Page 334 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 39 RNA Enveloped Viruses
323
Parainfluenza virus 4 rarely causes disease, except for the
common cold.
Most infections are diagnosed clinically. The diagnosis can
be made in the laboratory either by isolating the virus in
Clinical Findings
cell culture or by observing a fourfold or greater rise in
Parainfluenza viruses are best known as the main cause of
antibody titer. PCR assay can also be used.
croup in children younger than 5 years of age. Croup is
characterized by a harsh cough and hoarseness. In addition
to croup, these viruses cause a variety of respiratory dis-
eases such as the common cold, pharyngitis, laryngitis, Treatment & Prevention
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There is neither antiviral therapy nor a vaccine available.
otitis media, bronchitis, and pneumonia.

CORONAVIRUSES

CORONAVIRUS

Diseases
sol. Infection occurs worldwide and occurs early in life,
Coronaviruses are an important cause of the common cold,
as evidenced by finding antibody in more than half of
probably second only to rhinoviruses in frequency. In 2002, Coronaviruses are transmitted by the respiratory aero-
children. Outbreaks occur primarily in the winter on a
a new disease, an atypical pneumonia called severe acute
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2- to 3-year cycle.
respiratory syndrome (SARS), emerged. In 2012, another
SARS originated in China in November 2002 and spread
severe pneumonia called Middle East respiratory syndrome
rapidly to other countries. As of this writing, there have
emerged.
mately 9%. Human-to-human transmission occurs, and
some patients with SARS are thought to be “super-spread-
Important Properties
ers.” Early in the outbreak, many hospital personnel were
Coronaviruses have a nonsegmented, single-stranded,
affected, but respiratory infection control procedures have
positive-polarity RNA genome. They are enveloped
greatly reduced the spread within hospitals. There are
viruses with a helical nucleocapsid. There is no virion
many animal coronaviruses, and they are suspected of
polymerase. In the electron microscope, prominent club-
being the source of CoV-SARS. The horseshoe bat appears
shaped spikes in the form of a corona (halo) can be seen.
to be the natural reservoir for CoV-SARS, with the civet cat
There are two serotypes called 229E and OC43. The
serving as an intermediate host.
genome sequence of the coronavirus that caused the SARS
In 2012–2013, a new human coronavirus caused an out-
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(CoV-SARS) outbreak is different from that of the exist-
ing human strains. The genome sequence of different
other countries in the region. The disease is called Middle
isolates of CoV-SARS is very similar, so the antigenicity of
East respiratory syndrome (MERS), and the virus is called
the virus is likely to be quite stable. The receptor for the
MERS coronavirus (MERS-CoV). Its closest relative is a bat
SARS coronavirus on the surface of cells is angiotensin-
coronavirus, and bats are thought to be a reservoir. Close
converting enzyme-2.
contact with camels appears to be the mode of transmission
to humans. The risk of person-to-person transmission is
low but has occurred in hospitals with inadequate infection
Summary of Replicative Cycle
control. Another name for the virus is human coronavirus-
The virus adsorbs to cells via its surface spikes (hemag-
glutinin), after which it enters the cytoplasm, where it is
uncoated. The positive-strand genome is translated into
Pathogenesis & Immunity
two large polypeptides, which are self-cleaved by the EMC (HCoV-EMC).
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Coronavirus infection is typically limited to the mucosal
virus-encoded protease. Two of these peptides aggregate
cells of the respiratory tract. Approximately 50% of infec-
to form the RNA polymerase that replicates the genome.
tions are asymptomatic, and it is unclear what role they
In addition, mRNAs are synthesized and then translated
into the structural proteins. The virus is assembled and
tion appears to be brief, and reinfection can occur.
obtains its envelope from the endoplasmic reticulum,
Pneumonia caused by SARS coronavirus is character-
not from the plasma membrane. Replication occurs in
ized by diffuse edema resulting in hypoxia. The binding of
the cytoplasm.
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