Poliovirus

Taxobox | color=violet
name = "Poliovirus"
image_caption = TEM micrograph of poliovirus virions.


virus_group = iv
familia = "Picornaviridae"
genus = "Enterovirus"
species = Poliovirus

Poliovirus, the causative agent of poliomyelitis, is a human enterovirus and member of the family of Picornaviridae.cite book | author = Ryan KJ, Ray CG (editors) | title = Sherris Medical Microbiology | edition = 4th ed. | publisher = McGraw Hill | year = 2004 | isbn = 0838585299 ] Poliovirus is composed of a RNA genome and a protein capsid. The genome is single-stranded positive-sense RNA genome that is about 7500 nucleotides long.cite journal |author=Hogle J |title=Poliovirus cell entry: common structural themes in viral cell entry pathways |url = http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=12142481 |journal=Annu Rev Microbiol |volume=56 |issue= |pages=677–702 |year=2002 |pmid = 12142481 |doi=10.1146/annurev.micro.56.012302.160757] The viral particle is about 300 Ångström in diameter with icosahedral symmetry. Because of its short genome and its simple composition—only RNA and a non-enveloped icosahedral protein coat that encapsulates it—poliovirus is widely regarded as the simplest significant virus.cite book |author=Goodsell DS |title=The machinery of life |publisher=Copernicus |location=New York |year=1998 |isbn=0-387-98273-6 |oclc= |doi=]

Poliovirus was first isolated in 1909 by Karl Landsteiner and Erwin Popper.cite book |author = Paul JR |title=A History of Poliomyelitis |publisher=Yale University Press |location=New Haven, Conn |year=1971 |pages= |isbn=0-300-01324-8 | series = (Yale studies in the history of science and medicine)] In 1981, the poliovirus genome was published by two different teams of researchers— by Vincent Racaniello and David Baltimore at MITcite journal |author=Racaniello and Baltimore |title=Molecular cloning of poliovirus cDNA and determination of the complete nucleotide sequence of the viral genome |journal=Proceedings of the National Academy Science USA |volume=78 |issue=8 |pages=4887–4891 |year=1981 |pmid = 6272282 |doi=10.1073/pnas.78.8.4887] and by Naomi Kitamura and others at the State University of New York, Stony Brook.cite journal |author=Kitamura N, Semler B, Rothberg P, "et al" |title=Primary structure, gene organization and polypeptide expression of poliovirus RNA |journal=Nature |volume=291 |issue=5816 |pages=547–53 |year=1981 |pmid = 6264310 |doi=10.1038/291547a0] Poliovirus is one of the most well-characterized viruses, and has become a useful model system for understanding the biology of RNA viruses.

Life cycle

Poliovirus infects human cells by binding to an immunoglobulin-like receptor, CD155, (also known as the "poliovirus receptor" (PVR))cite journal |author=Mendelsohn Cl, Wimmer E, Racaniello VR|title=Cellular receptor for poliovirus: molecular cloning, nucleotide sequence, and expression of a new member of the immunoglobin superfamily |journal=Cell |volume=56 |issue=5 |pages=855–865 |year=1989 |pmid = 2538245 |doi=10.1016/0092-8674(89)90690-9] cite journal |author=He Y, Mueller S, Chipman P, "et al" |title=Complexes of poliovirus serotypes with their common cellular receptor, CD155 | url= http://jvi.asm.org/cgi/content/full/77/8/4827?view=long&pmid=12663789| journal=J Virol |volume=77 |issue=8 |pages=4827–35 |year=2003 |pmid = 12663789 |doi=10.1128/JVI.77.8.4827-4835.2003] on the cell surface.cite journal |author=Dunnebacke TH, Levinthal JD, Williams RC|title=Entry and release of poliovirus as observed by electron microscopy of cultured cells |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=4309884| journal=Journal of Virology |volume=4 |issue=4 |pages=505–513 |year=1969 |pmid = 4309884] Interaction of poliovirus and CD155 facilitates an irreversible conformational change of the viral particle necessary for viral entry.cite journal |author=Kaplan G, Freistadat MS, Racaniello VR, |title=Neutralization of poliovirus by cell receptors expressed in insect cells |url= http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=2168959|journal=Journal of Virology |volume=60 |issue=10 |pages=4697–4702 |year=1990 |pmid = 2168959] cite journal |author=Gomez Yafal A, Kaplan G, Racaniello VR, Hogle, JM |title=Characterization of poliovirus conformational alteration mediated by soluble cell receptors |journal=Virology |volume=197 |issue=1 |pages=501–505 |year=1993 |pmid = 8212594 |doi=10.1006/viro.1993.1621] The precise mechanism poliovirus uses to enter the host cell has not been firmly established.cite book | title = Picornaviruses: The Enteroviruses: Polioviruses "in:" Baron's Medical Microbiology "(Baron S "et al", eds.)| edition = 4th ed. | publisher = Univ of Texas Medical Branch | year = 1996 | url= http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed.section.2862 | isbn = 0-9631172-1-1 ] Attached to the host cell membrane, entry of the viral nucleic acid was thought to occur one of two ways: via the formation of a pore in the plasma membrane through which the RNA is then “injected” into the host cell cytoplasm, or that the virus is taken up by receptor-mediated endocytosis.cite journal |author=Mueller S, Wimmer E, Cello J |title=Poliovirus and poliomyelitis: a tale of guts, brains, and an accidental event |journal=Virus Res |volume=111 |issue=2 |pages=175–93 |year=2005 |pmid = 15885840 |doi=10.1016/j.virusres.2005.04.008] Recent experimental evidence supports the latter hypothesis and suggests that poliovirus binds to CD155 and is taken up via endocytosis. Immediately after internalization of the particle, the viral RNA is released.cite journal |author=Brandenburg B, Lee LY, Lakadamyali M, Rust MJ, Zhuang X, Hogle JM, |title=Imaging poliovirus entry in live cells |doi= 10.1371/journal.pbio.0050183 |journal=PLOS Biology |volume=5 |issue=7 |pages=e183 |year=2007 |pmid = 17622193] However, any mechanism by which poliovirus enters the cell is very inefficient; as an infection is initiated only about 1% of the time.Charles Chan and Roberto Neisa. [http://www.brown.edu/Courses/Bio_160/Projects2000/Polio/TableofContents.html "Poliomyelitis".] Brown University.]

Poliovirus is a positive stranded RNA virus. Thus the genome enclosed within the viral particle can be used as messenger RNA and immediately translated by the host cell. On entry the virus hijacks the cell's translation machinery; causing inhibition of cellular protein synthesis in favor of virus–specific protein production. Unlike the host cell's mRNAs the 5' end of poliovirus RNA is extremely long—over 700 nucleotides—and is highly structured. This region of the viral genome is called internal ribosome entry site (IRES) and it directs translation of the viral RNA. Genetic mutations in this region prevent viral protein production.cite journal |author=Chen CY, Sarnow P |title=Initiation of protein synthesis by the eukaryotic translational apparatus on circular RNAs |journal=Science |volume=268 |issue=5209 |pages=415–417 |year=1995 |pmid = 7536344 |doi=10.1126/science.7536344] cite journal |author=Pelletier J, Sonenberg N |title=Internal initiation of translation of eukaryotic mRNA directed by a sequence derived from poliovirus RNA |journal=Nature |volume=334 |issue=6180 |pages=320–325 |year=1988 |pmid = 2839775 |doi=10.1038/334320a0] cite journal |author=Jang SK, Krausslich HG, Nicklin MJ, Duke GM, Palmenberg AC, Wimmer E |title=A segment of the 5' nontranslated region of encephalomyocarditis virus RNA directs internal entry of ribosome during in vitro translation |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=2839690 |journal=Journal of Virology |volume=62 |issue=8 |pages=2636–2643 |year=1988 |pmid = 2839690]

Poliovirus mRNA is translated as one long polypeptide. This polypeptide is then cleaved into approximately 10 individual viral proteins, including:

*"3D^pol", an RNA dependent RNA polymerase whose function is to copy and multiply the viral RNA genome.
*"2A^pro" and "3C^pro/3CD^pro", proteases which cleave the viral polypeptide.
*"VPg" (3B), a small protein that binds viral RNA and is necessary for synthesis of viral positive and negative strand RNA.
*"2BC, 2B, 2C, 3AB, 3A, 3B" proteins which comprise the protein complex needed for virus replication.
*"VP0, VP1, VP2, VP3, VP4" proteins of the viral capsid.

The assembly of new virus particles, (i.e. the packaging of progeny genome into a capsid which can survive outside the host cell) is poorly understood. Fully assembled poliovirus leaves the confines of its host cell 4 to 6 hours following initiation of infection in cultured mammalian cells.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 |doi=10.1146/annurev.micro.58.030603.123625] The mechanism of viral release from the cell is unclear, but each dying cell can release between 10,000 and 100,000 polio virions.

Origin and serotypes

Poliovirus is structurally similar to other human enteroviruses (coxsackieviruses and echoviruses), as well as to human rhinoviruses, which also use immunoglobulin-like molecules to recognize and enter host cells. Phylogenetic analysis of the RNA and protein sequences of poliovirus suggests that PV may have evolved from a C-cluster coxsackie A virus ancestor, that arose through a mutation within the capsid. [cite journal |author=Jiang P, Faase JA, Toyoda H, "et al" |title=Evidence for emergence of diverse polioviruses from C-cluster coxsackie A viruses and implications for global poliovirus eradication |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=104 |issue=22 |pages=9457–62 |year=2007 |pmid=17517601 |doi=10.1073/pnas.0700451104 |url=http://www.pnas.org/cgi/content/full/104/22/9457] The distinct speciation of poliovirus probably occurred as a result of change in cellular receptor specificity from intercellular adhesion molecule-1 (ICAM-1), used by C-cluster coxsackie A viruses, to CD155; leading to a change in pathogenicity, and allowing the virus to infect nervous tissue.

There are three serotypes of poliovirus, "PV1", "PV2" , and "PV3"; each with a slightly different capsid protein. Capsid proteins define cellular receptor specificity and virus antigenicity. "PV1" is the most common form encountered in nature, however all three forms are extremely infectious. Wild polioviruses can be found in approximately 10 countries. PV1 is highly localized to regions in India, Pakistan, Afghanistan, and Egypt, but following outbreaks of poliomeyletis in 2003–2004 it remains widespread in West and Central Africa. Wild poliovirus type 2 has probably been eradicated; it was last detected in October 1999 in Uttar Pradesh, India.cite journal |author= |title=Transmission of wild poliovirus type 2--apparent global interruption |journal=Wkly. Epidemiol. Rec. |volume=76 |issue=13 |pages=95–7 |year=2001 |month=March |pmid=11315462 |doi= |url=http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5012a2.htm] Wild PV3 is found in parts of only five countries (Nigeria, Niger, Pakistan, India, and Sudan).

Specific strains of each serotype are used to prepare vaccines against polio. Inactive polio vaccine (IPV) is prepared by formalin inactivation of three wild, virulent reference strains, Mahoney or Brunenders (PV1), MEF-1/Lansing (PV2), and Saukett/Leon (PV3). Oral polio vaccine (OPV) contains live attenuated (weakened) strains of the three serotypes of poliovirus. Passaging the virus strains in monkey kidney epithelial cells introduces mutations in the viral IRES, and hinders (or attenuates) the ability of the virus to infect nervous tissue.

Pathogenesis

The primary determinant of infection for any virus is its ability to enter a cell and produce additional infectious particles. The presence of CD155 is thought to define the animals and tissues that can be infected by poliovirus. CD155 is found only on the cells of humans, higher primates, and Old World monkeys. Poliovirus is however strictly a human pathogen, and does not naturally infect any other species (although chimpanzees and Old World monkeys can be experimentally infected). [cite journal |author=Mueller S, Wimmer E |title=Recruitment of nectin-3 to cell-cell junctions through trans-heterophilic interaction with CD155, a vitronectin and poliovirus receptor that localizes to alpha(v)beta3 integrin-containing membrane microdomains | url=http://www.jbc.org/cgi/content/full/278/33/31251| journal=J Biol Chem |volume=278 |issue=33 |pages=31251–60 |year=2003 |pmid=12759359 |doi=10.1074/jbc.M304166200]

Poliovirus is an enterovirus. Infection occurs via the fecal-oral route; meaning that one ingests the virus and it is within the alimentary tract that virus replication occurs.cite book |author = Bodian D and Horstmann DH |title=Polioviruse |publisher=Lippincott |location=Philadelphia, Penn |year=1969 |pages= 430-473] Virus is shed in the feces of infected individuals. In 95% of cases only a primary, transient presence of the virus in the bloodstream occurs (called a viremia) and the poliovirus infection is asymptomatic. In about 5% of cases, the virus spreads, and replicates in other sites such as brown fat, the reticuloendothelial tissues, and muscle. This sustained replication causes a secondary viremia, and leads to the development of minor symptoms such as fever, headache and sore throat. [cite journal |author=Sabin A |title=Pathogenesis of poliomyelitis; reappraisal in the light of new data |journal=Science |volume=123 |issue=3209 |pages=1151–7 |year=1956 |pmid=13337331 |doi=10.1126/science.123.3209.1151] Paralytic poliomyletis occurs in less than 1% of poliovirus infections. Paralytic disease occurs when the virus enters the central nervous system (CNS) and replicates in motor neurons within the spinal cord, brain stem, or motor cortex, resulting in the selective destruction of motor neurons; leading to either temporary or permanent paralysis and, in rare cases, to respiratory arrest and death. In many respects this neurological phase of infection is thought to be an accidental diversion of the normal gastrointestinal infection.

The mechanisms by which poliovirus enters the CNS are poorly understood. Three theories have been suggested to explain its entry, which are not mutually exclusive; all require that the virus first be present in the blood (viremia). One theory is that virus passes directly from the blood into the central nervous system by crossing the blood brain barrier, independent of CD155. [cite journal |author=Yang W, Terasaki T, Shiroki K, "et al"|title=Efficient delivery of circulating poliovirus to the central nervous system independently of poliovirus receptor |journal=Virology |volume=229 |issue=2 |pages=421–8 |year=1997 |pmid=9126254 |issn= |doi=10.1006/viro.1997.8450] A second hypothesis suggests that the virus is transported from the muscle to the spinal cord through nerve pathways by retrograde axonal transport. [cite journal |author=Ohka S. Yang WX, Terada E, Iwasaki K, Nomot A |title=Retrograde transport of intact poliovirus through the axon via the first transport system |journal=Virology |volume=250 |issue=1 |pages=67–75 |year=1998 |pmid=9770421 |issn= |doi=10.1006/viro.1998.9360] [cite journal |author=Ren R, Racaniello V |title=Poliovirus spreads from muscle to the central nervous system by neural pathways |journal=J Infect Dis |volume=166 |issue=4 |pages=747–52 |year=1992 |pmid=1326581 |issn=] A third hypothesis is that the virus is imported into the CNS by infected monocytes or macrophages.

Poliomyelitis is a disease of the central nervous system. However, CD155 is believed to be present on the surface of most or all human cells, so receptor expression does not explain why poliovirus preferentially infects certain tissues. This suggests that tissue tropism is determined after cellular infection. Recent work has suggested that the type I interferon response (specifically that of interferon alpha and beta) is an important factor that defines which types of cells support poliovirus replication. [cite journal |author=Ida-Hosonuma M, Iwasaki T, Yoshikawa T, Nagata N, Sato Y, Sata T, Yoneyama M, Fujita T, Taya C, Yonekawa H, Koike S |title=The alpha/beta interferon response controls tissue tropism and pathogencicity of poliovirus |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=15767446 |journal=Journal of Virology |volume=79 |issue=7 |pages=4460–4469 |year=2005 |pmid=15767446 |doi=10.1128/JVI.79.7.4460-4469.2005] In mice expressing CD155 but lacking the type I interferon receptor, poliovirus not only replicates in tissues where it normally would not, but these mice are also able to be infected orally with the virus. [cite journal |author=Ohka S, Igarashi H, Sakai M, Koike S, Nochi T, Kiyono A, Nomoto A |title=Esstablishment of a poliovirus oral infection system in human poliovirus receptor-expressing transgenic mice that are deficient in alpha/beta interferon receptor |journal=Journal of Virology |volume=81 |issue=15 |pages=7902–7912 |year=2007 |pmid=17507470 |doi=10.1128/JVI.02675-06]

Immune system avoidance

Poliovirus uses two key mechanisms to evade the immune system. First, it is capable of surviving the highly acidic conditions of the gastrointestinal tract, allowing the virus to infect the host and spread throughout the body via the lymphatic system. Second, because it can replicate very quickly, the virus overwhelms the host organs before an immune response can be mounted.cite journal |author=Racaniello V |title=One hundred years of poliovirus pathogenesis |journal=Virology |volume=344 |issue=1 |pages=9–16 |year=2006 |pmid = 16364730 |doi=10.1016/j.virol.2005.09.015]

Individuals who are exposed to poliovirus, either through infection or by immunization with polio vaccine, develop immunity. In immune individuals, antibodies against poliovirus are present in the tonsils and gastrointestinal tract (specifically IgA antibodies) and are able to block poliovirus replication; IgG and IgM antibodies against poliovirus can prevent the spread of the virus to motor neurons of the central nervous system. Infection with one serotype of poliovirus does not provide immunity against the other serotypes, however second attacks within the same individual are extremely rare.

PVR transgenic mouse

Although humans are the only known natural hosts of poliovirus, monkeys can be experimentally infected and they have long been used to study poliovirus. In 1990-91, a small animal model of poliomyelitis was developed by two laboratories. Mice were engineered to express a human receptor to poliovirus (hPVR).cite journal |author=Ren RB, Costantini F, Gorgacz EJ, Lee JJ, Racaniello VR |title=Transgenic mice expressing a human poliovirus receptor: a new model for poliomyelitis |journal=Cell |volume=63 |issue=2 |pages=353–62 |year=1990 |pmid=2170026|doi=10.1016/0092-8674(90)90168-E] cite journal |author=Koike S, Taya C, Kurata T, "et al" |title=Transgenic mice susceptible to poliovirus |url= http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=1846972 |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=88 |issue=3 |pages=951–5 |year=1991 |pmid=1846972|doi=10.1073/pnas.88.3.951]

Unlike normal mice, transgenic poliovirus receptor (TgPVR) mice are susceptible to poliovirus injected intravenously or intramuscularly, and when injected directly into the spinal cord or the brain.cite journal |author=Horie H, Koike S, Kurata T, "et al" |title=Transgenic mice carrying the human poliovirus receptor: new animal models for study of poliovirus neurovirulence | url= http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=8289371| journal=J. Virol. |volume=68 |issue=2 |pages=681–8 |year=1994 |pmid=8289371 |doi=] Upon infection, TgPVR mice show signs of paralysis that resemble those of poliomyelitis in humans and monkeys, and the central nervous systems of paralyzed mice are histocytochemically similar to those of humans and monkeys. This mouse model of human poliovirus infection has proven to be an invaluable tool in understanding poliovirus biology and pathogenicity.cite journal |author=Ohka S, Nomoto A |title=Recent insights into poliovirus pathogenesis |journal=Trends Microbiol. |volume=9 |issue=10 |pages=501–6 |year=2001 |pmid=11597452|doi=10.1016/S0966-842X(01)02200-4]

Three distinct types of TgPVR mice have been well studied:cite journal |author=Koike S, Taya C, Aoki J, "et al" |title=Characterization of three different transgenic mouse lines that carry human poliovirus receptor gene--influence of the transgene expression on pathogenesis |journal=Arch. Virol. |volume=139 |issue=3-4 |pages=351–63 |year=1994 |pmid=7832641|doi=10.1007/BF01310797]

*In TgPVR1 mice the transgene encoding the human PVR was incorporated into mouse chromosome 4. These mice express the highest levels of the transgene and the highest sensitivity to poliovirus. TgPVR1 mice are susceptible to poliovirus through the intraspinal, intracerebral, intramuscular, and intravenous pathways, but not through the oral route.
*TgPVR21 mice have incorporated the human PVR at chromosome 13. These mice are less susceptible to poliovirus infection through the intracerebral route, possibly because they express decreased levels of hPVR. TgPVR21 mice have been shown to be susceptible to poliovirus infection through intranasal inoculation, and may be useful as a mucosal infection model.cite journal |author=Nagata N, Iwasaki T, Ami Y, "et al" |title=A poliomyelitis model through mucosal infection in transgenic mice bearing human poliovirus receptor, TgPVR21 |journal=Virology |volume=321 |issue=1 |pages=87–100 |year=2004 |pmid=15033568 |doi=10.1016/j.virol.2003.12.008]
*In TgPVR5 mice the human transgene is located on chromosome 12. These mice exhibit the lowest levels of hPVR expression and are the least susceptible to poliovirus infection.

Recently a forth TgPVR mouse model was developed. These "cPVR" mice carry hPVR cDNA, driven by a β-actin promoter, and have proven susceptible to poliovirus through intracerebral, intramuscular, and intranasal routes. In addition, these mice are capable of developing the bulbar form of polio after intranasal inoculation.

The development of the TgPVR mouse has had a profound effect on oral poliovirus vaccine (OPV) production. Previously, monitoring the safety of OPV had to be performed using monkeys, because only primates are susceptible to the virus. In 1999 the World Health Organization approved the use of the TgPVR mouse as an alternative method of assessing the effectiveness of the vaccine against poliovirus type-3. In 2000 the mouse model was approved for tests of vaccines against type-1 and type-2 poliovirus.cite journal |author=Dragunsky E, Nomura T, Karpinski K, "et al" |title=Transgenic mice as an alternative to monkeys for neurovirulence testing of live oral poliovirus vaccine: validation by a WHO collaborative study |url= http://www.scielosp.org/scielo.php?script=sci_arttext&pid=S0042-96862003000400006&lng=en&nrm=iso&tlng=en |journal=Bull. World Health Organ. |volume=81 |issue=4 |pages=251–60 |year=2003 |pmid=12764491 |doi=]

Cloning and synthesis

In 1981 Racaniello and Baltimore used recombinant DNA technology to generate the first infectious clone of an animal RNA virus, poliovirus. DNA encoding the RNA genome of poliovirus was introduced into cultured mammalian cells and infectious poliovirus was produced. [cite journal |author=Racaniello V, Baltimore D |title=Cloned poliovirus complemenatry DNA is infectious in mammalian cells |journal=Science |volume=214 |issue=453 |pages=916–919 |year=1981 |pmid=6272391 |doi=10.1126/science.6272391] Creation of the infectious clone propelled understanding of poliovirus biology, and has become a standard technology used to study many other viruses.

In 2002 researchers at SUNY Stony Brook succeeded in synthesizing poliovirus from its chemical code, producing the world's first synthetic virus. [cite journal |author=Cello J, Paul AV, Wimmer E |title=Chemical synthesis of poliovirus cDNA: generation of infectious virus in the absence of natural template |journal=Science |volume=297 |issue=5583 |pages=1016–8 |year=2002 |pmid=12114528 |doi=10.1126/science.1072266] Scientists first converted poliovirus's published RNA sequence, 7741 bases long, into a DNA sequence, as DNA was easier to synthesize. Short fragments of this DNA sequence were obtained by mail-order, and assembled. The complete viral genome was then assembled by a gene synthesis company. This whole painstaking process took two years. Nineteen markers were incorporated into the synthesized DNA, so that it could be distinguished from natural poliovirus. Enzymes were used to convert the DNA back into RNA, its natural state. Other enzymes were then used to translate the RNA into a polypeptide, producing functional viral particle. The newly minted synthetic virus was injected into PVR transgenic mice, to determine if the synthetic version was able to cause disease. The synthetic virus was able to replicate, infect, and cause paralysis or death in mice. However, the synthetic version was between 1,000 and 10,000 times less lethal than the original virus. [cite journal |author=Couzin J |title=Virology. Active poliovirus baked from scratch |journal=Science |volume=297 |issue=5579 |pages=174–5 |year=2002 |pmid=12114601 |doi=10.1126/science.297.5579.174b]

References

External links

* [http://www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/00.052.0.01.001.htm ICTVdb virus classification]