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1 > Study Questions
Self-Study Modules on Tuberculosis
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Module
1: Transmission and Pathogenesis
Answers To Study Questions
1.1. In what year was each of the following discoveries
made?
- TB was proven to be contagious 1865
- The bacteria that causes TB was discovered 1882
- The first drug that could kill TB bacteria was discovered 1943
1.2. What organism causes TB? What are two other tuberculous
mycobacteria?
TB is caused by an organism called Mycobacterium
tuberculosis. M. bovis and M. africanum are
two other tuberculous mycobacteria.
1.3. How is TB spread?
TB is spread from person to person through the air. When
a person with infectious TB disease coughs or sneezes, tiny particles
containing M. tuberculosis may be expelled into the air.
These particles, called droplet nuclei, are about 1 to 5 microns
in diameter — less than 1/5000 of an inch. Droplet nuclei can remain
suspended in the air for several hours, depending on the environment.
1.4. When a person inhales air that contains droplets,
where do the droplet nuclei go?
Most of the larger droplets become lodged in the upper
respiratory tract, where infection is unlikely to develop. However,
the droplet nuclei may reach the small air sacs of the lung (the
alveoli), where infection begins.
1.5. After the tubercle bacilli reach the small air sacs
of the lung (the alveoli), where do they go?
At first, the tubercle bacilli multiply in the alveoli
and a small number 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 10 weeks, however, the body's immune system usually
intervenes, halting multiplication and preventing further spread.
1.6. In people with TB infection (but not TB disease),
how does the immune system keep the tubercle bacilli under control?
The immune system produces special immune cells that
surround the tubercle bacilli. The cells form a hard shell that
keeps the bacilli contained and under control.
1.7. How is TB infection detected?
TB infection is detected by the tuberculin skin test.
1.8. What are the major similarities and differences between
TB infection and TB disease? List characteristics of each.
TB infection
- Tubercle bacilli are in the body.
- The tuberculin skin test reaction is positive.
- Usually the chest x-ray is normal.
- Sputum smears (or smears from other specimens) and cultures
are negative.
- People with TB infection (but not TB disease)
- Do not have symptoms (are not sick)
- Are not infectious
- Are not counted as having a case of TB
TB disease
- Tubercle bacilli are in the body.
- The tuberculin skin test reaction is positive.
- Usually the chest x-ray is abnormal (if the disease is in
the lungs).
- Sputum smears (or smears from other specimens) and cultures
are usually positive for M. tuberculosis.
- People with TB disease
- Usually have symptoms (are sick)
- Are often infectious before treatment
- Are counted as having a case of TB
1.9. What happens if the immune system cannot keep the
tubercle bacilli under control and the bacilli begin to multiply
rapidly?
When this happens, TB disease develops. The risk that TB disease
will develop is higher for some people than for others.
1.10. What percentage of people who have TB infection (but
not HIV infection) develop TB disease?
In the United States, about 5% of the people who have recently
been infected with M. tuberculosis will develop TB disease
in the first year or two after infection. Another 5% will develop
disease later in their lives. In other words, about 10% of all
people who have TB infection will develop disease at some point.
The remaining 90% will stay infected, but free of disease, for
the rest of their lives.
1.11. What conditions appear to increase the risk that
TB infection will progress to disease?
- HIV infection
- Injection of illicit drugs
- Recent TB infection (within the past 2 years)
- Chest x-ray findings suggestive of previous TB
- Diabetes mellitus
- Silicosis
- Prolonged therapy with corticosteroids
- Immunosuppressive therapy
- Certain types of cancer (e.g., leukemia, Hodgkin's disease,
or cancer of the head and neck)
- Severe kidney disease
- Certain intestinal conditions
- Low body weight (10% or more below ideal)
1.12. How does being infected with both M. tuberculosis
and HIV affect the risk for TB disease?
Because their immune systems are weakened, 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. Studies suggest that 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.
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 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.
1.13. What site of the body is the most common site for
TB disease? What are some other common sites?
- Lungs are the most common site.
Other common sites:
- Larynx
- Lymph nodes
- Pleura (the membranes surrounding the lungs)
- Brain
- Kidneys
- Bones and joints
1.14. What is the classification system for TB based on?
What is it used for?
The current classification system is based on the pathogenesis
of TB. Many health departments and private health care providers
use this system when describing patients.
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