Page 349 - Review of Medical Microbiology and Immunology ( PDFDrive )

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mebooksfree.com mebooksfree.com mebooksfree.com Clinical Findings mebooksfree.com mebooksfree.com
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338
PART IV Clinical Virology
However, now it appears that an indirect mode, in which
respiratory droplets are deposited on the hands or on a
After an incubation period of 2 to 4 days, sneezing, nasal
surface such as a table and then transported by fingers to
discharge, sore throat, cough, and headache are common.
the nose or eyes, is also important.
A chilly sensation may occur, but there are few other sys-
The common cold is reputed to be the most common
temic symptoms. The illness lasts about 1 week. Note that
human infection, although data are difficult to obtain
because it is not a well-defined or notifiable disease.
enza C virus, and Coxsackie viruses also cause the common
Millions of days of work and school are lost each year as a
cold syndrome.
result of “colds.” Rhinoviruses occur worldwide, causing other viruses such as coronaviruses, adenoviruses, influ-
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disease particularly in the fall and winter. The reason for
Laboratory Diagnosis
this seasonal variation is unclear. Low temperatures per se
do not predispose to the common cold, but the crowding
that occurs at schools, for example, may enhance transmis-
secretions in cell culture, but this is rarely attempted. Sero-
sion during fall and winter. The frequency of colds is high
logic tests are not done.
in childhood and tapers off during adulthood, presumably
because of the acquisition of immunity.
A few serotypes of rhinoviruses are prevalent during
Treatment & Prevention
one season, only to be replaced by other serotypes during
No specific antiviral therapy is available. Vaccines appear
the following season. It appears that the population builds
impractical because of the large number of serotypes. Paper
up immunity to the prevalent serotypes but remains sus-
tissues impregnated with a combination of citric acid
ceptible to the others.
(which inactivates rhinoviruses) and sodium lauryl sulfate
(a detergent that inactivates enveloped viruses such as
Pathogenesis & Immunity
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influenza virus and respiratory syncytial virus) limit trans-
The portal of entry is the upper respiratory tract, and the
mission when used to remove viruses from fingers con-
taminated with respiratory secretions. High doses of
infection is limited to that region. Rhinoviruses rarely
cause lower respiratory tract disease, probably because they
grow poorly at 37°C.
colds. Lozenges containing zinc gluconate are available for
Immunity is serotype-specific and is a function of nasal
the treatment of the common cold, but their efficacy
secretory IgA rather than humoral antibody.
remains uncertain.
CALICIVIRUSES
Caliciviruses are small, nonenveloped viruses with single- Important Properties
stranded RNA of positive polarity. Although they share
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Norovirus has a nonsegmented, single-stranded, positive-
those features with picornaviruses, caliciviruses are dis-
polarity RNA genome. It is a nonenveloped virus with an
tinguished from picornaviruses by having a larger genome
and having distinctive spikes on the surface. Norovirus
the virion. In the electron microscope, 10 prominent spikes
is the main human pathogen in the calicivirus family.
and 32 cup-shaped depressions can be seen. There are
Sapporo virus, another calicivirus that causes gastroen-
many serotypes; the exact number is uncertain. Six geno-
teritis, is described in the chapter on Minor Viral Patho-
groups have been identified. Most human infections are
gens (Chapter 46).
caused by members of genogroup II.
NOROVIRUS
Disease
Norovirus replicates in a manner similar to that of poliovirus
(see earlier in this chapter).
Norovirus is one of the most common causes of viral gas- Summary of Replicative Cycle
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troenteritis in adults both in the United States and world-
wide. Norovirus is also the most common cause of viral
Transmission & Epidemiology
gastroenteritis in children in the United States because the
rotavirus vaccine has lowered the incidence of disease
involving the ingestion of contaminated seafood or water.
caused by that virus. Norwalk virus is an important norovi-
Outbreaks typically occur in group settings such as cruise
rus and is named for an outbreak of gastroenteritis in a
school in Norwalk, OH, in 1969.
ships (especially in the Caribbean region), schools, camps,
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