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Mutation of the Avian Flu Virus

All Type A Influenza viruses are well known for their propensity to easily mutate over time and space.  A mutation is defined as a change in genetic information.  Changes in genetic information can lead to changes in the actual structure of the virus, leading to new subtypes or strains.  For example, the Type A Influenza virus of humans undergo frequent mutations leading to permanent antigenic changes in the virus.  An antigen is a foreign substance, generally a protein that causes an immune response.  The Influenza Virus looks like a ball studded with “spikes” on the surface of the virus.  The spikes consist of molecules of two proteins (antigens), hemagglutinin and neuraminidase, on the surface of the virus. 

All Type A Influenza viruses, which includes those that infect humans and birds, mutate easily because the virus lack mechanisms that prevent changes in its genetic code during replication.  The virus also cannot repair any changes should there be any.  As a result of these uncorrected errors, the genetic composition of the virus changes (mutation) during replication, and the existing strain is replaced with a new mutant virus, properly known as an antigenic variant.  These constant, permanent and usually small changes in the antigenic composition of Influenza A Viruses are known as “antigenic drift”.

The danger in antigenic drift is that Influenza Viruses that are “mildly pathogenic” can mutate into forms that become “highly pathogenic”.  This has been shown during a 1983-84 epidemic of Avian Flu Virus in the United States.  During this period, Avian Flu Virus Subtype H5N2 initially caused low mortalities in poultry.  But within six months, the virus had mutated into a highly pathogenic form as demonstrated by high mortalities approaching 90%.   

The probability of the Flu virus mutating into a more pathogenic form also increases the longer it circulates.  This was shown in a study conducted earlier this year by the Harbin Veterinary Research Institute (Harbin, China) and the St Jude Children's Research Hospital (Memphis, U.S).  In their study, researchers reported results of experiments in which mice had been exposed to 21 isolates of Avian Flu Virus subtype H5N1 strains obtained from ducks in China between 1999 and 2002.  They found a pattern of progressively increasing pathogenicity over time, with the 2002 isolates being far more harmful then those in 1999. 

Another characteristic of Influenza viruses that cause great public health concern is the ability of different strains/subtypes of viruses to hybridize together.  This means that these viruses can swap and merge genetic information to create a new strain/subtype of flu virus.  This “hybridization” process is known as “antigenic shift”.  These antigenic shifts have long been thought to occur in areas where humans live in close proximity to domestic poultry and pigs.  Pigs are especially notorious for being susceptible to a wide range of different strains/subtypes of Flu viruses.  The WHO sees pigs, and recent evidence also implicate humans, as a “mixing vessel” for the scrambling of genetic material from human and avian viruses, resulting in the emergence of a new form of Flu virus.   

As populations have no immunity to the new subtype, and as no existing vaccines can confer protection, antigenic shift has historically resulted in highly lethal pandemics.  For this to happen, the novel subtype needs to have genes from human influenza viruses that make it readily transmissible from person to person for a sustainable period.  The most notable example of this is the Great Spanish Influenza epidemic of 1918-19 that resulted in the deaths of 40-50 million people worldwide.  The Flu virus causing this great tragedy is believed to derive from an antigenic shift resulting from the interactions between Avian and Human Flu Viruses. 

Influenza has been largely controlled in the world because of the creation of very effective vaccines against the virus.  The vaccines contain small doses of the virus’ antigens, so when injected in your body your immune system responds to these antigens by producing antibodies against them.  A vaccine essentially “primes” your immune system so that it will react swiftly and powerfully when, and if, the actual Influenza virus infects you.  Thus, mutation of the Flu Virus may render the vaccine ineffective since the mutated virus will have new antigenic types, which the vaccine doesn’t provide in its formulation.  That is why the WHO is constantly monitoring the global influenza situation and makes annual adjustments in the composition of its influenza vaccines.  These activities have been a cornerstone of the WHO Global Influenza Program since its inception in 1947.