The rapid expansion of genotype D1.1 A(H5N1) influenza viruses in wild birds across North America during the 2024 migratory season.

In late 2021, clade 2.3.4.4b high pathogenicity avian influenza (HPAI) A(H5N1) viruses were introduced into North America, where they rapidly reassorted with local avian influenza viruses. In September 2024, we detected a novel reassortant virus that was subsequently ascribed to the D1.1 genotype. Leveraging an extensive network of active and passive avian wildlife influenza surveillance approaches within Canada and the U.S., we documented the emergence and rapid spread of D1.1 viruses in wild birds during the 2024 fall migratory season. The D1.1 viruses formed a monophyletic group and rapidly replaced earlier A(H5) genotypes across multiple flyways, as shown by phylodynamic analysis. The rapid dissemination of D1.1 in wild birds coincided with detections in other hosts including 17 human cases, four of which were severe or fatal, and at least two A(H5N1) introductions into dairy cattle. Importantly, none of the mammalian adaptive markers, such as PB2 E627K or HA Q226H, detected in human cases were detected in viruses from wild birds, and candidate vaccine viruses retained antigenic cross-reactivity with D1.1 strains isolated in this study.