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Chapter 29 : Poxviruses†
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Smallpox vaccine, Orthopoxvirus vaccinia, was used extensively for routine vaccination against variola virus. Historically poxvirus infections were laboratory confirmed by a combination of approaches including pock morphology on chicken embryo chorioallantoic membranes (CAMs), serologic reactivity, and electron microscopy (EM). Although EM can distinguish parapoxviruses from other poxviruses, the diagnostic method is constrained by the inability to differentiate between species within genera of poxviruses. Real-time PCR assays currently allow rapid and definitive diagnosis of the species of poxvirus causing an infection but still rely upon time-consuming processing of samples. The poxvirus family is divided into two subfamilies: Entomopoxvirinae (poxviruses of insects) and Chordopoxvirinae (poxviruses of vertebrates). The vertebrate poxviruses were further subclassified into genera by comparing cross-protection in animal studies, cross-neutralization of virion infectivity in cell culture, and through the analysis of genetic polymorphisms in genomic viral DNA. Molluscum contagiosum virus has one of the most limited host cell tropisms of any virus, replicating only in the human keratinocyte of the epidermis. The parapoxviruses, including orf, bovine papular stomatitis, pseudocowpox (milker’s nodule), and sealpox viruses cause occupational infections of humans, with orf infections being the most common. The genus Yatapoxvirus has two members, tanapoxvirus and Yaba monkey tumor virus, which are serologically related. Tanapox virus infection may occur via scratches or possibly via arthropod vectors. Current strategies for preventing human poxvirus infections are ones that stress awareness of the potential for infection and possible behavioral modifications to reduce risk of infection.
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, monkeypox, or variola. Cells were observed daily, and photographs were taken. The number of hours postinfection (hpi) and characteristics of cytopathic effect are denoted. Magnification, ×10.
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.
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 Fields Virology, 5th ed. (D. M. Knipe and P. M. Howley [ed.], Lippincott Williams & Wilkins [Moss, 2007]).
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.
Monkeypox rash. A 7-year-old Zairian girl is shown 2 days after the onset of the rash. Courtesy of M. Szczeniowski.
Primary and secondary lesions of cowpox. The primary lesion is at the early eschar stage (probably 2 to 3 weeks after infection), whereas the secondary lesion (below) is at the early vesicular stage. Provided by M. White; reprinted with permission from Baxby et al., 1994.
Vaccinia-like lesions. Lesions found on the hands of a milker within Minas Gerais State, Brazil. Courtesy of Bovine Vaccinia Investigation Group, Federal University of Minas Gerais State, Brazil.
Molluscum contagiosum lesions. A giant molluscum lesion next to a more typical lesion. Note the umbilicated center. Courtesy of J. Burnett.
A typical orf lesion at the target stage of development. Courtesy of Andrew Mercer.
Historic standards for diagnosis of poxviruses a
Poxviruses of the vertebrates a
Genera of vertebrate poxviruses which infect humans a