DUHS CME - Ojha Institute of Chest Diseases





Transmission & Pathogenesis



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TB spread from person to person through the air.


Spread (Source & Mode)

TB is spread from person to person through the air.

When a person with infectious TB disease.
or Sings,

Tiny particles containing M. tuberculosis may be expelled into the air.
These particles, called droplet nuclei, are about 1 to 5 microns in diameter.
Droplet nuclei can remain suspended in the air for several hours, depending on the environment; another person inhales air that contains these droplet nuclei.

Transmission is the spread of an organism such as M. tuberculosis from one person to another.

Not everyone who is an infectious TB patient becomes infected with M. tuberculosis.

The probability that TB will be transmitted depends on four factors:

  • How infectious or contagious is the TB patient?
  • In what kind of environment did the exposure occur?
  • How long did the exposure last?
  • How virulent (strong) are the tubercle bacilli?

Close contacts of TB patients are at highest risk of becoming infected

Close contacts of TB patients are at highest risk of becoming infected with M. tuberculosis.

Close contacts are more likely to become infected with M. tuberculosis than contacts that spent less time with a person while the person was infectious.

Close contacts (persons with prolonged, frequent, or intense contact) are at highest risk of becoming infected (22 percent infection rate). They may be

  • Family members
  • Roommates
  • Friends
  • Coworkers
  • Others at risk include:
    • Residents and employees of high-risk congregate settings
    • Health care workers who serve high-risk clients
    • Medically underserved, low-income populations
    • Children exposed to adults in high-risk categories
    • Drug abusers.

The best way to stop transmission is to start giving them the standard TB treatment as soon as possible. The length of time required for a TB patient to become noninfectious after starting TB therapy varies. However, once the standard TB therapy is started, and as long as the patient follows the prescribed treatment regimen, the infectiousness of the TB patient can rapidly decline.

Drug-resistant TB can be transmitted in the same way as drug-susceptible TB. However, drug-resistant TB is more difficult to treat because it can survive in a patient’s body even after treatment with the first-line drugs is started. Also, because it takes longer to diagnose drug-resistant TB, these patients may be infectious for a longer period of time. This may result in more people being infected.

Drug-resistant TB can be caused in two different ways: primary and secondary (acquired).

Primary resistance is caused by person-to-person transmission of drug-resistant organisms.

Secondary resistance  develops during TB treatment, either because the patient was not treated with the appropriate treatment regimen or because the patient did not follow the treatment regimen as prescribed.

In other words, if patients do not take all of their pills, or if they do not take their pills as often as prescribed, they could develop secondary drug-resistant TB. Patients with drug-resistant TB should be closely monitored to see if they are responding to treatment, they should remain in isolation until it is shown that they are no longer infectious.


Potential Transmitters of M. tuberculosis


  • TB patients with cough
  • Persons with sputum positive for acid-fast bacilli
  • Persons not on chemotherapy
  • Persons just started on chemotherapy
  • Persons with a poor response to chemotherapy


Two Stages

The process of catching tuberculosis involves two stages:

first, a person has to become infected.

second, the infection has to progress to disease.

To become infected, a person has to come in close contact with another person having active tuberculosis (not latent TB infection). In other words, the person has to breathe the same air in which the person with active disease (persons with TB germs present in the sputum) coughs or sneezes.

The process of infection progresses to disease in about ten percent of those infected, and it can happen any time during the remainder of their lives. Although the chance of progression to disease diminishes with the passage of time, tuberculosis can develop more easily if the immune system weakens, as drugs.

In people with both HIV and TB infection, as many as eight percent can happens with malnutrition, AIDS, diabetes, cancer, or treatment with immunosuppressant develop TB each year.




Infection begins when droplet nuclei reach the alveoli.

  • When a person inhales air that contains water droplets containing M. tuberculosis, most of the larger droplets become lodged in the upper respiratory tract (the nose and throat), where infection is unlikely to develop.
  • However, smaller droplet nuclei may reach the small air sacs of the lung (the alveoli), where infection may begin.
  • In the alveoli, some of the tubercle bacilli are killed, but a few multiply in the alveoli, enter the bloodstream, and spread throughout the body.
  • Bacilli may reach any part of the body, including areas where TB disease is more likely to develop. These areas include the upper portions of the lungs, as well as the kidneys, the brain, and bone.
  • Within 2 to 8 weeks, however, the body’s immune system usually intervenes, halting multiplication and preventing further spread.
  • The immune system is the system of cells and tissues in the body that protect the body from foreign substances.
  • At this point, the person has latent TB infection (LTBI).
  • When the immune system is weakened, the body may not be able to control the multiplication and spread of tubercle bacilli.
  • For this reason, people who are infected with both M. tuberculosis and HIV are much more likely to develop TB disease than people who are infected only with M. tuberculosis.
  • The risk of developing TB disease is 7% to 10% each year for people who are infected with both M. tuberculosis and HIV.
  • Whereas it is 10% over a lifetime for people infected only with M. tuberculosis.
  • For people with LTBI and diabetes, the risk is 3 times as high, or about 30% over a lifetime.
  • In an HIV-infected person, TB disease can develop in either of two ways.
    • First, a person with LTBI can become infected with HIV and then develop TB disease as the immune system is weakened.
    • Second, a person who has HIV infection can become infected with M. tuberculosis and then rapidly develop TB disease.



Latent TB Infection (LTBI)

Latent TB infection (LTBI) means that tubercle bacilli are in the body, but the body's immune system is keeping the bacilli under control and inactive. The immune system does this by producing special immune cells that surround the tubercle bacilli. The cells form a shell that acts as a fence and keeps the bacilli contained and inactive.

LTBI is detected by the Mantoux tuberculin skin test (TST) or an interferon-gamma release assay (IGRA) such as the QuantiFERON®-TB Gold test (QFT-G). Most people with LTBI have a positive TST or QFT-G result. Module 3, Targeted Testing and the Diagnosis of Latent Tuberculosis Infection and Tuberculosis Disease, discusses the TST and the QFT-G in more detail.

People who have LTBI but not TB disease are NOT infectious — in other words, they cannot spread the infection to other people. These people usually have a normal chest x-ray. It is important to remember that LTBI is not considered a case of TB. Major similarities and differences between LTBI and TB disease are shown in the Table below.

LTBI vs. TB Disease

Latent TB Infection (LTBI)
TB Disease (in the lungs)
Inactive tubercle bacilli in the body Active tubercle bacilli in the body
Tuberculin skin test or QuantiFERON®-TB Gold test results usually positive Tuberculin skin test or QuantiFERON®-TB Gold 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



Study Questions

Q-1     Write the difference between pulmonary, extra-pulmonary & milliary TB?

Q-2     What is drug-resistant TB, primary resistance & secondary resistance?

Q-3     How TB spread from person to person?

Q-4     What are the factors on which TB transmission depends?

Q-5     Who are the potential transmitters of TB?

Q-6     Which is the best way to stop TB transmission?

Q-7     What are the droplet nuclei, what happen when they are inhaled?

Q-8     Why HIV patients are more likely to develop TB?

Q-9     What is latent TB disease?

Q-10  Write the difference b/w latent TB infection & TB disease?



After working through learner will be able to understand

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