Prevention and Treatment

Vaccination against tuberculosis has been widely used in many parts of the world with varying success. The vaccinating agent, a living attenuated mycobacterium known as Bacille Calmette-Guerin, or BCG, is derived from M. bovis. Repeated subculture in the laboratory over many years resulted in selection of this strain of M. bovis, which has little virulence in humans but produces some immunity to tuberculosis. Use of the vaccine is discouraged in the United States, because people who receive the vaccine usually develop a positive tuberculin test. By causing a positive test, BCG vaccination eliminates an important way of diagnosing tuberculosis early in the disease when it can most easily be treated. BCG is not safe to use in severely immuno-compromised patients because the vaccine bacillus can spread throughout the body and cause disease. Safer and more effective genetically engineered vaccines are under development. Control oftuberculosis is aided by identifying unsuspected cases using skin tests and lung X-rays. Individuals with active disease are then treated, thus interrupting the spread of M. tuberculosis. People whose Mantoux tests have changed from negative to positive are also treated, even when no evidence of active disease exists. Treatment reduces the risk (estimated to be about 12%) that these people will develop active disease later in life.

Mutants resistant to antibacterial medications frequently occur among sensitive M. tuberculosis strains. Since mutants simul

23.5 Bacterial Infections of the Lower Respiratory System 583

1 mm

Figure 23.18 Stained Lung Tissue Showing a Tubercle, a Kind of Granuloma Caused by the Body's Reaction to Mycobacterium tuberculosis The innumerable dark dots around the outer portion of the picture are nuclei of lung tissue and inflammatory cells. Centrally and extending to the right of the photograph, most of the nuclei have disappeared because the cells are dead and the tissue has begun to liquefy. The photograph depicts a chest X-ray film of an individual with tuberculosis.

1 mm

Figure 23.18 Stained Lung Tissue Showing a Tubercle, a Kind of Granuloma Caused by the Body's Reaction to Mycobacterium tuberculosis The innumerable dark dots around the outer portion of the picture are nuclei of lung tissue and inflammatory cells. Centrally and extending to the right of the photograph, most of the nuclei have disappeared because the cells are dead and the tissue has begun to liquefy. The photograph depicts a chest X-ray film of an individual with tuberculosis.

taneously resistant to more than one antimicrobial medication occur with a very low frequency, two or more of the medications are always given together in treating tuberculosis. The combination of rifampin and isoniazid (INH) is favored because both drugs are bactericidal against actively growing organisms in cavities as well as metabolically inactive intracellular organisms. Because of the long generation times of M. tuberculosis and its resistance to destruction by body defenses, drug treatment of tuberculosis must generally be continued for a minimum of 6 months to cure the disease. During the prolonged treatment symptoms usually disappear, and many individuals become careless about taking their medications. Such negligence allows the rare resistant mutants to multiply and can lead to high rates of relapse. Up to two-thirds of the M. tuberculosis strains obtained from inadequately treated patients are resistant to one or more antitubercular medications. These resistant strains can infect others in the community. DOT (directly observed therapy) programs

Positive Mantoux Test
Figure 23.19 Tuberculin Test A positive test is the result of delayed hypersensitivity to Mycobacterium tuberculosis antigens injected into the skin, and it gives evidence of past or current infection with the bacterium.

have been highly effective in assuring that medications are taken properly. Nevertheless, the problem of drug-resistant M. tuberculosis strains has reached alarming proportions in some areas and has motivated the search for new vaccines against the disease. ■ antibiotic resistance, p. 521

The main features of tuberculosis are shown in table 23.9.

Legionnaires' Disease

Legionnaires' disease was unknown until 1976, when a number of people attending an American Legion Convention in Philadelphia developed a mysterious pneumonia that was fatal in many cases. Months of scientific investigation eventually paid off when the cause was discovered to be a previously unknown bacterium commonly present in the natural environment.

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