| Poxviridae | |
|---|---|
 | |
Virus classification | |
| (unranked): | Virus |
| Realm: | Varidnaviria |
| Kingdom: | Bamfordvirae |
| Phylum: | Nucleocytoviricota |
| Class: | Pokkesviricetes |
| Order: | Chitovirales |
| Family: | Poxviridae |
| Subfamilies | |
Poxviridae is a family of double-strandedDNAviruses.Vertebrates andarthropods serve as natural hosts. The family contains 22 genera that are assigned to two subfamilies:Chordopoxvirinae andEntomopoxvirinae.Entomopoxvirinae infect insects andChordopoxvirinae infect vertebrates. Diseases associated with this family includesmallpox.[1][2]
Four genera of poxviruses can infect humans:Orthopoxvirus,Parapoxvirus,Yatapoxvirus,Molluscipoxvirus.Orthopoxvirus:smallpox virus (variola),vaccinia virus,cowpox virus,Mpox virus;Parapoxvirus:orf virus,pseudocowpox,bovine papular stomatitis virus;Yatapoxvirus:tanapox virus,yaba monkey tumor virus;Molluscipoxvirus:molluscum contagiosum virus (MCV).[3] The most common are vaccinia (seen on the Indian subcontinent)[citation needed] and molluscum contagiosum, but Mpox infections are rising (seen in west and central African rainforest countries). The similarly named diseasechickenpox is not a true poxvirus and is caused by theherpesvirus,varicella zoster. Parapoxvirus and orthopoxvirus genera are zoonotic.
The name of the family,Poxviridae, is a legacy of the original grouping of viruses associated with diseases that produced poxes on the skin. Modernviral classification is based on phenotypic characteristics; morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. Thesmallpox virus remains the most notable member of the family.[citation needed]
Diseases caused by pox viruses, especially smallpox, have been known about for centuries. One of the earliest suspected cases is that of EgyptianpharaohRamses V who is thought to have died from smallpox circa 1150 years BCE.[4][5] Smallpox was thought to have been transferred to Europe around the early 8th century and then to theAmericas in the early 16th century, resulting in the deaths of 3.2 millionAztecs within two years of introduction. This death toll can be attributed to the indigenous population's complete lack of exposure to the virus over millennia.[citation needed]
A century afterEdward Jenner showed that the less potent cowpox could be used to effectively vaccinate against the more deadly smallpox, a worldwide effort to vaccinate everyone against smallpox began with the ultimate goal to rid the world of the plague-like epidemic.[citation needed] The last case of endemic smallpox occurred in Somalia in 1977. Extensive searches over two years detected no further cases, and in 1979 theWorld Health Organization (WHO) declared the disease officially eradicated.[citation needed]
In 1986, all virus samples were destroyed or transferred to two approved WHO reference labs: at the headquarters of the federalCenters for Disease Control and Prevention (the C.D.C.) inAtlanta, Georgia (the United States) and at the Institute of Virus Preparations in Moscow.[6] After theSeptember 11 attacks in 2001, the American and UK governments have had increased concern over the use of smallpox, or a smallpox-like disease, in bioterrorism. However, several poxviruses including vaccinia virus, myxoma virus, tanapox virus and raccoon pox virus are currently being investigated for their therapeutic potential in various human cancers in preclinical and clinical studies.[7][8][9]
Poxviridae viral particles (virions) are generallyenveloped (external enveloped virion), though theintracellular mature virion form of the virus, which contains different envelope, is also infectious. They vary in their shape depending upon the species but are generally shaped like a brick or as an oval form similar to a rounded brick because they are wrapped by the endoplasmic reticulum. The virion is exceptionally large, its size is around 200nm in diameter and 300nm in length and carries itsgenome in a single, linear, double-stranded segment of DNA.[10] By comparison,rhinoviruses are 1/10 as large as a typicalPoxviridae virion.[11] On the outer surface membrane it has randomly arranged tubules.
Phylogenetic analysis of 26 differentchordopoxvirus genomes has shown that the central region of the genome is conserved and contains ~90 genes.[12] The termini in contrast are not conserved between species. Of this group Avipoxvirus is the most divergent. The next most divergent is Molluscipoxvirus. Capripoxvirus, Leporipoxvirus, Suipoxvirus and Yatapoxvirus genera cluster together: Capripoxvirus and Suipoxvirus share a common ancestor and are distinct from the genus Orthopoxvirus. Within the Othopoxvirus genus Cowpox virus strain Brighton Red, Ectromelia virus and Mpox virus do not group closely with any other member.Variola virus andCamelpox virus form a subgroup. Vaccinia virus is most closely related to CPV-GRI-90.[citation needed]
TheGC-content of family member genomes differ considerably.[13] Avipoxvirus, capripoxvirus, cervidpoxvirus, orthopoxvirus, suipoxvirus, yatapoxvirus and one Entomopox genus (Betaentomopoxvirus) along with several other unclassified Entomopoxviruses have a low G+C content while others - Molluscipoxvirus, Orthopoxvirus, Parapoxvirus and some unclassified Chordopoxvirus - have a relatively high G+C content. The reasons for these differences are not known.[citation needed]
Replication of the poxvirus involves several stages.[14] The replication can be diveded on early, intermadieate and to late phase. The virus first binds to a receptor on the host cell surface; the receptors for the poxvirus are thought to beglycosaminoglycans.[citation needed] After binding to the receptor, the virus enters the cell where it uncoats.[citation needed] Uncoating of the virus is a two step process.[citation needed] Firstly the outer membrane is removed as the particle enters the cell; secondly the virus particle (without the outer membrane) fuses with the cellular membrane to release the core into the cytoplasm.[citation needed] The pox viral genes are expressed in two phases.[citation needed] The early genes encode the non-structural protein, including proteins necessary for replication of the viral genome, and are expressed before the genome is replicated.[citation needed] The late genes are expressed after the genome has been replicated and encode the structural proteins to make the virus particle.[citation needed] The assembly of the virus particle occurs in five stages of maturation that lead to the final exocytosis of the new enveloped virion.[citation needed] After the genome has been replicated, the immature virion assembles the A5 protein to create the intracellular mature virion.[citation needed] The protein aligns and the brick-shaped envelope of the intracellular enveloped virion.[citation needed] These particles are then fused to the cell plasma to form the cell-associated enveloped virion, which encounters the microtubules and prepares to exit the cell as an extracellular enveloped virion.[citation needed] The assembly of the virus particle occurs in the cytoplasm of the cell and is a complex process that is currently being researched to understand each stage in more depth.[citation needed] Considering the fact that this virus is large and complex, replication is relatively quick taking approximately 12 hours until the host cell dies by the release of viruses.[citation needed]
The replication of poxvirus is unusual for a virus with double-strandedDNA genome because it occurs in the cytoplasm,[15] although this is typical of other large DNA viruses.[16] Poxvirus encodes its own machinery for genome transcription, a DNA dependent RNA polymerase,[17] which makes replication in the cytoplasm possible. Most double-stranded DNA viruses require the host cell's DNA-dependent RNA polymerase to perform transcription. These host polymerases are found in thenucleus, and therefore most double-stranded DNA viruses carry out a part of their infection cycle within the host cell's nucleus.[citation needed]
The intermediate phase of replication is critical because, on that stage, the virus affects the host's normal function and modifies it more optimally to itself. For example, the virus can inhibit host apoptosis and block the antiviral state. On the replication, poxviruses have their enzymes for example vaccinia virus has decapping enzymes D9 and D10. Decapping enzymes that belong to the Nudix hydrolase superfamily those it used to remove mRNA 5'cap from viral and host mRNA. By removing 5'cap from the mRNA the virus reduces the accumulation of viral dsRNA and inhibit immune response.
The ancestor of the poxviruses is not known but structural studies suggest it may have been anadenovirus or a species related to both the poxviruses and the adenoviruses.[18]
Based on the genome organisation and DNA replication mechanism a phylogenetic relationships may exist between the rudiviruses (Rudiviridae) and the large eukaryal DNA viruses: the African swine fever virus (Asfarviridae), Chlorella viruses (Phycodnaviridae) and poxviruses (Poxviridae).[19]
The mutation rate in poxvirus genomes has been estimated to be 0.9–1.2 x 10−6 substitutions per site per year.[20] A second estimate puts this rate at 0.5–7 × 10−6 nucleotide substitutions per site per year.[21] A third estimate places the rate at 4–6 × 10−6.[22]
The last common ancestor of the extant poxviruses that infect vertebrates existed0.5 million years ago. The genusAvipoxvirus diverged from the ancestor 249 ± 69 thousand years ago. The ancestor of the genusOrthopoxvirus was next to diverge from the other clades at0.3 million years ago. A second estimate of this divergence time places this event at 166,000 ± 43,000 years ago.[21] The division of theOrthopoxvirus into the extant genera occurred ~14,000 years ago. The genusLeporipoxvirus diverged ~137,000 ± 35,000 years ago. This was followed by the ancestor of the genusYatapoxvirus. The last common ancestor of theCapripoxvirus andSuipoxvirus diverged 111,000 ± 29,000 years ago.[citation needed]
An isolate from a fish – salmon gill poxvirus – appears to be the earliest branch in theChordopoxvirinae.[23] A new systematic has been proposed recently after findings of a new squirrel poxvirus in Berlin, Germany.[24]
The date of the appearance of smallpox is not settled. It most likely evolved from a rodent virus between 68,000 and 16,000 years ago.[25][26] The wide range of dates is due to the different records used to calibrate the molecular clock. One clade was the variola major strains (the more clinically severe form of smallpox) which spread from Asia between 400 and 1,600 years ago. A second clade included both alastrim minor (a phenotypically mild smallpox) described from the American continents and isolates from West Africa which diverged from an ancestral strain between 1,400 and 6,300 years before present. This clade further diverged into two subclades at least 800 years ago.[citation needed]
A second estimate has placed the separation of variola fromTaterapox at 3000–4000 years ago.[22] This is consistent with archaeological and historical evidence regarding the appearance of smallpox as a human disease which suggests a relatively recent origin. However, if the mutation rate is assumed to be similar to that of theherpesviruses the divergence date between variola from Taterapox has been estimated to be 50,000 years ago.[22] While this is consistent with the other published estimates it suggests that the archaeological and historical evidence is very incomplete. Better estimates of mutation rates in these viruses are needed.[citation needed]
The species in the subfamilyChordopoxvirinae infectvertebrates and those in the subfamilyEntomopoxvirinae infectinsects.
The following subfamilies and genera are recognized (-virinae denotes subfamily and -virus denotes genus):[2]
Subfamily:Chordopoxvirinae
Subfamily:Entomopoxvirinae
The prototypical poxvirus isvaccinia virus, known for its role in the eradication of smallpox. The vaccinia virus is an effective tool for foreign protein expression, as it elicits a strong host immune-response. The vaccinia virus enters cells primarily by cell fusion, although currently the receptor responsible is unknown.[citation needed]
Vaccinia contains three classes of genes: early, intermediate and late. These genes are transcribed by viral RNA polymerase and associated transcription factors. Vaccinia replicates its genome in the cytoplasm of infected cells, and after late-stage gene expression undergoes virion morphogenesis, which produces intracellular mature virions contained within an envelope membrane. The origin of the envelope membrane is still unknown. The intracellular mature virions are then transported to the Golgi apparatus where it is wrapped with an additional two membranes, becoming the intracellular enveloped virus. This is transported along cytoskeletal microtubules to reach the cell periphery, where it fuses with the plasma membrane to become the cell-associated enveloped virus. This triggers actin tails on cell surfaces or is released as external enveloped virion.[citation needed]
By special permission of the late President Anwar el Sadat, I was allowed to examine the front upper half of Ramses V's unwrapped mummy in the Cairo Museum in 1979. …Inspection of the mummy revealed a rash of elevated "pustules," each about two to four millimeters in diameter, …(An attempt to prove that this rash was caused by smallpox by electron-microscopic examination of tiny pieces of tissue that had fallen on the shroud was unsuccessful. I was not permitted to excise one of the postules.) …The appearance of the larger pustules and the apparent distribution of the rash are similar to smallpox rashes I have seen in more recent victims
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