Chapter 33 : Poxviruses

Morphology and structure of a poxvirus virion. (A) Electron micrograph of a negative-stained M form of a Molluscum contagiosum virus virion. Magnification, ×120,000. Note the textured surface. (B) Electron micrograph of a thin section of a Cowpox virus virion. N, nucleosome; L, lateral body; M, membrane. Note the immature forms of the virus in various stages of morphogenesis in the upper portion of the photograph. Magnification, ×120,000.

Replication cycle of Vaccinia virus. A virion, containing a double-stranded DNA genome, enzymes, and transcription factors, attaches to a cell (1) and fuses with the cell membrane, releasing a core into the cytoplasm (2). The core synthesizes early mRNA that is translated into a variety of proteins, including growth factors, immune defense molecules, enzymes, and factors for DNA replication and intermediate transcription (3). Uncoating occurs (4), and the DNA is replicated to form concatemeric molecules (5). Intermediate genes in the progeny DNA are transcribed, and the mRNA is translated to form late transcription factors (6). The late genes are transcribed, and the mRNA is translated to form virion structural proteins, enzymes, and early transcription factors (7). Assembly begins with the formation of discrete membrane structures (8). The concatemeric DNA intermediates are resolved into unit genomes and packaged in immature virions (IV) (9). Maturation proceeds to the formation of infectious intracellular MV (10). The MVs are wrapped by modified trans Golgi and endosomal cisternae (11), and the wrapped virions (WV) are transported to the periphery of the cell along microtubules (12). Fusion of the WVs with the plasma membrane results in release of extracellular EV (13). The actin tail polymerizes in the cytoplasm beneath the EV (13). Although replication occurs entirely in the cytoplasm, nuclear and cytoplasmic cell factors may be involved in transcription and assembly. (Reprinted from Moss, 2007 with permission from the publisher.)

Electron micrograph of a thin section of a Molluscum contagiosum virus-infected cell or molluscum body. All of the cellular organelles are beyond recognition, having been pushed to the periphery of the cell by the masses of virions. Magnification, ×3,000.

Histologic section of a molluscum contagiosum lesion. A hematoxylin- and eosin-stained wax section of a skin biopsy specimen showing hyperkeratosis and acanthosis of the epidermis. Note the hyperplasia associated with the lesion causes severe invagination of the epidermis without loss of integrity of the basal layer. Arrows indicate molluscum bodies. Magnification, ×100.

Cytopathic effect of Orthopoxvirus infection within tissue cell culture. African Green Monkey Kidney cells (BSC-40) were infected at a low multiplicity (multiplicity of infection = 0.01) to mimic what might be found within a clinical specimen. Cells were either mock infected or infected with one of the following orthopoxviruses: Vaccinia virus, Monkeypox virus, or Variola virus. Cells were observed daily, and photographs were taken. The number of hours postinfection (hpi) and characteristics of cytopathic effect are denoted. Magnification, ×10.