Avian Influenza (Bird Flu)
The Avian Influenza virus is a small, infectious micro-organism that is an intracellular parasite. It takes over the metabolism of a cell in which it replicates itself. To enter the cell, the virus must bind onto a specific receptor on the surface of the host cell. This determines which cells are sensitive to infection and which species are vulnerable to infection, making the influenza virus highly host-specific. The avian influenza virus therefore has much more affinity for avian cells than human cells, and must adapt to a new species in order to jump across the species barrier.
All avian influenza strains have internal type A antigens in common. The subtypes that affect birds are characterised by external antigens; some are haemagglutinins H (16 different kinds from H1 to H16) and others neuraminidases N (9 different kinds from N1 to N9).
Out of 144 possible combinations, there are 23 known H – N associations – of which 15 are found in birds. A slight modification or recombination of viruses of different lineages is sufficient for the creation of potentially virulent strains. On many occasions such in Mexico and Italy, H5N2 and H7N1 strains became extremely virulent in just
a few months. A Dutch study showed clearly that the virus could survive in eggs for more than 17 days at ambient temperature