Key amino acid position 272 in neuraminidase determines the replication and virulence of H5N6 avian influenza virus in mammals

Clade 2.3.4.4b highly pathogenic H5N1 influenza viruses from birds in China replicate effectively in bovine cells and pose potential public health risk

In February 2024, H5N1 highly pathogenic avian influenza viruses (HPAIVs) of clade 2.3.4.4b were first reported in dairy cows in the USA. Subsequent multiple outbreaks on dairy farms and sporadic human infections have raised substantial public health concerns. In the same year, four H5N1 HPAIVs of clade 2.3.4.4b were isolated from ducks and geese in live poultry markets (LPMs) spanning seven provinces in China. Evolutionary analysis demonstrated that these viruses had undergone two genetic reassortments with H5 influenza viruses from wild birds in different countries. Except for 565/H5N1, the other three viruses exhibited over 99% genetic homology with avian-origin H5N1 HPAIVs from South Korea and Japan. Notably, 571/H5N1 demonstrated high replication efficiency in bovine-derived cells, particularly in bovine mammary epithelial (MAC-T) cells, and caused 16.7% (1/6) mortality in mice at a dose of 105 EID50/50 μL, indicating its zoonotic potential. Given the potential cross-species transmission risk of H5N1 HPAIVs to cattle herds, we collected 228 serum samples from 12 cattle farms across five provinces and conducted serological testing to investigate seroprevalence of H5N1 HPAIVs in Chinese cattle herds. All tested samples were negative, indicating no widespread infection in the sampled cattle populations. However, infections in cattle from other regions cannot be ruled out. Nevertheless, due to the high mutability of H5N1 HPAIVs, enhanced surveillance of avian influenza viruses is critical to ensure timely responses to potential outbreaks.

Molecular and ecological determinants of mammalian adaptability in avian influenza virus

The avian influenza virus (AIV) primarily affects birds and poses an increasing concern due to its growing adaptability to other hosts, heightening zoonotic risks. The adaptability is a key factor in AIV to infect multiple non-avian species, including humans, companion animals, aquatic mammals, carnivores, and other mammals. The virus is evolving through genetic mutations and reassortments, leading to the emergence of AIV strains with enhanced virulence and adaptability in mammals. This highlights the critical need to understand the genetic factors of AIV, including mutations in polymerase proteins, surface antigens, and other regulatory proteins, as well as the dynamics of AIV-host interactions and environmental factors such as temperature, humidity, water salinity, and pH that govern the cross-species adaptability of the virus. This review provides comprehensive insights into the molecular/genetic changes AIV undergoes to adapt in mammalian hosts including bovines, swine, equines, canines, and felines. The adaptive mutations in viral polymerase proteins, such as PB2-E627K, and receptor specificity shift facilitate the virus adaptability in mammals. Since AIVs interact with specific receptors on host cells, therefore the type and distribution of receptors are crucial in determining the host range of the virus and its adaptability by facilitating attachment and entry of the virus. This review examines sialic acid receptor distribution and binding patterns in various mammalian hosts, emphasizing how the presence and structure of specific receptors influence viral interaction, adaptation, and transmission. The review concludes that the differential distribution and expression of SA receptors are vital in the mammalian adaptability and tissue tropism of viral strains. Notably, during the adaptation to mammals, AIVs show a shift in preference from α-2,3 to α-2,6 receptors. This review further emphasizes the role of ecological determinants in the adaptation of viruses to mammalian hosts. Low temperatures, high humidity, and neutral to slightly acidic pH levels enhance virus stability, facilitating its persistence in the environment and spread among susceptible hosts. Overall, AIV remains a global health threat, necessitating coordinated efforts in research, surveillance, and public health strategies.

Continued evolution of H10N3 influenza virus with adaptive mutations poses an increased threat to mammals

The H10 subtype avian influenza virus (AIV) poses an ongoing threat to both birds and humans. Notably, fatal human cases of H10N3 and H10N8 infections have drawn public attention. In 2022, we isolated two H10N3 viruses (A/chicken/Shandong/0101/2022 and A/chicken/Shandong/0603/2022) from diseased chickens in China. Genome analysis revealed that these viruses were genetically associated with human-origin H10N3 virus, with internal genes originating from local H9N2 viruses. Compared to the H10N8 virus (A/chicken/Jiangxi/102/2013), the H10N3 viruses exhibited enhanced thermostability, increased viral release from erythrocytes, and accumulation of hemagglutinin (HA) protein. Additionally, we evaluated the pathogenicity of both H10N3 and H10N8 viruses in mice. We found that viral titers could be detected in the lungs and nasal turbinates of mice infected with the two H10N3 viruses, whereas H10N8 virus titers were detectable in the lungs and brains of mice. Notably, the proportion of double HA Q222R and G228S mutations in H10N3 viruses has increased since 2019. However, the functional roles of the Q222R and G228S double mutations in the HA gene of H10N3 viruses remain unknown and warrant further investigation. Our study highlights the potential public health risk posed by the H10N3 virus. A spillover event of AIV to humans could be a foretaste of a looming pandemic. Therefore, it is imperative to continuously monitor the evolution of the H10N3 influenza virus to ensure targeted prevention and control measures against influenza outbreaks.

Mutational antigenic landscape of prevailing H9N2 influenza virus hemagglutinin spectrum

H9N2 influenza viruses are globally endemic in birds, and a sharp increase in human infections with H9N2 occurred during 2021 to 2022. In this study, we assess the antigenic and pathogenic impact of 23 hemagglutinin (HA) amino acid mutations. Our study reveals that three specific mutations, labeled R164Q, N166D, and I220T, are responsible for the binding of antibodies with escape mutations. Variants containing R164Q and I220T mutations increase viral replication in avian and mammalian cells. Furthermore, T150A and I220T mutations are found to enhance viral replication in mice, indicating that these mutations may have the potential to adapt mammals. Structure analysis reveals that residues 164 and 220 bearing R164Q and I220T mutations increase interactions with the surrounding residues. Our findings enrich current knowledge about the risk assessment regarding which predominant HA immune-escape mutations of H9N2 viruses may pose the greatest threat to the emergence of pandemics in birds and humans.