Page 326 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 39 RNA Enveloped Viruses
snatching.” Most of the mRNAs move to the cytoplasm,
where they are translated into viral proteins. Some of the
ynx, trachea, and bronchi. Pneumonia, which involves the
viral mRNAs remain in the nucleus, where they serve as the
alveoli may also occur.
After the virus has been inhaled, the neuraminidase
template for the synthesis of the negative-strand RNA
degrades the protective mucus layer, allowing the virus to
genomes for the progeny virions. Replication of the prog-
eny genomes is performed by a different subunit of the viral
tract. The infection is limited primarily to this area because
RNA polymerase (acting as a replicase) from the subunit
the proteases that cleave the hemagglutinin are located in
that functioned earlier as a transcriptase that synthesized
the respiratory tract.
the mRNAs. Two newly synthesized proteins, NP protein gain access to the cells of the upper and lower respiratory
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and matrix protein, bind to the progeny RNA genome in
Despite systemic symptoms, viremia rarely occurs. The
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systemic symptoms, such as severe myalgias, are due to
the nucleus, and that complex is transported to the
cytokines circulating in the blood. There is necrosis of the
cytoplasm.
The helical ribonucleoprotein assembles in the cyto-
plasm, matrix protein mediates the interaction of the
virus pneumonia, which can complicate influenza, is inter-
nucleocapsid with the envelope, and the virion is released
stitial in location.
from the cell by budding from the outer cell membrane at
Immunity depends mainly on secretory IgA in the respi-
the site where the hemagglutinin and neuraminidase are
ratory tract. IgG is also produced but is less protective.
Cytotoxic T cells also play a protective role.
located. The neuraminidase releases the virus by cleaving
neuraminic acid on the cell surface at the site of the bud-
ding progeny virions. Influenza virus, hepatitis delta virus,
and retroviruses are the only RNA viruses that have an
After an incubation period of 24 to 48 hours, fever, myal-
important stage of their replication take place in the Clinical Findings
gias, headache, sore throat, and cough develop suddenly.
nucleus.
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Severe myalgias (muscle pains) coupled with respiratory
tract symptoms are typical of influenza. Vomiting and diar-
Transmission & Epidemiology
rhea are rare. The symptoms usually resolve spontaneously
The virus is transmitted by airborne respiratory droplets.
The ability of influenza A virus to cause epidemics is
complicate the course. One of the well-known complications
dependent on antigenic changes in the hemagglutinin and
of influenza is pneumonia caused by either Staphylococcus
neuraminidase. As mentioned previously, influenza A virus
aureus or Streptococcus pneumoniae.
undergoes both major antigenic shifts as well as minor
Reye’s syndrome, characterized by encephalopathy and
liver degeneration, is a rare, life-threatening complication
antigenic drifts. Antigenic shift variants appear infre-
quently, whereas drift variants appear virtually every year.
The last major antigenic shift that caused a pandemic in
influenza B and chickenpox. Aspirin given to reduce fever
humans was in 1968 when H3N2 emerged. Epidemics and
in viral infections has been implicated in the pathogenesis
of Reye’s syndrome.
pandemics (worldwide epidemics) occur when the antige- in children following some viral infections, particularly
nicity of the virus has changed sufficiently that the preexist-
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ing immunity of many people is no longer effective. The
Laboratory Diagnosis
antigenicity of influenza B virus undergoes antigenic drift
Although most diagnoses of influenza are made on clinical
but not antigenic shift. The antigenic changes exhibited by
influenza B virus are less dramatic and less frequent than
monly used is an enzyme-linked immunosorbent assay
those of influenza A virus.
(ELISA) for viral antigen in respiratory secretions such as
Influenza occurs primarily in the winter months of
nasal or throat washings, nasal or throat swabs, or sputum.
December to February in the northern hemisphere, when
Several rapid ELISA tests suitable for a physician’s office
influenza and bacterial pneumonia secondary to influenza
laboratory are available. Two tests (FLU OIA and QuickVue
cause a significant number of deaths, especially in older
Influenza Test) are based on detection of viral antigen using
people. The morbidity of influenza in children younger
monoclonal antibodies, and a third test (ZSTATFLU) is
than 2 years is also very high, second only to the morbidity
based on detection of viral neuraminidase using a substrate
in the elderly. In the southern hemisphere (e.g., in Australia
of the enzyme that changes color when cleaved by neur-
and New Zealand), influenza occurs primarily in the winter
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months of June through August. In the tropics, influenza
treatment with the neuraminidase inhibitors should be
occurs year round with little seasonal variation.
instituted within 48 hours of the onset of symptoms. Other
tests such as direct fluorescent antibody and polymerase
Pathogenesis & Immunity
chain reaction (PCR) are also used.
Influenza virus infection causes inflammation of the
Influenza can also be diagnosed by the detection of anti-
mucosa of upper respiratory tract sites such as the nose and
bodies in the patient’s serum. A rise in antibody titer of at
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