The natural reservoir for nearly all influenza A viruses is wild aquatic birds (ducks, geese). Of the 18 hemagglutinin and 11 neuraminidase subtypes of influenza A viruses identified to date, nearly all (except for H17N10, H18N11 identified in bats) have been identified among birds.[35] Other animal species can also be infected by influenza A viruses, including pigs, marine mammals, horses, dogs, cats, and bats. Highly pathogenic avian influenza (HPAI) A viruses can cause asymptomatic infection to fatal disease in wild birds and domestic poultry. HPAI H5N1 virus was first identified in Scotland in 1959. However, the progenitor HPAI H5N1 virus to all Asian lineage HPAI H5N1 viruses circulating among birds was identified in 1996 from an infected goose in southern China.

Most human HPAI H5N1 cases are sporadic and associated with direct contact (e.g., touching) or very close exposure with sick or dead backyard poultry (usually chickens), and a seasonal variation observed in human cases parallels that of outbreaks in birds.[18][36][37][38] However, other risk factors include visiting a live poultry market[39][40][41] and prolonged, unprotected close contact with a human HPAI H5N1 case.[17] In some cases, a possible exposure risk was not identified, suggesting possible environmental exposure or close contact with an unknown infected person.[42] Clustering of HPAI H5N1 cases among blood-related family members suggests the potential for increased genetic susceptibility. However, human-to-human transmission remains rare.[43][44][45][46][47] Rare nosocomial transmission has also been documented.[1][2][48] There is no evidence of ongoing human-to-human transmission of HPAI H5N1 virus.

Experimental studies in ferrets have demonstrated that HPAI H5N1 virus can acquire traits that improve transmissibility via respiratory droplets, and thus increase the risk of human-to-human transmission.[49] Of the several amino acid substitutions associated with increased respiratory transmission in this mammalian model, some are already found in HPAI H5N1 viruses currently circulating among poultry.[50] The odds of spontaneous mutations resulting in improved transmissibility are very low.[51] A change in the current epidemiology of HPAI H5N1 human cases, including epidemiologically related clusters or unrelated cases, could suggest increased transmissibility from viral mutations and increased pandemic potential.[52] However, investigations of a large increase in human HPAI H5N1 cases in Egypt during 2014-2015 attributed increased diagnostic testing of exposed persons (and not viral mutations) as the likely cause of increased case detection.[53][54]

Influenza A viruses are subject to genetic reassortment. Previous pandemic viruses are believed to have emerged in human populations through mutation from a zoonotic reservoir (1918 H1N1), genetic reassortment between low pathogenic avian influenza and seasonal influenza A viruses (1957 H2N2, 1968 H3N2), and genetic reassortment between triple reassortant swine influenza A (H1N1) and other swine influenza A viruses (2009 H1N1).[55][56]


Highly pathogenic avian influenza (HPAI) H5N1 virus binds to receptors with sialic acids bound to galactose by alpha-2,3 linkages, which are primarily, but not entirely, distributed in the human lower respiratory tract.[57][58] Such receptors have also been reported in the human gastrointestinal tract.[59] Furthermore, specific structural conformation, not just receptor binding affinity, may be important in binding to receptors in the upper respiratory tract.[60] HPAI H5N1 virus obtained from human clinical samples with the ability to bind upper respiratory tract tissue has also been reported.[57][61] High and prolonged HPAI H5N1 viral replication in the lower respiratory tract induces proinflammatory cytokines and chemokines,[62][63] resulting in pulmonary capillary leak, diffuse alveolar damage, and acute lung injury, and can lead to development of ARDS. HPAI H5N1 viremia has been reported in fatal cases,[63] and dissemination of HPAI H5N1 virus to infect brain tissue; isolation from cerebrospinal fluid, gastrointestinal infection, and vertical transmission with evidence of virus in placenta and fetal lung cells have been documented.[20][64] Reactive hemophagocytosis has also been reported.[64]

Avian influenza A viruses, including HPAI H5N1 virus, can potentially be transmitted to humans through different modalities.

  • Direct contact (touching) or close exposure to infected sick or dead poultry or poultry products is thought to be the major risk for transmission of avian influenza A viruses to humans.[17]

  • Inhalation of aerosolized material (e.g., poultry feces) containing infectious HPAI H5N1 virus is a likely route of transmission from poultry to humans.

  • Self-inoculation of the mucous membranes after direct contact with material containing HPAI H5N1 virus (touching or cleaning infected birds) or indirect (fomite) contact transmission from surfaces contaminated with poultry feces or products containing HPAI H5N1 virus to mucous membranes has also been hypothesized.

  • Consumption of uncooked poultry products, including blood from infected birds, has been identified as a potential risk factor in field investigations, but whether transmission can occur by primary HPAI H5N1 virus infection of the human gastrointestinal tract is unknown.



Avian influenza A virus strains are classified as low pathogenic avian influenza (LPAI) or highly pathogenic avian influenza (HPAI) on the basis of molecular and pathogenicity criteria.

  • Most strains are LPAI viruses and cause asymptomatic infection or mild disease in poultry. LPAI H6N1, H7N2, H7N3, H7N7, H7N9, H9N2, H10N7, and H10N8 virus strains have infected humans causing disease ranging from conjunctivitis to nonfatal upper respiratory and lower respiratory tract disease, to severe lower respiratory tract disease and death (H7N9, H10N8).[3][4][5][6][7]

  • HPAI strains identified to date are of the H5 and H7 subtypes and can cause severe illness in poultry. HPAI virus infections in humans have ranged from asymptomatic to severe or fatal disease. Rare, sporadic human cases of HPAI virus infection have been detected with H5N1, H5N6, H7N3, and H7N7 viruses and have caused a wide spectrum of illness from conjunctivitis (H7N3, H7N7) to severe pneumonia, ARDS, and fatal outcomes (H7N7, H5N1, H5N6).[8][9][10][11] Asian lineage HPAI H7N9 viruses were detected and reported in the People‚Äôs Republic of China for the first time in February 2017.

Antigenic structure (clades)

In 2014, the World Health Organization/World Organisation for Animal Health/Food and Agriculture Organization H5N1 Evolution Working Group published a revision to HPAI H5N1 nomenclature.[12] According to this revised nomenclature system, circulating HPAI H5N1 virus strains among birds are classified into numerous clades, and subdivided into subclades and lineages.[13][14] Circulating HPAI H5N1 virus strains now include clades and subclades 1.1, 2.1, 2.2, 2.3, and 7.[15] Further subdivisions of antigenically distinct circulating subclades have also been described (e.g., 1.1.1, 1.1.2, 2.1.3, 2a, 2.2.1, 2.3.2, 2.3.4, and 7.2), and they continue to undergo antigenic drift.[12][16] These antigenic changes have important implications for vaccine development. Clades that have infected humans include 0, 1, 2, and 7.[17] HPAI H5N1 virus continues to cause rare, sporadic human infections, including fatal outcomes. Most human HPAI H5N1 virus infections since 2005 have been with clade 2 virus strains. HPAI H5N1 virus strains continue to evolve among infected birds.

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