Page 395 - Review of Medical Microbiology and Immunology ( PDFDrive )

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PART IV Clinical Virology
384
stage is usually accompanied by a lower level of viremia and
state.
a rise in the number of CD8-positive (cytotoxic) T cells
A syndrome called AIDS-related complex (ARC) can
directed against HIV.
occur during the latent period. The most frequent manifes-
Antibodies to HIV typically appear 10 to 14 days after
tations are persistent fevers, fatigue, weight loss, and
infection, and most patients will have seroconverted by 3 to
4 weeks after infection. Note that the inability to detect
antibodies prior to that time can result in “false-negative”
The late stage of HIV infection is AIDS, manifested by a
serologic tests (i.e., the person is infected, but antibodies
decline in the number of CD4 cells to below 200/μL and an
are not detectable at the time of the test). This has impor- lymphadenopathy. ARC often progresses to AIDS.
increase in the frequency and severity of opportunistic
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infections. Table 45–2 describes some of the common
tant implications because HIV can be transmitted to others
opportunistic infections and their causative organisms seen
during this period. If the antibody test is negative but HIV
in HIV-infected patients during the late, immunocompro-
infection is still suspected, then a polymerase chain reac-
tion (PCR)–based assay for viral RNA in the plasma should
be done.
The two most characteristic manifestations of AIDS are
Pneumocystis pneumonia and Kaposi’s sarcoma. However,
Of those who become seropositive during the acute
infection, approximately 87% are symptomatic (i.e., about
many other opportunistic infections occur with some fre-
13% experience an asymptomatic initial infection).
quency. These include viral infections such as dissemi-
nated herpes simplex, herpes zoster, and cytomegalovirus
After the initial viremia, a viral set point occurs, which
can differ from one person to another. The set point repre-
thy; fungal infections such as thrush (caused by Candida
sents the amount of virus produced (i.e., the viral load) and
tends to remain “set,” or constant, for years. The higher the
albicans), cryptococcal meningitis, and disseminated his-
set point at the end of the initial infection, the more likely infections and progressive multifocal leukoencephalopa-
toplasmosis; protozoal infections such as toxoplasmosis
and cryptosporidiosis; and disseminated bacterial infec-
the individual is to progress to symptomatic AIDS. It is
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estimated that an infected person can produce up to 10 bil-
tions such as those caused by Mycobacterium avium-
lion new virions each day. This viral load can be estimated
intracellulare and Mycobacterium tuberculosis. Many AIDS
by using an assay for viral RNA in the patient’s plasma.
(The assay detects the RNA in free virions in the plasma,
and neuropathy), which can be caused by either HIV
infection of the brain or by many of these opportunistic
not cell-associated virions.)
organisms.
The amount of viral RNA serves to guide treatment
decisions and the prognosis. For example, if a drug regimen
fails to reduce the viral load, the drugs should be changed.
Laboratory Diagnosis
As far as the prognosis is concerned, a patient with more
than 10,000 copies of viral RNA/mL of plasma is signifi-
by the detection of antibodies in the patient’s serum to the
cantly more likely to progress to AIDS than a patient with
p24 protein of HIV using the enzyme-linked immunosor-
fewer than 10,000 copies.
bent assay (ELISA) test. Because there are some false-
The number of CD4-positive T cells is another impor- The presumptive diagnosis of HIV infection is often made
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positive results with this test, the definitive diagnosis is
tant measure that guides the management of infected
made by Western blot (also known as Immunoblot)
patients. It is used to determine whether a patient needs
analysis, in which the viral proteins are displayed by acryl-
chemoprophylaxis against opportunistic organisms, to
determine whether a patient needs anti-HIV therapy, and
paper (the blot), and reacted with the patient’s serum. If
to determine the response to this therapy. The lower limit
antibodies are present in the patient’s serum, they will bind
of CD4 count considered as normal is 500 cells/μL. People
to the viral proteins (predominantly to the gp41 or p24
with this level or higher are usually asymptomatic. The
protein). Enzymatically labeled antibody to human IgG is
frequency and severity of opportunistic infections signifi-
then added. A color reaction reveals the presence of the
cantly increase when the CD4 counts fall below 200/μL. A
CD4 count of 200/μL or below is an AIDS-defining
depicts a Western blot (Immunoblot) test used to diagnose
condition.
HIV infection.
In the middle stage of HIV infection, a long latent
OraQuick is a rapid, screening immunoassay for HIV
period, measured in years, usually ensues. In untreated HIV antibody in the infected patient’s serum. Figure 64–9
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antibody that uses an oral swab sample in an ELISA-type
patients, the latent period typically lasts for 7 to 11 years.
test that can be done at home. Results are available in
The patient is asymptomatic during this period.
20 minutes. Positive results for HIV antibody require con-
Although the patient is asymptomatic and viremia is low
or absent, a large amount of HIV is being produced by
lymph node cells but remains sequestered within the
The PCR test is a very sensitive and specific technique
that can be used to detect HIV DNA within infected cells.
lymph nodes. This indicates that during this period of
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