Influenza a2 virus

Sequencing has confirmed that these viruses share a common genetic ancestry; however, they have genetically diverged, such that reassortment — the exchange of viral RNA segments between viruses — has been reported to occur within each genus, or type , but not across types. Influenza A viruses are further characterized by the subtype of their surface glycoproteins, the hemagglutinin HA and the neuraminidase NA. Influenza viruses have a standard nomenclature that includes virus type; species from which it was isolated if non-human ; location at which it was isolated; isolate number; isolate year; and, for influenza A viruses only, HA and NA subtype.

While many genetically distinct subtypes — 16 for HA and 9 for NA — have been found in circulating influenza A viruses, only three HA H1, H2, and H3 and two NA N1 and N2 subtypes have caused human epidemics, as defined by sustained, widespread, person-to-person transmission [ 1 ]. By electron microscopy, influenza A and B viruses are virtually indistinguishable.

They are spherical or filamentous in shape, with the spherical forms on the order of nm in diameter and the filamentous forms often in excess of nm in length. The influenza A virion is studded with glycoprotein spikes of HA and NA, in a ratio of approximately four to one, projecting from a host cell—derived lipid membrane [ 1 ]. A smaller number of matrix M2 ion channels traverse the lipid envelope, with an M2:HA ratio on the order of one M2 channel per 10 1 2 HA molecules [ 2 ].

The envelope and its three integral membrane proteins HA, NA, and M2 overlay a matrix of M1 protein, which encloses the virion core. However, influenza C virions are compositionally similar, with a glycoprotein-studded lipid envelope overlying a protein matrix and the RNP complex.

The influenza C viruses have only one major surface glycoprotein, the hemagglutinin-esterase-fusion HEF protein, which corresponds functionally to the HA and NA of influenza A and B viruses, and one minor envelope protein, CM2 [ 1 ]. See Table 1. The eight segments of influenza A and B viruses and the seven segments of influenza C virus are numbered in order of decreasing length. Segment 7 of both influenza A and B viruses code for the M1 matrix protein.

The genomic organization of influenza C viruses is generally similar to that of influenza A and B viruses; however, the HEF protein of influenza C replaces the HA and NA proteins, and thus the influenza C virus genome has one fewer segment than that of influenza A or B viruses.

Kindly provided by Megan L. The ends of each vRNA segment form a helical hairpin, which is bound by the heterotrimeric RNA polymerase complex; the remainder of the segment is coated with arginine-rich NP, the net positive charge of which binds the negatively charged phosphate backbone of the vRNA [ 13 — 15 ].

However, the extreme ends of all segments are highly conserved among all influenza virus segments; these partially complementary termini base-pair to function as the promoter for vRNA replication and transcription by the viral polymerase complex. The noncoding regions also include the mRNA polyadenylation signal and part of the packaging signals for virus assembly.

A segmented genome enables antigenic shift , in which an influenza A virus strain acquires the HA segment, and possibly the NA segment as well, from an influenza virus of a different subtype. This segment reassortment can happen in cells infected with different human and animal viruses, and the resulting virus may encode completely novel antigenic proteins to which the human population has no preexisting immunity.

Characterization of the reconstructed influenza virus indicated that its unique constellation of genes was responsible for its extreme virulence, with the HA, NA, and PB1 genes all contributing to its high pathogenicity [ 16 , 17 ].

Influenza viruses recognize N-acetylneuraminic sialic acid on the host cell surface. Sialic acids are nine-carbon acidic monosaccharides commonly found at the termini of many glycoconjugates. Thus, they are ubiquitous on many cell types and in many animal species.

The differential expression of sialic acids in the mammalian respiratory tract may help to explain the low infectivity but high pathogenicity of some avian strains. The lungs are not as accessible to airborne virus particles as is the upper respiratory tract nasopharynx, paranasal sinuses, trachea, and bronchi , so avian virus infection is relatively rare in humans.

The crystal structure of the HA molecule is a trimer with two structurally distinct regions: a stem, comprising a triple-stranded coiled-coil of alpha-helices, and a globular head of antiparallel beta-sheet, positioned atop the stem [ 25 ]. The head contains the sialic acid receptor binding site, which is surrounded by the predicted variable antigenic determinants, designated A, B, C, and D in the H3 subtype [ 26 ] and Sa, Sb, Ca1, Ca2, and Cb in the H1 subtype see Figure 1 [ 1 ].

During virus replication, the HA protein is cleaved by serine proteases into HA1 and HA2; this post-translational modification is necessary for virus infectivity. The HA2 portion is thought to mediate the fusion of virus envelope with cell membranes, while the HA1 portion contains the receptor binding and antigenic sites reviewed in [ 27 ]. Antibodies to HA neutralize virus infectivity, so virus strains evolve frequent amino acid changes at the antigenic sites; however, the stem-head configuration of the HA molecule remains conserved among strains and subtypes.

These relatively minor changes accumulate in a process called antigenic drift. Eventually, mutations in multiple antigenic sites result in a virus strain that is no longer effectively neutralized by host antibodies to the parental virus, and the host becomes susceptible again to productive infection by the drifted strain. The head contains the sialic acid receptor-binding site, which is surrounded by the five predicted antigenic sites Sa, Sb, Ca1, Ca2, and Cb. The stem comprises helices A and B and the fusion peptide, as shown.

Adapted from a figure, kindly provided by James Stevens and Ian Wilson, in [ 1 ]. Following attachment of the influenza virus HA protein or the HEF protein of influenza C virus to sialic acid, the virus is endocytosed.

The name starts with the virus type, followed by the place the virus was isolated, followed by the virus strain number often a sample identifier , the year isolated, and finally, the virus subtype. CDC follows an internationally accepted naming convention for influenza viruses.

This convention was accepted by WHO in and published in February in the Bulletin of the World Health Organization, 58 4 see A revision of the system of nomenclature for influenza viruses: a WHO Memorandum pdf icon[ KB, 7 pages]external icon pdf icon external icon.

The approach uses the following components:. Getting a flu vaccine can protect against these viruses as well as additional flu viruses that are antigenically similar to the viruses used to make the vaccine. Seasonal flu vaccines do not protect against influenza C or D viruses or against zoonotic animal-origin flu viruses that can cause human infections, such as variant or avian flu viruses.

In addition, flu vaccines will NOT protect against infection and illness caused by other viruses that also can cause influenza-like symptoms. There are many other viruses besides influenza that can result in influenza-like illness ILI that spread during flu season. To receive weekly email updates about Seasonal Flu, enter your email address:.

Skip directly to site content Skip directly to page options Skip directly to A-Z link. Influenza Flu. Section Navigation. Facebook Twitter LinkedIn Syndicate. Types of Influenza Viruses. Minus Related Pages. Full text is available as a scanned copy of the original print version.

Get a printable copy PDF file of the complete article K , or click on a page image below to browse page by page. Links to PubMed are also available for Selected References. These references are in PubMed. This may not be the complete list of references from this article. National Center for Biotechnology Information , U. Journal List J Hyg Lond v. J Hyg Lond. Oxford and C. Poultry infected with LPAI viruses may show no signs of disease or only exhibit mild illness such as ruffled feathers and a drop in egg production which may not be detected.

Infection of poultry with HPAI viruses can cause severe disease with high mortality. However, ducks can be infected without any signs of illness. There are genetic and antigenic differences between the influenza A virus subtypes that typically infect only birds and those that can infect birds and people. Avian influenza viruses rarely infect people. The most frequently identified subtypes of avian influenza that have caused human infections are H5, H7 and H9 viruses.

H7 virus infection in humans is uncommon. The most frequently identified H7 viruses associated with human infection are Asian lineage avian influenza A H7N9 viruses, which were first detected in China in While human infections are rare, these have commonly resulted in severe respiratory illness and death.