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Please improve this articleif you can. (September 2007) Orthomyxoviridae Virus classificationGroup: Group V ((-)ssRNA) Family: Orthomyxoviridae

Influenzavirus A
Influenzavirus B
Influenzavirus C


The Orthomyxoviridae (Derivation of name: orthos is Greek for straight; myxa is Greek for mucus)[1] are a family of RNA viruses that includes five genera: Influenzavirus A, Influenzavirus B, Influenzavirus C, Thogotovirus and Isavirus. The first three genera contain viruses that cause influenza in vertebrates, including birds (see also avian influenza), humans, and other mammals. Isaviruses infect salmon; thogotoviruses infect vertebrates and invertebrates, such as mosquitoes and sea lice.[2][3][4][5]

The three genera of Influenzavirus, which are identified by antigenic differences in their nucleoprotein and matrix protein infect vertebrates as follows:[2]



Structure of the influenza virion. The hemagglutinin (HA) and neuraminidase (NA) proteins are shown on the surface of the particle. The viral RNAs that make up the genome are shown as red coils inside the particle and bound to Ribonuclear Proteins (RNPs).

The virions have envelopes and occur in pleomorphic and filamentous forms. In general the virus's morphology is spherical with particles 50 to 120 nm in diameter, or filamentous virions 20 nm in diameter and 200 to 300 (-3000) nm long. There are some 500 distinct spike-like surface projections of the envelope each projecting 10 to 14 nm from the surface with some types (i.e. hemagglutinin esterase (HEF)) densely dispersed over the surface, and with others (i.e. hemagglutinin (HA)) spaced widely apart.

The major glycoprotein (HA) is interposed irregularly by clusters of neuraminidase (NA), with a ratio of HA to NA of about 4-5 to 1.

Lipoprotein membranes enclose the nucleocapsids; nucleoproteins of different size classes with a loop at each end; the arrangement within the virion is uncertain. The nucleocapsids are filamentous and fall in the range of 50 to 130 nm long and 9 to 15 nm in diameter. They have a helical symmetry.

Nucleic Acid

Viruses of this family contain 7 to 8 segments of linear negative-sense single stranded RNA.

The total genome length is 12000-15000 nucleotides (nt). The largest segment 2300-2500 nt; of second largest 2300-2500 nt; of third 2200-2300 nt; of fourth 1700-1800 nt; of fifth 1500-1600 nt; of sixth 1400-1500 nt; of seventh 1000-1100 nt; of eighth 800-900 nt. Genome sequence has terminal repeated sequences; repeated at both ends. Terminal repeats at the 5'-end 12-13 nucleotides long. Nucleotide sequences of 3'-terminus identical; the same in genera of same family; most on RNA (segments), or on all RNA species. Terminal repeats at the 3'-end 9-11 nucleotides long. Encapsidated nucleic acid is solely genomic. Each virion may contain defective interfering copies.

Classification and nomenclature

In a phylogenetic-based taxonomy the "RNA viruses" includes the "negative-sense ssRNA viruses" which includes the Order "Mononegavirales", and the Family "Orthomyxoviridae" (among others). The species and serotypes of Orthomyxoviridae are shown in the following table.

Orthomyxoviridae Genera, Species, And Serotypes
GenusSpecies(* indicates type species) Serotypesor Subtypes HostsInfluenzavirus AInfluenza A virus(*) H1N1, H1N2, H2N2, H3N1, H3N2, H3N8, H5N1, H5N2, H5N3, H5N8, H5N9, H7N1, H7N2, H7N3, H7N4, H7N7, H9N2, H10N7Human, pig, bird, horseInfluenzavirus BInfluenza B virus(*)
Human, sealInfluenzavirus CInfluenza C virus(*)
Human, pig IsavirusInfectious salmon anemiavirus (*)
Atlantic salmonThogotovirusThogoto virus(*)
Tick, Mosquito, Mammal(including Human) Dhori virusBatken virus
Dhori virus

Types of influenza virus

There are three genera of influenza virus: Influenzavirus A, Influenzavirus B and Influenzavirus C. Each genus includes only one species, or type: Influenza A virus, Influenza B virus, and Influenza C virus, respectively. Influenza A and C infect multiple species, while influenza B almost exclusively infects humans.[6][7]

Influenza A

Main article: Influenzavirus A

Influenza A viruses are further classified, based on the viral surface proteins hemagglutinin (HA or H) and neuraminidase (NA or N). Sixteen H subtypes (or serotypes) and nine N subtypes of influenza A virus have been identified.

Diagram of influenza nomenclature.

Further variation exists; thus, specific influenza strain isolates are identified by a standard nomenclature specifying virus type, geographical location where first isolated, sequential number of isolation, year of isolation, and HA and NA subtype.[8][9]

Examples of the nomenclature are:

  1. A/Moscow/10/99 (H3N2)
  2. B/Hong Kong/330/2001

The type A viruses are the most virulent human pathogens among the three influenza types and causes the most severe disease. The serotypes that have been confirmed in humans, ordered by the number of known human pandemic deaths, are:

Latest flu pandemics[11]Name of pandemic Date Deaths Subtype involved Asiatic (Russian) Flu 1889-90 1 million possibly H2N2Spanish Flu1918-20 40 million H1N1Asian Flu1957-58 1 to 1.5 million H2N2Hong Kong Flu1968-69 0.75 to 1 million H3N2

Influenza B

Main article: Influenzavirus B

Influenza B virus is almost exclusively a human pathogen, and is less common than influenza A. The only other animal known to be susceptible to influenza B infection is the seal.[12] This type of influenza mutates at a rate 2-3 times lower than type A[13] and consequently is less genetically diverse, with only one influenza B serotype.[6] As a result of this lack of antigenic diversity, a degree of immunity to influenza B is usually acquired at an early age. However, influenza B mutates enough that lasting immunity is not possible.[14] This reduced rate of antigenic change, combined with its limited host range (inhibiting cross species antigenic shift), ensures that pandemics of influenza B do not occur.[15]

Influenza C

Main article: Influenzavirus C

The influenza C virus infects humans and pigs, and can cause severe illness and local epidemics.[16] However, influenza C is less common than the other types and usually seems to cause mild disease in children.[17][18]

Structure and properties

An in-depth example can be found at H5N1 genetic structure

The following applies for Influenza A viruses, although other influenza strains are very similar in structure[19]:

The influenza A virus particle or virion is 80-120 nm in diameter and usually roughly spherical, although filamentous forms can occur.[20] Unusually for a virus, the influenza A genome is not a single piece of nucleic acid; instead, it contains eight pieces of segmented negative-sense RNA (13.5 kilobases total), which encode 11 proteins (HA, NA, NP, M1, M2, NS1, NEP, PA, PB1, PB1-F2, PB2).[21] The best-characterised of these viral proteins are hemagglutinin and neuraminidase, two large glycoproteins found on the outside of the viral particles. Neuraminidase is an enzyme involved in the release of progeny virus from infected cells, by cleaving sugars that bind the mature viral particles. By contrast, hemagglutinin is a lectin that mediates binding of the virus to target cells and entry of the viral genome into the target cell.[22] The hemagglutinin (H) and neuraminidase (N) proteins are targets for antiviral drugs.[23] These proteins are also recognised by antibodies, i.e. they are antigens.[11] The responses of antibodies to these proteins are used to classify the different serotypes of influenza A viruses, hence the H and N in H5N1.

Infection and replication

Invasion and replication of the influenza virus. The steps in this process are discussed in the text.

Typically, influenza is transmitted from infected mammals through the air by coughs or sneezes, creating aerosols containing the virus, and from infected birds through their droppings. Influenza can also be transmitted by saliva, nasal secretions, feces and blood. Infections occur through contact with these bodily fluids or with contaminated surfaces. Flu viruses can remain infectious for about one week at human body temperature, over 30 days at 0 °C (32 °F), and indefinitely at very low temperatures (such as lakes in northeast Siberia). They can be inactivated easily by disinfectants and detergents.[24][25][26]

The viruses bind to a cell through interactions between its hemagglutinin glycoprotein and sialic acid sugars on the surfaces of epithelial cells in the lung and throat (Stage 1 in infection figure).[27] The cell imports the virus by endocytosis. In the acidic endosome, part of the haemagglutinin protein fuses the viral envelope with the vacuole's membrane, releasing the viral RNA (vRNA) molecules, accessory proteins and RNA-dependent RNA polymerase into the cytoplasm (Stage 2).[28] These proteins and vRNA form a complex that is transported into the cell nucleus, where the RNA-dependent RNA transcriptase begins transcribing complementary positive-sense vRNA (Steps 3a and b).[29] The vRNA is either exported into the cytoplasm and translated (step 4), or remains in the nucleus. Newly-synthesised viral proteins are either secreted through the Golgi apparatus onto the cell surface (in the case of neuraminidase and hemagglutinin, step 5b) or transported back into the nucleus to bind vRNA and form new viral genome particles (step 5a). Other viral proteins have multiple actions in the host cell, including degrading cellular mRNA and using the released nucleotides for vRNA synthesis and also inhibiting translation of host-cell mRNAs.[30]

Negative-sense vRNAs that form the genomes of future viruses, RNA-dependent RNA transcriptase, and other viral proteins are assembled into a virion. Hemagglutinin and neuraminidase molecules cluster into a bulge in the cell membrane. The vRNA and viral core proteins leave the nucleus and enter this membrane protrusion (step 6). The mature virus buds off from the cell in a sphere of host phospholipid membrane, acquiring hemagglutinin and neuraminidase with this membrane coat (step 7).[31] As before, the viruses adhere to the cell through hemagglutinin; the mature viruses detach once their neuraminidase has cleaved sialic acid residues from the host cell.[27] After the release of new influenza virus, the host cell dies.

Because of the absence of RNA proofreading enzymes, the RNA-dependent RNA transcriptase makes a single nucleotide insertion error roughly every 10 thousand nucleotides, which is the approximate length of the influenza vRNA. Hence, nearly every newly-manufactured influenza virus will contain mutation in its genome.[32] The separation of the genome into eight separate segments of vRNA allows mixing or reassortment of the genes if more than one variety of influenza virus has infected the same cell. The resulting alteration in the genome segments packaged in to viral progeny confers new behavior, sometimes the ability to infect new host species or to overcome protective immunity of host populations to its old genome (in which case it is called an antigenic shift).[11]

See also

Viruses templates

v • d • eBaltimore(virus classification) I: dsDNA virusesAdenovirus, Herpesvirus, PoxvirusII: ssDNA virusesParvovirusIII: dsRNA virusesReovirusV: (-)ssRNA virusesOrthomyxovirus, RhabdovirusVI: ssRNA-RT virusesRetrovirusVII: dsDNA-RT virusesHepadnavirus v • d • eViraldiseases (A80-B34, 042-079) Viral infections of the CNSPoliomyelitis(Post-polio syndrome) - Subacute sclerosing panencephalitis- Progressive multifocal leukoencephalopathy- Rabies- Encephalitis lethargica- Lymphocytic choriomeningitis- Tick-borne meningoencephalitis- Tropical spastic paraparesisArthropod-borne viral fevers and
viral haemorrhagic feversMosquito (Dengue fever- Chikungunya- Rift Valley fever- Yellow fever- O'nyong'nyong- West Nile- Japanese Encephalitis- St. Louis Encephalitis- Murray Valley encephalitis- Ross River) Tick (Crimean-Congo hemorrhagic fever- Omsk hemorrhagic fever- Kyasanur forest disease- Alkurma- Powassan) Zoonoticviruses (Vector) Menangle- Nipah- BDVRat (Lassa fever- Venezuelan hemorrhagic fever- Junin- Argentine hemorrhagic fever- Bolivian hemorrhagic fever- Puumala- Andes- Sin Nombre- Haantan) Bat (Australian bat lyssavirus- Ebola- Marburg hemorrhagic fever- Mokola- Duvenhage) Viral infections characterized by
skinand mucous membranelesionsHerpes simplex- Chickenpox- Herpes zoster- Smallpox- Monkeypox- Measles- Rubella- Cowpox- Vaccinia- Molluscum contagiosum- Roseola- Fifth disease- Hand, foot and mouth disease- Foot-and-mouth disease- KSHV- WartViral hepatitisHepatitis A- Hepatitis B- Hepatitis C- Hepatitis D- Hepatitis E- Hepatitis GViral infections of the respiratory systemAvian influenza- Acute viral nasopharyngitis- Infectious mononucleosis- Influenza- Severe acute respiratory syndrome- Viral pneumonia- Human parainfluenza viruses- RSV- hMPVSexually transmitted HIV(AIDS, AIDS dementia complex) - HPV(Genital warts, Cervical cancer) - Adult T-cell leukemiaViral gastroenteritisRotavirus- Norovirus- Astrovirus- Coronavirus- AdenovirusViruses and cancers HTLV(induces leukemia) - VSV(oncolytic) Other viral diseases Cytomegalovirus- Mumps- Bornholm disease v • d • eOther virustopics Table of clinically important viruses, Bacteriophage, Virus cancer link, Laboratory diagnosis of virus, Antiviral drug, Neurotropic virus, Oncovirus v • d • eInfluenzaInfluenza Research- Vaccine- Avian influenza- Treatment- Genome sequencing- SeasonInfluenza viruses Orthomyxoviridae - Influenza A- Influenza B- Influenza CSubtypes of Influenza A virus H1N1- H1N2- H2N2- H3N1- H3N2- H3N8- H5N1- H5N2- H5N3- H5N8- H5N9- H7N1- H7N2- H7N3- H7N4- H7N7- H9N2- H10N7H5N1Genetic structure- Transmission and infection- Global spread- Clinical Trials- Human mortalityAntiviral drugsArbidol- adamantane derivatives(Amantadine, Rimantadine) - neuraminidase inhibitors(Oseltamivir, Peramivir, Zanamivir)
Experimental (Peramivir) Influenza vaccinesFluMist- FluzoneInfluenza pandemicsAsian Flu- Hong Kong Flu- Spanish flu- Fujian flu- Pandemic Severity IndexOutbreaks of Avian influenzaCroatia (2005)- India (2006)- UK (2007)- West Bengal (2008)Influenza in non-human mammals Canine influenza- Equine influenza(2007 Australian outbreak) - Swine flu


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