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| __NOTOC__
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| {{Polio}}
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| {{CMG}}
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| ==Overview==
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| Polio is a highly infectious disease caused by Poliovirus that invades the [[nervous system]]. Poliovirus are small (27–30 nm), nonenveloped viruses with [[capsid]]s enclosing a single-stranded, positive-sense [[RNA]] genome about 7,500 nucleotides long. Person-to-person spread of poliovirus via the fecal-oral route is the most important route of transmission, although the oral-oral route may account for some cases.
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| ==Taxonomy==
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| ==Biology===
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| Poliovirus is a member of the enterovirus subgroup, family Picornaviridae. Enteroviruses are transient inhabitants of the gastrointestinal tract, and are stable at acid pH. Picornaviruses are small, ether-insensitive viruses with an RNA genome.
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| There are three poliovirus serotypes (P1, P2, and P3). There is minimal heterotypic immunity between the three serotypes. That is, immunity to one serotype does not produce significant immunity to the other serotypes. The poliovirus is rapidly inactivated by heat, formaldehyde, chlorine, and ultraviolet light.
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| [[Image:Polio EM PHIL 1875 lores.PNG|thumb|left|A [[Transmission electron microscopy|TEM]] [[micrograph]] of poliovirus]]
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| {{main|Poliovirus}}
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| Poliomyelitis is caused by infection with a member of the [[genus]] ''[[enterovirus]]'' known as [[poliovirus]] (PV). This group of [[RNA virus]]es prefers to inhabit the [[gastrointestinal tract]]. PV [[pathogen|infects and causes disease]] in humans alone. Its [[Virus#Structure|structure]] is very simple, composed of a single [[sense (molecular biology)|(+) sense]] [[RNA]] [[genome]] enclosed in a protein shell called a [[capsid]]. In addition to protecting the virus’s genetic material, the capsid proteins enable poliovirus to infect certain types of cells. Three [[serovar|serotype]]s of poliovirus have been identified—poliovirus type 1 (PV1), type 2 (PV2), and type 3 (PV3)—each with a slightly different capsid protein.<ref>{{cite book |author=Katz, Samuel L.; Gershon, Anne A.; Krugman, Saul; Hotez, Peter J. |title=Krugman's infectious diseases of children |publisher=Mosby |location=St. Louis |year=2004 |pages=81–97 |isbn=0-323-01756-8 |oclc= |doi=}}</ref> All three are extremely [[virulence|virulent]] and produce the same disease symptoms. PV1 is the most commonly encountered form, and the one most closely associated with paralysis.
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| Individuals who are exposed to the virus, either through infection or by [[immunization]] with polio vaccine, develop [[immunity (medical)|immunity]]. In immune individuals, [[IgA]] [[antibodies]] against poliovirus are present in the [[tonsil]]s and gastrointestinal tract and are able to block virus replication; [[IgG]] and [[IgM]] antibodies against PV can prevent the spread of the virus to motor neurons of the [[central nervous system]]. Infection or vaccination with one serotype of poliovirus does not provide immunity against the other serotypes, and full immunity requires exposure to each serotype.
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| ===Transmission===
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| Poliomyelitis is highly contagious and spreads easily from human-to-human contact.<ref name=Kew_2005>{{cite journal |author=Kew O, Sutter R, de Gourville E, Dowdle W, Pallansch M |title=Vaccine-derived polioviruses and the endgame strategy for global polio eradication |journal=Annu Rev Microbiol |volume=59 |issue= |pages=587–635 |year=2005 |pmid=16153180}}</ref> In endemic areas, wild polioviruses can infect virtually the entire human population.<ref name=McGraw>{{cite book |author = Parker SP (ed.) | title = McGraw-Hill Concise Encyclopedia of Science & Technology |publisher=McGraw-Hill |location=New York |year=1998 | isbn=0-07-052659-1| page= 67}}</ref> It is seasonal in temperate climates, with peak transmission occurring in summer and autumn. These seasonal differences are far less pronounced in tropical areas. The time between first exposure and first symptoms, known as the [[incubation period]], is usually 6 to 20 days, with a maximum range of 3 to 35 days.<ref name=Racaniello>{{cite journal |author=Racaniello V |title=One hundred years of poliovirus pathogenesis |journal=[[Virology (journal)|Virology]] |volume=344 |issue=1 |pages=9–16 |year=2006 |pmid = 16364730}}</ref> Virus particles are excreted in the [[feces]] for several weeks following initial infection. The disease is [[Transmission (medicine)|transmitted]] primarily via the [[fecal-oral route]], by ingesting contaminated food or water. It is occasionally transmitted via the oral-oral route,<ref name= Ohri>{{cite journal |last= Ohri |first=Linda K. |coauthors= Jonathan G. Marquess |year=1999 |title= Polio: Will We Soon Vanquish an Old Enemy? |journal= Drug Benefit Trends |volume= 11 |issue= 6|pages=41–54 |id= |url=http://www.medscape.com/viewarticle/416890 |accessdate= 2007-11-06 }} (Available free on [[Medscape]]; registration required.)</ref> a mode especially visible in areas with good sanitation and hygiene. Polio is most infectious between 7–10 days before and 7–10 days after the appearance of symptoms, but transmission is possible as long as the virus remains in the saliva or feces.
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| Factors that increase the risk of polio infection or affect the severity of the disease include [[immune deficiency]],<ref>{{cite journal |author=Davis L, Bodian D, Price D, Butler I, Vickers J |title=Chronic progressive poliomyelitis secondary to vaccination of an immunodeficient child |journal=[[New England Journal of Medicine|N Engl J Med]] |volume=297 |issue=5 |pages=241–5 |year=1977 |pmid = 195206}}</ref> [[malnutrition]],<ref>{{cite journal |author=Chandra R |title=Reduced secretory antibody response to live attenuated measles and poliovirus vaccines in malnourished children| url= http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=1131622|journal=[[British Medical Journal|Br Med J]] |volume=2 |issue=5971 |pages=583–5 |year=1975 |pmid=1131622}}</ref> [[tonsillectomy]],<ref>{{cite journal |author=Miller A |title=Incidence of poliomyelitis; the effect of tonsillectomy and other operations on the nose and throat | url= http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=12978882 |journal=Calif Med |volume=77 |issue=1 |pages=19–21 |year=1952 |pmid=12978882}}</ref> physical activity immediately following the onset of paralysis,<ref>{{cite journal |author=Horstmann D |title=Acute poliomyelitis relation of physical activity at the time of onset to the course of the disease |journal=[[Journal of the American Medical Association|J Am Med Assoc]] |volume=142 |issue=4 |pages=236–41 |year=1950 |pmid=15400610}}</ref> skeletal muscle injury due to [[intramuscular injection|injection]] of vaccines or therapeutic agents,<ref>{{cite journal |author=Gromeier M, Wimmer E |title=Mechanism of injury-provoked poliomyelitis |url= http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=9573275| journal=J. Virol. |volume=72 |issue=6 |pages=5056–60 |year=1998 |pmid=9573275 |doi=}}</ref> and [[pregnancy]].<ref name= Evans_1960>{{cite journal |author=Evans C |title=Factors influencing the occurrence of illness during naturally acquired poliomyelitis virus infections | url=http://mmbr.asm.org/cgi/reprint/24/4/341.pdf | format = PDF | journal=Bacteriol Rev |volume=24 |issue=4 |pages=341–52 |year=1960 |pmid=13697553}}</ref> Although the virus can cross the [[placenta]] during pregnancy, the fetus does not appear to be affected by either maternal infection or polio vaccination.<ref name=UK>{{cite book |author=Joint Committee on Vaccination and Immunisation (Salisbury A, Ramsay M, Noakes K (eds.) |title = Chapter 26:Poliomyelitis. ''in:'' Immunisation Against Infectious Disease, 2006 | url=http://www.immunisation.nhs.uk/files/GB_26_polio.pdf | format = PDF |publisher=Stationery Office |location=Edinburgh |year=2006 |pages = 313–29 |isbn = 0-11-322528-8}}</ref> Maternal antibodies also cross the [[placenta]], providing [[passive immunity]] that protects the infant from polio infection during the first few months of life.<ref>{{cite journal |author=Sauerbrei A, Groh A, Bischoff A, Prager J, Wutzler P |title=Antibodies against vaccine-preventable diseases in pregnant women and their offspring in the eastern part of Germany |journal=Med Microbiol Immunol |volume=190 |issue=4 |pages=167–72 |year=2002 |pmid=12005329}}</ref>
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| ===Reservoir===
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| Humans are the only known reservoir of poliovirus, which is transmitted most frequently by persons with inapparent infections. There is no asymptomatic carrier state except in immune deficient persons.
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| ==References==
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| {{Reflist|2}}
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| [[Category:Needs overview]]
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| [[Category:Disease]]
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| [[Category:Infectious disease]]
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| [[Category:Primary care]]
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| {{WH}}
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| {{WS}}
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