Families of RNA Viruses

Family: Picornaviridae (Picorn a viruses)

Genus' Enterovirus (enteroviruses)

Genus: Hepatovirus (hepatitis A-Iike viruses)

Genus: Rhinovirits (rhinoviruses)

The Picornavirtdae (pica, "micro-micro"; rrn, sigla for ribonucleic acid) comprise small nonenveloped icosahedral viruses 25-30 nm in diameter, which contain a single molecule of plus sense ssRNA (7.5-8.5 kb), and replicate in the cytoplasm (see Table 2-2). The genus Enterovirus includes 3 polio-viruses, 32 human echoviruses, 29 coxsackieviruses, and a few other human enteroviruses. Most of these viruses usually produce inapparent enteric infections, but the polioviruses may also cause paralysis; other enteroviruses are sometimes associated with meningoencephalitis, rashes, carditis, myositis, conjunctivitis, and mild upper respiratory tract disease. The only human pathogen in the genus Hepatovirus is human hepatitis A virus. The genus Rhhiovirus includes well over 100 serotypes that affect humans; they are the most frequent viruses causing the common cold.

Family: CaHciviridae (Caliciviruses)

Genus: Caticivirus (caliciviruses)

The caliciviruses (caSix, cup) are icosahedral viruses whose virions are 35-40 nm in diameter and have 32 cup-shaped depressions on the surface. The genome consists of one molecule of plus sense ssRNA, size 8 kb. The Nor-walk agent and related caliciviruses are important causes of gastroenteritis, and one cause of human hepatitis transmitted by the fecal-oral route, hepatitis E viius, is a calicivirus.

Family: Astroviridae (Astroviruses)

Genus: Astrovnus (astroviruses)

Astrovirus (astron, star) is a name accorded to small spherical virions with a characteristic star-shaped outline by negative staining. These viruses have been found in the feces of humans, calves, and lambs suffering from enteritis. The genome consists of one molecule of ssRNA about the same size as that of the picornaviruses, but unlike picornaviruses and caliciviruses they possess only two capsid proteins.

Picornavirus Shape

Family: Togaviridae (Togaviruses)

Conns. Alphavnus (formerly "group A" arboviruses) Genus: Rubivntts (rubella virus)

The togaviruses (toga, cloak) are small spherical enveloped viruses 60-70 nm in diameter, containing plus sense ssRNA (12 kb) enclosed within an icosahedral core. They replicate in the cytoplasm and mature by budding from cell membranes. The genus Alphavirus contains many species, all of which are mosquito-transmitted. Important human pathogens include eastern, western and Venezuelan equine encephalitis viruses, Ross River virus, and chikungunya virus. In nature the alphaviruses usually produce inap-parent viremic infections of birds, mammals, or reptiles. When humans are bitten by an infected mosquito the usual consequence is an inapparent infection, but generalized disease, often associated with arthritis or encephalitis, can result.

The only non-arthropod-borne togavirus is rubella virus (genus Rubi-virus), a human pathogen important for its ability to cause congenital defects in the fetus when pregnant women are infected.

Family: Flaviviridae (Flaviviruses)

Genus: Flavivirus (formeily "group B" arboviruses) Genus: Hepatitis C (hepatitis C virus)

Flaviviruses (flavus, yellow) have an enveloped icosahedral virion 40-50 nm in diameter and a genome of 10 kb. Viruses of the largest genus, Flavwirus, are arthropod-borne (mosquitoes and ticks), but hepatitis C virus is transmitted sexually and via human blood. The genus Flavivirus contains several important human pathogens including the viruses of yellow fever, dengue, and St. Louis, Japanese, Murray Valley, and Russian tick-borne en-cephalitides.

Family: Coronaviridae (Coronaviruses)

Genus: Coromvirus (coronaviruses of mammals and birds)

The coronaviruses (coraim, crown) are somewhat pleomorphic viruses 75-160 nm in diameter, with widely spaced, pear-shaped peplomers embedded in a lipoprotein envelope. The envelope lacks a matrix protein, and it encloses a core of helical symmetry with a single linear molecule of plus sense ssRNA, 27-33 kb. Some coronaviruses cause common colds in humans, while others have been visualized in human feces.

Family: Paratnyxoviridae (Paramyxoviruses)

Genus: Paramyxovirus (parainfluenzaviruses) Genus: Morbilhvirus (measleslike viruses) Genus: Ruhulavirus (mumps virus) Subfamily: Ptieumruirime

Genus: Pneitmovmis (respiratory syncytial viruses)

The paramyxoviruses (para, by the side of; myxa, mucus) have a large, pleomorphic, enveloped virion 150-300 nm in diameter, with a helical nucle-ocapsid. The genome consists of a single linear molecule of minus sense ssRNA (15-16 kb), The envelope contains two glycoproteins, a hemagglutinin (in most species with neuraminidase activity also) and a fusion protein.

Human pathogens in the genus Paraiinfxoi'iiit^ include four types of para-influenzaviruses, which cause respiratory disease Measles (genus MorhiHh z'inis) is an important generalized infection associated with a rash, and mumps virus is the only human pathogen in the genus Riibulavuas. Respiratory syncytial virus (subfamily Pneuniovimme, genus Pveumomrus) is a major cause of respiratory disease in infants.

Family: Rhabdoviridae (Rhabdoviruses)

Genus: Vesiculovirus (vesicular stomatitis-like viruses)

Genus: Lyssaviru? (rabies-like viruses)

The rhabdoviruses (rhabdos, rod) are bullet-shaped viruses, about 180 by 75 nm, containing a single molecule of minus sense ssRNA (13—16 kb). The helical capsid is enclosed within a shell to which is closely applied an envelope with embedded peplomers. The virion matures at the plasma membrane. Animal pathogens in the genus Vesiculovirus include vesicular stomatitis, Chandipura, Piry, and Isfahan viruses, each of which is an occasional human pathogen. The genus Li/ssaoirus includes rabies virus and several serologically related viruses from Africa, which may cause severe disease in humans following animal bites.

Family: FUoviridae (Filoviruses)

Genus' Filovirus (Marburg, Ebola, and Reston viruses)

The virions of filoviruses resemble those of rhabdoviruses but are pleomorphic and sometimes very long {filo, threadlike), maximum infectivity being associated with a particle 790-970 nm long and 80 nm wide. The genome is a single molecule of minus sense ssRNA, 12 7 kb. Marburg and Ebola viruses cause sporadic infections and occasional nosocomial epidemics of severe hemorrhagic fever in humans in Africa. In 1989 another filovirus, called Reston virus, was isolated from monkeys imported from the Philippines, but it caused only subclinical infections in animal handlers.

Family: Orthomyxaviridae (Influenza viruses)

Genus: influenzavmts A, B (influenza A and B viruses)

Genus, hifluenziwims C (influenza C virus)

Genus: unnamed Thogoto-like viruses (tick-borne orthomyxoviruses)

The orthomyxoviruses (orfhos, straight; myxa, mucus) are spherical RNA viruses 80-120 nm in diameter, with a helical nucleocapsid enclosed within an envelope acquired by budding from the plasma membrane. The genome consists of seven (influenza C virus) or eight (influenza A and B viruses) segments of minus sense ssRNA (total size, f3.6 kb). The envelope is studded with spikes, which are of two kinds, a hemagglutinin and a neuraminidase in influenza A and B viruses, and of one kind, hemagglutinin-esterase, in influenza C virus.

Influenza A virus infects birds, horses, swine, mink, seals, and whales, as well as humans; influenza B virus is a human pathogen only. Influenza C virus infects humans and swine, but rarely causes serious disease. Influenza A viruses of birds and humans undergo genetic reassortment to generate novel subtypes ("antigenic shift") which cause major pandemics of human influenza The tick-borne orthomyxoviruses, Dhori and Thogoto, which occasionally infecl humans, have been allocated to a separate genus, so far unnamed

Family: Arettaviridae (Arenaviruses)

Genus Arenavirus (arenaviruses)

Arenaviruses {arena, sand) are so named because of the presence of particles resembling ribosomes (hence grains of sand) within the pleomorphic 110-130 nm enveloped virions. The genome consists of two segments of minus sense or ambisense ssRNA (total size 10-14 kb), each held in a circular configuration by hydrogen bonds.

Arenaviruses cause natural inapparent infections of rodents, and humans occasionally develop a serious generalized disease following accidental exposure to arenavirus-infected rodent urine. Lymphocytic choriomeningitis virus is an important laboratory model for the study of persistent infections and may cause human disease; Lassa, Machupo, }unin, and Guanari to viruses may cause severe hemorrhagic fever and (like the filoviruses) are Biosafety Level 4 pathogens.

Family: Bttnyaviridae (Bunyaviruses)

Genus: Bunyavmts (Bunyamwera supergroup)

Genus: Phlebomrus (sandfly fever viruses)

Genus: Nairovirus (Nairobi sheep disease-like viruses)

Genus: Hantavirus (hemorrhagic fever with renal syndrome viruses)

The bunyaviruses (Bunyamwera, locality in Uganda), numbering over 100, comprise the largest srngle group of arboviruses. The enveloped virions are 90-120 nm in diameter, within which there are three tubular nucleocap-sids, each in the form of a circle. The genome consists of three molecules of minus sense (or in Phiebovirus, ambisense) ssRNA (total size 13.5-21 kb), each held in a circular configuration by hydrogen bonds. The bunyaviruses replicate in the cytoplasm and bud from Golgi membranes. Because of their segmented genome, closely related bunyaviruses readily undergo genetic reas-sortment.

All members of the family Butiyaviridae except the hantaviruses are arboviruses which have wild anmial reservoir hosts; some are Iransovarially transmitted in mosquitoes with a high frequency. The hantaviruses, which are enzootic in rodents, cause hemorrhagic fever with renal syndrome or pulmonary disease in humans The genus Phiebovirus includes sandfly fever virus (transmitted by Ph)ci>olo»ms) and Rift Valley fever virus, a mosquito-transmitted virus that is an important pathogen of sheep and humans. The genus Bnm/ax'irits, most members of which are mosquito-transmitted, includes the California group arboviruses, some of which occasionally cause encephalitis in humans. Members of the genus Nairovirus are tick-borne and include the virus of Crimean-Congo hemorrhagic fever.

Family: Reoviridae (Reoviruses)

Genus: Orthoreovitus (reoviruses of animals) Genus: Orbivirits (orbiviruses) Genus: Rotavirus (rotaviruses) Genus Coifivirus (Colorado tick fever virus)

"I he family name Reovindne is a sigla, respnatory enteric orphan virus, reflecting the fact that members ul (he first discovered genus, O/f/ionwiY/is, were found in both the respiratory and enteric tract ol humans and most animals, but were not associated with any disease (orphan viruses were "viruses in search of a disease") The distinctive feature of (he family is that the virions contain dsRNA, in 10-12 segments: 10 [()r//?om«'fnis (22 kbp total) and Orbwirtis (18 kbp)], 11 [Rotmuius (16-21 kbp)or 12 [Colt¡vims (27 kbp)]. The virion is a nonenveloped icosahedron 60-80 nm in diameter Orbiviruses (orbi, ring) are arboviruses, some of which cause disease in humans, as does Colorado tick fever virus (genus Cnitivtrus) The rotaviruses (tota, wheel) include viruses that are important causes of diarrhea in humans and some domestic animals.

Family: Retroviridae (Retroviruses)

Genus: Lentivirus (HIV-Iike viruses, maodi/visna-like viruses) Genus: Spumaviius (foamy viruses)

Genus1 Unnamed, HTLV-I3LV viruses (includes human T-cell leukemia viruses)

The name Retroviridae (tetra, backwards) is used for a large family of enveloped viruses 80-100 nm in diameler, with a complex structure and an unusual enzyme, reverse transcriptase. Uniquely among viruses, the genome is diploid, consisting of an inverted dimer of plus sense ssRNA, 7—10 kb in size. In the life cycle of the exogenous retroviruses the dsDNA copy of the viral genome transcribed by the viral reverse transcriptase is circularized and integrates into the cellular DNA as an essential part of the replication cycle. Proviral DNA of endogenous retroviruses is found in Ihe DNA of all normal cells of many species of animals and may under certain circumstances be induced to produce virus.

This important family is subdivided into seven genera, only two of which have been given official generic names, and only three of which contain viruses known to infect humans. The viruses of medical importance are the human T-cell lymphotropic viruses (HTLV-BLV viruses), which cause leukemia, and the human immunodeficiency viruses (genus Lentivirus; lenti, slow), which produce the acquired immunodeficiency syndrome (AIDS). Spuma-viruses (foamy agents) have been isolated from humans but appear to be completely benign. Retroviruses of other genera have been important research tools for unraveling the mechanisms of oncogenesis and the nature of oncogenes.

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