Biomedical Engineering Reference
In-Depth Information
Acute infections
result when the virions remain localized, typically with short-lived
symptoms. Cell and tissue damage results when new virions are released and the infected
host cell dies. The host's defense mechanisms usually eliminate the virus over a period of
days or months, bringing about the host's immunity to future infection. Acute infections
include measles, mumps, and influenza (the flu) [13,17].
Persistent infections
result when virions persist within the host organism, yet there are no
disease symptoms. Several categories of persistent infections exist, manifestations of
which are influenced by the process of replication as outlined above. In reality, persistent
infections are usually the result of an acute infection. For instance, the infectious and
disease phases of the measles are relatively short lived, yet in one-in-300,000 people late
complications result in a fatal brain disorder known as subacute sclerosing panecephalitis
(SSPE). SSPE, which is the result of a defective viral replication with brain cells, usually
occurs within 10 years of having measles [9].
Latent persistent infections
exhibit an extended, noninfectious stage between the original
and subsequent disease. Chicken pox and shingles are the best examples of latent
infection, both of which are caused by varicellazoster virus (VZV). Shingles or “zoster”
appears after the virus is reactivated by unknown circumstances. Another example of
latent virus is the herpes virus, both herpes simplex type 1 and type 2 [10].
Persistent infections may result in continuous or “chronic” infectious stages following a
relatively brief period of disease. Examples of this type of infection are hepatitis B and C.
Initial symptoms include nausea, fever, and jaundice; while the patient typically recovers
from these early symptoms, he or she remains infectious. Over a period of time, the dis-
ease gets manifested as hepatitis, cirrhosis of the liver, or cancer. The opposite counterpart
of a chronic infection is a “slow” infection in which no disease symptoms are initially
observed, yet the infected person becomes more infectious over a period of time, and ulti-
mately, a disease becomes apparent. An instance of the slow-infection type is AIDS, which
is caused by the HIV virus [13,10].
21.1.4
Detection Methods
In general, diagnostic tests can be grouped into three categories: (1) direct detection,
(2) indirect examination (virus isolation), and (3) serology [18]. In direct examination, the
clinical specimen is examined directly for the presence of virus particles, virus antigen, or
viral nucleic acids. Indirect examination is carried out in cell culture, eggs, or animals in
an attempt to grow the virus; this is called virus isolation. Serology actually constitutes by
far the bulk of the work of any virology laboratory. A serological diagnosis can be made
by the detection of rising titers of antibody between acute and convalescent stages of infec-
tion, or the detection of IgM. In general, the majority of common viral infections can be
diagnosed by serology. The specimen used for direction detection and virus isolation is
very important. A positive result from the site of disease would be of much greater diag-
nostic significance than those from other sites. For example, in the case of herpes simplex
encephalitis, a positive result from the CSF or the brain would be of much greater signifi-
cance than a positive result from an oral ulcer, since reactivation of oral herpes is common
during times of stress [18,19].
1.
Direct Examination of Specimen
Electron microscopy morphology or immune electron microscopy
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Light microscopy histological appearance, e.g., inclusion bodies
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Antigen detection immunofluorescence, ELISA, etc.
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Molecular techniques for the direct detection of viral genomes
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