Do migratory birds spread highly pathogenic avian influenza viruses?
There are many unanswered questions regarding the role of migratory birds in spreading highly pathogenic avian flu. Wild waterfowl have probably carried influenza A viruses, with no apparent harm, for centuries. They are known to carry viruses of the H5 and H7 subtypes, but usually of the low pathogenic form. Considerable evidence suggest that migratory birds can introduce to poultry low pathogenic H5 and H7 viruses, which then mutate to the highly pathogenic form. In the past, highly pathogenic viruses have been isolated from a few number of migratory birds, usually found dead near a poultry outbreak. Recent events make it likely that some migratory birds are now spreading the H5N1 virus in its highly pathogenic form.
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 a
