Page 200 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 21 Mycobacteria
An important component of prevention is the use of the
Treatment of latent (asymptomatic) infections consists
of INH taken for 6 to 9 months or INH plus rifapentine for
treatment for latent infections as described earlier. Groups
3 months. This approach is most often used in asymptom-
that should be screened with the PPD skin test include
atic patients whose PPD skin test recently converted to
people with HIV infection, close contacts of patients with
positive. The risk of symptomatic infection is greatest
within the first 2 years after infection, so INH is particu-
larly indicated for these “recent converters.” INH is also
and intravenous drug users, prison inmates, and foreign-
used in children exposed to patients with symptomatic
born individuals from countries with a high incidence of
tuberculosis.
tuberculosis. Patients who receive INH should be evaluated active tuberculosis, low-income populations, alcoholics
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Because there are some problems associated with PPD
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for drug-induced hepatitis, especially those over the age of
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35 years. Rifampin can be used in those exposed to INH-
skin tests, such as the measurement and the interpretation
of results and the inconvenience of the patient having to
resistant strains. A combination of rifampin and pyrazin-
amide should not be used because it causes a high rate of
severe liver injury.
detect latent infections was developed. This test, called
Quantiferon-TB (QFT), measures the amount of
Resistance to INH and other antituberculosis drugs is
being seen with increasing frequency in the United States,
interferon-γ released from the patient’s lymphocytes after
especially in immigrants from Southeast Asia and Latin
exposure to PPD in cell culture. QFT requires only a single
America. Strains of M. tuberculosis resistant to multiple
blood specimen and determines the amount of interferon-γ
drugs (MDR strains) have emerged, primarily in AIDS
patients. The most common pattern is resistance to both
BCG vaccine can be used to induce partial resistance to
INH and rifampin, but some isolates are resistant to three
tuberculosis. The vaccine contains a strain of live, attenu-
or more drugs. The treatment of MDR organisms usually by an enzyme-linked immunosorbent assay (ELISA) test.
ated M. bovis called bacillus Calmette-Guérin. The vaccine
involves the use of four or five drugs, including ciprofloxa-
is effective in preventing the appearance of tuberculosis as
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cin, amikacin, ethionamide, and cycloserine. The precise
a clinical disease, especially in children, although it does
recommendations depend on the resistance pattern of the
not prevent infection by M. tuberculosis. However, a major
isolate and are beyond the scope of this book.
can range from 0% to 70%. It is used primarily in areas of
In 2013, a new drug, bedaquiline, was approved for the
treatment of MDR strains. It should be used in combina-
the world where the incidence of the disease is high. It is
tion with other drugs, not as monotherapy. It is a diaryl-
not usually used in the United States because of its variable
quinoline that inhibits an ATP synthase unique to M.
effectiveness and because the incidence of the disease is low
enough that it is not cost-effective.
tuberculosis.
The skin test reactivity induced by the vaccine given to
Previous treatment for tuberculosis predisposes to the
selection of these MDR organisms. Noncompliance (i.e.,
the failure of patients to complete the full course of ther-
test reaction in adults is not altered by the vaccine. For
example, skin test reactions of 10 mm or more should not
apy) is a major factor in allowing the resistant organisms to
survive. One approach to the problem of noncompliance is children wanes with time, and the interpretation of the skin
be attributed to the vaccine unless it was administered
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directly observed therapy (DOT), in which health care
recently. In the United States, use of the vaccine is limited
to young children who are in close contact with individuals
workers observe the patient taking the medication.
The strains of M. tuberculosis resistant to INH, rifampin, a
with active tuberculosis and to military personnel. BCG
fluoroquinolone, and at least one additional drug are called
extensively drug-resistant (XDR) strains. XDR strains emerged
ple because the live BCG organisms can cause disseminated
in 2005 among HIV-infected patients in South Africa.
disease.
BCG vaccine is also used to treat bladder cancer. The
vaccine is instilled into the bladder and serves to nonspe-
Prevention
cifically stimulate cell-mediated immunity, which can
The incidence of tuberculosis began to decrease markedly
even before the advent of drug therapy in the 1940s. This is
Pasteurization of milk and destruction of infected cattle
attributed to better housing and nutrition, which have
are important in preventing intestinal tuberculosis.
improved host resistance. At present, prevention of the inhibit the growth of the carcinoma cells.
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spread of the organism depends largely on the prompt
identification and adequate treatment of patients who are
ATYPICAL MYCOBACTERIA
coughing up the organism. The use of masks and other
respiratory isolation procedures to prevent spread to medi-
cal personnel is also important. Contact tracing of indi-
cal, because they differ in certain respects from the proto-
type, M. tuberculosis. For example, atypical mycobacteria
viduals exposed to patients with active pulmonary disease
are widespread in the environment and are not pathogenic
who are coughing should be done.
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