Smallpox virus On the Web
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Smallpox is caused by the variola virus, a dsDNA virus of the Poxviridae family. There are two forms of this virus with different virulences, as evidenced by their respective death rates. The virus survives in the cold and aerosoled environments, which explains its oral transmission among humans. Humans are the viruses only host which likely facilitated its eradication. Unlike other DNA viruses, smallpox replicates within the cytoplasm, to which it shows tropism.
Variola virus, also known as smallpox virus, is an orthopoxvirus, from the family Poxviridae, the largest viruses to infect humans. It is a 200-400 nm dsDNA virus, lacking icosahedral symmetry. The other viruses of the family Poxviridae include:
There are 2 forms of variola virus:
- Variola major
- Variola minor
Unlike other DNA viruses, poxviruses replicate within the cytoplasm of the host cell. In order to replicate, poxviruses produce a variety of specialized proteins, not produced by other DNA viruses, the most important of which is a viral-associated DNA-dependent RNA polymerase.
The date of the origin of the smallpox virus is not settled. It most likely evolved from a rodent virus between 68,000 and 16,000 years ago. This broad range of dates is due to the different records used to calibrate the molecular clock. It appears that the smallpox virus derived from a remote zoonosis from another animal host, that is today extinct.
Little is known about the mechanism responsible for host species tropism of smallpox virus. The virus is known to bind mammalian cells unspecifically. There appears to be no particular extracellular receptors involved in viral internalization and initial transcription. However, intracellular availability of trans-acting factors and viral capacity to block host cells antiviral response, such as the interferon pathway, are though to be important intracellular factors, determining viral tropism. The overall immune response by the host towards the virus, will be the key determinant of the infection's outcome and potential transmission to other hosts.
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