The Influenza A virus (IAV) belongs to the family Orthomyxoviridae and the genus Influenzavirus. Its genome consists of eight single-stranded, negative-sense RNA segments. IAV exhibits high antigenic variability, particularly in the hemagglutinin (HA) and neuraminidase (NA) glycoproteins found on its viral envelope. Based on the antigenic differences of these two glycoproteins, IAV is currently classified into 18 HA subtypes (H1–H18) and 11 NA subtypes (N1–N11)[1]. Human infections are primarily caused by H1 and H3 strains (i.e., H1N1 and H3N2). Except for the H17N10 and H18N11 subtypes, which have only been detected in bats, the remaining 16 HA subtypes and 9 NA subtypes can be isolated from wild birds, which are widely regarded as the natural reservoir of IAVs[2, 3].
IAV is characterized by a short incubation period, rapid onset, seasonal prevalence, and a tendency to cause sporadic pandemics. It exhibits relatively strict host specificity[4], with avian influenza viruses rarely infecting mammals and humans. However, under specific conditions, mutations leading to antigenic drift or genetic reassortment causing antigenic shift can enable the virus to cross species barriers, infecting humans and other mammals such as horses, pigs, dogs, cats, seals, and minks, posing significant challenges and threats to public health and societal development.
Since the 20th century, four influenza pandemics have occurred[5]: the 1918 "Spanish Flu" (H1N1), the 1957 "Asian Flu" (H2N2), the 1968 "Hong Kong Flu" (H3N2), and the 2009 H1N1 pandemic. In recent years, avian influenza viruses such as H5N1/N6, H7N9, H6N1, H9N2, and H3N8 have demonstrated the ability to breach host barriers and cause sporadic human infections[6], further complicating global influenza prevention and control efforts.
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[1] ZHANG N, WANG L, DENG X, et al. Recent advances in the detection of respiratory virus infection in humans [J]. Journal of medical virology, 2020, 92(4): 408-17.
[2] TONG S, LI Y, RIVAILLER P, et al. A distinct lineage of influenza A virus from bats [J]. Proc Natl Acad Sci U S A, 2012, 109(11): 4269-74.
[3] TONG S, ZHU X, LI Y, et al. New world bats harbor diverse influenza A viruses [J]. PLoS Pathog, 2013, 9(10): e1003657.
[4] PINTO R M, BAKSHI S, LYTRAS S, et al. BTN3A3 evasion promotes the zoonotic potential of influenza A viruses [J]. Nature, 2023, 619(7969): 338-47.
[5] PAULES C, SUBBARAO K. Influenza [J]. Lancet, 2017, 390(10095): 697-708.
[6] 邹淑梅,唐静,王大燕.甲型流感病毒血凝素蛋白受体研究进展 [J]. 中国病毒病杂志, 2024, 14(05): 488-494.