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The Spreading of the Bird Flu in Nature
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The birds play an important role in spreading flu, because the subtypes of hemagglutinin (H1, H2 and H3) and neuraminidase (N1 and N2), widely circulating among people, are found in wild fowl. It is considered that the initial reservoir of type A flu is different migratory birds, belonging to the Anseriformes (wild ducks and geese) and the Charadriiformes (herons, dotterels and terns). Most frequently they have 24 combinations of hemagglutinin and neuraminidase: ?1N1 - H2N2 - H2N3 - H3N2 - H3N8 - H4N2 - H4N4 - H4N6 - H4N8 - H5N1 - H5N2 - H5N9 - H6N1 - H6N2 - H6N5 - H6N9 - H7N1 - H7N2 - H7N3 - H7N7 - H9N2 - H9N8 - H10N7 - H11N9. The subtypes ?5 and ?7 are the most pathogenic for birds.
Waterfowl transfer the virus in bowels and excrete it into the environment through saliva, respiratory and fecal material. The most common way of spreading the virus is the fecal-oral route. In wild ducks, the flu viruses replicate mostly in the cells lining the intestine tract, cause no visible disease signs and are excreted into the environment in high concentration.
Studies on the gene lineages in various avian species show separate sublineages of influenza in Eurasia and the Americas, indicating that migratory birds moving between these continents (latitudinal migration) have little or no role in the transmission of influenza, while birds that migrate longitudinally appear to play a key role in the continuing process of virus evolution.
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