Human parainfluenza viruses epidemiology and demographics

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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