Rna Viruses

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Fig. 2-1 Shapes and sizes of viruses of families that include human pathogens. The virions are drawn to scale, but artistic license has been used in representing then structure In some, the cross-sectional structure of capsid and envelope is shown, with a representation of the genome, with the very small virions, only size and symmetry are depicted

Genus. Erylhroi'irus (human parvovirus B19) Genus: Deprmlmnrns (adeno-associated viruses)

Parvoviruses (parvus, small) are about 20 nm in diameter, have icosahedral symmetry, and possess a genome of ssDNA, 5 kb (Table 2-1). The virions are relatively heat stable. Most species have a narrow host range and replicate preferentially in dividing cells. Members of the genus Parvovirus infect a number of species of animals, and one parvovirus (B19, the only member of Ihe genus Eiythrovirus) has been identified in humans. Members of the genus Depcndoviras are defective viruses, which depend an adenovirus (or, experimentally, a herpesvirus) for replication. Five serotypes of these "adeno-associaled viruses" occur in humans but are not known to cause disease.

Papovaviridae Diseases Caused

Family: Papovaviridae (Papova viruses)

Genus: Papillomavirus (papillomaviruses) Genus: Potifonmvirus (polyomaviruses)

The papovaviruses [sigla, from papilloma, polyoma, vacuolating agent (early name far SV40)| are small nonenveloped icosahedral viruses which replicate in the nucleus and may transform infected cells. In the virion the nucleic acid occurs as a cyclic double-stranded DNA molecule, which is infectious. There are two genera. Virions of Papillomavirus (wart viruses) are 55 nm in diameter and have a larger genome (8 kbp) which may persist in transformed cells in an episomal form. Virions of members of the genus Poly-own virus are 45 nm in diameter, have a smaller genome (5kbp), and may persist in cells via the integration of their genome into the host cell DNA.

Human papillomaviruses cause warts, and some of the several dozen types are associated with cancer of the cervix or the skin; the human polyomaviruses usually cause inapparent infections but may be reactivated by immunosuppression Murine polyoma virus and simian virus 40 (SV40, from rhesus monkeys) have been useful models for the laboratory study of viral oncogenesis

Family: Adertoviridac (Adenoviruses)

Genus: Masladetwvirus (mammalian adenoviruses)

The adenoviruses (adcnos, gland) are nonenveloped icosahedral viruses 70 nm in diameter, with a single linear dsDNA genome of 36-38 kbp. They replicate in the nucleus. Nearly 50 serologically distinct types of human adenoviruses are currently recognized; all share a group antigen with adenovirus serotypes infecting other mammals (the genus Mastadenovirus).

Human adenoviruses are associated with infections of the respiratory tract, the eye, and the intestinal tract. Many infections are characterized by prolonged persistence and may be reactivated by immunosuppression.

Family: Herpesviridae (Herpesviruses)

Subfamily: Alplmherpesvirmae (herpes simplex-like viruses) Genus Swiplexvirus (herpes simplex-like viruses) Genus: Vartcellovtrus (varicella-zoster virus) Subfamily: Bctaher;>csvirinae (cytomegaloviruses) Genus: Cytomegalovirus (human cytomegalovirus) Genus: Kospo/ouirns (human herpesvirus 6) Subfamily: Gammaherpesvirime (lymphoproliferative herpesviruses) Genus- Lywphocryptovirus (Epstein-Barr virus)

The herpesviruses (herpes, creeping) have enveloped virions about 150 nm in diameter, with icosahedral nudeocapsids about 100 nm m diameter. The genome is a single linear molecule of dsDNA, 125-229 kbp. The herpesviruses replicate in the nucleus and mature by budding through the nuclear membrane, thus acquiring an envelope. This large family includes several important human pathogens and has been divided into three subfamilies. Alphaherpesvmme includes herpes simplex types 1 and 2, varicella-zoster virus, and B virus of monkeys, which is pathogenic for humans. Betaherpes-virinae comprises the cytomegaloviruses, which are highly host-specific vi ruses of humans and other animals and produce low-grade, chronic infections Human cytomegalovirus is an important cause of congenital abnormalities. Human herpesvirus 6 has been allocated to the subfamily Betahcrpes-virnme, genus Roseoiovirus. Gamiwihcrpesvirinae includes the genus Lipupho-cryptovirus, which contains one species, Epstein-Barr (EI3) virus, the cause of infectious mononucleosis.

A feature of all herpesvirus infections is life-long persistence of the virus in the body, usually in latent form. Excretion, especially in saliva or genital secretions, may occur continuously or intermittently without disease, or episodes of recurrent clinical disease and recurrent excretion may occur years after the initial infection, especially following immunosuppression. There is evidence that some herpesviruses may have a role in human cancers, notably EB virus in nasopharyngeal cancer and Burkitt's lymphoma.

Family: Poxviridae (Poxviruses)

Subfamily: Chordopoxvirime (poxviruses of vertebrates) Genus: Orthopoxvirus (vaccinia virus subgroup) Genus: Parapoxvirus (orf virus subgroup) Genus: Mollusapoxvirus (molluscum contagiosum virus) Genus: Yalapoxvirus (yaba/tanapox virus subgroup) Genus: Avtpoxvirus (bird poxviruses)

The poxviruses (poc, pocc, pustule) are the largest and most complex viruses of vertebrates. The virions are brick-shaped, measuring about 250 by 200 by 200 nm in all genera that cause human infections except Parapoxvirus, the virions of which are ovoid and measure 260 by 160 nin. All poxviruses have an inner core which contains a single linear molecule of dsDNA, 130-250 kbp. Unlike most other DNA viruses of vertebrates, poxviruses replicate in the cytoplasm, mRNA being transcribed by a virion-associated transcriptase. A large number of other virion-associated enzymes are involved in DNA synthesis.

The family is divided into two subfamilies, one of which, Chordopoxvirime, comprises the poxviruses of vertebrates. This subfamily contains four genera that include human pathogens. The genus Orthopoxvirus includes cowpox, ectromelia (mousepox), rabbitpox (a variant of vaccinia virus), and monkeypox viruses. Variola virus, which caused human smallpox, and vaccinia virus, used to control that disease, also belong to this genus. Parapoxviius includes contagious pustular dermatitis virus of sheep and pseudocowpox (milker's node) virus, both of which produce skin lesions in humans. Molluscum contagiosum (genus Mollusapoxvirus) is a specifically human virus, and the genus Yafopoxvirus contains two viruses of African wildlife, yabapox-virus and tanapoxvirus, both of which may infect humans. Fowlpox virus and canarypox viruses {genus Avipoxvirus) are being investigated as possible vectors for human vaccines.

Family: Hepadnavtridae (Hepatitis B-Iike Viruses)

Genus- Orthohepadnavirua (mammalian hepatitis B-like viruses)

Human hepatitis B virus and related viruses of other animals, all highly host-specific, comprise the family Hepadnavtridae (hepar, liver; dna, sigla for deoxyribonucleic acid). The virions are spherical particles 42 nm in diameter, consisting of a 27-nm icosahedral core within a closely adherent outer capsid thai contains cellular lipids, glycoproteins, and a virus-specific surface antigen (HBsAg) The genome is a small, circular, partially double-stranded DNA molecule, which consists of a long (3.2 kb) and a short (1 7-2.8 kb) strand. Replication involves an RNA intermediate and requires a virus-coded reverse transcriptase.

The hepadnaviruses replicate in hcpatocytes and cause hepatitis, which may progress to a chronic carrier state, cinhosis, and primary hepatocellular carcinoma. The most important species is human hepatitis B virus, but hepadnaviruses also occur in woodchucks, ground squirrels, Pekin ducks, and herons.

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