Human parainfluenza viruses (HPIVs) commonly cause upper and lower respiratory illnesses. The symptoms of HPIVs are not severe enough to cause concern in healthy adults. However, they can be life-threatening in an infant, the elderly, or anyone with a compromised or weakened immune system.
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There are four types of parainfluenza viruses which cause respiratory infections. The exact type of infection, the symptoms, and the location of the infection depends on the type of virus:
- HPIV-1: the leading cause of croup in children
- HPIV-2: also causes croup in children, but it is detected with less frequency than HPIV-1
- HPIV-3: mainly associated with bronchiolitis and pneumonia
- HPIV-4 (includes subtypes 4A and 4B): not as well known, but may cause mild to severe respiratory tract illnesses
HPIVs are spread from person to person by direct contact or exposure to contaminated secretions from the nose or throat. Most children are infected with HPIV-3 by the age of two years and with HPIV-1 and HPIV-2 by the age of five years. HPIV-3 infections are a major cause of pneumonia and bronchiolitis in infected infants under 6 months old.
Laboratory diagnosis of parainfluenza viruses can be performed by isolation and detection of the virus in cell culture, or detection of viral antigens directly within respiratory tract secretions using immunofluorescence (IFA), enzyme immunoassays (EIA), fluroimmunoassays or polymerase chain reaction (PCR). Also, analysis of specific IgG antibodies showing a subsequent rise in titer following infection (using paired serum specimens) can demonstrate an acute infection. However, individual parainfluenza virus types are known to cross-react making subtyping difficult. Hemagglutination inhibition tests (HIT), complement fixation (CF) and neutralization tests (NT) can be performed to differentiate the HPIV types by evaluating the specific IgM, IgG and IgA antibody titers.
During the acute phase of infection, two thirds of patients show a high serum titer of specific HPIV IgM antibodies which persist 2-11 weeks. In 70 – 80% of patients, an increase in specific IgG antibodies (at least fourfold within 10 days) is found during primary infection with HPIV-1, 2 or 3. New EIA assays rely on purified viral envelope glycoprotein and nucleocapsid preparations. In differential diagnosis, tests for other paramyxoviruses like mumps, pneumonia and simian virus type 5 must be performed due to possible cross-reactions.
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