Poxviridae is a family of double-stranded DNA viruses. Vertebrates and arthropods serve as natural hosts. The family contains 28 genera that are assigned to two subfamilies: Chordopoxvirinae and Entomopoxvirinae. Entomopoxvirinae infect insects and Chordopoxvirinae infect vertebrates. Diseases associated with this family include smallpox. Four genera of poxviruses can infect humans: Orthopoxvirus, Parapoxvirus, Yatapoxvirus, Molluscipoxvirus. Only 2 are species-specific to humans: variola virus (VARV), and Molluscum contagiosum (MCV). The most common are the vaccinia virus (seen on the Indian subcontinent) and molluscum contagiosum virus, but Mpox infections are rising (seen in the West, and central African rainforest countries). The similarly named disease chickenpox is not caused by a poxvirus, but by varicella zoster, a herpesvirus. Parapoxvirus and Orthopoxvirus genera are zoonotic.
Etymology The name of the family, Poxviridae, is a legacy of the original grouping of viruses associated with diseases that produced poxes on 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 the most notable member of the family.
History
Diseases caused by poxviruses, 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 circa 1150 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, resulting in the deaths of 3.2 million Aztecs 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. A century after Edward Jenner (1798) 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. The last case of endemic smallpox occurred in Somalia in 1977. Extensive searches over two years detected no further cases, and in 1979 the World Health Organization (WHO) declared the disease officially eradicated. 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. After the September 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. The study of poxviruses within cell cultures, allowed for the discovery of Guanosine-5'-triphosphate 5'-methyl-(GTP) capping, 2'-O-methylation, and 3' polyadenylation; vital molecules in the stability and effective translation of mRNA into proteins.
Microbiology
Structure
Poxviridae viral particles (virions) are generally enveloped (external enveloped virion), although the intracellular 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 brick- or oval-shaped. The virion is exceptionally large, its size ranges from 220-450nm long, 140-260nm wide, and 140-260nm thick, this is likely due to poxviruses encoding all of the vitally needed proteins for their self-sufficient DNA replication and transcription. By comparison, rhinoviruses are 1/10 as large as a typical Poxviridae virion. The genome is a single, linear, double-stranded segment of DNA. On the outer surface membrane it has randomly arranged tubules.
Genome Phylogenetic analysis of 26 different Chordopoxvirinae genomes has shown that the central region of the genome is conserved, and contains ~90 genes. 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 and Camelpox virus form a subgroup. Vaccinia virus is most closely related to CPV-GRI-90. The GC-content of family member genomes differ considerably. 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. The ends of their double-stranded DNA (dsDNA) genomes are closed with a hairpin-like structure which is AT-rich, and incompletely base-paired. This structure can exist in two forms: inverted, and complementary. The DNA genome varies from 130-230 kbp. The genome encodes for many important proteins for viral replication. For vaccinia these are: a helicase-primase (viral gene, D5), single-stranded DNA-binding protein (viral gene, I3), protein kinase (viral gene, B1), 117-kDa DNA polymerase (viral gene, E9), a processivity factor (viral gene, A20) and an uracil DNA glycosylase (viral gene, E4). The 117-kDa DNA polymerase is found within all of the currently sequenced Poxviruses (~30 strains).
Replication
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