Tuberculosis Flashcards
How long do TB Bacilli take to form a colony on a culture?
How is it spread?
After how many weeks of treatment does sputum infectivity become minimal?
- 2-6 weeks
- Infected droplets
- 2 weeks (Can leave hospital after this time)
Describe the Pathogenesis of infection with Mycobacterium tuberculosis
- Alveolar macrophages phagocytose MTB but are not able to kill them
- The macrophages initiate cell-mediated immunity, which leads to emergence of Activated Macrophages which have enhanced ability to kill MTB. This takes 6 weeks to develop
How does Tuberculosis look histologically?
What cells may be seen?
- Granuloma with central caseation (aka tubercles)
- Lymphocytes
- Epitheloid Histiocytes
- Langhans giant cells
Primary TB occurs on first exposure to MTB.
What usually follows deposition of MTB in the alveoli?
Development of a Sub-Pleural focus of Tubercles called the Primary/ Ghon’s Focus, in any lung zone
Which lymph nodes do bacilli drain to from the Primary/ Gohn’s Focus?
What is the Primary Complex?
Hilar lymph nodes
The Primary Focus + the Hilar Lymph nodes
Most primary TB infections will heal with/ without calcification of the Primary Complex.
How can the bacilli spread before healing occurs?
Enters lymphatic drainage then venous drainage, spreading to other parts of the lung as well as to other organs
With cell-mediated immunity, the infection is contained and the primary complex heals, but some bacilli are still alive in lungs/ other organs.
What is this called?
What 2 tests can be used to characterise this?
Latent Tuberculosis
- QuantiFERON test/ IGRA (Interferon Gamma Release Assay)
- Tuberculin skin test
When does TB reactivation usually occur?
When immune mechanism are impaired/ fail
E.g old age, malnutrition, HIV, Immunosuppression
Describe the basis of the IGRA/ QuantiFERON test
Based on ability of MTB Antigens to stimulated host production of Interferon Gamma
- Patient’s blood lymphocytes are cultured with MTB Antigens
- If exposed to TB before, T lymphocytes produce Interferon Gamma
Why can the IGRA/ QuantiFERON test be used to distinguish Latent TB from previous BCG or Exposure to atypical mycobacteria?
Antigens used in test are not present in;
- Atypical mycobacteria
- Bacilli used in the TB vaccine (BCG)
Describe the Tuberculin skin test
- Tuberculin is injected intra-dermally (a protein derived from mycobacteria)
- Skin reaction 48-72hrs later indicates previous exposure to TB (Type IV Hypersensitivity)
(Immunity and hypersensitivity develop at the same time in a naturally infected person)
What percentage of people with TB have a risk of developing active disease?
10%
5% Primary, 5% Post-primary/ Latent reactivation
Where is Post-Primary Pulmonary TB most often seen?
List 5 conditions/ pathologies that can result from this
In the Upper Lung Zones
(Higher pAO2 may predispose to reactivation)
- Cavity formation
- Haemorrhage
- Spread to rest of lung
- Pleural effusion
- Miliary TB
How can Post-Primary Pulmonary TB lead to Cavity formation?
- Softening and liquefaction of the caseous material
- Discharged into a Bronchus
(Fibrous tissue forms around these lesions but is unable to limit extension)
How can Post-Primary Pulmonary TB lead to Haemorrhage?
- Exension of the caseous process into vessels in cavity walls
- Causes Haemoptysis
