Poxviridae

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

The only poxvirus not in genus Orthopoxvirus known to specifically infect humans is the molluscum contagiosum virus (MCV).^[1]

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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. The virion 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.^[2] By comparison, Rhinovirus is 1/10th as large as a typical Poxviridae virion.^[3]

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 currently unknown. 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 only 12 hours approximately. The replication of this virus is unusual for a virus with double stranded DNA genome because it encodes its own machinery for genome replication and therefore the replication occurs in the cytoplasm. Most viruses with a double stranded DNA genome replicate in the nucleus and use the host cells genome replication machinery.

Taxonomy

The name of the family, Poxviridae, is a legacy of the original grouping of viruses associated with diseases that produced poxs in the skin. Modern viral classification is based on the shape and molecular features of viruses, and the smallpox virus remains as the most notable member of the family.

The following genera are currently included here:

• Subfamily Chordopoxvirinae
  • Genus Orthopoxvirus; type species: Vaccinia virus; diseases: cowpox, vaccinia, smallpox
  • Genus Parapoxvirus; type species: Orf virus
  • Genus Avipoxvirus; type species: Fowlpox virus
  • Genus Capripoxvirus; type species: Sheeppox virus
  • Genus Leporipoxvirus; type species: Myxoma virus
  • Genus Suipoxvirus; type species: Swinepox virus
  • Genus Molluscipoxvirus; type species: Molluscum contagiosum virus
  • Genus Yatapoxvirus; type species: Yaba monkey tumor virus
• Subfamily Entomopoxvirinae
  • Genus Entomopoxvirus A; type species: Melolontha melolontha entomopoxvirus
  • Genus Entomopoxvirus B; type species: Amsacta moorei entomopoxvirus
  • Genus Entomopoxvirus C; type species: Chironomus luridus entomopoxvirus

Examples

Vaccinia virus

Main article: Vaccinia virus

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 elicite 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.

History

Viruses, especially smallpox have been known about for centuries. One of the earliest documented evidence is of the Egyptian pharaoh Ramses V who is known to have died from smallpox nearly 2000 years BC. Smallpox was thought to have been transferred to Europe around the early 700s and then to the Americas in the early 1500s. 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 can attributed to lack of sensitization to the virus as a child and therefore the Aztecs had no immunity. After Edward Jenner showed that you could use the less potent cow pox to effectively vaccinate against the more deadly smallpox, a worldwide effort to vaccinate everyone against smallpox was started (a century later) with the final goal to rid the world of what had become a plague-like epidemic. The World Health Organization (WHO) declared the virus officially eradicated in 1977, with samples retained at laboratories within the two then global superpowers, United States and the Soviet Union. Post September 11 2001 the American and UK governments have had increased concern over the use of smallpox or small pox like disease, in bio-terrorism.

References

1. ^ (August 1996) "Pathogenic Molluscum Contagiosum Virus Sequenced". Antiviral Agents Bulletin: 196-7. Biotechnology Information Institute. Retrieved on 2006-07-16.
2. ^ International Committee on Taxonomy of Viruses (2004-06-15). ICTVdb Descriptions: 58. Poxviridae. Retrieved on 2005-02-26.
3. ^ How Big is a ... ? at Cells Alive!. Retrieved 2005-02-26.