Tuberculosis and non-tuberculous mycobacterium Flashcards
What is tuberculosis?
A contagious infectious disease caused by the bacterium Mycobacterium tuberculosis (MTB).
What is latent tuberculosis?
When a person is infected with Mycobacterium tuberculosis (MTB), but the bacteria remain in an inactive, dormant state within the body.
How is Mycobacterium tuberculosis transmitted?
It is transmitted by airborne route exclusively, especially to people in close proximity to the affected individual
How does age influence the presentation of tuberculosis?
TB is more severe in younger children.
Infants and young children are more susceptible to developing severe forms of TB, and to have progression of the disease.
Name some key features of the Mycobacterium tuberculosis (4)
- Shape: aerobic, slightly curved non-motile rod
- Grows best in high oxygen tension, but can survive in adverse conditions for a long time in a dormant state
- Very thick waxy outer layer which protects it and gives acid fast property
- Only known host is human
What is the probability of transmission of MTB?
It is the probability of inhaling a viable bacterium, which is proportional to the concentration of the bacterium in air.
What is the probability of MTB transmission in a small room vs a big room?
Small room
* greater concentration
* greater probability of transmission
Big room
* smaller concentration
* smaller probability of transmission
How can MTB be eliminated from the environment?
- MTB can be killed by sunlight or drying
- Ventilation can remove airborne bacteria
What are markers of TB contagiousness?
Cavities on chest X-ray and positive acid-fast smear
What are the two possible results of exposure to MTB?
Either new infection or no infection.
This is determined by innate immunity - mechanism poorly understood
Define primary infection
When exposure results in new infection
How does the initial (innate) immune response respond to TB.
It is largely ineffective
What is our body’s primary defence mechanism for MTB?
Cell-mediated immunity: Special immune cells form a hard shell called granuloma around the MTB to contain the infection.
Following primary infection, cell-mediated immunity (CMI) determines…
Whether the disease develops or the infection becomes dormant.
CMI can be defective in… (4)
- the very young and very old
- patients with HIV infection
- patient with other immuno-compromising conditions
- patients with co-morbidities (diabetes, renal failure)
Development of effective CMI occurs by weeks 4-7 after infection. What happens if CMI is defective?
Primary TB infection is not controlled and symptomatic disease develops within 3-6 months.
What is disseminated TB or TB meningitis?
Severe, life-threatening forms of TB.
Who is most likely to develop more severe forms of TB, including disseminated TB or TB meningitis?
Very young children
If CMI is effective, what happens to TB?
It enters a latent or dormant state, because the TB bacteria are contained within granulomas.
Explain why the latent state of TB is a dynamic state.
The latent state results from homeostasis between MTB constantly trying to “escape” the granulomas and host defences constantly trying to “hold them back”. This is an active immune process.
How long can dormant TB bacteria remain viable?
40 years or more!
What are some clinical test features of latent TB? (4)
- patient is asymptomatic
- interferon gamma release assay is positive
- tuberculin skin test is positive (5mm+)
- chest X-ray is normal
In 5-10% of cases of latent infection, active TB develops at some point. What are some risk factors for this? (6)
- interval since infection
- uncontrolled HIV/AIDS
- age (infants and young children, elderly)
- smoking, alcohol use
- malnourishment (underweight)
- diabetes (renal failure and silicosis)
When is the highest risk for an active infection to develop from latent TB?
Within the first 1-2 years after infection
