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Avian Influenza (Bird Flu)
(Released July 2007)

  by Sujata Suri  


Key Citations



Viral structure and taxonomy

viruses in green
Fig. 2 Electron micrograph of the Influenza virus (~200nm in diameter)
A virus, (in Latin, toxin or poison), is a sub-microscopic, acellular particle that cannot survive in the absence of a living cell/host cell. Viruses cannot reproduce on their own as they are dependent on host metabolic machinery and ribosomes to replicate and reproduce. Since antibiotics do not harm a virus, treatment for viral diseases such as flu mainly helps ease the symptoms rather than to kill the viruses. Most viruses cause generally mild diseases like the common cold and some even don't cause any symptoms and may go unnoticed, but some cause diseases that can be severe and deadly like Avian influenza, AIDS, and some forms of cancer.

Influenza viruses are pleomorphic (variable), mostly spherical or ovoid and filamentous, ssRNA (single-stranded RNA) enveloped viruses with a helical symmetry (Fig. 2). They are covered over by lipid/lipoprotein envelope. The viral envelope has lipoprotein membranes that enclose nucleocapsids and nucleoproteins.

diagram of influenza virus
Fig.3 The influenza (flu) virus

The diameter of each enveloped virus ranges from 50-120 nanometers (nm) and filamentous virions are 20nm in diameter and 200-300nm long (Fig. 3). The genome is in the form of eight negative-sense ssRNA fragments (seven for Type C). The total genome length is 12000-15000 nucleotides (nt), the largest segment being 23-25 and the smallest being 800-900 nt. All have terminal repeats at their 5' end and 3' end about 9-13 nucleotides long (Fig. 4). The 5' and 3' terminal sequences of all RNA strands are highly conserved. The longest RNA strand is closely associated with the nucleoprotein to form helical symmetry.

replication of 3 types of rna
Fig. 4 Different types of mRNAs formed in the host nucleus by influenza viruses

There are some 500 distinct spike-like surface proteins of the viral envelope, each projecting 10-14 nm from the surface. There are mainly four types of glycoproteins /antigens:

  1. Hemagglutinin (HA) is a 135Å trimer, a major glycoprotein present on the viral surface as rod-shaped projections, and mediates the attachment of the virus to the cellular receptor. There are 16 types of HA reported.
  2. Neuraminidase (NA) is a 60Å tetramer, a kind of glycoprotein, and 9 types have been reported. The ratio of HA to NA is about 4-5 to 1.
  3. Nucleocapsid protein (NP) It coats the RNA strands.
  4. Matrix protein (M) The inner side of the viral envelope is lined by the matrix protein.

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