A. Picornaviridae

Picornaviruses are small (28–30 nm), ether-resistant viruses exhibiting cubic symmetry. The RNA genome is single stranded and positive sense (ie, it can serve as an mRNA) and is 7.2–8.4 kb in size. The groups infecting humans are entero viruses (polioviruses, coxsackieviruses, echoviruses, parecho viruses, and rhinoviruses [more than 100 serotypes causing common colds]) and hepatovirus (hepatitis A). Rhinoviruses are acid labile and have a high density; other enteroviruses are generally acid stable and have a lower density. Picornaviruses infecting animals include foot-and-mouth disease of cattle and encephalomyocarditis of rodents.

B. Astroviridae

Astroviruses are similar in size to picornaviruses (28–30 nm), but particles display a distinctive star-shaped outline on their surfaces. The genome is linear, positive-sense, single stranded RNA, 6.8–7.0 kb in size. These agents are associated with gastroenteritis in humans and neurological disease in some animals.

C. Caliciviridae

Caliciviruses are similar to picornaviruses but slightly larger (27–40 nm). The particles appear to have cup-shaped depressions on their surfaces. The genome is single-stranded, positive sense RNA, 7.3–8.3 kb in size; the virion has no envelope. Important human pathogens are the noroviruses (eg, Norwalk virus), the cause of epidemic acute gastroenteritis. Other agents infect cats and sea lions as well as primates.

D. Hepeviridae

Hepeviruses are similar to caliciviruses. The particles are small (32–34 nm) and ether resistant. The genome is single-stranded, positive-sense RNA, 7.2 kb in size. It lacks a genome-linked protein (VPg). Human hepatitis E virus belongs to this group.

E. Picobirnaviridae

Picobirnaviruses are small (35–40 nm) nonenveloped viruses with icosahedral structure. The genome is linear, double stranded, segmented (bipartite) RNA (two segments), totaling about 4 kb. Human disease associations remain unclear.

F. Reoviridae

Reoviruses are medium-sized (60–80 nm), ether-resistant, nonenveloped viruses having icosahedral symmetry. Particles have two or three protein shells with channels extending from the surface to the core; short spikes extend from the virion surface. The genome is linear, double-stranded, segmented RNA (10–12 segments), totaling 18–30 kbp in size. Individual RNA segments range in size from 200 to 3000 bp. Replication occurs in the cytoplasm; genome segment reassortment occurs readily. Reoviruses of humans include rotaviruses, which have a distinctive wheel-shaped appearance and cause gastroenteritis. Antigenically similar reoviruses infect many animals. The genus Coltivirus includes Colorado tick fever virus of humans

G. Arboviruses and Rodent-Borne Viruses

 Arboviruses and rodent-borne viruses are ecologic groupings (not a virus family) of viruses with diverse physical and chemical properties. The arboviruses (there are more than 350 of them) have a complex cycle involving arthropods as vectors that transmit the viruses to vertebrate hosts by their bite. Viral replication does not seem to harm the infected arthropod. Arboviruses infect humans, mammals, birds, and reptiles and use mosquitoes and ticks as vectors. Human pathogens include dengue, yellow fever, West Nile fever, and encephalitis viruses. Rodent-borne viruses establish persistent infections in rodents and are transmitted without an arthropod vector. Human diseases include hantavirus infections and Lassa fever. The viruses in these ecologic groupings belong to several virus families, including arenaviridae, bun yaviridae, flaviviridae, reoviridae, rhabdoviridae, and toga viridae .

H. Togaviridae

Many arboviruses that are major human pathogens, called alphaviruses—as well as rubella virus—belong to this group. They have a lipid-containing envelope and are ether sensitive, and their genome is single-stranded, positive-sense RNA, 9.7–11.8 kb in size. The enveloped virion measures 65–70 nm. The virus particles mature by budding from host cell membranes. An example is eastern equine encephalitis virus. Rubella virus has no arthropod vector.

I. Flaviviridae

Flaviviruses are enveloped viruses, 40–60 nm in diameter, containing single-stranded, positive-sense RNA. Genome sizes vary from 9.5 to 12 kb. Mature virions accumulate within cisternae of the endoplasmic reticulum. This group of arboviruses includes yellow fever virus and dengue viruses. Most members are transmitted by blood-sucking arthropods. Hepatitis C virus is a flavivirus with no known vector.

J. Arenaviridae

Arenaviruses are pleomorphic, enveloped viruses ranging in size from 60 to 300 nm (mean, 110–130 nm). The genome is segmented, circular, single-stranded RNA that is negative sense and ambisense, 10–14 kb in total size. Replication occurs in the cytoplasm with assembly via budding on the plasma membrane. The virions incorporate host cell ribosomes during maturation, which gives the particles a “sandy” appearance. Most members of this family are unique to tropical America (ie, the Tacaribe complex). Arenaviruses pathogenic for humans cause chronic infections in rodents. Lassa fever virus of Africa is one example. These viruses require maximum containment conditions in the laboratory.

 K. Coronaviridae

Coronaviruses are enveloped 120- to 160-nm particles containing an unsegmented genome of positive-sense, single stranded RNA, 27–32 kb in size. Coronaviruses resemble orthomyxoviruses but have petal-shaped surface projections arranged in a fringe, similar to a solar corona. Coronavirus nucleocapsids develop in the cytoplasm and mature by bud ding into cytoplasmic vesicles. These viruses have narrow host ranges. Classically, human coronaviruses cause mild acute upper respiratory tract illnesses—“colds”—but more recently discovered coronaviruses cause severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS). Toroviruses, which cause gastroenteritis, form a dis tinct genus. Coronaviruses of animals readily establish persistent infections and include mouse hepatitis virus and avian infectious bronchitis virus.

 L. Retroviridae

Retroviruses are spherical, enveloped viruses (80–110 nm in diameter) whose genome contains two copies of linear, positive-sense, single-stranded RNA. Each monomer RNA is 7–11 kb in size. Particles contain a helical nucleocapsid within an icosahedral capsid. Replication is unique; the virion contains a reverse transcriptase enzyme that produces a DNA copy of the RNA genome. This DNA becomes circularized and integrated into host chromosomal DNA. The virus is then replicated from the integrated “provirus” DNA copy. Virion assembly occurs by budding from plasma membranes. Hosts remain chronically infected. Retroviruses are widely distributed; there are also endogenous proviruses resulting from ancient infections of germ cells transmitted as inherited genes in most species. Leukemia and sarcoma viruses of animals and humans (see Chapter 43), foamy viruses of primates, and lentiviruses (human immunodeficiency viruses; visna of sheep) are included in this group. Retroviruses cause acquired immunodeficiency syn drome (AIDS) and make possible the identification of cellular oncogenes.

 M. Orthomyxoviridae

 Orthomyxoviruses are medium-sized, 80- to 120-nm enveloped viruses exhibiting helical symmetry. Particles are either round or filamentous, with surface projections that contain hemagglutinin or neuraminidase activity. The genome is linear, segmented, negative-sense, single-stranded RNA, totaling 10–13.6 kb in size. Segments range from 890 to 2350 nucleotides each. The virus matures by budding at the cell membrane. Orthomyxoviruses include influenza viruses that infect humans or animals. The segmented nature of the viral genome permits ready genetic reassortment when two influenza viruses infect the same cell, presumably fostering the high rate of natural variation among influenza viruses. Viral reassortment and transmission from other species is thought to explain the emergence of new human pandemic strains of influenza A viruses.

 N. Bunyaviridae

Bunyaviruses are spherical or pleomorphic, 80- to 120-nm enveloped particles. The genome is made up of a triple segmented, single-stranded, negative-sense or ambisense RNA, 11–19 kb in overall size. Virion particles contain three circular, helically symmetric nucleocapsids about 2.5 nm in diameter and 200–3000 nm in length. Replication occurs in the cytoplasm, and an envelope is acquired by budding into the Golgi. The majority of these viruses are transmitted to vertebrates by arthropods (arboviruses). Hantaviruses are transmitted not by arthropods but by persistently infected rodents via aerosols of contaminated excreta. They cause hemorrhagic fevers and nephropathy as well as a severe pulmonary syndrome.

O. Bornaviridae

 Bornaviruses are enveloped, spherical (70–130 nm) viruses. The genome is linear, single-stranded, nonsegmented, negative-sense RNA, 8.5–10.5 kb in size. Unique among non segmented, negative-sense RNA viruses, replication and transcription of the viral genome occur in the nucleus. Borna disease virus is neurotropic in animals; a postulated association with neuropsychiatric disorders of humans is unproven.

P. Rhabdoviridae

Rhabdoviruses are enveloped virions resembling a bullet, flat at one end and round at the other, measuring about 75 × 180 nm. The envelope has 10-nm spikes. The genome is linear, single-stranded, nonsegmented, negative-sense RNA, 11–15 kb in size. Particles are formed by budding from the cell membrane. Viruses have broad host ranges. Rabies virus is a member of this group.

Q. Paramyxoviridae

Paramyxoviruses are similar to but larger (150–300 nm) than orthomyxoviruses. Particles are pleomorphic. The internal nucleocapsid measures 13–18 nm, and the linear, single stranded, nonsegmented, negative-sense RNA is 16–20 kb in size. Both the nucleocapsid and the hemagglutinin are formed in the cytoplasm. Those infecting humans include mumps, measles, parainfluenza, metapneumovirus, and respiratory syncytial viruses. These viruses have narrow host ranges. In contrast to influenza viruses, paramyxoviruses are genetically stable.

 R. Filoviridae

Filoviruses are enveloped, pleomorphic viruses that may appear very long and threadlike. They typically are 80 nm wide and about 1000 nm long. The envelope contains large peplomers. The genome is linear, negative-sense, single stranded RNA, 18–19 kb in size. Marburg and Ebola viruses cause severe hemorrhagic fever in Africa. These viruses require maximum containment conditions (Biosafety Level 4) for handling.

S. Emerging Viruses

Novel viruses are being discovered with increasing frequency; most belong to existing families but rarely agents are not classifiable. Some of these are associated with human disease, while many affect other species.

T. Viroids

Viroids are small infectious agents that cause diseases of plants. Viroids are agents that do not fit the definition of classic viruses. They are nucleic acid molecules without a protein coat. Plant viroids are single-stranded, covalently closed circular RNA molecules consisting of about 360 nucleotides and with a highly base-paired rodlike structure. Viroids replicate by an entirely novel mechanism. Viroid RNA does not encode any protein products; the devastating plant diseases induced by viroids occur by an unknown mechanism. Hepatitis D virus in humans has properties similar to viroids.

 U. Prions

 Prions are infectious particles composed solely of protein with no detectable nucleic acid. They are highly resistant to inactivation by heat, formaldehyde, and ultraviolet light that inactivate viruses. The infectious prion protein is misfolded and able to change the conformation of the native prion protein which is encoded by a single cellular gene. Prion diseases, called “transmissible spongiform encephalopathies,” include scrapie in sheep, mad cow disease in cattle, and kuru and Creutzfeldt-Jakob disease in humans.

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مواضيع ذات صلة

Chemical Composition of Viruses

Survey of DNA-Containing Virus Families

Universal System of Virus Taxonomy

Human parainfluenza viruses (HPIV)

Complications of Influenza virus

Influenza virus

Chikungunya virus

West Nile virus

Transmission Cycle of Japanese encephalitis virus

Lab Diagnosis of Dengue virus

Pathogenesis of Dengue virus

Rotavirus