Clade 2.3.4.4 H5 North American Highly Pathogenic Avian Influenza Viruses Infect, but Do Not Cause Clinical Signs in, American Black Ducks (Anas rubripes).

Highly pathogenic avian influenza virus (HPAIV) from the goose/Guangdong/1996 clade 2.3.4.4 H5 lineage spread from Asia into North America in 2014, most likely by wild bird migrations. Although several variants of the virus were detected, H5N8 and H5N2 were the most widespread in North American wild birds and domestic poultry. In early 2015, the H5N2 virus spread through commercial poultry in the Midwest, and >50 million chickens and turkeys died or had to be culled. Related H5 HPAIVs are still endemic in much of the Eastern Hemisphere. The wild bird species that were involved with dissemination of the virus in North America are not known. Dabbling ducks, especially mallards (Anas platyrhynchos), typically have the highest detection rates for avian influenza viruses. To better characterize the wild avian species that could spread the virus, American black ducks (Anas rubripes), which are closely related to mallards, were challenged with the North American H5N2 and H5N8 index HPAIV isolates: A/Northern Pintail/WA/40964/2014 H5N2 and A/Gyrfalcon/WA/41088/2014 H5N8. Although the American black ducks could be infected with low doses of both isolates (≤102 50% egg infective doses), ducks shed the H5N2 longer than the H5N8 (10 vs. 7 days) and the titers of virus shed were higher. Although there were too few ducks available on which to draw definitive conclusions, this suggests that American black ducks could serve as a more efficient reservoir for the H5N2 virus than the H5N8 virus.