Classification and Major Species
The family Poxviridae is divided into two subfamilies. Medically important is the subfamily Chordopoxvirinae (vertebrate poxviruses). The other subfamily, Entomopoxvirinae, infects only insects.
Key genera and pathogens:
- Orthopoxvirus: the most significant genus. Includes Variola virus, Vaccinia virus, Cowpox virus, and mpox virus.
- Molluscipoxvirus: Molluscum contagiosum virus (strictly anthroponotic pathogen).
- Parapoxvirus: Orf virus (causes contagious pustular dermatitis, transmitted from sheep) and pseudocowpox virus.
- Yatapoxvirus: Tanapox viruses (cause fever and single vesicular lesions in humans).
- Specific animal viruses: Avipoxvirus (birds), Capripoxvirus (sheep and goats), Leporipoxvirus (lagomorphs, causes myxoma), and Suipoxvirus (pigs).
Virion Ultrastructure
Poxviruses are exceptionally large for viruses, measuring 230 × 400 nm. They have an ovoid or brick-like shape. Unlike the vast majority of viruses, poxviruses can be visualized using standard light microscopy with special staining techniques (e.g., Morozov silver staining). These structures are known as elementary bodies or Passchen bodies.
Virion structure:
- Core (nucleoid): has a characteristic dumbbell shape. It contains linear double-stranded DNA with hairpin loops (covalently closed ends) and over 30 structural proteins.
- Lateral bodies: two structures flanking the core.
- Membranes: the genome is enclosed by the core membrane; outside of this, an outer membrane with tubular elements forms in the cytoplasm. The outer envelope is acquired by the virion as it buds from the cell.
Orthopoxviruses also synthesize hemagglutinin, although it is non-virion-associated (not part of the structural virion itself).
Replication Cycle
A unique feature of poxviruses among all DNA viruses is that their replication occurs exclusively in the cell cytoplasm. The virus is completely autonomous and independent of host nuclear mechanisms because it encodes its own enzymes (including viral DNA polymerase) for nucleic acid synthesis.
Main stages:
- Entry into the cell via a phagocytic vacuole.
- Removal of the outer membrane inside the vacuole.
- Virion-associated enzymes initiate the transcription of early genes, leading to the synthesis of "uncoating protein".
- The core membrane disintegrates, releasing DNA into the cytoplasm where replication takes place.
During the assembly of new virions in the cytoplasm, characteristic inclusions called Guarnieri bodies (viral replication factories) are formed. Exit from the cell occurs via budding (with the virus hijacking modified Golgi membranes) or cellular lysis.
Smallpox and its Pathogenesis
Variola virus is the causative agent of a severe quarantine infection. The source of infection was exclusively infected humans, with transmission occurring via airborne and contact routes.
Pathogenesis involves several phases:
- Invasion through the upper respiratory mucosa (rarely skin).
- Multiplication in regional lymph nodes.
- Hematogenic dissemination (viremia) spreading to the skin and lymphoid tissue.
- Secondary replication leading to rash formation.
Skin infection induced robust, lifelong immunity mediated by virus-neutralizing antibodies and cellular factors. Smallpox exhibited high environmental stability: the virus persisted for long periods in pustular crusts, resisting ether and low temperatures, but was instantly destroyed by boiling (100 °C).
Other Pathogenic Poxviruses
- Vaccinia virus: a distinct species of uncertain exact origin (likely derived from cowpox virus). It shares cross-reactive antigens with Variola virus, allowing its use as a live vaccine. It exists exclusively as laboratory strains.
- Molluscum contagiosum virus: infects children and adults. Causes pearly-pink papules (1–5 mm) that release a cheesy core when squeezed. Transmitted by contact (including microtrauma), with potential autoinoculation.
- Mpox virus: causes an infectious disease characterized by intoxication, fever, and a pustular-papular rash.