The Evolution of the Influenza Virus

Influenza or flu is a highly contagious respiratory disease of mammals and avian species that usually occurs during the fall and winter months and is caused by an RNA virus belonging to the Orthomyxoviridae family (Earn, Dushoff and Levin, 2002). The capacity of this virus to form new strains by undergoing rapid mutation while at the same time eluding the body’s immune mechanisms – the antibodies, makes it a major public health problem that is currently managed through annual vaccinations.

The flu virus is subdivided into A, B and C types as well as subtypes which can be differentiated by their primary surface proteins. From an epidemiologist’s perspective, Type A is significant due to its presence in various animal species and its capability of going through rapid evolution or antigenic drift and shift (Earn, Dushoff and Levin, 2002). On the other hand, Type B is limited to humans on the whole and remains an essential cause of illness while insufficient knowledge has been generated regarding type C.

Being an RNA virus, the influenza virus copies its genetic material in ribonucleic acid, a molecule that is used by most species as messengers in the transcription and transfer of information from DNA in order for the production of proteins to occur. As such, RNA is far less stable than DNA, generating about 1 error for every 10,000 nucleotides every time it is reproduced, incapable of resolving such errors and under ideal situations will even react with itself (epidemic. org, 2008).

These mutations produce variations in the hemagglutinin (HA) and neuraminidase (NA) proteins of the virus and this characteristic, compounded by the fact that the flu virus reproduces itself a hundred times, enables it to undergo an evolutionary process that is much faster than other organisms (epidemic. org, 2008). Although its rapid mutations are generally regarded as a downside, the high rate of reproduction ensures the virus’ survival so that when a mutation favorable to the virus occurs, it is quickly subjected to selection and reproduction.

Mutations which produce various subtype strains of the flu virus prevent organisms to create long term immunity against it. Immunity against an antigen, a substance found on the HA protein surface of the flu virus, begins when receptors on the surface of host antibodies recognize viral proteins as foreign and activate immune responses towards eliminating the virus (primary response), a process which could take days (Public Library of Science, 2008). This allows the virus to infect cells and the host exhibits the symptoms of the disease.

Antibody memory permits a faster and more effective immune response during subsequent exposures to the same virus (memory response). However, when minor mutations in the HA protein of a flu strain occur, the antibodies that will bind to this protein will not recognize it producing a primary response while the antibodies that will bind to the NA will still recognize it and provide the body with a degree of immunity (Public Library of Science, 2008). Thus, even if people contract the illness, it is not as severe as to cause high mortality.

This manner of mutation is referred to as antigenic drift and is the cause of annual flu epidemics. On the other hand, fatal strains result when both HA and NA proteins mutate under conditions where a flu strain in a human cell crosses with another strain from an animal species so that the resulting RNA undergoes complete reassortment (UCLA. edu, 2008). This is called antigenic shift. Because the result is an entirely different subtype, humans have no existing immunity against it causing rapid death, transmission and eventually, pandemics.

The most severe pandemics of influenza is known as the Great Pandemic of 1918, also known as the Spanish Flu, where the dead was estimated to reach fifty million individuals, half a million of which was recorded in the U. S. (Potter, 2001). The Asian Flu occurred in 1957 and resulted in two million dead (70,000 in the U. S. ) and the Hong Kong Flu in 1968 caused 700,000 deaths with 34,000 in the U. S. (Potter, 2001). Different influenza subtypes caused these pandemics.

Beginning in 1997, virulent outbreaks of avian flu and avian influenza A (H5N1) virus have been noted among humans in Hong Kong and China and are significant because only 3 subtypes – the H1N1, H1N2 and H3N2, have been known to spread in humans before (Center for Disease Control, 2007). Most of those who contracted the disease had prolonged contact with fowl and is the first time that the virus spread directly to humans, posing a serious threat due to the lack of human immunity against these viruses.

Widely coordinated researches are being conducted in order to ascertain the pattern by which flu viruses evolve and led to world wide surveillance efforts. The type of influenza strains in circulation as well as trends regarding the illness are collected by 122 national influenza centers in 94 countries then collated data undergo analysis by WHO Collaborating Centers for Reference and Research on Influenza (Center for Disease Control, 2008).

On the basis of this data, specialists issue a forecast estimating the possible influenza strains which will cause the next flu epidemic in each hemisphere along with recommendations as to what particular viral strains should be incorporated in vaccines. The Food and Drug Administration in the U. S. convenes an advisory committee in February of each year and using the forecast, decides with finality regarding the viral strains (Center for Disease Control, 2008).

Vaccination in the U. S. takes place beginning in September and from then up to January or even later, since the outbreak of flu is variable – usually occurring in October and peaking in January. Three influenza subtypes compose the vaccine – A (H3N2) virus, A (H1N1) virus and a B virus where shots contain dead viruses while nasal sprays contain live attenuated ones (Center for Disease Control, 2008). Only the strains are subject to change depending on the forecasts.

Immunity is developed two weeks after vaccination and its effectiveness depends on how well it matches the actual flu viruses that circulate in the season.

List of References

Center for Disease Control (2008). Key Facts About Avian Influenza (Bird Flu) and Avian Influenza A (H5N1) Virus, Key Facts About Seasonal Flu Vaccine 2008-2009 Season. Retrieved 21 October 2008 from http://www. cdc. gov/flu/avian/gen- info/facts. htm.

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