Senin, 17 Oktober 2011

Virus Famili Poxviridae

Poxviruses (members of the family Poxviridae) are viruses that can, as a family, infect both vertebrate and invertebrate animals.

Four genera of poxviruses may infect humans: orthopox, parapox, yatapox, molluscipox. Orthopox: smallpox virus (variola), vaccinia virus, cowpox virus, monkeypox virus; Parapox: orf virus, pseudocowpox, bovine papular stomatitis virus; Yatapox: tanapox virus, yaba monkey tumor virus; Molluscipox: molluscum contagiosum virus (MCV). The most common are vaccinia (seen on Indian subcontinent) and molluscum contagiousum, but monkeypox infections are rising (seen in west and central African rainforest countries).
Structure

Poxviridae viral particles (virions) are generally enveloped (external enveloped virion- EEV), though the intracellular mature virion (IMV) 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 200 nm in diameter and 300 nm in length and carries its genome in a single, linear, double-stranded segment of DNA. By comparison, Rhinovirus is 1/10 as large as a typical Poxviridae virion.

Replication
Replication of the poxvirus involves several stages. The first thing the virus does is to bind to a receptor on the host cell surface; the receptors for the poxvirus are thought to be Glycosaminoglycans (GAGs). After binding to the receptor, the virus enters the cell where it uncoats. Uncoating of the virus is a two step process. Firstly the outer membrane is removed as the particle enters the cell; secondly the virus particle (without the outer membrane) is uncoated further to release the core into the cytoplasm. The pox viral genes are expressed in two phases. The early genes are expressed first. These genes encode the non-structural protein, including proteins necessary for replication of the viral genome, and are expressed before the genome is replicated. The late genes are expressed after the genome has been replicated and encode the structural proteins to make the virus particle. The assembly of the virus particle occurs in the cytoskeleton of the cell and is a complex process that is poorly understood but is currently being researched. 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.

The replication of poxvirus is unusual for a virus with double-stranded DNA genome (dsDNA) because it occurs in the cytoplasm[4] Poxvirus encodes its own machinery for genome transcription, a DNA dependent RNA polymerase, which makes replication in the cytoplasm possible. Most dsDNA viruses require the host cell's proteins to perform transcription. These host proteins are found in the nucleus, and therefore most dsDNA viruses carry out a part of their infection cycle within the host cell's nucleus.

axonomy
The name of the family, Poxviridae, is a legacy of the original grouping of viruses associated with diseases that produced poxes in the skin. Modern viral classification is based on phenotypic characteristics; morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. The smallpox virus remains as the most notable member of the family.

The following genera are currently included here:

    Subfamily Chordopoxvirinae
        Genus Avipoxvirus; type species: Fowlpox virus; species: Canarypox virus, Fowlpox virus, Juncopox virus, Mynahpox virus, Pigeonpox virus, Psittacinepox virus, Quailpox virus, Sparrowpox virus, Starlingpox virus, Turkeypox virus
        Genus Capripoxvirus; type species: Sheeppox virus; species: Goatpox virus, Lumpy skin disease virus, Sheeppox virus
        Genus Cervidpoxvirus; type species: Deerpox virus W-848-83; species: Deerpox virus W-848-83
        Genus Leporipoxvirus; type species: Myxoma virus; species: Hare fibroma virus, Myxoma virus, Rabbit fibroma virus, Squirrel fibroma virus
        Genus Molluscipoxvirus; type species: Molluscum contagiosum virus; species: Molluscum contagiosum virus
        Genus Orthopoxvirus; type species: Vaccinia virus; species: Camelpox virus, Cowpox virus, Ectromelia virus, Monkeypox virus, Raccoonpox virus, Taterapox virus, Vaccinia virus, Variola virus, Volepox virus
        Genus Parapoxvirus; type species: Orf virus; species: Bovine papular stomatitis virus, Orf virus, Parapoxvirus of red deer in New Zealand, Pseudocowpox virus
        Genus Suipoxvirus; type species: Swinepox virus; species: Swinepox virus
        Genus Unassigned; species: Squirrel poxvirus
        Genus Yatapoxvirus; type species: Yaba monkey tumor virus; species: Tanapox virus, Yaba monkey tumor virus
    Subfamily Entomopoxvirinae
        Genus Alphaentomopoxvirus; type species: Melolontha melolontha entomopoxvirus; species: Anomala cuprea entomopoxvirus, Aphodius tasmaniae entomopoxvirus, Demodema boranensis entomopoxvirus, Dermolepida albohirtum entomopoxvirus, Figulus subleavis entomopoxvirus, Geotrupes sylvaticus entomopoxvirus, Melolontha melolontha entomopoxvirus
        Genus Betaentomopoxvirus; type species: Amsacta moorei entomopoxvirus 'L'; species: Acrobasis zelleri entomopoxvirus 'L', Amsacta moorei entomopoxvirus 'L', Arphia conspersa entomopoxvirus 'O', Choristoneura biennis entomopoxvirus 'L', Choristoneura conflicta entomopoxvirus 'L', Choristoneura diversuma entomopoxvirus 'L', Choristoneura fumiferana entomopoxvirus 'L', Chorizagrotis auxiliars entomopoxvirus 'L', Heliothis armigera entomopoxvirus 'L', Locusta migratoria entomopoxvirus 'O', Oedaleus senigalensis entomopoxvirus 'O', Operophtera brumata entomopoxvirus 'L', Schistocera gregaria entomopoxvirus 'O'
        Genus Gammaentomopoxvirus; type species: Chironomus luridus entomopoxvirus; species: Aedes aegypti entomopoxvirus, Camptochironomus tentans entomopoxvirus, Chironomus attenuatus entomopoxvirus, Chironomus luridus entomopoxvirus, Chironomus plumosus entomopoxvirus, Goeldichironomus haloprasimus entomopoxvirus
        Genus Unassigned; species: Diachasmimorpha entomopoxvirus

The prototype of poxvirus family is vaccinia virus, which has been used as a successful vaccine to eradicate smallpox virus. Vaccinia virus is also used as an effective tool for foreign protein expression to elicit strong host immune response. Vaccinia virus enters cells mainly by cell fusion, although currently the receptor is not known. Virus contains three classes of genes, early, intermediate and late, that are transcribed by viral RNA polymerase and associated transcription factors. Vaccinia virus replicates its genome in cytoplasm of the infected cells and after late gene expression virion morphogenesis produces IMV that contains envelope, although the origin of the envelope membrane is still unknown. IMV is transported to Golgi to be wrapped additional two membrane to become intracellular enveloped virus (IEV). IEV transports along microtubules to reach cell periphery and fuse with plasma membrane to become cell-associated enveloped virus (CEV) that triggers actin tails on cell surfaces or is releared as EEV.
Diseases caused by pox viruses, especially smallpox, have been known about for centuries. One of the earliest suspected cases is that of Egyptian pharaoh Ramses V who is thought to have died from smallpox nearly 2000 years BCE.

Smallpox was thought to have been transferred to Europe around the early 8th century and then to the Americas in the early 16th century. It is widely accepted that the main defeat of the Aztecs was due to a smallpox epidemic and within two years over 3.2 million Aztecs died. This death toll can be attributed to the American population's complete lack of sensitization to the virus as children. A century after Edward Jenner showed that the less potent cow pox 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. The World Health Organization (WHO) declared the virus officially eradicated in 1977 and after nine years, in 1986, all virus samples were destroyed or transferred to two approved WHO reference labs: at the headquarters of the federal Centers for Disease Control and Prevention (the C.D.C.) in Atlanta, Georgia (the United States) and at the Institute of Virus Preparations in Moscow.  Post September 11, 2001 the American and UK governments have had increased concern over the use of smallpox, or a small pox like disease, in bio-terrorism.

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