Zoonotic Potential of Influenza A Viruses: A Comprehensive Overview

Abstract

Influenza A viruses (IAVs) possess a great zoonotic potential as they are able to infect different avian and mammalian animal hosts, from which they can be transmitted to humans. This is based on the ability of IAV to gradually change their genome by mutation or even reassemble their genome segments during co-infection of the host cell with different IAV strains, resulting in a high genetic diversity. Variants of circulating or newly emerging IAVs continue to trigger global health threats annually for both humans and animals. Here, we provide an introduction on IAVs, highlighting the mechanisms of viral evolution, the host spectrum, and the animal/human interface. Pathogenicity determinants of IAVs in mammals, with special emphasis on newly emerging IAVs with pandemic potential, are discussed. Finally, an overview is provided on various approaches for the prevention of human IAV infections.

Keywords: Influenza A virus; evolution; pandemic; pathogenicity; zoonosis.

Figure 1

Schematic structure of influenza A virus (IAV). The envelope of the IAV particle, which is derived from the host cell plasma membrane, contains three trans-membrane proteins; two surface glycoproteins designated as hemagglutinin (HA) and neuraminidase (NA) and the proton channel matrix protein 2 (M2). The matrix protein 1 (M1) underlies the inner surface of the viral envelope and associates with NEP and viral ribonucleoprotein complexes (vRNPs). The eight vRNPs comprise eight negative-strand RNA segments associated with the nucleoprotein (NP) and three RdRp polymerase subunits (PA, PB1, PB2).

Figure 2

Overview of minigenome assay for IAV. The DNA of four expression plasmids, encoding the PB1, PB2, PA, and NP of IAV under control of polymerase II promoter (Pol-II), are co-transfected into host cell with reporter plasmid, which is carrying the ORF of the reporter gene (e.g., Luciferase, CAT, GFP) in a negative polarity and flanked by the NCRs of an IAV segment. Transcription is controlled by a Pol-I promoter (Pol-I) and a Pol-I terminator (T) to express a specific vRNA-like Pol-I-transcript of the reporter gene. The expression of the viral proteins together with the vRNA-like reporter gene transcript results in the in vitro reconstitution of RNP complexes. The RNP complexes generate the corresponding mRNA, which is then translated into the reporter protein of unique enzymatic, fluorescent or chemiluminescence activity. The represented regions in plasmid constructs are not drawn to scale.

Figure 3

Evolution mechanisms of IAV. (A) Antigenic Drift: Gradual accumulation of mutations in the genome of IAVs leads to emergence of new virus variants. Mutations in the HA (blue) and NA (red) can affect the antigenic epitopes leading to antigenically new variants. (B) Antigenic Shift: The exchange/reassortment of genetic segments between two or more invading IAVs in a host cell can lead to emergence of (antigenically) distinct new subtype(s).

Figure 4

Timeline showing influenza pandemics and epidemics caused by IAVs. The “Spanish Flu” of 1918 was the most devastating influenza pandemic in the 20th century and was likely caused by a zoonotic transmission of an H1N1-type IAV from poultry to humans. This strain disappeared in 1957 when the influenza virus A/H2N2, a reassortant of the H1N1 virus and other avian IAVs, appeared and led to the second influenza pandemic—the “Asian Flu.” In 1968, H3N2, a novel reassortant strain between the H2N2-type and an H3-type virus, displaced the H2N2 strain in the human population and led to the “Hong Kong Flu”—the third influenza pandemic. In 1977 the H1N1 strain reemerged, resulting in the “Russian Flu”. In 2009, a new H1N1 reassortant was transmitted from swine to humans leading to the first pandemic of the 21st century—the “Swine Flu.” In parallel, different avian influenza A virus strains (H5-, H6-, H7-, H9-, and H10-types) have occasionally crossed the host barriers causing mild-to-fatal infections in humans.

Figure 5

Documented human cases and fatalities caused by zoonotic AIVs. Zoonotic events by H5Nx, H6N1, H7Nx, H9N2, and H10Nx viruses were reported in the indicated countries. In brackets the number of confirmed cases against the number of fatalities until July 2018 countries are indicated [44,91,92,93]. Compared to other human isolates of AIVs (black), H5N1 (red), and H7N9 (blue) demonstrated increased zoonotic potential.

Figure 6

H9N2-type influenza A viruses donate their internal genes to other IAVs. Recent studies revealed the contribution of the internal genes of H9N2 in the genesis of various, recently evolved AIV strains with zoonotic potential [135,138,139]. Poultry (chicken pictogram) served as mixing hosts for emergence of these influenza reassortants, which are then transmitted naturally to humans (human pictogram), or evaluated experimentally in ferrets (ferret pictogram), leading to infections/deaths.

Figure 7

The targets of anti-influenza agents that are currently licensed or under clinically investigation. Before attachment of the influenza virus particle (IVP) to the host cell, specific neutralizing monoclonal antibodies (mAbs) against conserved domains in HA can prevent viral infection. Enzymatic destruction of the receptor determinant can further prevent IVP-binding to the target cells. After binding of the IVP to host cell sialic-acid receptors the viral life-cycle is continued by receptor-mediated endocytosis, HA-mediated fusion of the viral membrane with vesicular membrane, vRNP uncoating and release into the cytosol. The viral genome is then replicated/transcribed in the nucleus. After the viral mRNA has been translated into proteins some undergo post-translational processing in the cytosol or support genome replication in the nucleus. Newly formed vRNPs are exported from the nucleus and finally progeny virions are assembled and released by budding from the infected cell to infect new cells. These different processes are potential targets for the currently licensed antiviral drugs and others, which are in clinical trials including CR6261, CR8020, MEDI8852, MHAA4549A, VIS-410 (neutralizing Abs); DAS181 (sialidase); Umifenovir (fusion inhibitor); adamantanes (M2 channel blockers); Favipiravir and Ribavirin (RdRp inhibitors); VX-787 (PB2 cap-binding inhibitor); S-033188 (PA endonuclease inhibitor); AVI-7100 (inhibits M1/M2 mRNA-splicing); Nitazoxanide (HA maturation inhibitor); and Oseltamivir, Peramivir, Zanamivir, and Laninamivir (Neuraminidase inhibitors). In addition to its NF-κB inhibition effect, LASAG antagonizes the nuclear export of viral genomes and thereby blocks the assembly and release of mature influenza virus.