Tuberculin reaction
(AKA: the TB Test):
- An example of a Type IV Delayed-Type
Hypersensitivity Reaction
- Reddening and induration begins 8-12
hours after injection
- peaks at 24-72 hours
- Histology: perivascular cuffing
of mononuclear cells in deep and superficial dermis
- Induration is caused by fibrin deposition
in interstitium
- Reaction mediated by CD4+ T- cells:
memory T-cells release lymphokines to amplify
response, specifically macrophage chemotactic and activating factors
- tuberculosis ABBR: TB. An infectious
disease caused by the tubercle bacillus,
Mycobacterium tuberculosis, and characterized pathologically by inflammatory
infiltrations, formation of tubercles, caseation, necrosis, abscesses, fibrosis, and
calcification.
It most commonly affects the respiratory
system, but other parts of the body such
as the gastrointestinal and genitourinary tracts, bones, joints, nervous system,
lymph nodes, and skin may also become infected. Fish, amphibians, birds, and
mammals (esp. cattle) are subject to the disease. Three types of the tubercle
bacillus exist: human, bovine, and avian. Humans may become infected by any
of the three types, but in the U.S. the human type predominates. Infection usually
is acquired from contact with an infected person or an infected cow or through
drinking contaminated milk. In the U.S., about 10 to 15 million persons have
been infected with tuberculosis. Worldwide, about 1.7 billion people harbor the
infection.
Tuberculosis usually affects the lungs, but the disease may spread to other
organs including the gastrointestinal and genitourinary tracts, bones, joints,
nervous system, lymph nodes, and skin. Macrophages surround the bacilli in an
attempt to engulf them but cannot, producing granulomas with a soft, cheesy
(caseous) core. From this state, lesions may heal by fibrosis and calcification
and the disease may exist in an arrested or inactive stage. Reactivation or
exacerbation of the disease or reinfection gives rise to the chronic progressive
form.
The incidence of TB declined steadily from the 1950s to about 1990, when the
acquired immunodeficiency syndrome epidemic, an increase in the homeless
population, an increase in immigrants from endemic areas, and a decrease
inpublic surveillance caused a resurgence of the disease. Populations at greatest
risk for TB include patients with human immunodeficiency virus (HIV), Asian
and other refugees, the urban homeless, alcoholics and other substance abusers,
persons incarcerated in prisons and psychiatric facilities, nursing home
residents, patients taking immunosuppressive drugs, and people with chronic
respiratory disorders, diabetes mellitus, renal failure, or malnutrition. People
from these risk groups should be assessed for TB if they develop pneumonia; all
health care workers should be tested annually.
INCUBATION PERIOD: Approx. 4 to 12
weeks will elapse between the time of
infection and the time a demonstrable primary lesion or positive tuberculin skin test
occurs.
SYMPTOMS: Pulmonary TB produces
chronic cough, sputum production, fevers,
sweats, and weight loss. TB may also cause neurological disease (meningitis), bone
infections, urinary bleeding, and other symptoms if it spreads to other organs.
DIAGNOSIS: A positive tuberculin
skin test indicates the patient has had a
tuberculous infection; however, unless repeated tests indicate a recent change from
negative, it is impossible to tell how recently the infection occurred. A presumptive
diagnosis of active disease is made by finding acid- fast bacilli in stained smears
from sputum or other body fluids. The diagnosis is confirmed by isolating
Mycobacterium tuberculosis on culture or rapid nucleic acid test probes.
TREATMENT: Regimens for TB
have been developed for patients depending on
their HIV status, the prevalence of multidrug-resistant disease in the community,
drug allergies, and drug interactions. Uncomplicated TB in the non-HIV infected
patient is typically treated with a four- drug regimen for 2 months (isoniazid,
rifampin, ethambutol, and pyrazinamide), followed by isoniazid and rifampin for an
additional 4 months. Some experts recommend a longer course of therapy in patients
co- infected with HIV. To ensure compliance and prevent the evolution of drug-
resistant strains of Mycobacteria, directly observed therapy should be used.
Multidrug-resistant TB is more difficult to treat successfully. It has a mortality rate
as high as 80% and may require treatment for as long as 2 years.
CAUTION: All patients with
HIV should be tested for TB, and all patients with TB
should be tested for HIV, because about one fourth of all patients with one disease
may be infected with the other.
avian tuberculosis
Tuberculosis of birds caused by Mycobacterium avium.
bovine tuberculosis
Tuberculosis of cattle caused by Mycobacterium bovis.
endogenous tuberculosis
Tuberculosis that reactivates after a previous infection.
exogenous tuberculosis
Tuberculosis originating from a source outside the body.
hematogenous tuberculosis
The spread of tuberculosis from a primary site to
another site via the bloodstream.
miliary tuberculosis
Tuberculosis that spreads throughout the body via the
bloodstream. It may be fatal.
multidrug resistant
tuberculosisABBR: MDR-TB. Mycobacterium
tuberculosisbacilli that are resistant to therapy with at least two standard
antitubercular drugs (esp. isoniazid and rifampin, the two drugs that have formed the
cornerstone of therapy for tuberculosis).
MDR-TB must be treated with at least three antitubercular drugs to which the
organism is presumed or proven to be sensitive.
open tuberculosis
Tuberculosis in which the tubercle bacilli are present in bodily
secretions that leave the body.
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