REVIEW Global Patterns of Influenza A Virus in Wild Birds

The outbreak of highly pathogenic avian influenza of the H5N1 subtype in Asia, which has subsequently spread to Russia, the Middle East, Europe, and Africa, has put increased focus on the role of wild birds in the persistence of influenza viruses. The ecology, epidemiology, genetics, and evolution of pathogens cannot be fully understood without taking into account the ecology of their hosts. Here, we review our current knowledge on global patterns of influenza virus infections in wild birds, discuss these patterns in the context of host ecology and in particular birds ’ behavior, and identify some important gaps in our current knowledge. I nfluenza A viruses have been isolated from many species, including humans, pigs, horses, mink, felids, marine mammals, and a wide range of domestic birds, but wildfowl and shorebirds are thought to form the virus res-ervoir in nature. The influenza A virus genome consists of eight segments of negative-stranded RNA, which code for 11 proteins. Influenza viruses are classified on the basis of two of these proteins expressed on the surface of virus particles; the hemagglutinin (HA) and neur-aminidase (NA) glycoproteins (1). In wild birds and poultry throughout the world, influenza viruses representing 16 HA and 9 NA antigen-ic subtypes have been detected (2), which can be found in numerous combinations (also called subtypes, e.g., H1N1, H16N3). The HA protein is initially synthesized as a single polypeptide precursor (HA0), which is cleaved into H