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Latent Tuberculosis (TB)

Deadly bacterial infection with risks for reactivation

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Updated May 29, 2014

Tuberculosis, or “TB,” is an airborne contagious disease that can be fatal if left untreated. It is a disease that starts by infection in the lungs, but can spread to other organs, including the kidneys, brain, and spine. Still a leading killer worldwide, TB is less prevalent in the U.S. than it used to be, but infections have been on the rise in recent years.

What is latent TB

About 90% of people who get infected with TB develop a latent TB infection, which means the infecting bacteria are alive in the body, but inactive. People who have latent infections do not have TB symptoms and cannot spread the infection to others, but they are at risk of developing an active infection that is both symptomatic and contagious. About 3% to 5% of latent TB become active TB in the first year, and about 5-15% after that.

How do I know if I have latent TB?

If you suspect that you have latent TB, you can take a TB skin test in which a health care worker will inject a small amount of TB extracts under the skin on your forearm. An immune reaction to these extracts will result in swelling that can be detected within 2 to 3 days. Sometimes the test may be given too soon after exposure to TB for an immune response to develop. Therefore, a second skin test is usually given at 8 to 10 weeks after exposure.

I don’t have symptoms of TB. Should I be tested?

Overall rates of TV are low in the U.S. However, reasons for getting tested include:

  • Exposure to people with known or suspected active TB, including hospitals, homeless shelters and prisons, or recent travel to countries where TB is endemic, such as Latin America, Africa, Asia, Eastern Europe, and Russia
  • Having a condition where your immune system is weakened (such as HIV/AIDS), which puts you at higher risk for active TB
  • Working with or around TB patients
  • Having symptoms of active TB

Who is at risk for active TB?

People at highest risk for active TB include those with HIV/AIDS, the very young and very old, people who are immunocompromised (including cancer patients treated with chemotherapy), injection drug users, and people whose infections occurred recently (within 2 years).

Can latent TB be treated?

Latent TB can be treated with a 6- to 9-month course of isoniazid, an antibiotic that kills the bacteria that causes TB. There are also alternative regimens for special circumstances. Some doctors will recommend prophylactic treatment for children or HIV patients who have had recent exposure to people with contagious TB, since they are at higher risk for developing active TB. Treatment of latent TB significantly reduces the likelihood of developing active TB.

How does latent TB become active TB?

TB becomes latent when an infected person’s immune system isn't strong enough to keep the infectious bacteria in check. Presence of the Mycobacterium tuberculosis bacteria causes an immune response in which many types of white blood cells are recruited to sites where the bacteria are growing. They form a walled off lesion, known as a “tubercle” or “granuloma." The bacteria within the tubercle can survive for decades, and conditions leading to a weakened immune response can allow the bacteria to break out of the lesion and reactivate to develop into active TB.

Sources

Centers for Disease Control. Division of Tuberculosis Elimination. http://www.cdc.gov/tb/default

Mims CA, et al. Medical Microbiology. ©1993. Mosby-Year Book Europe Limited. London.

Salyers AA and Whitt DD. Bacterial Pathogenesis: A Molecular Approach. ©1994. American Society for Microbiology. Washington, D.C.

World Health Organization. Tuberculosis. http://www.who.int/tb/en/

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