What Does H5N1 Mean When We Talk About Avian Influenza Virus?

Main Category: Bird Flu / Avian Flu
Also Included In: Flu / Cold / SARS
Article Date: 16 Feb 2006 - 6:00 PDT

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Many different subtypes of influenza exist. The subtypes are named after the combination of variants of two of the virus proteins.

Each influenza A virus has one of 16 H subtypes and one of 9 N subtypes. Only avian viruses which have the combination of H5 and H7 subtypes are known to cause the highly pathogenic form of the disease.

However, not all viruses of the H5 and H7 subtypes are highly pathogenic and not all will cause severe disease in poultry.

Low pathogenic avian influenza viruses appear to have the ability to become highly pathogenic by mutation, which appears to occur only after the H5 or H7 viruses have moved from their natural wild bird hosts into domestic poultry. Mutation is a process where there is an alteration in the genes of a cell or virus which is transmitted to the offspring.

Influenza A viruses contain 8 distinct RNA genes and reassortment of these can occur if two different viruses infect the same animal so that viruses emerge with a set of genes made up of some of the genes from one virus and some from the other. In the 20th Century there were 4 pandemics of influenza due to the emergence of new and genetically distinct strains in humans: 1918 (H1N1), 1957 (H2N2), 1968 (H3N2) and 1977 (H1N1). The 1957 and 1968 pandemic viruses differed from the preceding viruses in humans by the substitution of genes that came from avian viruses, suggesting they arose by genetic reassortment of viruses of human and avian origin.

Mutation and re-assortment happen continuously in the family of influenza viruses, but as can be seen pandemics have only occurred four times in 100 years and are unevenly spaced over time.

http://www.defra.gov.uk

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