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Virus classification
Group V ((−)ssRNA)

Influenzavirus A
Influenzavirus B
Influenzavirus C

The Orthomyxoviridae (orthos, Greek for "straight"; myxa, Greek for "mucus")[1] are a family of RNA viruses. They include 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 can be told apart by the structure of their proteins. They infect vertebrates, as follows:[2]

Types[change | change source]

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[change | change source]

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

Diagram of influenza nomenclature.

Further variation exists. 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:

Flu pandemics[11]
Name Year Deaths (millions) Subtype involved
Asiatic (Russian) Flu 1889-90 1 possibly H2N2
Spanish Flu 1918-20 40 H1N1
Asian Flu 1957-58 1-1.5 H2N2
Hong Kong Flu 1968-69 0.75 H3N2

Influenza B[change | change source]

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, people usually get a degree of immunity to influenza B 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[change | change source]

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]

References[change | change source]

  1. International Committee on Taxonomy of Viruses Index of Viruses - Orthomyxoviridae (2006). In: ICTVdB - The Universal Virus Database, version 4. Büchen-Osmond, C (Ed), Columbia University, New York, USA.
  2. 2.0 2.1 Büchen-Osmond, C., ed. (2006). "Index of Viruses - Orthomyxoviridae (2006). In: ICTVdB - The Universal Virus Database, version 4". Columbia University, New York, USA.
  3. Jones LD, Nuttall PA (1989). "Non-viraemic transmission of Thogoto virus: influence of time and distance". Trans. R. Soc. Trop. Med. Hyg. 83 (5): 712–4. doi:10.1016/0035-9203(89)90405-7. PMID 2617637.
  4. Barry Ely (1999). "Infectious Salmon Anaemia". Mill Hill Essays. National Institute for Medical Research. Archived from the original on 2007-08-24. Retrieved 2007-09-14.
  5. Raynard RS, Murray AG, Gregory A (2001). "Infectious salmon anaemia virus in wild fish from Scotland". Dis. Aquat. Org. 46 (2): 93–100. doi:10.3354/dao046093. PMID 11678233.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  6. 6.0 6.1 Hay A, Gregory V, Douglas A, Lin Y (2001-12-29). "The evolution of human influenza viruses" (PDF). Philos Trans R Soc Lond B Biol Sci. 356 (1416): 1861–70. doi:10.1098/rstb.2001.0999. PMC 1088562. PMID 11779385.{{cite journal}}: CS1 maint: multiple names: authors list (link)[permanent dead link]
  7. "Avian Influenza (Bird Flu)". Centers for Disease Control and Prevention. Retrieved 2007-09-15.
  8. Atkinson W, Hamborsky J, McIntyre L, Wolfe S, ed. (2007). Epidemiology and Prevention of Vaccine-Preventable Diseases (10th ed.). Washington DC: Centers for Disease Control and Prevention.{{cite book}}: CS1 maint: multiple names: editors list (link)
  9. "Avian Influenza (Bird Flu): Implications for Human Disease". Center for Infectious Disease Research & Policy, University of Minnesota. 2007-06-27. Archived from the original on 2013-06-17. Retrieved 2007-09-14.
  10. Fouchier R, Schneeberger P, Rozendaal F, Broekman J, Kemink S, Munster V, Kuiken T, Rimmelzwaan G, Schutten M, Van Doornum G, Koch G, Bosman A, Koopmans M, Osterhaus A (2004). "Avian influenza A virus (H7N7) associated with human conjunctivitis and a fatal case of acute respiratory distress syndrome". Proc Natl Acad Sci USA. 101 (5): 1356–61. Bibcode:2004PNAS..101.1356F. doi:10.1073/pnas.0308352100. PMC 337057. PMID 14745020.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  11. Hilleman M (2002-08-19). "Realities and enigmas of human viral influenza: pathogenesis, epidemiology and control". Vaccine. 20 (25–26): 3068–87. doi:10.1016/S0264-410X(02)00254-2. PMID 12163258.
  12. Osterhaus A, Rimmelzwaan G, Martina B, Bestebroer T, Fouchier R (2000). "Influenza B virus in seals". Science. 288 (5468): 1051–3. Bibcode:2000Sci...288.1051O. doi:10.1126/science.288.5468.1051. PMID 10807575.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  13. Nobusawa E, Sato K (April 2006). "Comparison of the mutation rates of human influenza A and B viruses". J Virol. 80 (7): 3675–8. doi:10.1128/JVI.80.7.3675-3678.2006. PMC 1440390. PMID 16537638.
  14. Webster R, Bean W, Gorman O, Chambers T, Kawaoka Y (1992-03-01). "Evolution and ecology of influenza A viruses". Microbiol Rev. 56 (1): 152–79. doi:10.1128/mr.56.1.152-179.1992. PMC 372859. PMID 1579108.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  15. Zambon M (November 1999). "Epidemiology and pathogenesis of influenza". J Antimicrob Chemother. 44 Suppl B: 3–9. doi:10.1093/jac/44.suppl_2.3. PMID 10877456.
  16. Matsuzaki Y, Sugawara K, Mizuta K, Tsuchiya E, Muraki Y, Hongo S, Suzuki H, Nakamura K (2002). "Antigenic and genetic characterization of influenza C viruses which caused two outbreaks in Yamagata City, Japan, in 1996 and 1998". J Clin Microbiol. 40 (2): 422–9. doi:10.1128/JCM.40.2.422-429.2002. PMC 153379. PMID 11825952.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  17. Matsuzaki Y, Katsushima N, Nagai Y, Shoji M, Itagaki T, Sakamoto M, Kitaoka S, Mizuta K, Nishimura H (May 1, 2006). "Clinical features of influenza C virus infection in children". J Infect Dis. 193 (9): 1229–35. doi:10.1086/502973. PMID 16586359.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  18. Katagiri S, Ohizumi A, Homma M (July 1983). "An outbreak of type C influenza in a children's home". J Infect Dis. 148 (1): 51–6. doi:10.1093/infdis/148.1.51. PMID 6309999.{{cite journal}}: CS1 maint: multiple names: authors list (link)

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