Highly pathogenic avian influenza (HPAI) H5N1 clade 2.3.4.4b virus has caused the death of millions of domestic birds and thousands of wild birds in the U.S. since January, 2022^1–4 Throughout this outbreak, spillovers to mammals have been frequently documented^5–12. We report spillover of HPAI H5N1 virus in dairy cattle herds across several states in the U.S. The affected cows displayed clinical signs encompassing decreased feed intake, altered fecal consistency, respiratory distress, and decreased milk production with abnormal milk. Infectious virus and viral RNA were consistently detected in milk from affected cows. Viral distribution in tissues via immunohistochemistry and in situ hybridization revealed a distinct tropism of the virus for the epithelial cells lining the alveoli of the mammary gland in cows. Whole viral genome sequences recovered from dairy cows, birds, domestic cats, and a raccoon from affected farms indicated multidirectional interspecies transmissions. Epidemiologic and genomic data revealed efficient cow-to-cow transmission after apparently healthy cows from an affected farm were transported to a premise in a different state. These results demonstrate the transmission of HPAI H5N1 clade 2.3.4.4b virus at a non-traditional interface underscoring the ability of the virus to cross species barriers.

自 2022 年 1 月以来，高致病性禽流感 (HPAI) H5N1 分支 2.3.4.4b 病毒已导致美国数百万只家禽和数千只野生鸟类死亡 ^1–4 在这次疫情爆发期间，对哺乳动物的溢出效应经常被记录 ^5–12 。我们报告了 HPAI H5N1 病毒在美国几个州的奶牛群中蔓延。受影响的奶牛表现出临床症状，包括采食量减少、粪便稠度改变、呼吸窘迫以及产奶量下降且产奶量异常。在受影响奶牛的牛奶中始终检测到传染性病毒和病毒 RNA。通过免疫组织化学和原位杂交分析病毒在组织中的分布，揭示了病毒对牛乳腺肺泡内衬上皮细胞的独特趋向性。从受影响农场的奶牛、鸟类、家猫和浣熊身上恢复的整个病毒基因组序列表明存在多向种间传播。流行病学和基因组数据显示，在受影响农场的明显健康的奶牛被运送到不同州的场所后，奶牛之间存在有效的传播。这些结果证明 HPAI H5N1 进化枝 2.3.4.4b 病毒在非传统界面上传播，强调了该病毒跨越物种障碍的能力。