What is the role of influenza virus in the etiology of viral pneumonia?

Updated: Mar 24, 2021
  • Author: Zab Mosenifar, MD, FACP, FCCP; Chief Editor: John J Oppenheimer, MD  more...
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Answer

The influenza viruses are enveloped, single-stranded, RNA viruses of the family Orthomyxoviridae and are the most common viral cause of pneumonia. Three serotypes of influenza virus exist: A, B, and C.

Influenza type A can alter surface antigens and infect livestock. This characteristic may account for its ability to create a reservoir for infection and cause epidemics in humans. The virus is spread by means of small-particle aerosol and targets the columnar epithelial cells along the entire respiratory tract.

Influenza type B causes illness that usually is seen in relatively closed populations such as boarding schools. Influenza type C is less common and occurs as sporadic cases.

Influenza type A is usually the most virulent pathogen. The influenza virus has two envelope glycoproteins, hemagglutinin (H) and neuraminidase (N), which are important for a number of reasons. The hemagglutinin initiates infectivity by binding to cellular sialic acid residues, whereas the N protein cleaves newly synthesized virus from sialic acid on cell surfaces, thus allowing spread of the virus to other cells.

The influenza virus maintains its infectivity by undergoing antigenic drift (small number of amino acid substitutions) and shift (large number of amino acid substitutions) due to changes in the protein structure of the surface protein, hemagglutinin. Epidemics occur when a viral drift occurs, and pandemics are seen with viral shift (two influenza A viruses exchange H or N genes during infection of the same hosts) because most people have no prior immunity to the virus.

Two influenza types have emerged of particular importance: H5N1 avian influenza strain and the novel H1N1 swine influenza strain.


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