Infection 7: Tb Flashcards
Non-mycobacterium Tb
These are other types that don’t affect human being, but only if they are immunocomprimised like e.g in AIDS.
- M. kansasi
- M. avium, occurring in unpasteurized cow’s milk
- M. ulcerans
- M. fortuitum
Infection acquired by inhalation (M.kansasii and M.avium infection of lungs), ingestion (disseminated M.avium disease via bowel), or direct contact (M.marinum)
List the clinically relevant mycobacterium and the relevant diseases they cause
M. tuberculosis= Tuberculosis
M. bovis= Bovine tuberculosis
(M. bovis BCG - attenuated vaccination strain)
M. leprae= Leprosy
M. ulcerans= Buruli ulcer
List the features of Mycobacterium tuberculosis [4]
- Bacterium
- Hydrophobic
- High lipid content of cell wall
- Gram stain difficult - Slow-growing (generation time ~22h)
- Special microscopy stains
Ziehl–Neelsen
Auramine fluorescence
“Acid-Fast Bacilli” (AFBs)
What are the common presenting features of Tb?
Cough (>3wks) Weight loss Fatigue Fever Night sweats Hemoptysis (1/3rd) Difficulty breathing
How is Tb diagnosed?
- Culture (lengthy)
- Smear positive - Acid fast (Ziehl-Neelsen) staining
- Molecular methods (PCR)
- Sputum
How is Tb transmitted?
Droplet [aerosol transmission] nuclei containing 1-3 bacilli
Reach alveolar space
5-200 bacilli required to establish infection (could be as low a 1 or 2 bacilli)
Describe the process of granuloma formation
Form of chronic inflammation characterised by focal accumulation of activated macrophages
What do granulomas do?
Granuloma forms – protecting bacteria and host
Latent infection – low number of bacteria, no clinical disease
Granuloma breaks down – bacteria escape and replicate
Active TB disease
What is a granuloma?
Aggregates of activated macrophages that assume an epithelioid appearance.
Describe the events leading to granuloma formation
- Offending Ag is captured by a macrophage, processed, and prepared for presentation. Macrophage secretes IL-1.
- Naive CD4 T cell recognizes peptide from the offending substance (Class II MHC)
- CD4 T cells differentiate into Th1 via IL-12. The Th1 cell differentiates and secretes IFN-gamma (increases macrophage strength), IL-2 (causes T-cell proliferation), and TNF-alpha (increase blood flow to area and promotes attachment of passing mononuclear cells.
Discuss sequelae (consequence) of granulomas.
- Fibrosis= activated macrophages produce fibronectin, and both alveolar macrophage growth factor (stimulates collagen) AND platelet derived growth factor stimulate fibrosis.
- Necrosis (caseous)= usually at center of granuloma
- Resolution
What are the sites of Tb disease
Brain Larynx Lymph node Pleura Bone Kidney Lung Spine (Pott's disease)
What are the typical features seen on CXR in Tb?
Infiltrates (collections of fluid and cells in lung tissue)
Cavities (hollow spaces within lung)
Previous Ghon complex (calcified) and
left hilar lymphadenopathy with lung infiltration
What are the typical features seen on Lung CT in Tb?
‘tree-in-bud’ pattern
Bronchial wall thickening and decreased volume in the right lobe
How may GI tuberculosis spread?
Primary infection due to unpasteurized milk
Secondary infection:
- 1° complex elsewhere with reinfection
- ingestion of expectorated, infected sputum - contiguous spread from organs
