Emergence and spread of highly pathogenic avian influenza A(H5N1) in dairy cattle have raised significant concerns among researchers and health officials. A recent study conducted by scientists at the United States Department of Agriculture’s National Animal Disease Center, along with various academic, state, and federal collaborators, has shed light on the transmission dynamics of this virus in cattle populations.
The study, published in Science, revealed that a reassortment event in wild birds preceded a single transmission to cattle, leading to asymptomatic or presymptomatic spread of the virus across the United States. Genetic analysis of viral samples collected from dairy farms in multiple states confirmed the presence of H5N1 clade 2.3.4.4b genotype B3.13 influenza A virus in milk samples, with limited detection in nasal swabs.
Phylogenetic analysis indicated that the virus sequences isolated from cattle clustered within a single group, suggesting a single wild bird-to-cattle spillover event in late 2023. The virus persisted in cattle, with evidence of transmission to other animal species, including poultry, raccoons, cats, and various wild bird species. Transmission chains were reconstructed using genomic analysis software, highlighting the spread of the virus through asymptomatic or presymptomatic cattle movement across multiple states.
The study also identified low-frequency genetic variants associated with virulence, mammalian adaptation, and expanded host range in cattle isolates. These variants, while not dominant, raise concerns about the virus’s potential to become more transmissible or pathogenic in mammals. Monitoring of cattle populations is crucial to assess the risk of zoonotic spillover and prevent potential transmission to humans.
The findings underscore the need for enhanced surveillance and monitoring of dairy cattle to mitigate the risk of virus spread and potential pandemic threats. A National Milk Testing Strategy and testing of cattle before interstate movement are recommended to prevent further dissemination of the virus. Continued research and vigilance are essential to safeguard animal and public health in the face of emerging infectious diseases like highly pathogenic avian influenza A(H5N1).
