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About the Avian Flu Virus

Avian influenza is an infectious disease of birds caused by type A strains of the influenza virus.  The disease, which was first identified in Italy more than 100 years ago, occurs worldwide.  The Avian viruses, along with the other influenza viruses, make up the virus family Orthomyxoviridae.  All of these influenza viruses are negative-stranded, segmented RNA.  The virus is approximately 80-120nm in diameter.  The “type” of influenza virus (A, B, or C) is determined by the antigenic character of the M (matrix) protein of the virus envelope and the nucleoprotein within the virus particle.  The M-protein makes up 35-45% of the entire virus particle mass.     

The Avian Flu virus particle, like all other influenza viruses, is characterized by possessing an envelope with glycoprotein projections with two types of surface antigens – hemagglutinin (HA) and neuraminidase (NA).  These proteins have hemagglutinating and neuraminidase activity respectively.  Hemagglutination refers to the virus’ ability to clump red blood cells together.  This facilitates the attachment of the virus to a cell, where it can go on to invade and replicate within it.  Neuraminidase is an enzyme that breaks the bonds holding a virus particle to the outside of an infected cell.  Neuraminidase activity refers to the breaking of these bonds, which allows the virus particle to be set free so it can infect other cells and spread the infection.  

Fifteen subtypes of influenza virus are known to infect birds, thus providing an extensive reservoir of influenza viruses potentially circulating in bird populations.  However, there are substantial genetic differences between the subtypes that typically infect both people and birds.  Within subtypes of avian influenza viruses there also are different strains.  The two surface antigens, HA and NA, are the basis of describing the serologic identity of the influenza viruses using the letters H and N with the appropriate numbers in the virus designation (e.g. H7N2).  There are now 15 hemagglutinin and 9 neuraminidase antigens described among the Type A influenza viruses.

All birds are thought to be susceptible to infection with avian influenza, though some species are more resistant to infection than others. Infection causes a wide spectrum of symptoms in birds, ranging from mild illness to a highly contagious and rapidly fatal disease resulting in severe epidemics. The latter is known as “highly pathogenic avian influenza”. This form is characterized by sudden onset, severe illness, and rapid death, with a mortality that can approach 100%.

Avian influenza can be distinguished as “mildly pathogenic” and “highly pathogenic” forms, on the basis of genetic features of the virus and the severity of the illness they cause in birds.  To date, all outbreaks of the highly pathogenic form have been caused by influenza A viruses of subtypes H5 and H7.  Avian Influenza H9 virus has been identified only in a “mildly pathogenic” form.