Poxviridae

Introduction

Poxviridae (Anglo-Saxon “pock”, meaning pustule) is a virus family that infects both vertebrates and invertebrates. Throughout human history, Poxviruses have held great clinical significance.  Perhaps the most famous of these viruses is variola (Latin, meaning “spotted”), more commonly known as smallpox.  According to estimates given by the World Health Organization, 300-500 million deaths resulted from smallpox alone in the 20th century. While humans serve as the natural reservoir for several poxviruses (including smallpox), animals frequently serve as the reservoir for human infections through the process of zoonosis.

The study of poxviruses has yielded important developments in the field of virology. Indeed, the family boasts the first successful vaccine ever created – the smallpox vaccine, created by Edward Jenner in 1798.  The implementation of this vaccine also led to the first successful viral eradication effort.

Through vaccination, careful monitoring, and public health campaigns around the world, the virus was eliminated from the natural world in 1977.


Timeline

Thom_Jenner-smallpoxlg910: the first medical report on smallpox, Treatise on Smallpox and Measles, was published by the Arabian scholar Rhazes

1520: introduction of smallpox to the New World via slave ships

1721: the practice of variolation, invented a few thousand years earlier, was introduced in England and subsequently became widespread.  The practice involves infecting individuals through an alternate route of transmission (skin-skin contact vs. respiratory) by rubbing pustule matter on the skin of normal individuals, resulting in a mild infection.

1798: Edward Jenner publishes An Inquiry into the Causes and Effects of the 

Variolae Vaccinae; A Disease Discovered in Some of the Western Countries of England, particularly Gloucestershire, and Known by the Name of the Cow Pox, a paper that recognizes the immunity conferred against smallpox from cowpox infection, effectively creating the first-ever vaccine.

1802: The safety and effectiveness of the smallpox vaccine was confirmed by the American physician Benjamin Waterhouse

1949: The last known case of smallpox in the United States (Rio Grande Valley, Texas)

1958: Monkeypox discovered in monkeys, and subsequently found to exist in African rodents

1966: The Worldwide Smallpox Eradication Programme was established by the World Health Assembly

1970: Monkeypox reported in humans

1977: Ali Maalin becomes the last person to naturally contract variola (variola minor)

in the world in Somalia. Depicted on right. Retrieved from vaccinenewsdaily.com

1978: Janet Parker and her mother die of smallpox, as a result of a laboratory accident at the University of Birmingham Medical School in England under Professor Henry Bedson

e5e336811ab20b968943583ae7cac7191979: Eradication of smallpox declared by the World Health Organization.

1996: Recommendation by the World Health Assembly for the remaining laboratory stocks of smallpox to be destroyed

2003: Outbreak of monkeypox reported in the Midwest of the United States; was found to be attributed to prairie dogs

2014: The World Health Organization postpones destruction of remaining laboratory stocks of smallpox; will reconvene in 2017 to discuss the matter again


Classification

Poxviruses share several features that characterize them as a member of Poxviridae: a monopartite, linear genome consisting of double-stranded DNA, a linear genome, a “complex” morphology composed of a biconcave core with an ovoid or brick-shaped nucleocapsid, and an extremely large size relative to other human viruses (200-350nm).  Poxviridae is further subdivided into two subfamilies: chordopoxvirinae (infects humans and other animals) and entomopoxvirinae (infects arthropods).


Nucleocapsid Morphology of Poxviruses

imgres-1Interestingly enough, Poxviridae is the only virus family that contains both non-enveloped and enveloped members, prompting some scholars to argue for the subdivision of Poxviridae as two distinct viral families. Under the current taxonomic classification system however, two of the genuses in the family are used to distinguish the presence of an envelope in poxviruses: orthopoxviruses (enveloped) and parapoxviruses (non-enveloped).  Other genuses in the family include: avipoxvirus, capripoxvirus, leporipoxvirus, suipoxvirus, molluscipoxvirus, and yatapoxvirus.


Viral Replication

UntitledThe process of viral replication in poxviruses begins with viral attachment to surface cell receptors, followed by entry of the virus via endocytosis by the host cell.  The cell tropism for poxviruses are typically epidermal cells.  Once in the host cell, the virus uncoats its viral DNA in the cytoplasm, and begins the process of replication in viral replication factories called virosomes.

Poxviruses are the only DNA virus that replicate in the cytoplasm and the only DNA virus with its own transcriptional machinery. As a result, they can direct replication soon after cell entry, but also must encode a significant amount of information in their genome.  In all, the virus encodes more than 100 genes, including the enzymes DNA polymerase and DNA-dependent RNA transcriptase.  Enzymes like poly(A) polymerase and mRNA capping enzymes further aid in viral replication by equipping the virus with a poly(A) tail and an mRNA cap in preparation for translation.

Poxviruses have three kinetic classes that allow transcription of certain genes to be prioritized and transcribed sequentially.  The early class allows for transcription of several critical genes, including the uncoating enzyme, critical for the start of viral translation.

Following transcription, translation of the transcript takes place in the host cell cytoplasm, where viruses assemble in viral “factories”.  Once assembled, poxviruses facilitate release in one of two ways.  Enveloped poxviruses bud from the host cell, while non-enveloped viruses trigger cell lysis.  Replication depicted on right.


Clinical Presentation

Poxviruses are generally transmitted via respiratory secretions or skin-skin contact.

Poxviruses for which humans are the natural reservoir:

  1. Variola (Orthopoxvirus)

Variola actually exists in two forms: variola major and variola minor (alastrim).  Variola major is the more severe form of smallpox, with a mortality rate of 10-20% compared to ~1% in variola minor.  Thanks to eradication efforts in the 1960s and 1970s, the virus is no longer naturally found in the world, and can only be found in stocks kept in laboratories in the U.S.

(CDC in Atlanta) and Russia (Vector in Koltsovo).

Symptoms: Pustules, high fever, chills, vomiting, can cause blindness if lesions occur on eyes.

smallpox-baby

Prevention: Vaccination with the live-attenuated vaccinia vaccine, derived from vaccinia (cowpox)

Treatment: 1. Cidofovir, a nucleotide analog that inhibits proper viral replication 2. CMX-001, an inhibitor of viral polymerase 3. Tecovirament, a drug that prevents viral egress from the host cell.

Drug Profile: Cidofovir

Drug name (brand name) Cidofovir (Vistide)
Drug Class Nucleotide analog
Mechanism of Action Inhibits replication through the inhibition of DNA polymerase
Side Effects Nausea, vomiting, headache, chills
Other viral targets Cytomegalovirus (licensed), HSV (not licensed)
Method of Delivery Intravenous injection
  1. Molluscum Contagiosum (Molluscipoxvirus)

molluscum-contagiosum-pictures-3Molluscum Contagiosum is a poxvirus that is transmitted by direct skin contact or sexual contact that results in skin-colored papules on the skin. It is a mild condition, and symptoms typically resolve themselves within a few days-weeks. While the immune system is usually able to fight off the virus before symptoms develop, HIV infected individuals as well as other immunocompromised individuals show higher rates of disease.

Symptoms: skin-colored, smooth, waxy papules (2-10mm diameter)

Prevention: abstinence, safe sex, avoiding direct contact with those presenting papules

Treatment: liquid nitrogen on papules

Table 1 Poxviruses for which humans are not the natural reservoir (zoonoses)

Virus Classification (Genera) Symptoms (all have cutaneous lesions) Geographic Distribution Other risk factors Prevention and Treatment
Cowpox Orthopoxvirus Flu-like malaise, fever, edema, redness of skin and mucous membranes, swollen lymph nodes. UK, Europe, Asia (near former USSR) Exposure to cows (often milkers), and small rodents, cats Avoid exposure to cows, and small rodents; antipyretics, pain-relievers
Monkeypox Orthopoxvirus Rash, fever, respiratory symptoms Western and Central Africa Exposure to monkeys, exotic animals from Africa Avoid exposure to monkeys, and exotic animals imported from Africa
Buffalopox Orthopoxvirus Mild.  Lesions limited usually to hands/arms Indian subcontinent Exposure to water buffalo (often milkers) Avoid exposure to water buffalo; pain-relievers
Orf Papaxovirus Mild. Few lesions, usually on hands Worldwide Exposure to sheep and goats Avoid exposure to sheep and goats; pain-relievers
Cantagalo Orthopoxvirus Mild. Lesions limited to hands/arms. South America Exposure to cattle and rodents Avoid exposure to cattle and rodents; pain-relievers
Tanapox Yatapoxvirus Few lesions, usually on legs and trunk.  Malaise and swollen lymph nodes. Flood plains of Kenya Exposure to monkeys Avoid exposure to monkeys; pain-relievers

*Varicella zoster virus, more commonly known as chickenpox, is actually a misnomer, as a member of Herpesviridae

Of particular note above is monkeypox, an emerging virus.  First detected in African monkeys in 1958, the virus has been responsible for several human outbreaks.  In 1970, the first human case of monkeypox was reported in Zaire. Sustained human to human transmission occurred for the first time in the Republic of Congo in 2003, and an outbreak in the United States occurred in 2003. The suspected culprit behind the U.S. outbreak was the Gambian giant rate, which was believed to have infected the victim’s prairie dog.


10 Things to Remember About Pox Virus

  1. 1st virus family to have a vaccine (variola/smallpox)
  2. 1st virus family to have a member eradicated (variola/smallpox)
  3. Only human virus family with enveloped and non-enveloped members
  4. Very wide host range and many zoonoses
  5. Monkeypox is an emerging virus that requires close monitoring
  6. Smallpox vaccine was made by Edward Jenner
  7. Only to viruses in the family with a natural human reservoir (variola and molluscum contagiosum)
  8. Only DNA virus to replicate in the cytoplasm
  9. Only DNA virus with its own transcriptional machinery (this is because it replicates in the cytoplasm and cannot use host cell polymerases in the nucleus to create mRNA)
  10. Three kinetic classes allow transcription of certain proteins to be prioritized

Recent Discoveries

  • It is known that the vaccinia virus is able to evade the immune system, specifically evading detection by MHC class II complexes. Recently, it was discovered that inhibition of MHCII antigen presentation can be partially resolved by ectopically expressing CD74, a chaperone protein that is found at significantly reduced levels upon vaccinia infection.
  • Decapping enzymes created by poxviruses increase their virulence by preventing the build-up of dsRNA, which is normally detected by the host innate immune system
  • Poxvirus-based vaccines continue to influence the development of other vaccines, including MVA-CHIKV, a vaccine that protects mice against Chikungunya infection
  • Analysis of a vaccinia virus protein, a viral membrane assembly protein called H7, has found a new binding fold critical for viral replication that can be a target for new drugs.
  • A novel zoonotic poxvirus has been found in two patients in the United States.  The new poxvirus bears 88% similarity to the Parapoxvirus genus and 78% similarity to the Molluscipox genus
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