How might HPAI be spread?
Wild waterfowl are the natural reservoir of all influenza A viruses. They have probably carried influenza viruses, including H5 and H7 subtypes in the low pathogenic form with no apparent harm for centuries. Evidence now suggests that migratory birds can introduce low pathogenic H5 and H7 viruses into poultry flocks and these then mutate to the highly pathogenic form. HPAI viruses have been isolated from migratory birds on rare occasions after these birds have been found dead within flight range of a poultry outbreak. Recent events indicate that some migratory birds are now spreading the H5N1 virus in the highly pathogenic form and will continue to spread it.
