Abstract
Highly pathogenic avian influenza viruses of the H5N1 subtype have recently undergone a major geographic expansion. At the same time, these viruses have substantially broadened their host range. Typically, H5N1 circulates in poultry, waterfowl, and shorebirds, but the breadth of avian species affected has increased and spillover to mammals has become common. The recent changes in H5N1 ecology have brought die-offs in wild birds and marine mammals, a sustained outbreak in US dairy cattle, and recurring spillover into domestic cats. Maintenance of H5N1 in wild birds in North America has furthermore produced a persistent risk to US poultry. Circulation of H5N1 in domestic animals has resulted in human infections, especially among farm workers and veterinarians, events that heighten the potential for a pandemic. Through their central position in One Health, veterinarians have a critical role to play in mitigating the impact of H5N1 on animal health, agriculture, and public health.