Mycobacterial disease Flashcards
What are the distinguishing features of mycobacteria?
- Unusual waxy cell wall:
•High lipid content
- Slow growing
•Different media requirements
- Intra - cellular pathogens
What are the staining characteristics of mycobacteria?
•Poor take up of standard Gram’s stains
–(Gram positive: Ghost cells)
•Retain certain stains without decolourisation by acid / alcohol:
–“acid fast bacilli” (AFBs).
- Ziehl Neelsen (ZN)
- Phenol auramine.
Which types of mycobacterium cause tuberculosis?
–M. tuberculosis
–M. bovis
What mycobacterium causes leprosy?
M. leprae
How does the transmission of TB occur?
Inhalation of infected respiratory droplets
What are the features of primary TB?
- 1o acquisition / infection & body’s reaction.
- Periphery of lung midzone: most common site
- Inhaled bacilli phagocytosed by macrophages
- Hilar lymph nodes – “Ghon focus”
- Intracellular multiplication
- Dissemination via lymphatic system / bloodstream
What is the body’s response to mycobacterial invasion?
–Granuloma
–Cell - mediated immune response
–Central area of epithelioid cells, giant cells.
–Surrounding lymphocytic cell infiltration.
–Central area caseous necrosis.
–Fibrosis / calcification of lesions
–Bacilli slowly die / may remain viable 20 years
What is the clinical presentation of primary TB?
“Influenza - like” syndrome
What happens after the primary phase of the disease?
Dissemination or latency.
Why might TB reactivate?
–Lowered immunity
–Western countries : over 50 year old, men
–Malnutrition
–Alcoholism
–Debilitating illness
–HIV infection.
–Silicosis, chronic renal failure, gastrectomy..
–Anti TNFα blockade (e.g. infliximab)
What are the features of reactivated TB?
- Coalescing tubercles, central caseous necrosis
- Cavitation
–High organism load - risk of transmission.
- Lung apices: highest oxygen tension.
- Symptomatic:
–Chronic productive cough
•Haemoptysis
–Weight loss, fever, night sweats.
What is extra-pulmonary tuberculosis?
- Disseminated (miliary TB)
- Very young / old; immunocompromised
- 1o disease
- 2o - erosion of necrotic tubercle into blood vessel
- Widespread infection, including meningitis.
What are the common extr-pulmonary sites?
–Pleura
–Lymph nodes
–Kidneys , epididymis
–Bone
–Intestines
–Brain / meninges
–Pericardium
What are the clinical features of TB meningitis?
–Often insidious onset
–Unidentified fever
–Personality change
–Focal neurological deficit
•Basilar inflammation.
–Mild headache / meningism.
–May lack constitutional quartet
•(fever, night sweats, anorexia, weight loss)
What tests are used to perform the diagnosis of TB?
–Index of suspicion
–Clinical
–Radiology: Chest X - ray.
–Histology
–Skin testing
–(Blood test: Interferon- γ release assay: IGRA)
