Summary. TB is an airborne communicable disease caused by Mycobacterium tuberculosis, or the tubercle bacillus. It is spread primarily by tiny airborne particles (droplet nuclei) expelled by a person who has infectious TB. If another person inhales air containing these droplet nuclei, transmission may occur. Infection begins with the multiplication of tubercle bacilli in alveolar macrophages, some of which spread through the bloodstream; however, the immune system response usually prevents the development of disease. Persons who are infected but whodo not have TB disease are asymptomatic and not infectious; such persons usually have a positive reaction to the tuberculin skin test. About 10% of infected persons will develop TB disease at some time in their lives, but the risk is considerably higher for persons who are immunosuppressed, especially those with HIV infection. Although the majority of TB cases are pulmonary, TB can occur in almost any anatomical site or as disseminated disease.
In the United States, the vast majority of TB cases are caused by Mycobacterium tuberculosis, sometimes referred to as the tubercle bacillus. M. tuberculosis and three very closely related mycobacterial species (M. bovis, M. africanum, and M. microti) can cause tuberculous disease, and they compose what is known as the M. tuberculosis complex. M. bovis and M. africanum are very rare in the United States; M. microti does not cause disease in humans. Mycobacteria that do not cause TB are called nontuberculous mycobacteria.
TB is spread from person to person through the air. When a person with pulmonary or laryngeal TB coughs or sneezes, droplet nuclei containing M. tuberculosis are expelled into the air. These tiny particles (1-5 microns in diameter) can remain suspended in the air for several hours.
If another person inhales air containing droplet nuclei, transmission may occur. The probability that TB will be transmitted depends on three factors: the infectiousness of the person with TB, the environment in which exposure occurred, and the duration of exposure. (See Infection Control for more information on infectiousness.)
The best way to stop transmission is to isolate patients with infectious TB immediately and to start effective TB therapy. Infectiousness declines very rapidly after adequate therapy is started, as long as the patient adheres to the prescribed regimen.
Persons at the highest risk of becoming infected with M. tuberculosis are close contacts--persons who often spend time with someone who has infectious TB. Close contacts may be family members, roommates, friends, coworkers, or others. These persons are at risk for TB infection because they are more likely to be exposed to TB. Infection rates have been relatively stable since 1987, ranging from 21% to 23% for the contacts of infectious TB patients.
For contacts of persons with drug-resistant TB, infection rates seem to be similar. However, because they may have a poor response to treatment, patients with drug-resistant disease are often infectious for longer periods and therefore have the potential to infect more contacts. HIV-infected persons with TB disease are not considered more infectious than non-HIV-infected persons with TB disease.
Extrapulmonary TB is rarely contagious; however, transmission from extrapulmonary sites has been reported during aerosol-producing procedures, such as autopsies and tissue irrigation.
When a person inhales air containing particles expelled by an infectious person, most of the larger particles become lodged in the upper respiratory tract, where infection is unlikely to develop. However, the droplet nuclei may reach the alveoli, where infection begins.
Initially, the tubercle bacilli multiply in the alveolar macrophages. A small number spread through the lymphatic channels to regional lymph nodes and then through the bloodstream to more distant tissues and organs, including areas in which TB disease is most likely to develop: the apices of the lungs, the kidneys, the brain, and bone. Within 2 to 10 weeks after infection, the immune system usually intervenes, halting the multiplication of the tubercle bacilli and preventing further spread.
The tuberculin skin test is used to identify persons who have been infected with M. tuberculosis. Most infected people have a positive reaction to the tuberculin skin test within 2 to 10 weeks after infection. Persons who are infected with M. tuberculosis but who do not have TB disease are not infectious to others. TB infection in a person who does not have TB disease is not considered a case of TB and is often referred to as latent TB infection.
TB infection progresses to disease when tubercle bacilli overcome the defenses of the immune system and begin to multiply. Infection can progress to disease very quickly or many years after infection. In the United States, in approximately 5% of persons who have been recently infected with M. tuberculosis, TB disease will develop in the first year or two after infection. In another 5%, disease will develop later in their lives. In other words, in approximately 10% of persons infected with M. tuberculosis, disease will develop at some point. The remaining 90% will stay infected, but free of disease, for the rest of their lives.
Some medical conditions increase the risk that TB infection will progress to disease. The risk may be approximately 3 times greater (as with diabetes) to more than 100 times greater (as with HIV infection) for persons who have these conditions than for those who do not. Some of these conditions include:
Compared with immunocompetent persons who are infected with M. tuberculosis, infected persons who are immunosuppressed are at considerably greater risk of developing TB disease. For example, studies suggest that the risk of developing TB disease is 7% to 10% each year for persons who are infected with both M. tuberculosis and HIV, whereas it is 10% over a lifetime for persons infected only with M. tuberculosis.
In an HIV-infected person, TB disease can develop in either of two ways. First, a person who has TB infection can become infected with HIV, and then TB disease can develop as the immune system is weakened. Second, a person who has HIV infection can become infected with M. tuberculosis , and TB disease can then rapidly develop.
The lungs are the most common site for TB disease; approximately 85% of TB cases are pulmonary. Patients with pulmonary TB usually have a cough and an abnormal chest radiograph, and they should be considered infectious.
However, TB is a systemic disease and may also occur as a pleural effusion; in the central nervous, lymphatic, or genitourinary systems; in the bones and joints; or as disseminated disease (miliary TB). Extrapulmonary TB is more common in persons with HIV infection than in persons without HIV infection; lymphatic TB and miliary disease are particularly common. In HIV-infected persons, extrapulmonary TB is often accompanied by pulmonary TB.
Pulmonary TB and extrapulmonary TB are among the conditions included in the 1993 AIDS surveillance case definition. Any HIV-infected person with a diagnosis of TB disease should be reported as having TB and AIDS.
The current clinical classification system for TB is based on the pathogenesis of the disease. A patient should not have a class 5 classification for more than 3 months.
All persons with class 3 or class 5 TB should be reported promptly to the state or local health department. Health care providers should comply with state and local laws and regulations requiring the reporting of TB.
| Class | Type | Description |
| 0 | No TB exposure Not infected |
No history of exposure Negative reaction to tuberculin skin test |
| 1 | TB Exposure No Evidence of infection |
History of exposure Negative reaction to tuberculin skin test |
| 2 | TB infection No disease |
Positive reaction to tuberculin skin test Negative bacteriologic studies (if done) No clinical or radiographic evidence of TB |
| 3 | Current TB disease | M. tuberculosis cultured (if done) or Positive reaction to tuberculin skin test and Clinical or radiographic evidence of current disease |
| 4 | Previous TB disease | History of episode(s) of TB or Abnormal but stable radiographic findings Positive reaction to the tuberculin skin test Negative bacteriologic studies (if done) and No clinical or radiographic evidence of current disease |
| 5 | TB suspected | Diagnosis pending |