Classification and major representatives
The family Poxviridae (from English pox – pustule, ulcer) is divided into two subfamilies. Medically important is the subfamily Chordopoxvirinae (poxviruses of vertebrates). The other subfamily, Entomopoxvirinae, affects only insects.
Key genera and pathogens:
- Orthopoxvirus: the most significant genus. Includes the viruses of smallpox, vaccinia, cowpox, and monkeypox.
- Molluscipoxvirus: molluscum contagiosum virus (strictly anthroponotic pathogen).
- Parapoxvirus: Orf virus (causes pustular dermatitis, transmitted from sheep) and pseudocowpox virus.
- Yatapoxvirus: tanapoxviruses (monkey tumors, cause fever and single vesicular lesions in humans).
- Specific animal viruses: Avipoxvirus (birds), Capripoxvirus (sheep and goats), Leporipoxvirus (lagomorphs, causes myxoma), Suipoxvirus (pigs).
Virion ultrastructure
Poxviruses are distinguished by their giant size for viruses — 230×400 nm. They have an ovoid or brick-like shape. Unlike the vast majority of viruses, poxviruses can be seen under a conventional light microscope if special staining methods are used (e.g., Morozov silver impregnation). These structures are called Paschen elementary bodies.
Virion structure:
- Core (nucleoid): has a characteristic dumbbell shape. Contains linear double-stranded DNA whose ends form "hairpins" (covalently closed), and more than 30 structural proteins.
- Lateral bodies: two structures between which the core is sandwiched.
- Envelopes: the core is surrounded by a core membrane; on top of it, the outer membrane with filamentous (tubular) elements forms in the cytoplasm. The outer envelope is acquired by the virion upon exit from the cell.
Orthopoxviruses also synthesize hemagglutinin, but it is non-virion (not part of the structure of the viral particle itself).
Replication cycle
A unique feature of poxviruses among all DNA-containing viruses is that their reproduction occurs exclusively in the cytoplasm of the cell. The virus is completely autonomous and does not depend on host nuclear mechanisms, as it has its own enzymes (including viral DNA polymerase) for nucleic acid synthesis.
Main steps:
- Entry into the cell occurs via a phagocytic vacuole.
- Inside the vacuole, the outer membrane is removed.
- Using the virus's own enzymes, transcription of early genes is triggered, leading to the synthesis of the "uncoating protein".
- The core membrane is disrupted, and DNA is released into the cytoplasm where replication takes place.
During the assembly of new virions in the cytoplasm, characteristic inclusions are formed — Guarnieri bodies (viral replication factories). Exit from the cell occurs by budding (whereby the virus captures modified membranes of the Golgi apparatus) or by cell lysis.
Smallpox and its pathogenesis
Variola virus (Variola virus) is the causative agent of a particularly dangerous quarantine infection. The source of infection was exclusively an infected human, and transmission mechanisms included aerosol and contact-household routes.
The pathogenesis of the disease includes several phases:
- Invasion through the mucous membranes of the upper respiratory tract (less commonly the skin).
- Multiplication in regional lymph nodes.
- Hematogenous dissemination (viremia) with seeding to the skin and lymphoid tissue.
- Secondary replication leading to the formation of a rash.
The infection conferred a robust, lifelong immunity mediated by virus-neutralizing antibodies and cellular factors. Smallpox was characterized by high persistence in the external environment: the virus survived for a long time in pustule crusts, was resistant to ether and low temperatures, but was instantly killed by boiling (100 °C).
Other pathogenic poxviruses
- Vaccinia virus: an independent species of unknown exact origin (likely derived from cowpox virus). It shares common antigens with variola virus, due to which it was used as a live vaccine. It exists only in the form of laboratory strains.
- Molluscum contagiosum virus: infects children and adults. Causes the appearance of pearly pink papules (1–5 mm) from which a cheesy mass is expressed upon pressure. Transmitted by contact (including via microtraumas); autoinoculation is possible.
- Monkeypox virus: causes an infectious disease accompanied by intoxication, fever, and a pustular-papular rash.
Briefly: answers to topical questions
What antigens are identified in the smallpox virus?
The major antigens identified in the smallpox virus are:
- Nucleoprotein antigens.
- Soluble antigens.
- Hemagglutinin.
Orthopoxviruses synthesize hemagglutinin, which is not part of the virion. Variola virus shares common antigens with vaccinia virus, which is important for immunoprophylaxis.
What laboratory diagnostic methods are used for poxvirus infections?
The following microbiological diagnostic methods are used for smallpox:
- Indication methods: electron microscopy, IFA, precipitin test (AGID), microscopy to detect intracellular inclusions – Guarnieri bodies.
- Virological method: inoculation of chicken embryos on the chorioallantoic membrane or inoculation of cell cultures; identification is performed using neutralization test, complement fixation test, hemagglutination inhibition test.
- Serological diagnostics: hemagglutination inhibition test, complement fixation test, indirect hemagglutination assay, neutralization test.
Studies involving smallpox are conducted with strict adherence to safety regulations for particularly dangerous infections. For monkeypox virus, diagnostics are indicated as analogous to the methods used for smallpox.
What enzymes are contained in the core of the poxvirus virion?
In poxviruses, the entire reproduction cycle takes place in the cell cytoplasm. The virus is autonomous because it possesses its own enzymes for mRNA and DNA synthesis and does not depend on the cell's nuclear mechanisms.
The sources indicate:
- genome replication is carried out by viral DNA polymerase;
- transcription of early genes occurs with the help of virion enzymes;
- these processes ensure the reproduction of poxviruses in the cell cytoplasm.
What types of vaccines are used for the prevention of smallpox?
A live smallpox vaccine is used for the specific prophylaxis of smallpox.
The sources indicate methods of production:
- from calf rash scrapings;
- by cultivating the vaccinia virus in chicken embryos.
Administration methods:
- scarification, percutaneous;
- using a jet injector.
A domestic oral tablet vaccine is also described: it is not inferior in efficacy to the percutaneous one and is less reactogenic.
Mandatory vaccination has been discontinued since 1980 due to the global eradication of smallpox.
The "Five P's" rule helps to remember the dynamics of smallpox rash development: Spot → Papule → Vesicle → Pustule → Drying (crust formation).
Class and exam questions
Why can poxviruses replicate in the cytoplasm if their genome is DNA?
Poxviruses are autonomous: they carry their own enzymes within the virion (e.g., DNA polymerase) for mRNA synthesis and DNA replication, so they do not require host cell nuclear enzymes.
What are Guarnieri bodies and Paschen bodies?
Paschen bodies are individual poxvirus virions visible under a light microscope with special staining. Guarnieri bodies are intracellular cytoplasmic inclusions ("virus replication factories") in the infected cell.
In what year was smallpox eradicated?
The last case of natural infection was recorded in 1977 in Somalia, and official global eradication was declared following the last laboratory-associated cases in 1978. Mandatory vaccination was discontinued in 1980.
What else to study on this topic
- Global Smallpox Eradication Programme: Contribution of Russian Scientists and Vaccine Development
- Microbiological Diagnostics: Cultivation on the Chorioallantoic Membrane
- Clinical Forms of Variola Vera: From Varioloid to Pustular-Hemorrhagic
- Serological Diagnostics of Poxviruses (HAI, CF test, Neutralization Test)
- Molluscum Contagiosum Virus: Epidemiology, Risk Groups, and Pathogenesis
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