The role of migratory birds in the spread of highly pathogenic avian influenza is not fully understood.
- Wild waterfowl are considered the natural reservoir of all influenza A viruses. They have probably carried influenza viruses, with no apparent harm, for centuries.
- They are known to carry viruses of the H5 and H7 subtypes, but usually in the low pathogenic form.
- Considerable circumstantial evidence suggests that migratory birds can introduce low pathogenic H5 and H7 viruses to poultry flocks, which then mutate to the highly pathogenic form.
- In the past, highly pathogenic viruses have been isolated from migratory birds on very rare occasions involving a few birds, usually found dead within the flight range of a poultry outbreak.
- This finding long suggested that wild waterfowl are not agents for the onward transmission of these viruses.
- Recent events make it likely that some migratory birds are now directly spreading the H5N1 virus in its highly pathogenic form. Further spread to new areas is expected.
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