Page 269 - Review of Medical Microbiology and Immunology ( PDFDrive )

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PART III Basic Virology
258
and (7) immunosuppression, as in acquired immunodefi-
ciency syndrome (AIDS).
the trigeminal ganglion, whereas herpes simplex virus type
There are three types of persistent viral infections of
2, which causes infections primarily of the genitals, is latent
clinical importance. They are distinguished primarily by
in the lumbar and sacral ganglia. Varicella-zoster virus,
whether virus is usually produced by the infected cells and
another member of the herpesvirus family, causes varicella
by the timing of the appearance both of the virus and of the
latent, primarily in the trigeminal or thoracic ganglion
symptoms of disease.
cells. It can recur in the form of the painful vesicles of zos-
ter (shingles), usually on the face or trunk.
Chronic-Carrier Infections (chickenpox) as its initial manifestation and then remains
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mebooksfree.com mebooksfree.com mebooksfree.com The term slow refers to the prolonged period between the mebooksfree.com
Some patients who have been infected with certain viruses
Slow Virus Infections
continue to produce significant amounts of the virus for
long periods. This carrier state can follow an asymptom-
atic infection as well as the actual disease and can itself
initial infection and the onset of disease, which is usually
either be asymptomatic or result in chronic illness. Impor-
measured in years. In instances in which the cause has been
tant clinical examples are chronic hepatitis, which occurs in
identified, the virus has been shown to have a normal, not
hepatitis B and hepatitis C virus carriers, and neonatal
prolonged, growth cycle. It is not, therefore, that virus
rubella virus and CMV infections, in which carriers can
growth is slow; rather, the incubation period and the pro-
produce virus for years.
gression of the disease are prolonged. Two of these diseases
Latent Infections
ing panencephalitis, which follows several years after measles
virus infections, and progressive multifocal leukoencepha-
In these infections, best illustrated by the herpesvirus
lopathy (PML), which is caused by JC virus, a papovavirus.
group, the patient recovers from the initial infection and are caused by conventional viruses, namely, subacute scleros-
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PML occurs primarily in patients who have lymphomas or
virus production stops. Subsequently, the symptoms may
recur, accompanied by the production of virus. In herpes
are immunosuppressed. Other slow infections in humans
simplex virus infections, the virus enters the latent state in
(e.g., Creutzfeldt-Jakob disease and kuru) may be caused by
the cells of the sensory ganglia. The molecular nature of the
virus infections are described in Chapter 44.
latent state is unknown. Herpes simplex virus type 1, which
PEARLS
The Infected Cell
• Death of infected cells is probably caused by inhibition of cel- The Infected Patient
• Viral infection in the person typically has four stages: incuba-
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lular protein synthesis. Translation of viral mRNA into viral pro-
tion period, prodromal period, specific-illness period, and
teins preempts the ribosomes preventing synthesis of cellular
recovery period.
proteins.
• The main portals of entry are the respiratory, gastrointestinal,
and genital tracts, but through the skin, across the placenta,
• Inclusion bodies are aggregates of virions in specific locations
and via blood are important as well.
in the cell that are useful for laboratory diagnosis. Two impor-
tant examples are Negri bodies in the cytoplasm of rabies
virus-infected cells and owl’s eye inclusions in the nucleus of
sion; all other modes of transmission (e.g., fecal–oral, respiratory
cytomegalovirus-infected cells.
aerosol, insect bite) are horizontal transmission. Transmission
can be from human to human or from animal to human.
• Multinucleated giant cells form when cells are infected with
certain viruses, notably herpesviruses and paramyxoviruses
such as respiratory syncytial virus.
from the portal of entry via the blood to various organs). How-
• Cytopathic effect (CPE) is a visual or functional change in • Most serious viral infection are systemic (i.e., the virus travels
ever, some are localized to the portal of entry, such as the com-
infected cells typically associated with the death of cells.
mon cold, which involves only the upper respiratory tract.
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mebooksfree.com mebooksfree.com mebooksfree.com • The symptoms of viral diseases are usually caused by death of mebooksfree.com
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• Malignant transformation occurs when cells are infected with
Pathogenesis
oncogenic viruses. Transformed cells are capable of unre-
strained growth.
• Some virus-infected cells appear visually and functionally nor-
the infected cells and a consequent loss of function. For
mal, yet are producing large numbers of progeny viruses.
example, poliovirus kills neurons, resulting in paralysis.
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