Poxviridae

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Poxviridae
Electron micrograph of smallpox virus.jpg
Virus classification OOjs UI icon edit-ltr.svg
(unranked): Virus
Realm: Varidnaviria
Kingdom: Bamfordvirae
Phylum: Nucleocytoviricota
Class: Pokkesviricetes
Order: Chitovirales
Family:Poxviridae
Subfamilies

See text

Poxviridae is a family of double-stranded DNA viruses. Vertebrates and arthropods serve as natural hosts. There are currently 83 species in this family, divided among 22 genera, which are divided into two subfamilies. Diseases associated with this family include smallpox. [1] [2]

Contents

Four genera of poxviruses may infect humans: Orthopoxvirus , Parapoxvirus , Yatapoxvirus , Molluscipoxvirus . Orthopoxvirus: smallpox virus (variola), vaccinia virus, cowpox virus, Mpox virus; Parapoxvirus: orf virus, pseudocowpox, bovine papular stomatitis virus; Yatapoxvirus: tanapox virus, yaba monkey tumor virus; Molluscipoxvirus: molluscum contagiosum virus (MCV). [3] The most common are vaccinia (seen on the Indian subcontinent)[ citation needed ] and molluscum contagiosum, but Mpox infections are rising (seen in west and central African rainforest countries). The similarly named disease chickenpox is not a true poxvirus and is caused by the herpesvirus varicella zoster.

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.[ citation needed ]

History

A) Electron micrograph of poxvirus particles in synovium of a big brown bat, northwestern United States. B) Negative staining of poxvirus particles in cell culture supernatant. Scale bar = 100 nm. Electron micrograph of poxvirus particles in synovium of a big brown bat and negative staining of poxvirus particles in cell culture supernatant.jpg
A) Electron micrograph of poxvirus particles in synovium of a big brown bat, northwestern United States. B) Negative staining of poxvirus particles in cell culture supernatant. Scale bar = 100 nm.

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 circa 1150 years BCE. [4] [5] 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.[ citation needed ]

A century after Edward Jenner 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.[ citation needed ] 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.[ citation needed ]

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. [6] 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. [7] [8] [9]

Microbiology

Structure

Poxviridae virion Poxviridae virion.jpg
Poxviridae virion

Poxviridae viral particles (virions) are generally enveloped (external enveloped virion), though 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 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. [10] By comparison, rhinoviruses are 1/10 as large as a typical Poxviridae virion. [11]

Genome

Phylogenetic analysis of 26 different chordopoxvirus genomes has shown that the central region of the genome is conserved and contains ~90 genes. [12] 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.[ citation needed ]

The GC-content of family member genomes differ considerably. [13] 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.[ citation needed ]

Replication

Poxviridae replication cycle Poxviridae cycle.jpg
Poxviridae replication cycle

Replication of the poxvirus involves several stages. [14] The virus first binds to a receptor on the host cell surface; the receptors for the poxvirus are thought to be glycosaminoglycans.[ citation needed ] After binding to the receptor, the virus enters the cell where it uncoats.[ citation needed ] Uncoating of the virus is a two step process.[ citation needed ] Firstly the outer membrane is removed as the particle enters the cell; secondly the virus particle (without the outer membrane) fuses with the cellular membrane to release the core into the cytoplasm.[ citation needed ] The pox viral genes are expressed in two phases.[ citation needed ] The early genes encode the non-structural protein, including proteins necessary for replication of the viral genome, and are expressed before the genome is replicated.[ citation needed ] The late genes are expressed after the genome has been replicated and encode the structural proteins to make the virus particle.[ citation needed ] The assembly of the virus particle occurs in five stages of maturation that lead to the final exocytosis of the new enveloped virion.[ citation needed ] After the genome has been replicated, the immature virion assembles the A5 protein to create the intracellular mature virion.[ citation needed ] The protein aligns and the brick-shaped envelope of the intracellular enveloped virion.[ citation needed ] These particles are then fused to the cell plasma to form the cell-associated enveloped virion, which encounters the microtubules and prepares to exit the cell as an extracellular enveloped virion.[ citation needed ] The assembly of the virus particle occurs in the cytoplasm of the cell and is a complex process that is currently being researched to understand each stage in more depth.[ citation needed ] 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.[ citation needed ]

The replication of poxvirus is unusual for a virus with double-stranded DNA genome because it occurs in the cytoplasm, [15] although this is typical of other large DNA viruses. [16] Poxvirus encodes its own machinery for genome transcription, a DNA dependent RNA polymerase, [17] which makes replication in the cytoplasm possible. Most double-stranded DNA viruses require the host cell's DNA-dependent RNA polymerase to perform transcription. These host polymerases are found in the nucleus, and therefore most double-stranded DNA viruses carry out a part of their infection cycle within the host cell's nucleus.[ citation needed ]

Evolution

Phylogenetic tree of Poxviridae and distribution of cGAMP nucleases across member species and genera Journal.ppat.1009372.g001.png
Phylogenetic tree of Poxviridae and distribution of cGAMP nucleases across member species and genera

The ancestor of the poxviruses is not known but structural studies suggest it may have been an adenovirus or a species related to both the poxviruses and the adenoviruses. [18]

Based on the genome organisation and DNA replication mechanism a phylogenetic relationships may exist between the rudiviruses ( Rudiviridae ) and the large eukaryal DNA viruses: the African swine fever virus ( Asfarviridae ), Chlorella viruses ( Phycodnaviridae ) and poxviruses (Poxviridae). [19]

The mutation rate in poxvirus genomes has been estimated to be 0.9–1.2 x 10−6 substitutions per site per year. [20] A second estimate puts this rate at 0.5–7 × 10−6 nucleotide substitutions per site per year. [21] A third estimate places the rate at 4–6 × 10−6. [22]

The last common ancestor of the extant poxviruses that infect vertebrates existed 0.5  million years ago. The genus Avipoxvirus diverged from the ancestor 249 ± 69 thousand years ago. The ancestor of the genus Orthopoxvirus was next to diverge from the other clades at 0.3  million years ago. A second estimate of this divergence time places this event at 166,000 ± 43,000 years ago. [21] The division of the Orthopoxvirus into the extant genera occurred ~14,000 years ago. The genus Leporipoxvirus diverged ~137,000 ± 35,000 years ago. This was followed by the ancestor of the genus Yatapoxvirus. The last common ancestor of the Capripoxvirus and Suipoxvirus diverged 111,000 ± 29,000 years ago.[ citation needed ]

An isolate from a fish – salmon gill poxvirus – appears to be the earliest branch in the Chordopoxvirinae. [23] A new systematic has been proposed recently after findings of a new squirrel poxvirus in Berlin, Germany. [24]

Smallpox

The date of the appearance of smallpox is not settled. It most likely evolved from a rodent virus between 68,000 and 16,000 years ago. [25] [26] The wide range of dates is due to the different records used to calibrate the molecular clock. One clade was the variola major strains (the more clinically severe form of smallpox) which spread from Asia between 400 and 1,600 years ago. A second clade included both alastrim minor (a phenotypically mild smallpox) described from the American continents and isolates from West Africa which diverged from an ancestral strain between 1,400 and 6,300 years before present. This clade further diverged into two subclades at least 800 years ago.[ citation needed ]

A second estimate has placed the separation of variola from Taterapox at 3000–4000 years ago. [22] This is consistent with archaeological and historical evidence regarding the appearance of smallpox as a human disease which suggests a relatively recent origin. However, if the mutation rate is assumed to be similar to that of the herpesviruses the divergence date between variola from Taterapox has been estimated to be 50,000 years ago. [22] While this is consistent with the other published estimates it suggests that the archaeological and historical evidence is very incomplete. Better estimates of mutation rates in these viruses are needed.[ citation needed ]

Taxonomy

The species in the subfamily Chordopoxvirinae infect vertebrates and those in the subfamily Entomopoxvirinae infect insects. There are ten recognised genera in the Chordopoxvirinae and three in the Entomopoxvirinae.

The following subfamilies and genera are recognized (-virinae denotes subfamily and -virus denotes genus): [2]

Chordopoxvirinae

Entomopoxvirinae

Both subfamilies also contain a number of unclassified species for which new genera may be created in the future.

Vaccinia virus

The prototypical poxvirus is vaccinia virus, known for its role in the eradication of smallpox. The vaccinia virus is an effective tool for foreign protein expression, as it elicits a strong host immune-response. The vaccinia virus enters cells primarily by cell fusion, although currently the receptor responsible is unknown.[ citation needed ]

Vaccinia contains three classes of genes: early, intermediate and late. These genes are transcribed by viral RNA polymerase and associated transcription factors. Vaccinia replicates its genome in the cytoplasm of infected cells, and after late-stage gene expression undergoes virion morphogenesis, which produces intracellular mature virions contained within an envelope membrane. The origin of the envelope membrane is still unknown. The intracellular mature virions are then transported to the Golgi apparatus where it is wrapped with an additional two membranes, becoming the intracellular enveloped virus. This is transported along cytoskeletal microtubules to reach the cell periphery, where it fuses with the plasma membrane to become the cell-associated enveloped virus. This triggers actin tails on cell surfaces or is released as external enveloped virion.[ citation needed ]

See also

Related Research Articles

<span class="mw-page-title-main">Cowpox</span> Disease of humans and animals

Cowpox is an infectious disease caused by the cowpox virus (CPXV). It presents with large blisters in the skin, a fever and swollen glands, historically typically following contact with an infected cow, though in the last several decades more often from infected cats. The hands and face are most frequently affected and the spots are generally very painful.

<span class="mw-page-title-main">Vaccinia</span> Strain of poxvirus

Vaccinia virus is a large, complex, enveloped virus belonging to the poxvirus family. It has a linear, double-stranded DNA genome approximately 190 kbp in length, which encodes approximately 250 genes. The dimensions of the virion are roughly 360 × 270 × 250 nm, with a mass of approximately 5–10 fg. The vaccinia virus is the source of the modern smallpox vaccine, which the World Health Organization (WHO) used to eradicate smallpox in a global vaccination campaign in 1958–1977. Although smallpox no longer exists in the wild, vaccinia virus is still studied widely by scientists as a tool for gene therapy and genetic engineering.

<i>Parapoxvirus</i> Genus of viruses

Parapoxvirus is a genus of viruses, in the family Poxviridae, in the subfamily Chordopoxvirinae. Like all members of the family Poxviridae, they are oval, relatively large, double-stranded DNA viruses. Parapoxviruses have a unique spiral coat that distinguishes them from other poxviruses. Parapoxviruses infect vertebrates, including a wide selection of mammals, and humans.

Orthopoxvirus is a genus of viruses in the family Poxviridae and subfamily Chordopoxvirinae. Vertebrates, including mammals and humans, and arthropods serve as natural hosts. There are 12 species in this genus. Diseases associated with this genus include smallpox, cowpox, horsepox, camelpox, and mpox. The most widely known member of the genus is Variola virus, which causes smallpox. It was eradicated globally by 1977, through the use of Vaccinia virus as a vaccine. The most recently described species is the Alaskapox virus, first isolated in 2015.

<span class="mw-page-title-main">Camelpox</span> Disease of camels caused by the Camelpox virus

Camelpox is a disease of camels caused by the camelpox virus (CMPV) of the family Poxviridae, subfamily Chordopoxvirinae, and the genus Orthopoxvirus. It causes skin lesions and a generalized infection. Approximately 25% of young camels that become infected will die from the disease, while infection in older camels is generally more mild. Although rare, the infection may spread to the hands of those that work closely with camels.

<i>Molluscum contagiosum virus</i> Species of virus

Molluscum contagiosum virus (MCV) is a species of DNA poxvirus that causes the human skin infection molluscum contagiosum. Molluscum contagiosum affects about 200,000 people a year, about 1% of all diagnosed skin diseases. Diagnosis is based on the size and shape of the skin lesions and can be confirmed with a biopsy, as the virus cannot be routinely cultured. Molluscum contagiosum virus is the only species in the genus Molluscipoxvirus. MCV is a member of the subfamily Chordopoxvirinae of family Poxviridae. Other commonly known viruses that reside in the subfamily Chordopoxvirinae are variola virus and monkeypox virus.

Avipoxvirus is a genus of viruses within the family Poxviridae. Poxviridae is the family of viruses which cause the afflicted organism to have poxes as a symptom. Poxviruses have generally large genomes, and other such examples include smallpox and monkeypox. Members of the genus Avipoxvirus infect specifically birds. Avipoxviruses are unable to complete their replication cycle in non-avian species. Although it is comparably slow-spreading, Avipoxvirus is known to cause symptoms like pustules full of pus lining the skin and diphtheria-like symptoms. These diphtheria-like symptoms might include diphtheric necrotic membranes lining the mouth and the upper respiratory tract. Like other avian viruses, it can be transmitted through vectors mechanically such as through mosquitoes. There is no evidence that this virus can infect humans.

Turkeypox virus is a virus of the family Poxviridae and the genus Avipoxvirus that causes turkeypox. It is one of the most common diseases in the wild turkey population. Turkeypox, like all avipoxviruses, is transmitted either through skin contact or by arthropods acting as mechanical vectors.

<span class="mw-page-title-main">Viral disease</span> Animal or plant disease resulting from a viral infection

A viral disease occurs when an organism's body is invaded by pathogenic viruses, and infectious virus particles (virions) attach to and enter susceptible cells.

<span class="mw-page-title-main">Smallpox</span> Eradicated viral disease

Smallpox was an infectious disease caused by variola virus, which belongs to the genus Orthopoxvirus. The last naturally occurring case was diagnosed in October 1977, and the World Health Organization (WHO) certified the global eradication of the disease in 1980, making smallpox the only human disease to have been eradicated to date.

Yatapoxvirus is a genus of viruses, in the family Poxviridae, in the subfamily Chordopoxvirinae. Monkeys and baboons serve as natural hosts. There are two species in this genus. Diseases associated with this genus include: histiocytomas, tumor-like mass of mononuclear cells.

Chordopoxvirinae is a subfamily of viruses in the family Poxviridae. Vertebrates and arthropods serve as natural hosts. Currently, 52 species are placed in this subfamily, divided among 18 genera. Diseases associated with this subfamily include smallpox.

Capripoxvirus is a genus of viruses in the subfamily Chordopoxvirinae and the family Poxviridae. Capripoxviruses are among the most serious of all animal poxviruses. All CaPV are notifiable diseases to the OIE. Sheep, goat, and cattle serve as natural hosts. These viruses cause negative economic consequences by damaging hides and wool and forcing the establishment of trade restrictions in response to an outbreak. The genus consists of three species: sheeppox virus (SPPV), goatpox virus (GTPV), and lumpy skin disease virus (LSDV). They share no serological relationship with camel pox, horse pox, or avian poxes. Capripoxviruses for sheeppox and goatpox infect only sheep and goat respectively. However, it is probable that North American relatives, the mountain goat and mountain sheep, may be susceptible to the strains but has not been experimentally proven. Lumpy skin disease virus affects primarily cattle, but studies have been shown that giraffes and impala are also susceptible to LSDV. Humans cannot be infected by Capripoxviruses.

<i>Monkeypox virus</i> Species of double-stranded DNA virus

The monkeypox virus, is a species of double-stranded DNA virus that causes mpox disease in humans and other mammals. The monkeypox virus is a zoonotic virus belonging to the orthopoxvirus genus, making it closely related to the variola, cowpox, and vaccinia viruses. MPV is oval-shaped with a lipoprotein outer membrane. The genome is approximately 190 kb.

Orthopoxvirus inclusion bodies are aggregates of stainable protein produced by poxvirus virions in the cell nuclei and/or cytoplasm of epithelial cells in humans. They are important as sites of viral replication.

Leporipoxvirus is a genus of viruses, in the family Poxviridae, in the subfamily Chordopoxvirinae. Lagomorphs and squirrels serve as natural hosts. There are four species in this genus. Diseases associated with this genus include: myxomatosis.

Suipoxvirus is a genus of viruses in the family Poxviridae and subfamily Chordopoxvirinae. Swine serve as natural hosts. There is only one species in this genus: Swinepox virus. Diseases associated with this genus include asymptomatic skin disease.

Crocodylidpoxvirus is a genus of viruses, in the family Poxviridae, in the subfamily Chordopoxvirinae. Crocodiles serve as natural hosts. There is only one species in this genus: Nile crocodilepox virus. Diseases associated with this genus include: nodular skin lesions in young animals. Symptoms vary from a nonfatal dermatitis to more severe disease characterized by ophthalmia, rhinitis resulting in asphyxia, and debilitating illness with stunting and high mortality.

Raccoonpox virus (RCN) is a double-stranded DNA virus and a member of the orthopoxviruses in the family Poxviridae and subfamily Chordopoxvirinae which consists of eight genera: Avipoxvirus, Capripoxvirus, Leporipoxvirus, Molluscipoxvirus, Orthopoxvirus, Parapoxvirus, Suipoxvirus and Yatapoxvirus Vertebrates are the natural host of Chordopoxvirinae subfamily viruses. More specifically, raccoons are the natural hosts of RCN. RCN was isolated in 1961 from the upper respiratory tissues of 2 raccoons in a group of 92 observably healthy raccoons trapped close to Aberdeen, Maryland.

<i>Red deerpox virus</i> Species of virus

Red deerpox virus (RDPV) is a species of virus in the genus Parapoxvirus. It has been reported in deer in New Zealand, and in wild ruminants in Italy.

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