Tuberculosis and Infection Control
An Infectious Disease with Worldwide Public Health Implications
Mar 7, 2009
Tuberculosis (TB) is one of the most common infectious diseases in the world. It is estimated that at least 2 billion people are infected with TB, and several million of these people have active TB, with or without resulting death. Rates of TB vary from country to country and are generally proportional to the country's overall conditions.
Given the high prevalence of TB, the importance of protection against this infectious disease cannot be stressed enough. It is an issue for individuals who have TB and for those who risk contracting it.
Etiology
TB is caused by a group of bacteria called Mycobacteria. In most cases, Mycobacterium tuberculosis is the pathogen involved, and Mycobacterium bovis, Mycobacterium africanum, and Mycobacterium microti occasionally cause disease similar to TB. These microorganisms uniquely stain with acid-fast stain and are, therefore, called acid-fast bacteria.
Pathophysiology
The bacteria is introduced into the human body by inhalation of droplets, which come from people with TB who cough, sneeze, or perform other forceful respiratory movements. Crowded and suboptimal conditions, such as prisons and poor infrastructure, are major risk factors for transmission of TB. Once TB enters the body, the immune system attacks it. About 95% of the time, the immune system traps TB, keeping it dormant for years. After this primary infection, TB can be reactivated under conditions of weakened immunity, including but not limited to HIV infection, chemotherapy, immunosuppressant drugs, and advancing age. During active disease, TB destroys lung tissue through a delayed hypersensitivity reaction, producing cavitary lung lesions. TB may also appear at sites outside the lung, with or without concurrent pulmonary involvement.
Clinical Manifestations
Patients with pulmonary TB present with cough accompanied by sputum, fever, night sweats, weight loss, and loss of appetite. Extrapulmonary TB outside the lungs produces symptoms and signs that depend on the location of disease, such as abdominal pain with TB peritonitis. When TB is distributed throughout the body via the bloodstream, causing fever, weakness, and malaise, it is called miliary TB.
Diagnosis
The diagnosis of TB requires these clinical manifestations plus various tests. A chest x-ray demonstrating abnormalities such as cavitary lesions may suggest TB, but it can mimic other lung conditions. Analysis of the sputum is important to demonstrate the presence of Mycobacteria. Another vital test is the tuberculin skin test, also called the Mantoux test or purified protein derivative (PPD) test, in which antigens are injected under the skin of the forearm. If there is wide enough hardness (induration) around the injection site (measurement cutoffs depend on the patient's risk factors), it is highly suggestive of TB, particularly if the patient previously had a negative skin test indicating no prior history of TB exposure.
Treatment
The treatment of TB requires up to four first-line drugs: isoniazid (INH), rifampin (RIF), pyrazinamide (PZA), and ethambutol (EMB). Other drugs for TB, such as streptomycin, are considered second-line. Total duration of treatment is six to nine months.
Because TB has great potential for transmission to others, the disease must be reported to public health authorities. Efforts are also made by these officials to evaluate the patient's natural settings and close contacts. One important and beneficial measure is directly observed therapy (DOT) to make sure that patients are correcting taking all drugs for TB as instructed.
Prevention
Patients at risk for contracting TB, such as healthcare workers and immigrants from certain areas, can be screened with the tuberculin skin test. Positive skin test results prompt a chest x-ray, which, if positive, is followed by evaluation for active TB. Treatment of such cases entails medication such as isoniazid.
A TB vaccine called Bacille Calmette-Guérin (BCG) is available but only has 50% efficacy overall. It is administered around the world but not in the United States. BCG may produce positive results on tuberculin skin testing.
Final Words
TB is an infectious disease for which public health officials remain constantly vigilant. Knowledge of the condition can help minimize one's risk for TB and prevent transmission to others.
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