Human Infections Caused by Parainfluenza Viruses and Laboratory diagnosis
Pathogenesis, clinical findings and immunity
According to antigenic structure there are 5 distinct serotypes of parainfluenza viruses corresponding to 5 viral species.
Types 1-4 of parainfluenza viruses are able to cause respiratory diseases in infants and young children (parainfluenza disease). Type 5 is not pathogenic for humans.
The source of infection is the sick person.
Parainfluenza is transmitted by airborne mechanism or by direct contact. Incubation period varies within 2-7 days. Viruses propagate in the respiratory epithelium.
Very often they induce mild upper respiratory tract infections. But in infants types 1 and 2 parainfluenza viruses provoke severe infections, affecting larynx and trachea. Larynx swelling and respiratory obstruction causes the croup syndrome (laryngotracheitis with obstruction and acute respiratory failure). Bronchial hyperreactivity takes part in croup pathogenesis.
Type 3 parainfluenza virus is the predominant agent of lower respiratory tract inflammation, resulting in severe bronchiolitis and pneumonia.
Type 4 of parainfluenza viruses is not so harmful; it causes inapparent or mild respiratory infections.
Maternal antibodies can’t prevent infant’s parainfluenza infection. The immunity is predominantly mediated by growing secretory antibodies of IgA class that block local viral propagation. Nevertheless, reinfections readily occur.
The specimens are obtained from nasopharingeal washings, nasal and throat swabs, etc.
For accelerated parainfluenza diagnostics various rapid methods are elaborated. Immunofluorescence assay of nasal swab and ELISA are frequently performed for primary virus detection.
Also PCR is used for identification of parainfluenza species in clinical specimens.
Virus isolation is made by inoculation of virus-containing material into the primary human cell lines or monkey kidney cell cultures.
Parainfluenza viruses develop indistinct cytopathic effect but reveal the remarkable hemadsorption activity with guinea pig erythrocytes that is used for viral detection.
Virus type identification is performed by hemadsorption or hemagglutination inhibition tests.
Serological diagnosis is made with paired sera tests. Hemagglutination or hemadsorbtion inhibition tests and ELISA are adopted for clinical use.
Principles of disease treatment and prophylaxis
Potent antiviral agent ribavirin is used for treatment of severe clinical forms of parainfluenza infection.
Effective parainfluenza vaccines are not designed yet.