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TB Resources

Latent vs Active TB

When someone gets exposed to TB, one of four possible outcomes can occur: The body kills and eradicates the bacteria and…

post by Visual Epidemiology

When someone gets exposed to TB, one of four possible outcomes can occur:

  1. The body kills and eradicates the bacteria and nothing ever happens
  2. The body cannot control the TB and the bacteria replicate and turn into a clinical case of TB (called active TB)
  3. The body “controls” the bacteria. Although TB is still in the person, it is not active TB (this is called latent TB)
  4. Latent TB infection can later progress to active TB infection (typically when immune system is compromised)

What is Latent TB Infection (LTBI)?

People can be exposed to the TB bacteria, but have an immune system strong enough to contain the disease. When someone is exposed to TB but is able to contain it (i.e. the bacteria does not spread), this person is said to have latent TB. People with latent TB infection are not infectious and cannot spread TB infection to others. Also, people with latent TB infection do not feel sick and do not have any symptoms. They have the bacteria inside them, but do not have active TB disease. Though they may test positive for TB diagnostics, they are not considered a medical case of tuberculosis. However, sometimes if the body is unable to continue keeping the latent TB in check, it can eventually multiply and turn into a case of active TB.

Active TB is a typical case of TB, primarily found in the lungs but can also be found in other areas of the body.

 Latent TB (LTBI)   Active TB 
Inactive, contained tubercle bacilli in the body Active, multiplying tubercle bacilli in the body
TST or blood test results usually positive TST or blood test results usually positive
Chest x-ray usually normal Chest x-ray usually abnormal
Sputum smears and cultures negative Sputum smears and cultures may be positive
No symptoms Symptoms such as cough, fever, weight loss
Not infectious Often infectious before treatment
Not a case of TB A case of TB

Overall, without treatment, about 5 to 10% of infected persons will progress from latent TB to active TB at some time in their lives. About half of people who develop active TB will do so within the first two years of infection. For persons whose immune systems are weak, especially those with HIV infection, the risk of developing TB disease is considerably higher than for persons with normal immune systems. Of special concern are persons infected by someone with extensively drug-resistant TB (XDR TB) who later develop TB disease; these persons will have XDR TB, not regular TB disease.

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