biostudies-literatureUnknown14(1)Xiao NLo Ying Shek Chi Wai FoundationInnovation and Technology Commissionthe Hong Kong Hainan Commercial AssociationThe newly emerged avian influenza A H5N1 Clade 2.3.4.4b can infect dairy cows and shed live virus in their milk. Sporadic cattle-to-human infections have been reported, highlighting the urgent need to understand its pathogenesis in mammals. Using both non-lactating and lactating BALB/c mice, we examined the viral tissue tropism, histopathological damages, and host immune responses upon intranasal inoculation with a reverse-genetic virus constructed based on A/dairy cattle/Texas/24-008749-003/2024 (Cattle-H5N1) and comparing with an older reference Clade 1 virus, A/Vietnam/1194/2004 virus (VNM1194-H5N1). Cattle-H5N1 was highly lethal in mice (mLD₅₀ = 1.48PFU) with broad tissue tropism and produced higher titer in respiratory tissue and multiple extrapulmonary organs than VNM1194-Emerging microbes & infections2475836https://www.ebi.ac.uk/biostudies/studies/S-EPMC11915741biostudies-literatureReverse genetics-derived cattle H5N1 virus from Clade 2.3.4.4b shows enhanced systemic infectivity and pathogenicity than an older Clade 1 H5N1 virus in BALB/c mice.PMC11915741Yuan SLi CCai JChan JFZhang AJChung HCChen HYe ZXiao NWang PLam AHChu HChen YYuen KYOong XYLee ACSong WKok KHfalseReverse genetics-derived cattle H5N1 virus from Clade 2.3.4.4b shows enhanced systemic infectivity and pathogenicity than an older Clade 1 H5N1 virus in BALB/c mice.The newly emerged avian influenza A H5N1 Clade 2.3.4.4b can infect dairy cows and shed live virus in their milk. Sporadic cattle-to-human infections have been reported, highlighting the urgent need to understand its pathogenesis in mammals. Using both non-lactating and lactating BALB/c mice, we examined the viral tissue tropism, histopathological damages, and host immune responses upon intranasal inoculation with a reverse-genetic virus constructed based on A/dairy cattle/Texas/24-008749-003/2024 (Cattle-H5N1) and comparing with an older reference Clade 1 virus, A/Vietnam/1194/2004 virus (VNM1194-H5N1). Cattle-H5N1 was highly lethal in mice (mLD₅₀ = 1.48PFU) with broad tissue tropism and produced higher titer in respiratory tissue and multiple extrapulmonary organs than VNM1194-2025-01-01T00:00:00Z2025 Dec2026-04-08T19:52:35.649Z2026-04-08T14:31:14.947ZS-EPMC119157414003577410.1080/22221751.2025.2475836