbered years.

hPIV-3 has been closely associated with bronchiolitis and pneumonia and principally targets those aged <1 year.

hPIV-4 remains infrequently detected. However, it is now believed to be more common than previously thought, but is less likely to cause severe disease. By the age of 10, the majority of children are sero-positive for hPIV-4 infection which may be indicative of a large proportion of asymptomatic or mild infections.

Important epidemiological factors that are associated with a higher risk of infection and mortality are those who are immuno-compromised and may be taken ill with more extreme forms of LRI. Associations between hPIV and neurologic disease are known, for example hospitalisation with certain types of hPIV has a strong association with febrile seizures. hPIV-4B has the strongest association (up to 62%) followed by hPIV-3 and 1.

hPIV has also been linked with rare cases of virally caused meningitis and Guillain-Barré syndrome.

HPIVs are spread person to person by 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.

Overall, hPIV remains best known for its effects on the respiratory system and this appears to be where the majority of the focus has been upon.