What is tuberculosis?
Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis. TB primarily affects the lungs (pulmonary TB). Other parts of the body may become involved as well, however, either as part of the initial infection or, more commonly, as part of a late widespread infection. Photo source: National Institute of Allergies and Infectious Diseases
TB is spread when a person with active, infectious TB disease coughs, sneezes, or spits, releasing droplets containing TB bacteria into the air, which are then inhaled by someone else. A single cough from a person with active pulmonary TB may contain 3,000 droplets with TB bacteria. If as few as 10 of those bacteria are inhaled by someone nearby, that person can become infected. An individual with active pulmonary TB disease may infect 10-15 other people through close contact over the course of a year.
Not everyone who is exposed to TB bacteria will develop an infection, and not all of those who become infected will develop symptoms of TB disease. Those who are infected but have no symptoms are said to have a latent TB infection. Those who do become sick have an active infection.
- People with latent TB usually have strong natural defense (immune) systems that prevent the TB bacteria from multiplying. They do not have TB symptoms and cannot spread TB to others. It is possible, however, for a person with a latent infection to develop an active infection in which TB bacteria are "reactivated" and start multiplying. This happens to about 10% of people with untreated latent TB infections who are otherwise in good health. A greater percentage of people with compromised immune systems, such as those suffering from malnutrition, tobacco users, those with diabetes, and especially those with HIV infection, will develop active TB disease.
- An active TB infection means the TB bacteria are multiplying, either in the lungs or elsewhere in the body. This will cause a person to have symptoms of TB disease. Active TB is highly contagious.
Both latent and active TB can be treated and cured with a combination of antibiotics that must be taken for several months. Without treatment, active TB disease may cause serious illness or even death.
TB has been a leading cause of death for thousands of years. In the days before the discovery of antibiotics, it was called consumption, and those who contracted it were put into long-term hospitals called sanatoriums for the rest of their lives. Although its frequency has decreased dramatically in the United States, the World Health Organization (WHO) estimates that one-third of the world's population has latent TB. Worldwide, active TB disease is the most common cause of infectious disease-related death, especially among those who are living with HIV, killing about 1.5 million people a year.
In the U.S., there were more than 9,400 reported cases of TB in 2014. This represents about 2.6 cases per 100,000 persons, the lowest recorded since reporting began in 1953. There was resurgence in new cases in the early 1990s, but the rate of TB infections has been steadily declining since then. Nevertheless, certain populations within the U.S. are at higher risk of infection. These include:
- International travelers
- African Americans continue to be disproportionately affected.
- Those living in correctional facilities and other group living situations
- The homeless
While the numbers of new cases of active TB have again declined in the U.S. due to constant vigilance by the medical community, TB remains a significant national and global public health problem.
One of the many challenges in managing TB worldwide is the number of new cases that are cannot be cured with the standard antibiotics used to treat the disease. This is known as multidrug-resistant tuberculosis (MDR-TB).
- Inappropriate or incorrect use of the standard antibiotics, or use of poor quality medicines, can cause MDR-TB.
- Once identified, MDR-TB can be treated and cured using a different combination of antibiotics. However, these "second-line" treatment options are limited and the recommended antibiotics may not be always available. Second-line therapeutic agents require a longer course of treatment (up to 2 years) and are more costly than standard treatment regimens, and the drugs may be associated with more severe adverse side effects.
- Some cases of TB are resistant to even the most effective second-line anti-TB drugs. This is known as extensively drug-resistant TB (XDR-TB) and is an emerging public health concern. XDR-TB has been defined by WHO and the U.S. Centers for Disease Control and Prevention (CDC) as M. tuberculosis that is resistant to the drugs isoniazid and rifampin, to the drug class fluoroquinolone, and to at least one of three injectable "second-line" drugs (amikacin, kanamycin, or capreomycin). Cases of XDR-TB are being closely monitored by the world medical community and measures are being taken in hopes of limiting its spread.