The role of wild birds in the spread of HPAI H5N1

There is much debate about the relative roles of poultry movements and wild bird movements in the spread of highly pathogenic avian influenza H5N1. This article looks at the problem from an ornithologic perspective. Outbreaks in wild birds are examined in relation to three scenarios of possible wild bird involvement in virus transmission. These scenarios are examined separately for five phases of the outbreak that began in 1997 and which has recently become more dynamic in terms of virus spread. Most outbreaks in wild birds seem to reflect local acquisition of infection from a contaminated source, followed by rapid death nearby. Outbreaks in Europe in early 2006 indicate that the virus can be spread further by wild birds and thus that they can become infected and travel varying distances before dying, and probably passing the infection to other wild birds before death. There is only limited evidence that some wild birds can carry the virus asymptomatically, and no evidence from wild bird outbreaks that they have done so over long distances on seasonal migration routes. Other potential sources of infection and evidence for asymptomatic infection in wild birds are discussed, and the need for more ornithologic input into epidemiological studies of HPAI H5N1 is highlighted.