Rna Viruses

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picornaviruses

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These are the smallest (28 nm in diameter) RNA viruses. They have single-stranded, linear, nonsegmented, positive-polarity RNA within a naked icosahedral capsid. The name "picorna" is derived from pico (small), RNA-containing. There are two groups of human pathogens: (1) enteroviruses such as poliovirus, Coxsackie virus, echovirus, and hepatitis A virus and (2) rhinoviruses.

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caliciviruses

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These are naked viruses (38 nm in diameter) with an icosahedral capsid. They have single-stranded, linear, nonsegmented, positive-polarity RNA. There are two human pathogens: Norwalk virus and hepatitis E virus. Taxonomists have recently placed hepatitis E virus into its own genus called hepevirus.

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reoviruses

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These are naked viruses (75 nm in diameter) with two icosahedral capsid coats. They have 10 or 11 segments of double-stranded linear RNA. The name is an acronym of respiratory enteric orphan, because they were originally found in the respiratory and enteric tracts and were not associated with any human disease. The main human pathogen is rotavirus, which causes diarrhea mainly in infants. The rotavirus genome has 11 segments of double-stranded RNA.

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Flaviruses

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These are enveloped viruses with an icosahedral capsid and single-stranded, linear, nonsegmented, positive-polarity RNA. The flaviviruses include hepatitis C virus, yellow fever virus, dengue virus, West Nile virus, and St. Louis and Japanese encephalitis viruses.

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Togaviruses

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These are enveloped viruses with an icosahedral capsid and single-stranded, linear, nonsegmented, positive-polarity RNA. There are two major groups of human pathogens: the alphaviruses and rubiviruses. The alphavirus group includes eastern and western encephalitis viruses; the rubivirus group consists only of rubella virus.

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Retroviruses

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These are enveloped viruses with an icosahedral capsid and two identical strands (said to be "diploid") of single-stranded, linear, positive-polarity RNA. The term "retro" pertains to the reverse transcription of the RNA genome into DNA. There are two medically important groups: (1) the oncovirus group, which contains the sarcoma and leukemia viruses, e.g., human T-cell leukemia virus (HTLV) and (2) the lentivirus ("slow virus") group, which includes human immunodeficiency virus (HIV) and certain animal pathogens, e.g., visna virus. A third group, spumaviruses, is described in Chapter 46.

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Orthomoxyviruses

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These viruses (myxoviruses) are enveloped, with a helical nucleocapsid and eight segments of linear, single-stranded, negative-polarity RNA. The term "myxo" refers to the affinity of these viruses for mucins, and "ortho" is added to distinguish them from the paramyxoviruses. Influenza virus is the main human pathogen.

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Paramyxoviruses

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These are enveloped viruses with a helical nucleocapsid and single-stranded, linear, nonsegmented, negative-polarity RNA. The important human pathogens are measles, mumps, parainfluenza, and respiratory syncytial viruses.

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Rhabdoviruses

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These are bullet-shaped enveloped viruses with a helical nucleocapsid and a single-stranded, linear, nonsegmented, negative-polarity RNA. The term "rhabdo" refers to the bullet shape. Rabies virus is the only important human pathogen.

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Filoviruses

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These are enveloped viruses with a helical nucleocapsid and single-stranded, linear, nonsegmented, negative-polarity RNA. They are highly pleomorphic, long filaments that are 80 nm in diameter but can be thousands of nanometers long. The term "filo" means "thread" and refers to the long filaments. The two human pathogens are Ebola virus and Marburg virus.

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Coronaviruses

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These are enveloped viruses with a helical nucleocapsid and a single-stranded, linear, nonsegmented, positive-polarity RNA. The term "corona" refers to the prominent halo of spikes protruding from the envelope. Coronaviruses cause respiratory tract infections, such as the common cold and SARS (severe acute respiratory syndrome), in humans.

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arenaviruses

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These are enveloped viruses with a helical nucleocapsid and a single-stranded, circular, negative-polarity RNA in two segments. (A part of both segments is positive-polarity RNA, and the term "ambisense RNA" is used to describe this unusual genome.) The term "arena" means "sand" and refers to granules on the virion surface that are nonfunctional ribosomes. Two human pathogens are lymphocytic choriomeningitis virus and Lassa fever virus.

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bunyaviruses

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These are enveloped viruses with a helical nucleocapsid and a single-stranded, circular, negative-polarity RNA in three segments. Some bunyaviruses contain ambisense RNA in their genome (see arenaviruses above.) The term "bunya" refers to the prototype, Bunyamwera virus, which is named for the place in Africa where it was isolated. These viruses cause encephalitis and various fevers such as Korean hemorrhagic fever. Hantaviruses, such as Sin Nombre virus (see Chapter 46), are members of this family.

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delataviruses

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Hepatitis delta virus (HDV) is the only member of this genus. It is an enveloped virus with an RNA genome that is a single-stranded, negative-polarity, covalently closed circle. The symmetry of the nucleocapsid is uncertain. It is a defective virus because it cannot replicate unless hepatitis B virus (HBV) is present within the same cell. HBV is required because it encodes hepatitis B surface antigen (HBsAg), which serves as the outer protein coat of HDV. The RNA genome of HDV encodes only one protein, the internal core protein called delta antigen.