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What is bird flu?
Bird flu is present in birds throughout the world. It is caused by various subtypes of the Influenza A virus. Wild water birds are natural hosts for all Influenza A subtypes and are able to carry the virus in their intestines without getting ill. They continuously shed virus in their saliva and feces. Domestic birds are exposed to the virus through contact with wild birds, including their dropping or other infected domestic birds. The severity of symptoms among infected domesticated birds like chickens, turkeys or ducks vary from minor to fatal, depending on the specific subtype of Influenza A virus involved.
H5N1 is an Influenza A subtype that occurs mainly in birds. It is very contagious and causes serious, often fatal infections in susceptible birds. Bird flu outbreaks caused by Influenza A H5N1 was first noticed among poultry in Asia in late 2003. Outbreaks have been ongoing since then.
Human flu is also caused by Influenza A virus. The specific subtypes that have caused widespread illness in people are H3N2, H2N2, H1N1, and H1N2. H1N1 and H3N2 subtypes have also caused outbreaks in pigs.
How does bird flu affect people?
Although it is uncommon for bird flu to occur in people, several bird handlers in Hong Kong got infected with the H5N1 bird flu virus in 1997. About 30% of infected people died during this outbreak. Since then there has been a number of bird flu outbreaks among people, with various Influenza A subtypes involved. H5N1 outbreaks have been the most severe, with 30-50% of infected people dying as a result of the infection. Outbreaks have occurred mainly in Asia. Infected people have all been directly or indirectly exposed to infected birds or uncooked bird products. There has been no evidence of spread from person to person.
As of August 5, 2005, there have been 112 human cases of avian influenza A (H5N1) in Vietnam (90), Thailand (17), Cambodia (4), and Indonesia (1), resulting in 57 deaths reported since January 2004 (Source: World Health Organisation)
A world-wide (bird) flu pandemic – reason for concern or exaggerated hype?
If we look back in history, we notice three major flu pandemics that occurred in the last 100 years:
• The ‘Spanish flu’ in 1918-1919 caused by Influenza A H1N1
subtype, resulting in the death of about 50 million people worldwide
• The ‘Asian flu’ in 1957-8 caused by Influenza A H2N2 subtype
and resulting in 70, 000 deaths in the USA alone
• The ‘Hong Kong flu’ in 1968-9 caused by Influenza A H3N2
The subtypes responsible for the ‘Asian flu’ and the ‘Hong Kong flu’ both developed as new Influenza A subtypes following the accidental combination of genes from a bird flu virus and human flu virus. It is not known how the ‘Spanish flu’ subtype came into being.
Every time a new Influenza A subtype is formed, there is a possibility of a world-wide pandemic because no-one has been exposed to the new virus and therefore no-one has any immunity against infection. Pandemics are usually associated with high levels of illness, death and economic loss. Scientists agree that a fourth flu pandemic is imminent.
Human flu virus + bird flu virus = new Influenza A subtype?
One of the characteristics of the Influenza A virus is its lack of attention to detail when it comes to duplicating its genetic material during replication. The virus tends to make errors during the duplication process, resulting in slight changes to the genetic make-up of its offspring. This is called antigenic drift and helps the virus avoid its host’s defense mechanisms.
Another important characteristic of Influenza A viruses is the ability of different subtypes occurring in the same host to swaop genetic material or to merge in order to form a brand new subtype. Pigs are susceptible to both bird and human flu viruses and could serve as a perfect “mixing vessel”.
The main concern with the current outbreaks of bird flu among people is the potential for the development of a brand new subtype in an individual who happens to be infected with both the human flu and the bird flu virus at the same time. This new subtype would be able to spread from person to person in the absence of infected birds. The easy availability of air travel will allow the new Influenza A subtype to spread around the world in a matter of days, resulting in a devastating flu pandemic.
H5N1 is the most likely bird flu subtype to be involved in the development of a new human subtype. Unfortunately it is one of the more contagious, severe subtypes. It is also resistant to currently available anti-viral treatment.
How serious would this new (bird) flu pandemic be?
Thanks to advances in science, our knowledge and understanding of Influenza
A and its different subtypes is much better than it was during the last flu
pandemic. Outbreaks have been contained effectively up to now. The development
of a new vaccine is underway and contingency plans are in place. So, while we
may not be able to prevent the onset of a fourth pandemic, we should be able
to contain its spread and impact better than during previous pandemics. In spite
of this, a pandemic caused by a new human flu virus containing genetic material
from H5N1 bird flu subtype is considered one of the biggest health threats the
world is facing today.
Information source: Center for Disease Control and W.H.O.
