In March 2024, highly pathogenic avian influenza virus (HPAIV) clade 2.3.4.4b H5N1 infections in dairy cows were first reported from Texas, USA¹. Rapid dissemination to more than 190 farms in 13 states followed². Here, we provide results of two independent clade 2.3.4.4b experimental infection studies evaluating (i) oronasal susceptibility and transmission in calves to a US H5N1 bovine isolate genotype B3.13 (H5N1 B3.13) and (ii) susceptibility of lactating cows following direct mammary gland inoculation of either H5N1 B3.13 or a current EU H5N1 wild bird isolate genotype euDG (H5N1 euDG). Inoculation of the calves resulted in moderate nasal replication and shedding with no severe clinical signs or transmission to sentinel calves. In dairy cows, infection resulted in no nasal shedding, but severe acute mammary gland infection with necrotizing mastitis and high fever was observed for both H5N1 isolates. Milk production was rapidly and drastically reduced and the physical condition of the cows was severely compromised. Virus titers in milk rapidly peaked at 10⁸ TCID₅₀/mL, but systemic infection did not ensue. Notably, adaptive mutation PB2 E627K emerged after intramammary replication of H5N1 euDG. Our data suggest that in addition to H5N1 B3.13, other HPAIV H5N1 strains have the potential to replicate in the udder of cows and that milk and milking procedures, rather than respiratory spread, are likely the primary routes of H5N1 transmission between cattle.

2024 年 3 月，美国德克萨斯州首次报道了奶牛感染高致病性禽流感病毒 (HPAIV) 2.3.4.4b H5N1 ¹ 。随后迅速传播到 13 个州的 190 多个农场² 。在这里，我们提供了两项独立的进化枝 2.3.4.4b 实验感染研究的结果，评估 (i) 犊牛对美国 H5N1 牛分离株基因型 B3.13 (H5N1 B3.13) 的口鼻易感性和传播，以及 (ii) 泌乳牛的易感性直接乳腺接种 H5N1 B3.13 或当前欧盟 H5N1 野鸟分离基因型 euDG (H5N1 euDG) 后。小牛的接种导致适度的鼻腔复制和脱落，没有严重的临床症状或传播给前哨小牛。在奶牛中，感染不会导致鼻腔脱落，但两种 H5N1 分离株均观察到严重的急性乳腺感染，伴有坏死性乳腺炎和高烧。牛奶产量迅速大幅下降，奶牛的身体状况受到严重损害。牛奶中的病毒滴度迅速达到峰值10 ⁸ TCID ₅₀ /mL，但并未发生全身感染。值得注意的是，适应性突变 PB2 E627K 在 H5N1 euDG 的乳房内复制后出现。我们的数据表明，除了 H5N1 B3.13 之外，其他 HPAIV H5N1 菌株也有可能在奶牛的乳房中复制，并且牛奶和挤奶过程（而不是呼吸道传播）可能是 H5N1 在牛之间传播的主要途径。