Emergence and interstate spread of highly pathogenic avian influenza A(H5N1) in dairy cattle in the United States

Highly pathogenic avian influenza (HPAI) viruses have critical consequences for animal health and the agricultural economy—and may have pandemic potential. HPAI related to the goose/Guangdong 2.3.4.4 hemagglutinin (HA) H5NX phylogenetic clade has spread to nearly 100 countries, and it is recognized as a panzootic. HPAI virus circulation is ongoing in North America, and after a trans-Atlantic incursion in late 2021, the HPAI H5N1 clade 2.3.4.4b virus caused widespread outbreaks. The outbreaks resulted in extensive mortality events, culling of poultry when detected in agricultural systems, and interspecies transmission events into mammals. It is critical to determine how HPAI clade 2.3.4.4b evolves in wild birds and in nonhuman mammals after spillover to assess the potential for human infection and transmission.