Infection and transmission dynamics of bovine and human influenza A H5N1 viruses in mouse and hamster models

Here we investigated the pathogenesis and contact transmission of bovine- and human-derived highly pathogenic avian influenza (HPAI) H5N1 clade 2.3.4.4b genotype B3.13 viruses in mammalian models. Using reverse genetics, we rescued three naturally occurring viruses: rTX2/24 (bovine-derived), rTexas/37 and rMichigan/90 (both human-derived), and compared their infection dynamics, replication and pathogenicity with the wild-type bovine TX2/24 strain in vitro and in vivo. All four viruses demonstrated comparable replication kinetics in four mammalian cell lines. However, the rMichigan/90 strain exhibited significantly smaller plaques in bovine and human cells. In vivo studies showed that mice infected with any of the viruses succumbed to infection within 4-5 days; however, mice infected with the rMichigan/90 virus exhibited slightly lower viral replication and shedding compared to the other strains. Similarly, as in the mouse experiments, in hamsters, all viruses induced body weight loss and oral shedding, with robust virus replication observed in tissues, but the rMichigan/90 virus presented reduced replication and shedding. Contact transmission studies in hamsters revealed limited transmissibility for these viruses, with only one out of four animals inoculated with the rMichigan/90 virus transmitting it to a naive contact. These findings indicate that both bovine- and human-derived H5N1 genotype B3.13 viruses present high pathogenicity in mammals, though the overall transmissibility remains low. Significance StatementInfluenza A H5N1 virus spilled over from wild birds into dairy cattle in the US in 2024. Since then, an increased number of human infections - with at least 71 confirmed cases - were confirmed. In the present study we evaluated and compared the pathogenicity and transmissibility of a bovine and two human H5N1 viruses of the genotype B3.13. Our results show that while all viruses are pathogenic in mouse and hamster models, the bovine isolate TX2/24 presents a broader tissue tropism than the human derived viruses. Contact transmission studies revealed a limited ability of these viruses to transmit in the hamster contact transmission model. These findings highlight the high pathogenicity of H5N1 viruses in mammals and demonstrate that that their transmissibility potential remains low.