Page 306 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 37 DNA Enveloped Viruses
295
zoster often are treated with acyclovir because it can reduce
the duration and severity of symptoms. Immunocompro-
The cycle is similar to that of HSV (see page 290). One
mised children and adults with chickenpox, zoster, or dis-
unique feature of CMV replication is that some of its
seminated disease should be treated with acyclovir. Disease
“immediate early proteins” are translated from mRNAs
caused by acyclovir-resistant strains of VZV can be treated
brought into the infected cell by the parental virion rather
with foscarnet. Two drugs similar to acyclovir, famciclovir
(Famvir) and valacyclovir (Valtrex), can be used in patients
newly infected cell.
with zoster to accelerate healing of the lesions, but none of
these drugs can cure the latent state. There is some evi- than being translated from mRNAs synthesized in the
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dence that these drugs reduce the incidence of postzoster
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Transmission & Epidemiology
neuralgia.
CMV is transmitted by a variety of modes. Early in life, it
Prevention
and quite commonly in breast milk. In young children, its
most common mode of transmission is via saliva. Later in
There are two vaccines against VZV: one designed to pre-
life it is transmitted sexually; it is present in both semen
vent varicella, called Varivax, and the other designed to
and cervical secretions. It can also be transmitted during
prevent zoster, called Zostavax. Both contain live, attenu-
blood transfusions and organ transplants. CMV infection
ated VZV, but the zoster vaccine contains 14 times more
occurs worldwide, and more than 80% of adults have anti-
virus than the varicella vaccine. The zoster vaccine is effec-
tive in preventing the symptoms of zoster, but does not
eradicate the latent state of VZV.
The varicella vaccine is recommended for children body against this virus.
Pathogenesis & Immunity
between the ages of 1 and 12 years, whereas the zoster vac-
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Infection of the fetus can cause cytomegalic inclusion
cine is recommended for people older than 60 years and
disease, characterized by multinucleated giant cells with
who have had varicella. The varicella vaccine is given in
two doses, whereas the zoster vaccine is given in one dose.
affected, and widespread congenital abnormalities result.
Because these vaccines contain live virus, they should not
Infection of the fetus occurs mainly when a primary infec-
be given to immunocompromised people or pregnant
tion occurs in the pregnant woman (i.e., when she has no
women.
antibodies that will neutralize the virus before it can infect
Acyclovir is useful in preventing varicella and dissemi-
the fetus). The fetus usually will not be infected if the preg-
nated zoster in immunocompromised people exposed to
nant woman has antibodies against the virus. Congenital
the virus. Varicella-zoster immune globulin (VZIG),
abnormalities are more common when a fetus is infected
which contains a high titer of antibody to the virus, is also
during the first trimester than later in gestation, because
used for such prophylaxis.
the first trimester is when development of organs occurs
and the death of any precursor cells can result in congeni-
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tal defects.
CYTOMEGALOVIRUS (CMV)
Infections of children and adults are usually asymp-
tomatic, except in immunocompromised individuals.
Diseases
CMV causes cytomegalic inclusion disease (especially con-
be reactivated when cell-mediated immunity is decreased.
genital abnormalities) in neonates. It is the most common
CMV can also persist in kidneys for years. Reactivation of
cause of congenital abnormalities in the United States.
CMV from the latent state in cervical cells can result in
CMV is a very important cause of pneumonia and other
infection of the newborn during passage through the
diseases in immunocompromised patients such as recipi-
birth canal.
ents of bone marrow (stem cell) and solid organ trans-
CMV has a specific mechanism of “immune evasion”
plants. It also causes heterophil-negative mononucleosis in
that allows it to maintain the latent state for long periods.
immunocompetent individuals.
In CMV-infected cells, assembly of the major histocompat-
ibility complex (MHC) class I–viral peptide complex is
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unstable, so viral antigens are not displayed on the cell
Important Properties
surface and killing by cytotoxic T cells does not occur. In
CMV is structurally and morphologically similar to other
addition, CMV encodes several microRNAs, one of which
herpesviruses but is antigenically different. It has a single
serotype. Humans are the natural hosts; animal CMV
the class I MHC protein. This prevents viral proteins from
strains do not infect humans. Giant cells are formed, hence
being displayed on the infected cell surface, and killing by
the name cytomegalo.
cytotoxic T cells does not occur.
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