The emergence and spread of highly pathogenic avian influenza A (H5N1) in dairy cattle have been identified by scientists at the United States Department of Agriculture’s National Animal Disease Center, in collaboration with multiple academic, state, and federal partners. This discovery sheds light on the transmission dynamics and potential risks associated with this virus.
Genetic analysis conducted by the researchers confirms that a reassortment event in wild birds preceded a single transmission to cattle. Subsequently, asymptomatic or presymptomatic cattle played a role in spreading the virus across multiple states in the United States. Viral genome sequencing revealed low-frequency mutations that are linked to transmission efficiency and potential adaptation to mammalian hosts, raising concerns about future zoonotic spillover events.
Highly pathogenic avian influenza viruses, such as H5N1, pose a significant threat to animal health, the agricultural economy, and potentially human health due to their pandemic potential. The spread of viruses related to the goose/Guangdong 2.3.4.4 hemagglutinin H5NX clade to nearly 100 countries has created a global concern.
Following a trans-Atlantic introduction in late 2021, clade 2.3.4.4b H5N1 caused widespread outbreaks across North America, affecting wild birds, poultry, and mammals. Cross-species transmission events raised urgent concerns about the virus’s ability to adapt to different hosts. Continuous monitoring of the circulation of HPAI clade 2.3.4.4b in mammals is crucial to understand the risks of human infection and transmission.
The study, published in Science, titled “Emergence and interstate spread of highly pathogenic avian influenza A(H5N1) in dairy cattle in the United States,” involved genomic and epidemiological investigations to determine the source, spread, and implications of the outbreak among cattle. Samples were collected from multiple dairy and poultry farms, and whole-genome sequencing was performed to trace the source and spread of the infection.
Phylogenetic analysis revealed that the virus sequences isolated from cattle clustered within a single group, indicating a single spillover event from wild birds to cattle. The virus then spread to other animals, including poultry, raccoons, cats, and various bird species. The study also reported a case of a dairy worker infected with a slightly distinct genotype, likely originating from cattle exposure.
The findings suggest that continued transmission of H5N1 HPAI within dairy cattle populations increases the risk of zoonotic transmission to humans and other animals. Monitoring of cattle and other agricultural animals is essential for early detection and risk assessment. The study highlights the importance of implementing a National Milk Testing Strategy and testing cattle before interstate movement to prevent further spread of the virus.
In conclusion, the study underscores the need for ongoing surveillance and research to understand the evolutionary potential of the virus in different hosts and to mitigate the risks associated with the spread of highly pathogenic avian influenza viruses. Vigilance and proactive measures are essential to protect animal and human health from the threats posed by these viruses.
