Tuberculosis
is said to be making a comeback with a vengeance. What are the main causes for
this? Briefly describe for this disease:
(a)
the pathogenesis,
(b)
the laboratory diagnosis,
(c)
the current treatment, and
(d)
the principles of chemoprophylaxis.
Suggested
Answer:
There has been a resurgence of tuberculosis in the West due to the
growing problem of AIDS. HIV-infected individuals have a high incidence of
tuberculosis, characterized by frequent extrapulmonary disease. Both primary,
pulmonary infection and endogenous reactivation are seen in HIV-positive
individuals. Owing to the loss of T-cell function in these patients, the
tuberculin skin test may not be reliable and the chest radiograph may not show
the classic well-defined primary tubercle. Both of these observations make the
diagnosis of tuberculosis in HIV-infected patients more challenging.
(a) Virtually all M tuberculosis infections occur by airborne transmission of droplet nuclei containing a few viable, virulent organisms produced by a sputum-positive individual. The bacilli are deposited in the alveolar spaces of the lungs, where they are engulfed by alveolar macrophages. A portion of the infectious inoculum resists intracellular destruction and persists, eventually multiplying and killing the macrophage. The ability of virulent mycobacteria to survive within phagocytes justifies their designation as facultative intracellular pathogens. The mechanisms of intracellular survival are not clear and may vary from species to species. There is some evidence that M tuberculosis can prevent phagosome-lysosome fusion. Other studies have demonstrated that virulent mycobacteria can prevent acidification of the phagolysosome, perhaps by modulating the activity of a membrane proton pump. Most of the tissue destruction associated with tuberculosis results from cell-mediated hypersensitivity, however, rather than direct microbial aggression.
Eventually, the accumulating mycobacteria stimulate an inflammatory focus which matures into a granulomatous lesion characterized by a mononuclear cell infiltrate surrounding a core of degenerating epithelioid and multinucleated giant (Langhans) cells. This lesion (called a tubercle) may become enveloped by fibroblasts, and its center often progresses to caseous necrosis. Liquefaction of the caseous material and erosion of the tubercle into an adjacent airway may result in cavitation and the release of massive numbers of bacilli into the sputum. In the resistant host, the tubercle eventually becomes calcified.
Early in infection, mycobacteria may spread distally either indirectly through the lymphatics to the hilar or mediastinal lymph nodes and thence via the thoracic duct into the blood stream, or directly into the circulation by erosion of the developing tubercle into a pulmonary vessel. Extrapulmonary hematogenous dissemination results in the seeding of other organs (e.g., spleen, liver, and kidneys) and, eventually, reinoculation of the lungs. The resulting secondary lung lesions (as opposed to the initial site of implantation) may serve as the origin of reactivation of clinical disease years or decades later owing to the persistence of viable tubercle bacilli. Primary disease is usually characterized by a single lesion in the middle or lower right lobe with enlargement of the draining lymph nodes. Endogenous reactivation is often accompanied by a single (cavitary) lesion in the apical region, with unremarkable lymph nodes and multiple secondary tubercles. In parts of the world where bovine tuberculosis has not been eliminated and where dairy products are not properly treated, direct infection of the gastrointestinal tract may occur by ingestion of virulent M bovis organisms. The gut is also exposed occasionally in pulmonary tuberculosis when large numbers of viable M tuberculosis cells are coughed up and swallowed.
(b) Infection in an asymptomatic individual can be diagnosed with the help of the intradermal PPD skin test. Intradermal introduction of PPD into a previously infected, hypersensitive person results in the delayed (48-72 hr) appearance of an indurated (raised, hard) reaction with or without erythema. It is impossible to distinguish between present and past infection on the basis of a positive tuberculin test. Recent conversion of the reaction from negative to positive warrants clinical attention. Although multiple-puncture (or tine) tests once were popular for screening for tuberculin hypersensitivity, they are not as accurate as the Mantoux test and should not be used. The Mantoux test requires the intradermal injection of a measured volume (0.1 ml) containing a specified quantity (5 tuberculin units) of PPD. The transverse diameter of induration is measured 48 to 72 hours later. Test is positive if the diameter of induration is more than 10mm. In a person with symptoms suggestive of tuberculosis, clinical specimens (sputum, bronchial or gastric washings, pleural fluid, urine, or cerebrospinal fluid) should be stained and cultured for acid-fast bacilli. Culture and identification of mycobacteria in such specimens are mandatory for diagnosis.
Culture for mycobacteria involves inoculation of solid and broth media. For specimens such as sputum that are contaminated with normal bacterial flora, a selective medium containing antimicrobial agents should be inoculated. Sterile body fluids should be inoculated to solid media and a broth medium. Cultures are incubated at 35° to 37° C in an atmosphere of 5 to 10% CO2. For specimens from cutaneous sites a second set of cultures should be incubated at 30° C. All cultures should be examined weekly for 8 weeks.
The major advantage of culture on solid media is that it allows visualization of colony morphology and pigmentation, which is useful diagnostically for distinguishing colonies of M tuberculosis from those of some nontuberculous mycobacteria. However, they require 3 or 4 weeks. The more rapid broth systems (e.g. Bactec) require only 5 to 12 days, and rely upon the detection of 14C-labeled C02 produced by growing mycobacteria.
Commercial chemiluminescent DNA probes, gas-liquid chromatography, high-performance liquid chromatography, and thin-layer chromatography allow identification of a few species of mycobacteria within hours after sufficient growth is present on solid or in a liquid medium.
(c) Current treatment regimens employ several antimycobacterial agents to guarantee sufficient antibacterial activity in different cellular and extracellular locations: inside phagocytic cells, in granulomas and in collections of respiratory secretions. In individuals with clinical disease, short term (6-9 month) ambulatory therapy with so-called first-line anti-mycobacterial drugs, such as isoniazid, rifampin, pyrazinamide, and sometimes, ethambutol and streptomycin (seldom used now), results in disappearance of viable tubercle bacilli from the sputum, rendering the patient noninfectious. Pyrazinamide treatment is usually stopped after 2 months. In immunocompromised and AIDS patients, ethambutol is added and all four drugs are given for 9-12 months. Prompt therapy, even in the absence of other signs or symptoms, is thought to sterilize the tissues and prevent endogenous reactivation of tuberculosis later in life. Patient compliance is probably the single most important variable affecting treatment outcome. Noncompliance of patients in the major factor in allowing resistant organisms to survive. Second-line drugs are employed in face of drug resistance to the first-line drugs. They are ethionamide, amikacin, ofloxacin and cycloserine.
(d) Isoniazid is the single agent employed in the chemoprophylaxis of tuberculosis. Chemoprophylaxis of tuberculosis does not normally deal with mycobacterial infection at its source or at the transmission stage but interferes with the invasion of sterile tissues and secondary spread. Chemoprophylaxis with isoniazid for 6-9 months are prescribed for asymptomatic patients with a positive PPD skin test against the 5-15% risk of meningitis orm iliary diseemination; children exposed to patients with tuberculosis and immunocompromised patients with a positive PPD skin test.