Page 350 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 40 RNA Nonenveloped Viruses
hospitals, and nursing homes. Person-to-person transmis-
contains blood. The illness typically lasts 2 to 3 days, and
sion also occurs, especially in group settings. There are
many animal caliciviruses, but there is no evidence that
there are no long-term sequelae, except in some immu-
nocompromised patients in whom chronic gastroenteri-
they cause human infection.
tis can occur. In some outbreaks, certain patients
Infection is enhanced by several features of the virus:
low infectious dose, excretion of virus in the stool both
before the onset of symptoms and for several weeks after
such as headache, meningismus, photophobia, and
recovery, and resistance to inactivation by chlorination and
obtundation.
to drying in the environment. It is thought to remain infec- manifest signs of central nervous system involvement
tious for several days in water, uncooked food, and on
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Laboratory Diagnosis
environmental surfaces such as door handles.
A PCR-based test on the stool is performed when a specific
Pathogenesis & Immunity
Norovirus infection is typically limited to the mucosal cells
clinical one.
of the intestinal tract. Watery diarrhea without red cells or
white cells occurs. Many asymptomatic infections occur, as
Treatment & Prevention
determined by the detection of antibodies. Immunity fol-
lowing infection appears to be brief, and reinfection can
There is no antiviral therapy or vaccine available. Dehy-
occur. New strains appear every 2 to 4 years and cause
widespread infections.
ing and diarrhea may require oral rehydration or
intravenous fluids. Personal hygiene, such as handwash-
Clinical Findings dration and electrolyte imbalance caused by the vomit-
ing, and public health measures, such as proper sewage
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Disease is characterized by sudden onset of vomiting
disposal and disinfection of contaminated surfaces, are
helpful.
and diarrhea accompanied by low-grade fever and
REOVIRUSES
REO is an acronym for respiratory enteric orphan; when
the virus was discovered, it was isolated from the respira-
Rotavirus attaches to the cell surface at the site of the
tory and enteric tracts and was not associated with any
β-adrenergic receptor. After entry of the virion into the
disease. Rotaviruses are the most important human patho-
cell, the RNA-dependent RNA polymerase synthesizes
gens in the reovirus family.
mRNA from each of the 11 segments within the cyto-
plasm. The 11 mRNAs are translated into the corre-
ROTAVIRUS
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proteins. One of these, an RNA polymerase, synthesizes
Disease
minus strands that will become part of the genome of the
progeny virus. Capsid proteins form an incomplete cap-
Rotavirus is a common cause of viral gastroenteritis, espe-
sid around the minus strands, and then the plus strands
cially in young children.
of the progeny genome segments are synthesized. The
virus is released from the cytoplasm by lysis of the cell,
Important Properties
not by budding.
Rotavirus has a segmented, double-stranded RNA
genome surrounded by a double-layered icosahedral
Transmission & Epidemiology
capsid without an envelope. The rotavirus genome has
11 segments. The virion contains an RNA-dependent RNA
occurs worldwide, and by age 6 years, most children have
polymerase. A virion polymerase is required because
antibodies to at least one serotype.
human cells do not have an RNA polymerase that can Rotavirus is transmitted by the fecal–oral route. Infection
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synthesize mRNA from a double-stranded RNA template.
Pathogenesis & Immunity
Many domestic animals are infected with their own
strains of rotaviruses, but these are not a source of human
Rotavirus replicates in the mucosal cells of the small intes-
disease. There are at least six serotypes of human rotavirus.
The outer surface protein (also known as the viral hemag-
lytes into the bowel lumen. The consequent loss of salt,
glutinin) is the type-specific antigen and elicits protective
glucose, and water leads to diarrhea. No inflammation
antibody.
occurs, and the diarrhea is nonbloody. It is thought that
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