Influenza classification: Difference between revisions

Jump to navigation Jump to search
VisualWikitext
No edit summary
m (Bot: Removing from Primary care)
 
(23 intermediate revisions by 4 users not shown)
Line 1: Line 1:
__NOTOC__
__NOTOC__
{{Influenza}}
{{Influenza}}
{{CMG}}
'''For more information about non-human (variant) influenza viruses that may be transmitted to humans, see [[Zoonotic influenza]]'''<br><br>
{{CMG}}; {{AE}} {{AL}}
==Overview==
Influenza virus can be classified into 3 main types: A, B and C.  Based on the surface glycoproteins, [[hemagglutinin]] (HA) and [[neuraminidase]] (NA), influenza type A is further classified into several subtypes and can infect many species, including humans.  Type B can only infect humans and type C is not common and causes only mild illness.


==Classification==
==Classification==
[[Image:3D influenza virus.png|thumb|400px|left|Structure of the influenza [[wikt:virion|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).]]
There are three types of influenza viruses: A, B, and C. Only influenza A viruses are further classified by subtype on the basis of the two main surface glycoproteins [[hemagglutinin]] (HA) and [[neuraminidase]] (NA). Influenza A subtypes and B viruses are further classified by strains.<ref>{{cite web|url=http://www.cdc.gov/flu/avianflu/influenza-a-virus-subtypes.htm| title= CDC Influenza Viruses}}</ref>
[[Image:Infnomenclature.svg|thumb|350px|right|Diagram of influenza virus [[International Committee on Taxonomy of Viruses|nomenclature]] (for a [[Fujian flu]] virus)]]


The influenza virus is an [[RNA virus]] of the family [[Orthomyxoviridae]], which comprises the influenzaviruses, [[Isavirus]], and [[Thogotovirus]].<ref name=Kawaoka>{{cite book | author = Kawaoka Y (editor). | title = Influenza Virology: Current Topics | publisher = Caister Academic Press | year = 2006 | url=http://www.horizonpress.com/flu | isbn = 978-1-904455-06-6}}</ref> There are three types of influenza virus: [[Influenzavirus A]], [[Influenzavirus B]], and [[Influenzavirus C]]. Influenza A and C infect multiple species, while influenza B almost exclusively infects humans.<ref name=hay>{{cite journal | last = Hay | first = A | coauthors = Gregory V, Douglas A, Lin Y | title = The evolution of human influenza viruses | journal = Philos Trans R Soc Lond B Biol Sci | volume = 356 | issue = 1416 | pages = 1861–70 | year = 2001 | month=Dec 29 | id = PMID 11779385}}</ref>
{| style="border: 0px; font-size: 85%; margin: 3px; width:400px; float:right"
Wild aquatic birds are the natural hosts for a large variety of influenza A viruses. Occasionally viruses are transmitted to other species and may then cause devastating outbreaks in domestic poultry or give rise to human influenza [[pandemic]]s.<ref name=sobrino6>{{cite book |chapterurl=http://www.horizonpress.com/avir|author=Klenk et al|year=2008|chapter=Avian Influenza: Molecular Mechanisms of Pathogenesis and Host Range|title=Animal Viruses: Molecular Biology|publisher=Caister Academic Press|isbn = 978-1-904455-22-6}}</ref>
|valign=top|
The type A viruses are the most virulent human pathogens among the three influenza types and cause the most severe disease.  The Influenza A virus can be subdivided into different [[serovar|serotype]]s based on the [[antibody]] response to these viruses.<ref name=hay/> The serotypes that have been confirmed in humans, ordered by the number of known human pandemic deaths, are:
|+<small>'''Different hemagglutinin and neuraminidase subtypes and the species in which they have been detected'''</small>
! style="background: #4479BA; color:#FFF;  width: 200px;" | Species
! style="background: #4479BA; color:#FFF;  width: 200px;" | Hemagglutinin <BR>Subtypes
! style="background: #4479BA; color:#FFF;  width: 200px;" | Neuraminidase <BR>Subtypes
|-
| style="padding: 5px 5px; background: #DCDCDC;font-weight: bold" | Humans
| style="padding: 5px 5px; background: #F5F5F5;" | H1, H2, H3, H5, H6, H7, H9, H10
| style="padding: 5px 5px; background: #F5F5F5;" | N1, N2, N6, N7, N8, N9
|-
| style="padding: 5px 5px; background: #DCDCDC;font-weight: bold" | Poultry
| style="padding: 5px 5px; background: #F5F5F5;" | H1, H2, H3, H4, H5, H6, H7, H8, H9, H10, H11, H12, H13, H14, H15, H16
| style="padding: 5px 5px; background: #F5F5F5;" | N1, N2, N3, N4, N5, N6, N7, N8, N9
|-
| style="padding: 5px 5px; background: #DCDCDC;font-weight: bold" | Pigs
| style="padding: 5px 5px; background: #F5F5F5;" | H1, H2, H3, H4, H5, H9
| style="padding: 5px 5px; background: #F5F5F5;" | N1, N2
|-
| style="padding: 5px 5px; background: #DCDCDC;font-weight: bold" | Bats
| style="padding: 5px 5px; background: #F5F5F5;" | H17, H18
| style="padding: 5px 5px; background: #F5F5F5;" | N10, N11
|-
| style="padding: 0px 5px; background: #F5F5F5;" colspan=3| <small>'''Adapted from CDC''' <ref name="CDC Transmission">{{cite web|url=http://www.cdc.gov/flu/about/viruses/transmission.htm#subtypes| title=CDC Seasonal Influenza - Transmission of Influenza Viruses from Animals to People}} </ref></small>
|}


*[[H1N1]] caused "[[Spanish Flu]]."
===Influenza Type A and Its Subtypes===
*[[H2N2]] caused "Asian Flu."
*Influenza type A viruses can infect people, birds, pigs, horses, and other animals, but wild birds are the natural hosts for these viruses. Influenza type A viruses are divided into subtypes identified by two proteins on the surface of the virus: hemagglutinin (HA) and neuraminidase (NA).  
*[[H3N2]] caused "Hong Kong Flu."
*For example, an “H7N2 virus” designates an influenza A subtype that has an HA 7 protein and an NA 2 protein; similarly an “H5N1” virus has an HA 5 protein and an NA 1 protein.  
*[[H5N1]] is a [[pandemic]] threat in 2007–8 flu season.
*There are 16 known HA subtypes and 9 known NA subtypes; many different combinations of HA and NA proteins are possible.  
*[[H7N7]] has unusual [[zoonotic]] potential.<ref>{{cite journal | last = Fouchier | first = R | coauthors = 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 | title = Avian influenza A virus (H7N7) associated with human conjunctivitis and a fatal case of acute respiratory distress syndrome. | url= http://www.pnas.org/cgi/content/full/101/5/1356 | journal = Proc Natl Acad Sci U S A | volume = 101 | issue = 5 | pages = 1356–61 | year = 2004 | id = PMID 14745020}}</ref>
*Only some influenza A subtypes (i.e., H1N1, H1N2, and H3N2) are currently in general circulation among humans.  
*[[H1N2]] is endemic in humans and pigs.
*Other subtypes are found most commonly in other animal species. For example, H7N7 and H3N8 viruses cause illness in horses, and H3N8 also has recently been shown to cause illness in dogs.  
*[[H9N2]], [[H7N2]], [[H7N3]], [[H10N7]].
*Only influenza A viruses infect birds, and al  known subtypes of influenza A viruses can infect birds.  
*However, there are substantial genetic differences between the influenza A subtypes that typicaly infect birds and those that infect both people and birds.  
*Three prominent subtypes of the avian influenza A viruses that are known to infect both birds and people are A H5, A H7, and A H9.


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 [[pinniped|seal]].<ref>{{cite journal | last = Osterhaus | first = A | coauthors = Rimmelzwaan G, Martina B, Bestebroer T, Fouchier R | title = Influenza B virus in seals. | journal = Science | volume = 288 | issue = 5468 | pages = 1051–3 | year = 2000 | id = PMID 10807575}}</ref> This type of influenza mutates at a rate 2–3 times lower than type A<ref>{{cite journal | last = Nobusawa | first = E | coauthors = Sato K | title = Comparison of the mutation rates of human influenza A and B viruses | journal = J Virol | volume = 80 | issue = 7 | pages = 3675–8 | year = 2006 | month=Apr | id = PMID 16537638}}</ref> and consequently is less genetically diverse, with only one influenza B serotype.<ref name=hay/> As a result of this lack of [[antigen]]ic diversity, a degree of [[immunity (medical)|immunity]] to influenza B is usually acquired at an early age. However, influenza B mutates enough that lasting immunity is not possible.<ref name=webster>{{cite journal | first = Webster | last = R | coauthors = Bean W, Gorman O, Chambers T, Kawaoka Y | title = Evolution and ecology of influenza A viruses. | url= http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=1579108 |journal = Microbiol Rev | volume = 56 | issue = 1 | pages = 152–79 | year = 1992 | id = PMID 1579108}}</ref> 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.<ref name=Zambon>{{cite journal | last = Zambon | first = M | title = Epidemiology and pathogenesis of influenza. | journal = J Antimicrob Chemother | volume = 44 Suppl B | issue = | pages = 3–9 | year = 1999 | month=Nov | id = PMID 10877456 | url=http://jac.oxfordjournals.org/cgi/reprint/44/suppl_2/3}}</ref>  
====Influenza A H5====
*Nine potential subtypes of H5 are known (H5N1, H5N2, H5N3, H5N4, H5N5, H5N6, H5N7, H5N8, and H5N9). 
*Most H5 viruses identified worldwide in wild birds and poultry are LPAI (low pathogenic avian influenza) viruses.  
*Sporadic H5 virus infection of humans, such as with highly pathogenic avian influenza A (H5N1) viruses currently circulating among poultry in Asia and the Middle East have been reported in 15 countries, often resulting in severe pneumonia with approximately 60% mortality worldwide.
{|style="float:right"
|
<gallery>
Image:H7n9-reassortment-diagram-sm.jpg|'''Genetic Evolution of H7N9 Virus in China'''<br><small>Click on the image to expand. <br>Image obtained from CDC [http://www.cdc.gov/flu/avianflu/h7n9-images.htm]</small>
</gallery>
|}


The influenza C virus infects humans and pigs, and can cause severe illness and local epidemics.<ref name = "Matsuzaki">{{cite journal | last = Matsuzaki | first = Y | coauthors = Sugawara K, Mizuta K, Tsuchiya E, Muraki Y, Hongo S, Suzuki H, Nakamura K | title = Antigenic and genetic characterization of influenza C viruses which caused two outbreaks in Yamagata City, Japan, in 1996 and 1998 | url= http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=11825952 | journal = J Clin Microbiol | volume = 40 | issue = 2 | pages = 422–9 | year = 2002 | id = PMID 11825952}}</ref>  However, influenza C is less common than the other types and usually seems to cause mild disease in children.<ref>{{cite journal | last = Matsuzaki | first = Y | coauthors = Katsushima N, Nagai Y, Shoji M, Itagaki T, Sakamoto M, Kitaoka S, Mizuta K, Nishimura H | title = Clinical features of influenza C virus infection in children. | journal = J Infect Dis | volume = 193 | issue = 9 | pages = 1229–35 | year = 2006 | month=May 1 | id = PMID 16586359}}</ref><ref name = "Katagiri">{{cite journal | last = Katagiri | first = S | coauthors = Ohizumi A, Homma M | title = An outbreak of type C influenza in a children's home. | journal = J Infect Dis | volume = 148 | issue = 1 | pages = 51–6 | year = 1983 | month=Jul | id = PMID 6309999}}</ref>
====Influenza A H7 ====
*Nine potential subtypes of H7 are known (H7N1, H7N2, H7N3, H7N4, H7N5, H7N6, H7N7, H7N8, and H7N9).
*Most H7 viruses identified worldwide in wild birds and poultry are LPAI viruses.
*H7 virus infection in humans is uncommon, but has been documented in persons who have direct contact with infected birds, especially during outbreaks of H7 virus among poultry. Illness in humans may include conjunctivitis and/or upper respiratory tract symptoms.
* In humans, LPAI (H7N2, H7N3, H7N7) virus infections have caused mild to moderate illness.
* HPAI (high pathogenic avian influenza) virus infections, subtypes H7N3 and H7N7, have caused mild to severe and fatal illness.


===Structure and properties===
====Influenza A H9====  
The following applies for [[Influenzavirus A|Influenza A]] viruses, although other strains are very similar in structure:<ref>International Committee on Taxonomy of Viruses descriptions of: [http://www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/46000000.htm Orthomyxoviridae], [http://www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/46040000.htm Influenzavirus B] and [http://www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/00.046.0.02.htm Influenzavirus C]</ref>
*Nine potential subtypes of H9 are known (H9N1, H9N2, H9N3, H9N4, H9N5, H9N6, H9N7, H9N8, and H9N9)
*All H9 viruses identified worldwide in wild birds and poultry are LPAI viruses.
*H9N2 virus has been detected in bird populations in Asia, Europe, the Middle East and Africa.  
*Rare, sporadic H9N2 virus infections of humans have been reported to cause generally mild upper respiratory tract illness.


The influenza A virus particle or ''virion'' is 80–120 nm in diameter and usually roughly spherical, although filamentous forms can occur.<ref>{{cite web |author= International Committee on Taxonomy of Viruses |title=The Universal Virus Database, version 4: Influenza A |url=http://www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/00.046.0.01.htm}}</ref> 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 10 [[protein]]s (HA (hemagglutinin), NA (neuraminidase), NP (nucleoprotein), M1, M2, NS1, PA, PB1, PB1-F2, PB2).<ref name=Ghedin>{{cite journal | last = Ghedin | first = E | coauthors = Sengamalay N, Shumway M, Zaborsky J, Feldblyum T, Subbu V, Spiro D, Sitz J, Koo H, Bolotov P, Dernovoy D, Tatusova T, Bao Y, St George K, Taylor J, Lipman D, Fraser C, Taubenberger J, Salzberg S | title = Large-scale sequencing of human influenza reveals the dynamic nature of viral genome evolution. | journal = Nature | volume = 437 | issue = 7062 | pages = 1162–6 | year = 2005 | month=Oct 20 | id = PMID 16208317}}</ref> The best-characterised of these viral proteins are [[hemagglutinin]] and [[neuraminidase]], two large [[glycoprotein]]s 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.<ref>{{cite journal | last = Suzuki | first = Y | title = Sialobiology of influenza: molecular mechanism of host range variation of influenza viruses. | url= http://www.jstage.jst.go.jp/article/bpb/28/3/399/_pdf | journal = Biol Pharm Bull | volume = 28 | issue = 3 | pages = 399–408 | year = 2005 | id = PMID 15744059}}</ref> The hemagglutinin (HA or H) and neuraminidase (NA or N) proteins are targets for [[antiviral drugs]].<ref>{{cite journal | last = Wilson | first = J | coauthors = von Itzstein M | title = Recent strategies in the search for new anti-influenza therapies | journal = Curr Drug Targets | volume = 4 | issue = 5 | pages = 389–408 | year = 2003 | month=Jul | id = PMID 12816348}}</ref> These proteins are also recognised by [[antibody|antibodies]], i.e. they are [[antigen]]s. The responses of antibodies to these proteins are used to classify the different [[serovar|serotype]]s of influenza A viruses, hence the ''H'' and ''N'' in ''H5N1''.
===Influenza Type B===
*Influenza B viruses are usually found only in humans.  
*Unlike influenza A viruses, these viruses are not classified according to subtype.  
*Influenza B viruses can cause morbidity and mortality among humans, in general are associated with less severe epidemics than influenza A viruses.  
*Although influenza type B viruses can cause human epidemics, they have not caused pandemics.


===Infection and replication===
===Influenza Type C===
[[Image:Virus Replication.svg|thumb|400px|left|Host cell invasion and replication by the influenza virus. The steps in this process are discussed in the text.]]
*Influenza type C viruses cause mild illness in humans and do not cause epidemics or pandemics.  
*These viruses are not classified according to subtype.


Influenza viruses bind through [[hemagglutinin]] onto [[sialic acid]] sugars on the surfaces of [[epithelium|epithelial cells]]; typically in the nose, throat and [[lung]]s of mammals and [[intestine]]s of birds (Stage 1 in infection figure).<ref name=Wagner>{{cite journal | last = Wagner | first = R | coauthors = Matrosovich M, Klenk H | title = Functional balance between haemagglutinin and neuraminidase in influenza virus infections. | journal = Rev Med Virol | volume = 12 | issue = 3 | pages = 159–66 | year = 2002 | month=May–Jun| id = PMID 11987141}}</ref> 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 replicase|RNA-dependent RNA transcriptase]] into the [[cytoplasm]] (Stage 2).<ref>{{cite journal | last = Lakadamyali | first = M | coauthors = Rust M, Babcock H, Zhuang X | title = Visualizing infection of individual influenza viruses. | journal = Proc Natl Acad Sci U S A | volume = 100 | issue = 16 | pages = 9280–5 | year = 2003 | month=Aug 5 | id = PMID 12883000}}</ref> 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).<ref>{{cite journal | last = Cros | first = J | coauthors = Palese P | title = Trafficking of viral genomic RNA into and out of the nucleus: influenza, Thogoto and Borna disease viruses. | journal = Virus Res | volume = 95 | issue = 1–2 | pages = 3–12 | year = 2003 | month=Sep | id = PMID 12921991}}</ref> 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 [[nucleotide]]s for vRNA synthesis and also inhibiting [[translation (biology)|translation]] of host-cell mRNAs.<ref>{{cite journal | last = Kash | first = J | coauthors = Goodman A, Korth M, Katze M | title = Hijacking of the host-cell response and translational control during influenza virus infection. | journal = Virus Res | volume = 119 | issue = 1 | pages = 111–20 | year = 2006 | month=Jul | id = PMID 16630668}}</ref>
==Strains==  
 
*Influenza B viruses and subtypes of influenza A virus are further characterized into strains.  
Negative-sense vRNAs that form the [[genome]]s 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).<ref>{{cite journal | last = Nayak | first = D | coauthors = Hui E, Barman S | title = Assembly and budding of influenza virus. | journal = Virus Res | volume = 106 | issue = 2 | pages = 147–65 | year = 2004 | month =Dec | id = PMID 15567494}}</ref> 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.<ref name=Wagner/> After the release of new influenza virus, the host cell dies.
*There are many different strains of influenza B viruses and of influenza A subtypes.  
 
*New strains of influenza viruses appear and replace older strains; this process occurs through antigenic drift.  
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 is a mutant.<ref>{{cite journal | last = Drake | first = J | title = Rates of spontaneous mutation among RNA viruses. | journal = Proc Natl Acad Sci USA | volume = 90 | issue = 9 | pages = 4171–5 | year = 1993 | month=May 1 | id = PMID 8387212}}</ref> The separation of the genome into eight separate segments of vRNA allows mixing or ''reassortment'' of vRNAs if more than one viral line has infected a single cell. The resulting rapid change in viral genetics produces [[antigenic shift]]s and allow the virus to infect new host species and quickly overcome protective immunity. This is important in the emergence of pandemics, as discussed in [[#Epidemiology|Epidemiology]].
*When a new strain of human influenza virus emerges, antibody protection that may have developed after infection or vaccination with an older strain may not provide protection against the new strain. Therefore, the influenza vaccine is updated on a yearly basis to keep up with the changes in influenza viruses.


==References==
==References==
{{Reflist|2}}
{{Reflist|2}}
{{WH}}
{{WS}}


[[Category:Disease]]
[[Category:Disease]]
[[Category:Mature chapter]]
[[Category:Pulmonology]]
[[Category:Pulmonology]]
[[Category:Influenza| ]]
[[Category:Influenza| ]]
[[Category:Infectious disease]]
[[Category:Primary care]]
[[Category:Needs overview]]
{{WH}}
{{WS}}

Latest revision as of 22:24, 29 July 2020

Influenza Microchapters

Home

Patient Information

Overview

Historical Perspective

Classification

Pathophysiology

Causes

Differentiating Influenza from other Diseases

Epidemiology and Demographics

Risk Factors

Natural History, Complications and Prognosis

Diagnosis

History and Symptoms

Physical Examination

Laboratory Findings

Chest X Ray

Other Diagnostic Studies

Treatment

Medical Therapy

Primary Prevention

Cost-Effectiveness of Therapy

Future or Investigational Therapies

Case Studies

Case #1

Influenza classification On the Web

Most recent articles

Most cited articles

Review articles

CME Programs

Powerpoint slides

Images

American Roentgen Ray Society Images of Influenza classification

All Images
X-rays
Echo & Ultrasound
CT Images
MRI

Ongoing Trials at Clinical Trials.gov

US National Guidelines Clearinghouse

NICE Guidance

FDA on Influenza classification

CDC on Influenza classification

Influenza classification in the news

Blogs on Influenza classification

Directions to Hospitals Treating Influenza

Risk calculators and risk factors for Influenza classification

For more information about non-human (variant) influenza viruses that may be transmitted to humans, see Zoonotic influenza

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [2]; Associate Editor(s)-in-Chief: Alejandro Lemor, M.D. [3]

Overview

Influenza virus can be classified into 3 main types: A, B and C. Based on the surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA), influenza type A is further classified into several subtypes and can infect many species, including humans. Type B can only infect humans and type C is not common and causes only mild illness.

Classification

There are three types of influenza viruses: A, B, and C. Only influenza A viruses are further classified by subtype on the basis of the two main surface glycoproteins hemagglutinin (HA) and neuraminidase (NA). Influenza A subtypes and B viruses are further classified by strains.[1]

Different hemagglutinin and neuraminidase subtypes and the species in which they have been detected
Species Hemagglutinin
Subtypes 		Neuraminidase
Subtypes
Humans H1, H2, H3, H5, H6, H7, H9, H10 N1, N2, N6, N7, N8, N9
Poultry H1, H2, H3, H4, H5, H6, H7, H8, H9, H10, H11, H12, H13, H14, H15, H16 N1, N2, N3, N4, N5, N6, N7, N8, N9
Pigs H1, H2, H3, H4, H5, H9 N1, N2
Bats H17, H18 N10, N11
Adapted from CDC [2]

Influenza Type A and Its Subtypes

  • Influenza type A viruses can infect people, birds, pigs, horses, and other animals, but wild birds are the natural hosts for these viruses. Influenza type A viruses are divided into subtypes identified by two proteins on the surface of the virus: hemagglutinin (HA) and neuraminidase (NA).
  • For example, an “H7N2 virus” designates an influenza A subtype that has an HA 7 protein and an NA 2 protein; similarly an “H5N1” virus has an HA 5 protein and an NA 1 protein.
  • There are 16 known HA subtypes and 9 known NA subtypes; many different combinations of HA and NA proteins are possible.
  • Only some influenza A subtypes (i.e., H1N1, H1N2, and H3N2) are currently in general circulation among humans.
  • Other subtypes are found most commonly in other animal species. For example, H7N7 and H3N8 viruses cause illness in horses, and H3N8 also has recently been shown to cause illness in dogs.
  • Only influenza A viruses infect birds, and al known subtypes of influenza A viruses can infect birds.
  • However, there are substantial genetic differences between the influenza A subtypes that typicaly infect birds and those that infect both people and birds.
  • Three prominent subtypes of the avian influenza A viruses that are known to infect both birds and people are A H5, A H7, and A H9.

Influenza A H5

  • Nine potential subtypes of H5 are known (H5N1, H5N2, H5N3, H5N4, H5N5, H5N6, H5N7, H5N8, and H5N9).
  • Most H5 viruses identified worldwide in wild birds and poultry are LPAI (low pathogenic avian influenza) viruses.
  • Sporadic H5 virus infection of humans, such as with highly pathogenic avian influenza A (H5N1) viruses currently circulating among poultry in Asia and the Middle East have been reported in 15 countries, often resulting in severe pneumonia with approximately 60% mortality worldwide.
  • Genetic Evolution of H7N9 Virus in China Click on the image to expand. Image obtained from CDC [1]

    Genetic Evolution of H7N9 Virus in China
    Click on the image to expand.
    Image obtained from CDC [1]

Influenza A H7

  • Nine potential subtypes of H7 are known (H7N1, H7N2, H7N3, H7N4, H7N5, H7N6, H7N7, H7N8, and H7N9).
  • Most H7 viruses identified worldwide in wild birds and poultry are LPAI viruses.
  • H7 virus infection in humans is uncommon, but has been documented in persons who have direct contact with infected birds, especially during outbreaks of H7 virus among poultry. Illness in humans may include conjunctivitis and/or upper respiratory tract symptoms.
  • In humans, LPAI (H7N2, H7N3, H7N7) virus infections have caused mild to moderate illness.
  • HPAI (high pathogenic avian influenza) virus infections, subtypes H7N3 and H7N7, have caused mild to severe and fatal illness.

Influenza A H9

  • Nine potential subtypes of H9 are known (H9N1, H9N2, H9N3, H9N4, H9N5, H9N6, H9N7, H9N8, and H9N9)
  • All H9 viruses identified worldwide in wild birds and poultry are LPAI viruses.
  • H9N2 virus has been detected in bird populations in Asia, Europe, the Middle East and Africa.
  • Rare, sporadic H9N2 virus infections of humans have been reported to cause generally mild upper respiratory tract illness.

Influenza Type B

  • Influenza B viruses are usually found only in humans.
  • Unlike influenza A viruses, these viruses are not classified according to subtype.
  • Influenza B viruses can cause morbidity and mortality among humans, in general are associated with less severe epidemics than influenza A viruses.
  • Although influenza type B viruses can cause human epidemics, they have not caused pandemics.

Influenza Type C

  • Influenza type C viruses cause mild illness in humans and do not cause epidemics or pandemics.
  • These viruses are not classified according to subtype.

Strains

  • Influenza B viruses and subtypes of influenza A virus are further characterized into strains.
  • There are many different strains of influenza B viruses and of influenza A subtypes.
  • New strains of influenza viruses appear and replace older strains; this process occurs through antigenic drift.
  • When a new strain of human influenza virus emerges, antibody protection that may have developed after infection or vaccination with an older strain may not provide protection against the new strain. Therefore, the influenza vaccine is updated on a yearly basis to keep up with the changes in influenza viruses.

References

  1. ↑ "CDC Influenza Viruses".
  2. ↑ "CDC Seasonal Influenza - Transmission of Influenza Viruses from Animals to People".

Template:WH Template:WS

Retrieved from "https://www.wikidoc.org/index.php?title=Influenza_classification&oldid=1639886"