Page 262 - Review of Medical Microbiology and Immunology ( PDFDrive )

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mebooksfree.com mebooksfree.com mebooksfree.com CHAPTER 31 Classification of Medically Important Viruses 251 mebooksfree.com
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alphaviruses and rubiviruses. The alphavirus group
RNA VIRUSES
includes eastern and western encephalitis viruses; the rubi-
The 14 families of RNA viruses are described in Table 31–2.
virus group consists only of rubella virus.
The three naked icosahedral virus families are listed first
and are followed by the three enveloped icosahedral
Retroviruses
viruses. The remaining eight families are enveloped helical
viruses; the first five have single-stranded linear RNA as
two identical strands (said to be “diploid”) of single-
their genome, whereas the last three have single-stranded
stranded, linear, positive-polarity RNA. The term retro
circular RNA. These are enveloped viruses with an icosahedral capsid and
pertains to the reverse transcription of the RNA genome
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into DNA. There are two medically important groups: (1)
Picornaviruses
the oncovirus group, which contains the sarcoma and leu-
These are the smallest (28 nm in diameter) RNA viruses.
They have single-stranded, linear, nonsegmented, positive-
and (2) the lentivirus (“slow virus”) group, which includes
polarity RNA within a naked icosahedral capsid. The name
human immunodeficiency virus (HIV) and certain animal
“picorna” is derived from pico (small), RNA-containing.
pathogens (e.g., visna virus). A third group, spumaviruses,
There are two groups of human pathogens: (1) enterovi-
is described in Chapter 46.
ruses, such as poliovirus, Coxsackie virus, echovirus, and
hepatitis A virus; and (2) rhinoviruses.
Orthomyxoviruses
Hepeviruses
nucleocapsid and eight segments of linear, single-stranded,
These are naked viruses (30 nm in diameter) with an icosa-
negative-polarity RNA. The term myxo refers to the affinity
hedral nucleocapsid. They have single-stranded, linear, These viruses (myxoviruses) are enveloped, with a helical
of these viruses for mucins, and ortho is added to distin-
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nonsegmented, positive-polarity RNA. The main human
guish them from the paramyxoviruses. Influenza virus is
pathogen is hepatitis E virus.
the main human pathogen.
Caliciviruses
These are naked viruses (38 nm in diameter) with an icosa-
These are enveloped viruses with a helical nucleocapsid
hedral capsid. They have single-stranded, linear, nonseg-
and single-stranded, linear, nonsegmented, negative-polar-
mented, positive-polarity RNA. The main human pathogen
ity RNA. The important human pathogens are measles,
is norovirus.
mumps, parainfluenza, and respiratory syncytial viruses.
Reoviruses
These are naked viruses (75 nm in diameter) with two ico-
These are bullet-shaped enveloped viruses with a helical
sahedral capsid coats. They have 10 or 11 segments of Rhabdoviruses
nucleocapsid and a single-stranded, linear, nonsegmented,
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double-stranded linear RNA. The name is an acronym of
negative-polarity RNA. The term rhabdo refers to the bullet
respiratory enteric orphan, because they were originally
shape. Rabies virus is the only important human pathogen.
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
These are enveloped viruses with a helical nucleocapsid
infants. The rotavirus genome has 11 segments of double-
and single-stranded, linear, nonsegmented, negative-polar-
stranded RNA.
ity RNA. They are highly pleomorphic, long filaments that
are 80 nm in diameter but can be thousands of nanometers
Flaviviruses
long. The term filo means “thread” and refers to the long
These are enveloped viruses with an icosahedral capsid and
single-stranded, linear, nonsegmented, positive-polarity
Marburg virus.
RNA. The flaviviruses include hepatitis C virus, yellow
fever virus, dengue virus, West Nile virus, and St. Louis and filaments. The two human pathogens are Ebola virus and
Coronaviruses
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Japanese encephalitis viruses.
These are enveloped viruses with a helical nucleocapsid and
a single-stranded, linear, nonsegmented, positive-polarity
Togaviruses
protruding from the envelope. Coronaviruses cause respira-
These are enveloped viruses with an icosahedral capsid and
tory tract infections, such as the common cold and severe
single-stranded, linear, nonsegmented, positive-polarity
RNA. There are two major groups of human pathogens: the
acute respiratory syndrome (SARS), in humans.
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