Variola Virus and Smallpox Disease

Orthopoxvirus variola is the virus responsible for the well-known smallpox disease. It belongs to the Poxviridae family which is further split into the subfamilies Entomopoxivirinae which only affects insects, and Chordopoxivirinae which infects vertebrae (Hughes). It is in group one of the Baltimore Classification since it possesses double-stranded DNA. This group also includes viruses in the Herpesviridae family, certain bacteriophages, as well as the mimivirus. The linear genome consists of approximately 186 kb pair and, like all orthopoxviruses, is about 200 nm in diameter (Li; Riedel). Virus particles may be enveloped, but the majority will be nonenveloped when released from a lysed cell, ready and capable to affect another. Extracellular enveloped viruses evolve from their precursors intracellular enveloped virus and cell-associated enveloped virus and contain proteins that aid the virus in neutralizing host cell antibodies to enhance virus spread (Smith). Entrance into the host cell may be accomplished by fusion of endocytosis, contingent on the particular strain. Host cell cytoplasm is the site of poxvirus replication, therefore host nuclear enzymes are unavailable to the virus; to overcome this, DNA-dependent RNA polymerase enters the host with the virus (Hughes).

The wide host range of the Poxviridae family implies that they evolved from organisms that infected early life forms (Bray). The high accuracy of their DNA polymerase has resulted in the variola virus genome being 99.6% conserved amid its own isolates and 98% conserved when compared to its nearest phylogenetic relatives the camelpox virus and the taterapox virus (Bray; Li). This indicates a recent common ancestor. Yet while their sequence is similar, their stru...

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... risk of developing the disease. It was observed that those who had been infected with the clinically similar but less severe cowpox disease by milking cows were also immune to smallpox. This observation led Edward Jenner to his first ever vaccination technique. He inserted the cowpox virus obtained from the scabs of a woman into a boy, and then when the boy was inoculated some time later, he proved immune to smallpox (Fenner). It is impossible to contract smallpox from this inoculation of the less virulent related virus and allowed individuals a way to protect themselves without risk. Later, the smallpox vaccination was adapted by using a different live virus, the vaccinia virus which is more similar to variola than cowpox and therefore provides better cross-immunity. As the practice of vaccination gained popularity, it had a significant impact on life expectancy.