POXVIRUSES (MEMBERS OF THE FAMILY POXVIRIDAE) ARE VIRUSES THAT CAN AS A FAMILY, INFECT BOTH VERTEBRATE AND INVERTEBRATE ANIMALS.

THE SMALLPOX VIRUS REMAINS AS THE MOST NOTABLE MEMBER OF THE FAMILY.

Poxviridae

Four genera of poxviruses may infect

humans: orthopox, parapox, yatapox, molluscipox.

Taxonomy•Subfamily Chordopoxvirinae

•Human pathogenic genera • Genus Orthopoxvirus;

• diseases: cowpox, vaccinia, smallpox ,monkeypox

• Genus Parapoxvirus; type species: Orf virus • Genus Molluscipoxvirus; type species: Molluscum

contagiosum virus • Genus Yatapoxvirus; type species: Yaba monkey

tumor virus • Nonpathogenic genera

• Genus Avipoxvirus; type species: Fowlpox virus

• Genus Capripoxvirus; type species: Sheeppox virus

• Genus Leporipoxvirus; type species: Myxoma virus

• Genus Suipoxvirus; type species: Swinepox virus

•Subfamily Entomopoxvirinae • Genus Entomopoxvirus A• Genus Entomopoxvirus B• Genus Entomopoxvirus C

Structure

viral particles (virions) are generally enveloped (external enveloped virion- EEV), though the intracellular mature virion (IMV) form of the virus, which contains different envelope and is also infectious.generally shaped like a brick or as an oval form similar to a

rounded brick. 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.

Oval or "brick-shaped" particles 200-400nm long - can be visualized by the best light microscopes. The external surface is ridged in parallel rows, sometimes arranged helically. The particles are extremely complex, containing many proteins (more than 100) and detailed structure is not known.The extracellular forms contain 2 membranes (EEV - extracellular enveloped virions), intracellular particles only have an inner membrane (IMV - intracellular mature virions).

Thin sections in E.M. reveal that the outer surface is composed of lipid and protein which surrounds the core, which is biconcave (dumbbell-shaped), with two "lateral bodies" (function unknown). The core is composed of a tightly compressed nucleoprotein.

Replication

involves several stages

the virus does is to bind to a receptor on the host cell surfaceAfter binding to the receptor, the virus enters the cell where it uncoats. Uncoating of the virus is a two step process. Firstly the outer membrane is removed as the particle enters the cell; secondly the virus particle (without the outer membrane) is uncoated further to release the core into the cytoplasm.

The pox viral genes are expressed in two phases. The early genes are expressed first. These genes encode the non-structural protein, including proteins necessary for replication of the viral genome, and are expressed before the genome is replicated. The late genes are expressed after the genome has been replicated and encode the structural proteins to make the virus particle.

Poxviruses are unique among DNA viruses in that they replicate in the cytoplasm of the cell rather than in the nucleus. 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.

replication is relatively quick taking approximately 12 hours1. EntryIntracellular mature virion (IMV) particles bind to unknown receptor(s) and fuse with the cell membrane. Extracellular enveloped virion (EEV) particles bind to unknown receptor(s) and are endocytosed into the cell.2. Initial UncoatingThe viral core particle (CORE) containing the viral genome, the viral DNA-dependent RNA polymerase, and other enzymes is released into the cytoplasm.3. Early TranscriptionEarly genes (including those coding for immunomodulatory proteins, enzymes, and replication and transcription factors) are transcribed and translated immediately upon core particle entry into the cytoplasm of the cell.4. TranslocationThe viral core particle translocates to the outside of the cell nucleus.5. Secondary UncoatingThe viral nucleoprotein (NP) complex, which contains the viral genome, is released. At this point the viral genome is replicated as a concatemer and transcription and translation of intermediate genes (mainly coding for transcription factors) occurs.6. Late TranscriptionThe viral late genes (coding for structural proteins, enzymes, and transcription factors) are transcribed and translated.7. AssemblyConcatemeric intermediates are resolved into linear double-stranded DNA and packaged with late viral proteins into immature virions (IV).8. ReleaseIVs mature into IMVs via an undescribed mechanism which may include processing of the IV through the Golgi apparatus. The IMVs are transported to the periphery of the cell where they are released in one of three ways. IMVs released via cell lysis remain IMVs. Alternatively, IMVs can bud through to the cell surface, picking up a viral envelope from the cell plasma membrane. Lastly the IMV can bud through the plasma membrane picking up an envelope and becoming an EEV.

Poviruses are unique among DNA viruses because they replicate only in the cytoplasm of the host cell, outside of the nucleus

IMV consists of a single lipoprotein membrane, while the EEV is surrounded by two membrane layers

The IMV is the most abundant infectious form and is thought to be responsible for spread between hosts and the EEV is thought to be important for long range dissemination within the host organism.

HUMAN INFECTION

Cowpox - is acquired by humans usually by milking cows;  it then manifests as ulcerative lesions (sometimes called "milkers nodules") on the hands of dairy workers. It was noted to protect against smallpox and was used by Jenner as a vaccine strain to protect persons against smallpox.  Despite its name, rodents are the main reservoir of cowpox;  it spreads secondarily to cows and domestic cats.

Molluscum contagiosum - is a minor infectious warty papule of the skin with a central umbilication, transferred by direct contact

Molluscum contagiosum virion

Monkey pox - is a rare smallpox like disease of children in central Africa. It is acquired from monkeys or wild squirrels, but does occasionally spread from man to man in unvaccinated communities. Antigenically cross-reacts with other poxviruses. Sick monkeys have not been identified, but apparently healthy animals have antibodies. Pseudocowpox - occurs worldwide and is a disease primarily of cattle.  In humans it causes non-ulcerating "milker's nodes".

ORF - a worldwide occupational disease associated with handling sheep and goats afflicted with "scabby mouth".  In humans it manifests as a single painless, papulo-vesicular lesion on the hand, forearm or face.

Orf virus (parapox virus)

occurring primarily in sheep and goatsas contagious pustular dermatitis (CPD)

humans can contract this disorder through direct contact with infected sheep and goats or with fomites carrying the orf virus. It causes a purulent-appearing papule locally and generally no systemic symptoms. Infected locations can include the finger, hand, arm, face The papule may persist for 7 to 10 weeks and spontaneously resolves

Orf pocks on thumb

Vaccinia virus

large, complex, enveloped virus, closey related to the virus that causes cowpox

linear, double-stranded DNA genomeThe dimensions of the virion are roughly 360 × 270 × 250 nm.

well-known for its role as a vaccine that eradicated the smallpox disease(variola)

infection is very mild and is typically asymptomatic in healthy individuals, but it may cause a mild rash and fever.

Yaba monkey tumor virus: The type species of yatapoxvirus, a tumor-producing DNA virus discovered in monkeys in Yaba, Nigeria. It has been found to produce histiocytomas (proliferation of tissue macrophages) in monkeys and humans.

Smallpox (Orthopoxvirus infection )

an infectious disease unique to humans, caused by either of two virus variants, Variola major and Variola minor.

localizes in small blood vessels of the skin and in the mouth and throat.

in the skin, this results in a characteristic maculopapular rash, and later, raised fluid-filled blisters. V. major produces a more serious disease and has an overall mortality rate of 30–35%. V. minor causes a milder form of disease

Long-term complications of V. major infection include characteristic scars, commonly on the face, which occur in 65–85% of survivors.Blindness resulting from corneal

ulceration and scarring, and limb deformities due to arthritis and osteomyelitis are less common complications, seen in about 2–5% of cases.

Both enveloped and unenveloped virions are infectious.

Transmission

through inhalation of airborne variola virus, usually droplets expressed from the oral, nasal, or pharyngeal mucosa of an infected person.

through direct contact with infected bodily fluids or contaminated objects (fomites) such as bedding or clothing.The virus can cross the placenta, but the incidence of congenital smallpox is relatively low

Signs and symptoms

There are two clinical forms of smallpox

Variola major is the severe and most common form of smallpox, with a more extensive rash and higher fever.

Variola minor is a less common presentation of smallpox, and a much less severe disease, with historical death rates of 1% or less.[

The incubation period between contraction and the first obvious symptoms of the disease is around 12 days. Once inhaled, variola virus invades the oropharyngeal (mouth and throat) or the respiratory mucosa, migrates to regional lymph nodes, and begins to multiply. In the initial growth phase the virus seems to move from cell to cell, but around the 12th day, lysis of many infected cells occurs and the virus is found in the bloodstream in large numbers (this is called viremia), and a second wave of multiplication occurs in the spleen, bone marrow, and lymph nodes. The initial or prodromal symptoms are similar to other viral diseases such as influenza and the common cold: fever (at least 38.5 °C (101 °F)), muscle pain, malaise, headache, prostration, and as the digestive tract is commonly involved, nausea and vomiting and backache often occur. The prodrome, or preeruptive stage, usually lasts 2–4 days. By days 12–15 the first visible lesions—small reddish spots called enanthem—appear on mucous membranes of the mouth, tongue, palate, and throat, and temperature falls to near normal. These lesions rapidly enlarge and rupture, releasing large amounts of virus into the saliva.

By the sixth or seventh day, all the skin lesions have become pustules. Between 7 and 10 days the pustules mature and reach their maximum size. The pustules are sharply raised, typically round, tense, and firm to the touch. The pustules are deeply embedded in the dermis, giving them the feel of a small bead in the skin. Fluid slowly leaks from the pustules, and by the end of the second week the pustules deflate, and start to dry up, forming crusts (or scabs). By day 16-20 scabs have formed over all the lesions, which have started to flake off, leaving de-pigmented scars

the last natural case of smallpox was diagnosed in 1977

Flat small pox

In Flat-type smallpox (also called malignant smallpox) the lesions remain almost flush with the skin at the time when raised vesicles form in ordinary-type smallpox. is nearly always fatal.

accounted for 5%–10% of cases

severe prodromal phase that lasts 3–4 days, prolonged high fever

The skin lesions mature very slowly

Hemorrhagic

severe form of smallpox that is accompanied by extensive bleeding into the skin, mucous membranes, and gastrointestinal tract. This form developed in perhaps 2% of infections and occurred mostly in adults. In hemorrhagic smallpox the skin does not blister, but remains smooth. Instead, bleeding occurs under the skin, making the skin look charred and black, hence this form of the disease is also known as black pox. hemorrhaging appears on the second or third day as sub-conjunctival bleeding turns the whites of the eyes deep red. hemorrhages in the spleen, kidney, serosa, muscle, and, rarely, the epicardium, liver, testes, ovaries and bladder.

Diagnosis

poxviruses produce characteristic cytoplasmic inclusions, the most important of which are known as Guarnieri bodies, and are the sites of viral replication. Guarnieri bodies are readily identified in skin biopsies stained with hematoxylin and eosin, and appear as pink blobs.

The diagnosis of an orthopoxvirus infection can also be made rapidly by electron microscopic examination of pustular fluid or scabs. However, all orthopoxviruses exhibit identical brick-shaped virions by electron microscopy.

Definitive laboratory identification of variola virus involves growing the virus on chorioallantoic membrane (part of a chicken embryo) and examining the resulting pock lesions under defined temperature conditions.[

Smallpox virus pocks on the chorioallantoic membrane of a chick embryo

Strains may be characterized by polymerase chain reaction (PCR) or restriction fragment length polymorphism (RFLP) analysisSerologic tests and enzyme linked immunosorbent assays (ELISA), which measure variola virus-specific immunoglobulin and antigen have also been developed to assist in the diagnosis of infection.[

Complicationsmost commonly in the respiratory system and range from simple bronchitis to fatal pneumoniaPustules can form on the eyelid, conjunctiva, and cornea, leading to complications such as conjunctivitis, keratitis, corneal ulcerHemorrhagic smallpox can cause subconjunctival and retinal hemorrhages.

Treatment

Vaccination four to seven days after exposure likely offers some protection from disease or may modify the severity of disease

treatment of smallpox is primarily supportive, such as wound care and infection controlNo drug is currently approved for the treatment of smallpox

antiviral drug cidofovir might be useful as a therapeutic agent. The drug must be administered intravenously, however, and may cause serious renal toxicity.[

Smallpox vaccine

The process of vaccination was discovered by Edward Jenner in 1796, who acted upon his observation that milkmaids who caught the cowpox virus did not catch smallpox.[

Before the introduction of a vaccineA process called inoculation, also known as insufflation or variolation was practiced in India as early as 1000 BCVaccines that only contain

attenuated vaccinia viruses (an attenuated virus is one in which the pathogenicity has been decreased through serial passage) have been proposed