Influenza A Virus: Cause of Multispecies Disease and Zoonoses

Abstract

Influenza A viruses (IAV) in the family Orthomyxoviridae, including all avian influenza viruses (AIVs), are enveloped, pleomorphic, and possess eight separate RNA genomic segments ranging in size between 890 and 2341 nucleotides. As observed by, the persistent and sporadic outbreaks of various Influenza A viruses in poultry and humans, respectively, warns the likelihood of avian influenza viruses (AIVs) becoming the next influenza pandemic strain. Further, among the vast pool of AIVs in nature, the HPAI A/H5N1 virus is believed to represent the greatest threat for the next flu pandemic. Therefore, the pandemic potential of subtypes of AIVs should not be overlooked and the domestic and aquatic wild bird populations should be under surveillance to monitor interspecies transmission. Such monitoring would help in understanding the ecology of human influenza and controlling avian zoonoses. The HA and NA glycoproteins on the virus surface encoded by separate RNA segments are antigenically diverse, and divide the IAVs into 18 H and 11 N antigenic subtypes, respectively. Aquatic birds like wild water fowl and ducks are natural host for AIV subtypes of H-1 to H-16 and N-1 to N-9. Two new subtypes each of HA and NA (H17N10, H18N11) have been recently identified in bats. Isolation of new AIV subtypes from bats has added another angle, in addition to the role of wild aquatic birds, to the ecology and emergence of influenza/ flu epidemics/pandemics that can affect both terrestrial birds and human beings depending upon availability of receptors on host cells. Bats are likely ancient reservoir for a diverse pool of influenza virus. Influenza A viruses naturally circulate in a range of avian and mammalian species, including in humans. The Influenza A serotypes that have been confirmed in humans are, H1N1, H1N2 (endemic in humans, pigs and birds), H2N2, H3N2, H5N1, H6N1, H7N2, H7N3, H7N7, H7N9, H9N2, and H10N7. Although transmission of AIVs between pigs and humans have already been confirmed, direct transmission from avian to human beings and between human to human is seldom. Segmented nature of the viral RNA genome combined with its error-prone polymerase enzymes can produce novel virus strain(s) with expansion of host range, inter species transmission, higher virulence, multi organ involvement with potential to cause influenza pandemics. Introduction of influenza A viruses into poultry can cause severe illness often leading to high mortality. According to degree of pathogenicity, the avian influenza viruses (AIVs) are divided into two pathotypes; high pathogenic avian influenza (HPAI) and low pathogenic avian influenza (LPAI) virus. Some HPAI strains of the H5 (H5N1) and H7 (H7N1, H7N3, and H7N7) subtypes are highly lethal in chickens with involvement of several organs other than the respiratory system. In contrast, LPAI strains mainly affect intestinal and/or respiratory tracts. Several AIV subtypes have caused zoonotic infections in humans (LPAIs H6N1, H7N2, H7N3, H7N4, H7N7, H7N9, H9N2, H10N7, H10N8, and HPAIs H5N1, H5N6, H7N3, H7N7, H7N9). Transmission of HPAI H5 and H7 viruses into humans in the recent past has been of significance from the point of zoonoses. First AIV zoonoses was caused by H5N1 virus in 1997 in Hong Kong, then spread to many parts of the World. Enzootic cocirculation of H5N1, H9N2, and H7N9 viruses in poultry birds has given rise to many novel reassortants with other viruses such as H10N8, H10N6, H5N8, H5N6, and H7N6. The present compilation is about interspecies transmission of Influenza A viruses.