The 14 families of RNA viruses are described in Table 31–2. The three naked icosahedral virus families are listed first and are followed by the three enveloped icosahedral viruses. The remaining eight families are enveloped helical viruses; the first five have single-stranded linear RNA as their genome, whereas the last three have single-stranded circular RNA.
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.
These are naked viruses (30 nm in diameter) with an icosahedral nucleocapsid. They have single-stranded, linear, nonsegmented, positive-polarity RNA. The main human pathogen is hepatitis E virus.
These are naked viruses (38 nm in diameter) with an icosahedral capsid. They have single-stranded, linear, nonsegmented, positive-polarity RNA. The main human pathogen is norovirus.
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.
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, Zika virus, and St. Louis and Japanese encephalitis viruses.
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.
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. A virus-encoded RNA-dependent DNA polymerase (reverse transcriptase) performs this function. Only retroviruses, integrate a DNA copy of the RNA genome into cellular DNA as an obligatory step in viral replication. Retroviruses encode an integrase that performs this function.
A large part of the retroviral genome consists of three structural genes, gag, pol and env, in that order. Each gene is transcribed into a single mRNA that is translated into a polyprotein. The gag and pol precursor proteins are cleaved into functional proteins by a virion-encoded protease. For example, the POL polyprotein is cleaved to form reverse transcriptase, integrase, and RNAse H by the protease that is also part of that polyproteins.
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 not medically important and is described in Chapter 46. Spumaviruses are very unusual because although they are retroviruses, they have double-stranded DNA in the virion. They have a retroviral sequence of genes (namely gag, pol, env) and use reverse transcriptase to synthesize virion DNA at the end of the replicative cycle within the nascent progeny virions. Also, the virion DNA integrates into cellular DNA during viral replication.
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.
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.
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.
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.
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 severe acute respiratory syndrome (SARS), in humans.
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.
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). The term bunya refers to the prototype, Bunyamwera virus, which is named after the place in Africa where it was isolated. These viruses cause encephalitis and various fevers such as Korean hemorrhagic fever and Crimean-Congo hemorrhagic fever. Hantaviruses, such as Sin Nombre virus (see Chapter 46), are members of this family.
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 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.
The classification of viruses is based primarily on the nature of the genome and whether the virus has an envelope.
Poxviruses, herpesviruses, and hepadnaviruses are DNA viruses with an envelope, whereas adenoviruses, polyomaviruses, papillomaviruses, and parvoviruses are DNA viruses without an envelope (i.e., they are naked nucleocapsid viruses). Parvoviruses have single-stranded DNA, whereas all the other families of DNA viruses have double-stranded DNA. The DNA of hepadnaviruses (hepatitis B virus) is mostly double-stranded but has a single-stranded region.
Picornaviruses, hepeviruses, caliciviruses, and reoviruses are RNA viruses without an envelope, whereas all the other families of RNA viruses have an envelope. Reoviruses have double-stranded RNA; all the other families of RNA viruses have single-stranded RNA. Reoviruses and influenza viruses have segmented RNA; all the other families of RNA viruses have nonsegmented RNA. Picornaviruses, hepeviruses, caliciviruses, flaviviruses, togaviruses, retroviruses, and coronaviruses have positive-polarity RNA, whereas all the other families have negative-polarity RNA.