AIDS: Acquired Immune Deficiency System – An epidemic disease caused by human immunodeficiency virus that causes immune system failure. (Definition: Online Med. Dictionary, University of Newcastle Upon Tyne)

Hepatitis: Inflammation of the liver. (Definition: Online Med. Dictionary, University of Newcastle Upon Tyne)

Influenza: An acute, extremely contagious, commonly epidemic disease caused by a specific virus. (Definition: Macquarie)

Nucleoprotein: Structures containing both nucleic acid and protein. (Definition: Online Med. Dictionary, University of Newcastle Upon Tyne)

Matrix Proteins: Proteins of the outer layer of the cell wall of gram-negative bacteria. (Definition: Online Med. Dictionary, University of Newcastle Upon Tyne)

Haemagglutinin: An antigenic glycoprotein found on the surface of the Influenza virus and is responsible for binding the virus to the cell that is being infected. (Definition:Wikipedia)

Neuraminidase: An antigenic glycoprotein enzyme found on the surface of the Influenza virus. (Definition: Wikipedia)

Strain: A group of organisms within a species that differ in trivial ways from similar groups. (Definition: Princeton Word Net)

Ribonucleic Acid: A long linear polymer of nucleotides found in the nucleus but mainly in the cytoplasm of a cell where it is associated with microsomes. (Definition: Princeton WordNet)

Antigenic Shifts: Change in molecular structure of RNA/DNA in micro-organisms, especially, virus, which produces new strains of the micro-organism. (Definition: Online Med. Dictionary, University of Newcastle Upon Tyne)



Virus | Subtypes | Mutation | Spread

Defining Virus

A virus is defined as an infective agent that's smaller than a common micro-organism and requires living cells for multiplication.* Viruses cause diseases such as AIDS, hepatitis and influenza. Bird flu is triggered by the avian influenza virus.
* Delbridge, A et al. 2001. The Macquarie Dictionary: Revised Third Edition. Macquarie Library. Australia.

Types, Subtypes and Strains

There exist three different types of influenza viruses - types A, B and C, each of which are distinguished by their respective nucleoproteins and matrix proteins. Avian influenza viruses are type A influenza viruses. These cause illness in animals including humans, birds, pigs, horses and whales.
Influenza A viruses are then further sorted into subtypes dependent on the type of surface proteins on the virus. Two general surface proteins (viral glycoproteins) plaster the surfaces of influenza A viruses – hemagglutinins (HA) and neuraminidases (NA). It is the types of hemagglutinins and neuraminidases present that determine the virus subtype. For instance, the H5N1 virus contains type 5 HAs and type 1 NAs. (See Subtypes for more)
Differences also exist between viruses of the same subtype and these are further separated into strains of a virus. Varying strains of the same virus subtype may differ in pathogenicity and highly pathogenic strains often evolve from low pathogenic strains of the same subtype, as was the case with the H5N1 outbreak in Asia. New virus strains appear continually and each requires a different vaccine to provide effective immunity against it.

Virus Structure

Avian influenza viruses are more or less spherical (perhaps slightly elongated) in shape and contain eight strands of ribonucleic acid (RNA), each of which holds a single gene. The surface proteins on the virus resemble small spikes with roughly 500 of these being hemagglutinins and 100 being neuraminidases.

Miscellaneous Facts

Additional information on the avian influenza virus:
  • It has an 80-120nm diameter.
  • The virus stays active in water for more than 30 days at 0°C
  • At a water temperature of 22°C, it lives for 4 days.
  • Neuraminidases in the virus are responsible for boosting the efficiency of viral replication.
  • Tamiflu® and Relenza® are neuraminidase inhibitors – they counteract the boost of viral replication caused by neuraminidase.
  • Type A influenza viruses have the greatest range of hosts. Influenza B and C viruses infect humans only.
  • Survivors of bird flu infection will develop natural immunity against that particular bird flu strain and are unlikely to catch it again.



A Novel Virus

Novel viruses are those that have mutated from existing viruses (See Mutation) and formed a new subtype against which the human body has very little or no defence. Influenza A viruses, including bird flu viruses, are particularly prone to the antigenic shifts that cause these mutations and are currently the most likely candidate from which the next novel virus will evolve.

References

  1. Avian Influenza at the University of Maryland, 2005: (URL) University of Maryland.
  2. Influenza Viruses, 2005: (URL) CDC.
  3. Influenza, 2005: (URL) Wikipedia.
  4. Influenza Viruses: (URL) Virology Online.
  5. What We Know About The Flu Virus, 2005: (URL) NIAID.
  6. Delbridge, A et al. 1997, "Macquarie Dictionary: Revised Third Edition", Macquarie Library, Sydney. (A Definitions Source)
  7. Online Medical Dictionary, 2005: (URL) University of Newcastle Upon Tyne. (A Definitions Source)
  8. WordNet: (URL) Princeton University. (A Definitions Source)
  9. Wikipedia: (URL) (A Definitions Source)