Tuberculosis Flashcards
1
Q
Describe the microbiology of mycobacterium tuberculosis
A
- Obligate aerobe - needs oxygen to survive
- Long-chain fatty acids, complex waves and glycolipids in cell wall
- Contribute to structural rigidity - resists weak disinfections
- Prevents gram stain from working
- Acid fast - able to hold onto acid
- Relatively slow-growing compared to other bacteria
- Generation time (duplication time) 15-20 hours
- Takes long time to culture - 3-6 weeks
2
Q
How is mycobacterium tuberculosis transmitted
A
- Transmitted through respiratory droplets - coughing, sneezing
- Contagious but not easy to acquire infection - need long exposure
- At least 8 hours/day up to 6 months
- Infection commonly acquired in households, schools, prisons
- At least 8 hours/day up to 6 months
3
Q
Describe the pathogenesis of TB to the latent phase
A
- Inhaled respiratory droplets phagocytosed by alveolar macrophages
- Macrophages activated by T helper cells from the cell-mediated response
- Mycobacterium tuberculosis releases factors which enable it to survive the macrophages and duplicate within it
- Developed primary tuberculosis - asymptomatic
- After 3 weeks, cell mediated response forms granuloma around infected macrophages to prevent spread
- Infected cells that are surrounded die off, forming caseous necrosis
- Known as Ghon focus
- Infected macrophages can travel to lymph nodes and form caseous necrosis there as well
- Ghon focus - lymph node involvement = Ghon complex - mainly in lower lobes
- Tissue surrounded within granulomas undergo fibrosis and calcification, which can be seen on chest x-rays
- 5% of primary complexes progress to primary active disease, whilst the rest remain in the latent infection phase
- Latent infections remain dormant and contained - do not multiply
4
Q
Describe how the latent phase can be reactivated
A
- If the immune system becomes compromised, the primary complex can be reactivated and spread to other lobes
- Known as post primary tuberculosis
- Commonly spreads towards apex as oxygenation greatest in these areas - myobacterium tuberculosis is an obligate aerobe
- Memory T cells release cytokines to control outbreak, forming more areas of caseous necrosis
- Necrosis forms cavities, which can allow bacteria to spread to other lobes or through the vascular system to other organs (Miliary TB)
5
Q
State potential pathology that can occur from TB in areas other than the lung
A
Spreading of bacteria vascularlly can cause sterile pyuria (WBC in urine), meningitis, Pott disease (lumbar spine), Addison’s disease, hepatitis, lymph node infection
6
Q
Define primary infection, primary TB, post primary TB, extra pulmonary TB and Miliary TB
A
- Primary infection - infection due to primary complex (Ghon complex)
- Primary TB - when the primary complex does not heal and progresses to cause active TB
- Post primary TB - reactivation of latent TB
- Extra pulmonary TB - reactivation of latent TB at sites other than the lungs
- Miliary TB - vascular spread of Mycobacterium tuberculosis
7
Q
Explain the difference between latent TB and active TB
A
- Latent TB is asymptomatic
- Latent TB is inactive, active TB is multiplying
- Latent TB shows normal CXR
- Latent TB has negative sputum smears and cultures
- Latent TB not infectious
- Both show positive on TST and IFN gamma tests
8
Q
List the risk factors for reactivation of TB
A
- Immunocompromised patients
- Infection with HIV
- Prolonged therapy with corticosteroids or other immunosuppressive therapy
- Substance abuse
- TNF-a antagonists
- Organ transplant
- Severe kidney disease
- Diabetes mellitus
9
Q
List the risk factors for TB in UK
A
- Non-UK born
- Common in south Asian and sub-saharan Africa
- HIV and other immunocompromised states
- Homeless
- Drug users, prison
- Close contacts
10
Q
List the symptoms of TB
A
- Fever - produces inflammatory cells
- Night sweats
- Weight loss, anorexia
- Tiredness and malaise
- Cough
- Haemoptysis
- Breathlessness if pleural effusion - if infection reaches pleural membrane
11
Q
Describe the radiographic changes seen on TB
A
- Granulomas with caseous necrosis
- Granuloma is a collection of epithelioid histiocytes (macrophages), lymphocytes and giant cells
- Langhans giant cells present (horseshoe appearance where nuclei are lined along the peripheries
- Apex of the lung often has patchy consolidation
- Cavitation usually develops within consolidation
- Healing results in fibrosis
12
Q
State the tests used to diagnose TB
A
- Tuberculin test / Mantoux test
- Interferon gamma release assays (IGRA)
- Sputum testing
- Bronchoalveolar lavage
13
Q
Describe the tuberculin test and its effectiveness
A
- Tuberculin is injected subdermal
- If patient previously exposed to TB, then immune reaction occurs causing large area of redness
- Occurs 2-3 days layer
- Doesn’t differentiate between active and latent disease
- False positive - BCG, non TB
- False negatives - immunocompromised
14
Q
Describe the Interferon gamma release assays (IGRA) test and its effectiveness
A
- Detection of antigen specific IFN-gamma production
- More specific than other tests - will not give false positive from previous BCG vaccines
- Cannot different between active and latent TB
15
Q
Explain how sputum testing is done for TB
A
- 3 early morning samples - when lying down, secretions congregate in sputum
- Ziehl–Neelsen staining where TB bacteria are red
