Wednesday, April 29, 2009

Avian Flu (Avian Influenza) Viruses Introduction Information

As per the United States OSHA website, numerous stories have aired on radio and television or been published in various news media concerning avian influenza and in particular the H5N1 subtype. Unfortunately there is now much confusion about the different human diseases caused by influenza viruses.

Influenza A viruses can cause three distinct diseases in humans: avian, pandemic and seasonal influenza. Avian influenza in humans is rare and the most common route of infection is via direct or indirect contact with secretions (nasal, oral or fecal) from infected poultry. Transmission from human-to-human, if it exists, is extremely rare. However, avian influenza viruses have the potential to mutate or reassort and become pandemic viruses; those that can be readily transmitted between humans and those for which the population has little immunity. If these viruses spread throughout the world, the disease caused by them would be called pandemic influenza and the new viruses would be called pandemic influenza viruses. Previous pandemic influenza episodes have occurred in two or three waves of 6-8 week duration and spanned a 12-18 month period. After this period, the population will have built up immunity to the virus, either naturally or through vaccination. If the virus continues to circulate in the population and causes disease, it would become an influenza virus that causes seasonal influenza (more popularly called human influenza or the flu).

Influenza A viruses are subdivided into numerous subtypes. The subtypes are differentiated by variations in two viral surface proteins, hemagglutinin (H) and neuraminidase (N). Sixteen different H proteins and nine N proteins have been identified. Subtypes are designated by numbering particular combinations of these proteins (e.g., H5N1). Therefore, there are a total of 144 possible subtypes (16H x 9N) of influenza A viruses and all or most of these have been found in wild waterfowl. Interestingly only three of the 144 subtypes, H1N1, H2N2 and H3N2, have caused pandemic influenza in the 20th century. Only strains of H1N1 and H3N2 are currently circulating and causing seasonal influenza. Recently a number of different subtypes of influenza A viruses have emerged as agents of avian influenza in humans and these include H5N1, H7N2, H7N3, H7N7 and H9N2.

As of October 2006, H5N1 viruses have killed more than 150 people in ten different countries since the beginning of 2003. On the other hand, the H7N7 virus has been associated with a single human death but numerous cases of conjunctivitis (eye infection) in the Netherlands. The H7N2, H7N3 and H9N2 viruses have caused only mild disease in humans. While the number of human deaths caused by the H5N1 virus is small in comparison to the annual deaths attributed to human seasonal influenza viruses (~36,000/ year in the U.S.), it is of particular concern to the public health community because many scientists believe that this virus may continue to mutate or reassort and a strain may ultimately develop the ability to pass readily between humans. If this happens, the virus that emerges may cause the next major influenza pandemic.

As of October 2006, the highly pathogenic avian influenza (HPAI) H5N1 virus has not been detected in North or South America and it is important to understand that a pandemic influenza virus has not yet emerged and when, and if, it will emerge is impossible to predict.

For more information visit the OSHA website

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