The emergence of highly pathogenic avian influenza H5N1 in dairy cattle: implications for public health, animal health, and pandemic preparedness.

Abstract

BACKGROUND: The 2024 outbreak of Highly Pathogenic Avian Influenza (HPAI) H5N1 in U.S. dairy cattle represents a significant change in the behavior of zoonotic influenza viruses. Previously, H5N1 was primarily an avian pathogen with limited infection in mammals. The emergence of clade 2.3.4.4b H5N1 in dairy herds across multiple states reveals the virus's adaptation to mammalian hosts and highlights its potential for zoonotic transmission, raising important concerns for public health, veterinary medicine, and agriculture. RESULTS: The virus demonstrated unique tropism for mammary tissue, with high viral loads detected in milk. Genomic analysis identified mutations that enhance binding to mammalian receptors and facilitate systemic spread. Viral RNA was found in raw milk, posing food safety risks; however, standard pasteurization effectively inactivated the virus. Epidemiological data indicate the outbreak likely began with spillover from wild birds or contaminated fomites, followed by efficient local transmission within herds. Forty-one human cases linked to infected dairy operations were confirmed. The outbreak caused significant economic losses due to decreased milk production and trade restrictions. Although human-to-human transmission remains low, the zoonotic risk requires urgent attention. CONCLUSION: The 2024 HPAI H5N1 outbreak in U.S. dairy cattle highlights critical gaps in surveillance, biosecurity, and coordination across sectors. A One Health approach integrating veterinary, public health, and environmental efforts is essential. Recommendations include improved surveillance, stringent biosecurity measures, occupational safety protocols, focused research on viral evolution, and investment in diagnostics and vaccines. These actions are vital to reduce risks, protect public health, and ensure the sustainability of the dairy industry against future zoonotic influenza threats.