Poxviruses are the largest and most complex of viruses infecting humans. The family encompasses a large group of agents that are similar morphologically and share a common nucleoprotein antigen. Infections with most poxviruses are characterized by a rash, although lesions induced by some members of the family are markedly proliferative. The group includes variola virus, the etiologic agent of smallpox—the viral disease that has affected humans throughout recorded history.
Even though smallpox was declared eradicated from the world (in 1980) after an intensive campaign coordinated by the World Health Organization, there is concern that the virus could be reintroduced as a biologic weapon. There is a continuing need to be familiar with vaccinia virus (used for smallpox vaccinations) and its possible complications in humans. It is also necessary to be aware of other poxvirus diseases that may resemble smallpox and must be differentiated from it by laboratory means. Lastly, vaccinia virus is under intensive study as a vector for introducing active immunizing genes as live-virus vaccines for a variety of viral diseases of humans and domestic animals.
Important properties of the poxviruses are listed in Table 34-1.
Table 34-1 Important Properties of Poxviruses |Favorite Table|Download (.pdf)
Table 34-1 Important Properties of Poxviruses
|Virion: Complex structure, oval or brick shaped, 300–400 nm in length × 230 nm in diameter; external surface shows ridges; contains core and lateral bodies|
|Composition: DNA (3%), protein (90%), lipid (5%)|
|Genome: Double-stranded DNA, linear; size, 130–375 kbp; has terminal loops; has low G + C content (30–40%) except for Parapoxvirus (63%)|
|Proteins: Virions contain more than 100 polypeptides; many enzymes are present in core, including transcriptional system|
|Envelope: Virion assembly involves formation of multiple membranes|
|Replication: Cytoplasmic factories|
|Large and complex viruses; very resistant to inactivation|
|Virus-encoded proteins help evade host immune defense system|
|Smallpox was the first viral disease eradicated from the world|
Structure and Composition
Poxviruses are large enough to be seen as featureless particles by light microscopy. By electron microscopy, they appear to be brick-shaped or ellipsoid particles measuring about 300–400 × 230 nm. Their structure is complex and conforms to neither icosahedral nor helical symmetry. The external surface of particles contains ridges. An outer lipoprotein membrane, or envelope, encloses a core and two structures of unknown function called lateral bodies (Figure 34-1).
Electron micrographs of vaccinia (Orthopoxvirus) virions. A: Negatively stained particle showing ridges or tubular elements covering the surface (228,000×). (Reproduced with permission from Dales S: The uptake and development of vaccinia virus in strain L cells followed with labeled viral deoxyribonucleic acid. J Cell Biol 1963;18:51.) B: Thin section of vaccinia virion showing a central biconcave core, two lateral bodies, and an outer membrane (220,000×). (Reproduced with permission from Pogo BGT, Dales ...