TB Flashcards
What are the 2 different classifications of mycobacteria?
- Tubercle bacilli: (takes 2-12 weeks to grow)
- M tuberculosis
- M africanum
- M bovis
- M bovis BCG - M. Leprae:
- causative agent of leprosy
- no in vitro growth
What are the main problems with mycobacteria? (staining + treating)
- resistant to decolourisation with acid due to lipid rich cell walls
- grows very slowly
- causes chronic infection
- antibiotic resistance
- cell mediated immune response needed for infection (no antibody effect)
What is the pathophysiology of primary TB?
- mycobacteria inhaled and settle in alveoli, underneath the pleura (where O2 concentration is high)
- initiates inflammatory response (neutrophils) –> phagocytose bacteria and the bacteria is usually killed
- in the immunocompromised the neutrophils are unable to destroy the bacteria (bacteria has a waxy coat and this prevents penetration by toxic compunds)
- the neutrophils release cytokines which recruit macrophages and T-lymphocytes, and the neutrophils die
- when the neutrophils die this results in a caseous necrosis appearance where bacteria are able to survive
- macrophages form a ring around the caseous tissue and phagocytose the bacteria
- some of the bacteria still remains
- macrophages form giant/Langerhan cells
- more cytokines are released to attract lymphocytes
- wall of fibroblasts form
- accumulation of macrophages and other cells, forming a granuloma
- granuloma = ghon focus –> attempt to ‘wall off’ the bacteria
How does TB appear on CXR?
- opacities (bilateral): mainly in upper zone with a patchy nodular appearance
- cavitation (usually in apices)
- calcification (usually surrounding Ghon foci)
- hilar shadowing
- diffuse nodular shadowing in miliary TB
What is post-primary TB?
When a healthy individual becomes immunocompromised and the bacteria escape the Ghon focus can be caused by: - alcohol - diabetes - old age
What is primary progressive TB?
inability to mount a vigorous enough immune response –> bacteria spreads further, granuloma can enlarge
What is open/active TB?
live TB bacilli in sputum
What are the clinical signs of active TB?
Constitutional:
- fever
- malaise
- weight loss
- night sweats
Local:
- cough
- haemoptysis
- chest pain
How is TB diagnosed?
- Clinical signs
- CXR
- 3 resp samples:
- deep cough or induced sputum
- bronchoscopy and lavage - Culture
- Nucleic acid amplification test
What sites, other than the lungs can be affected by TB?
- genitourinary system: urogenital TB
- bones and joints: Potts disease
- pleura: TB pleurisy
- CNS: TB meningitis
- Lymphatic system
What stain is used for mycobacterium? How will normal cells appear compared to the bacterial cells?
Ziehl-Neeson
Normal cells: blue
Bacterial cells: pink ribbons
What is the Mantoux test? What do different results mean?
- Standard dose of tuberculin is injected intradermally
- -> tuberculin = glycerol extract of tubercle bacillus
- skin reaction viewed after 48-72 hours
- measure widest diameter of the induration
Results:
- Small (5-9mm) = high risk (immunocompromised)
- Medium (10-15mm) = medium risk (drug users, prisoners, homeless)
- Large (>15 mm) = sign of active infection in individuals with no known risk of TB
What is the BCG vaccine? How effective is it?
Strain of attenuated live mycobacterium bovis
Can be anywhere from 0-80% effective in preventing TB for a duration of 15 years
How is TB treated?
4-drug regimen:
- Rifampicin
- Isoniazid
- Pyrazinamide
- Ethambutol (or streptomycin)
How does treatment for TB change after 2 months?
- pyrazinamide can be stopped
- isoniazid plus rifampicin are continued daily or intermittent therapy for 4 months
- if pt shows resistance to isoniazid, then stop and continue with rifampicin, pyrazinamide and ethambutol for the entire 6 months
- therapy must be extended if pt has cavitary disease and remains culture positive for 2 months after treatment
