Page 264 - Review of Medical Microbiology and Immunology ( PDFDrive )

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P
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Pathogenesis

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CHAPTER C ONTENT S

Persistent Viral Infections
Introduction
Chronic-Carrier Infections
The Infected Cell
The Infected Patient
Latent Infections
Slow Virus Infections
Transmission & Portal of Entry
Localized or Disseminated Infections
Pathogenesis & Immunopathogenesis
Practice Questions: USMLE & Course Examinations
Virulence
Evasion of Host Defenses Self-Assessment Questions
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INTRODUCTION
infected brain neurons. Another important example is the
The ability of viruses to cause disease can be viewed on two
owl’s eye inclusion seen in the nucleus of cytomegalovirus-
distinct levels: (1) the changes that occur within individual
infected cells. Electron micrographs of inclusion bodies can
cells and (2) the process that takes place in the infected
also aid in the diagnosis when virus particles of typical
patient.
morphology are visualized.
Fusion of virus-infected cells produces multinucleated
THE INFECTED CELL
with herpesviruses and paramyxoviruses. Fusion occurs
as a result of cell membrane changes, which are probably
There are four main effects of virus infection on the cell: giant cells, which characteristically form after infection
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(1) death, (2) fusion of cells to form multinucleated cells,
caused by the insertion of viral proteins into the mem-
(3) malignant transformation, and (4) no apparent mor-
brane. The clinical diagnosis of herpesvirus skin infections
phologic or functional change.
is aided by the finding of multinucleated giant cells with
Death of the cell is probably due to inhibition of macro-
molecular synthesis. Inhibition of host cell protein synthe-
A hallmark of viral infection of the cell is the cytopathic
sis frequently occurs first and is probably the most
effect (CPE). This change in the appearance of the infected
cell usually begins with a rounding and darkening of the cell
important effect. Inhibition of DNA and RNA synthesis
may be a secondary effect. It is important to note that syn-
and culminates in either lysis (disintegration) or giant cell
thesis of cellular proteins is inhibited but viral protein
formation. Detection of virus in a clinical specimen fre-
synthesis still occurs. For example, poliovirus inactivates an
In addition, CPE is the basis for the plaque assay, an impor-
initiation factor (IF) required for cellular mRNA to be
tant method for quantifying the amount of virus in a
translated into cellular proteins, but poliovirus mRNA has
a special ribosome-initiating site that allows it to bypass the quently is based on the appearance of CPE in cell culture.
sample.
Infection with certain viruses causes malignant trans-
IF so that viral proteins can be synthesized.
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Infected cells frequently contain inclusion bodies,
formation, which is characterized by unrestrained growth,
prolonged survival, and morphologic changes such as focal
which are discrete areas containing viral proteins or viral
particles. They have a characteristic intranuclear or intra-
described in more detail in Chapter 43.
cytoplasmic location and appearance depending on the
Infection of the cell accompanied by virus production
virus. One of the best examples of inclusion bodies that can
can occur without morphologic or gross functional
assist in clinical diagnosis is that of Negri bodies, which are
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