Seasonal infection and antibody response to highly pathogenic H5N1 influenza A virus in ducks in the Mississippi and Central Flyways, United States.

Clade 2.3.4.4b A/goose/Guangdong/1/1996 (Gs/GD) H5N1 highly pathogenic (HP) influenza A virus (IAV) (HP H5N1) infection in Blue-winged Teal in the Mississippi Flyway in the United States during the 2022/2023 and 2023/2024 migratory seasons followed a seasonal pattern similar to low pathogenic IAV, with infection peaks in late summer and fall. To determine if this seasonal pattern persisted during the 2024/2025 migration season, we repeated testing of Blue-winged Teal from September 2024 to April 2025 in the Mississippi Flyway. Results from other dabbling duck species sampled concurrently with Blue-winged Teal during the 2023/2024 and 2024/2025 migratory seasons in the Mississippi and Central Flyways also were compared. For Blue-winged Teal in the Mississippi Flyway, a seasonal peak in infection was detected during late fall (November/December 2024) on wintering grounds in Louisiana and Texas corresponding to low H5 and N1 antibody prevalence in juvenile birds. Estimated antibody prevalence increased rapidly following this detection, and from January to April 2025, HP H5N1 infections were not detected. Virologic and serologic results in Mallard, Northern Pintail, American Wigeon, Northern Shoveler, and Green-winged Teal mirrored virologic and serologic results observed in Blue-winged Teal and were consistent between flyways.