écologie des virus influenza aviaires en Camargue - IRD

Chapitre IIIcombinations). Many variants will not be viable but on those that do survive to enter newhosts, selection will act during many stages of the infection process, within both environmentand host, offering a wide range of selection pressures in natural ecosystems. Under theconditions in which domestic birds are maintained, on the other hand, the range of hostspecies available for infection is considerably reduced but galliform birds, in addition towaterfowl, are readily available. Poultry host density is often considerably higher than thevirus would encounter in the wild, age structure is generally more uniform andenvironmental conditions are frequently kept more constant. In addition, wider opportunitiesfor viral transmission exist in the form of multiple physical transport mechanisms for bothliving poultry and their products. The latter include feces, feathers and meat, and physicaltransport can include cages, packaging, farm workers and their clothes, and vehicles used onfarm and over long distances. Under these circumstances, selection pressures differ greatlyfrom those encountered by the virus in their natural, primarily aquatic, ecosystems. AIVstrains that have evolved to survive under these domestic conditions are highly likely to bemal­adapted to natural ecosystems and hosts. In particular, HP AIV, which often demonstratehigh and rapid lethality in their hosts, evolve reliance on a high and sustained host contactrate that is rare under natural conditions.HP AIV H5N1 infections in wild birds appear to retain their high virulence, leadingto infections in such birds being discovered almost entirely in sick or dead birds.Significantly, there have been no reported instances of LP AIV H5N1 viruses showinggenetic affinities to the Asiatic lineage H5N1 following outbreaks of the latter in wild birds.Although there have been examples of mutual host­pathogen co­evolution when new highlyvirulent viruses enter new wild hosts, e.g. the amelioration of myxomatosis virulence in wild126