9. TB Flashcards
what is the causative organism of TB and how is it identified
- Mycobacterium tuberculosis
- identified via:
1. sputum smear - acid-fast so stains red with Ziehl-Neelson stain
2. culture (Lowenstein-Jensen medium) - gold standard for diagnosis (although slow growing,
up to 6 wks though 1-3 wks with modern automated systems) and required for drug sensitivity testing
how is Mtb transmitted
person to person by infected droplets
what is a primary complex
result of immune response mounted against Mtb - includes:
- GHON’S FOCUS - containment of Mtb with caseating granuloma
- associated lymph node
Ghon focus is macroscopic feature - visible on Xrays
describe the process of primary infection
- Mtb inhaled and deposted in alveoli are phagocytosed by alveolar macrophages…
- Mtb prevents phagolysosome fusion but macrophages still initiate cell-mediated immunity…
- over 6 wks, Th cell response mounted against Mtb… produce IFNy… activates macrophages to become bactericidal and produce TNF… recruit monocytes…
- monocytes differentiate into ‘epithelioid hisitocytes’ - form caseating granulomas, i.e. GHON’S FOCUS
(in maj. of Ps, before healing occurs,
describe the microscopic appearance of a TB granuloma
caseous necrosis core surrounded by epithelioid marcophages, Langerhans giant cells and lymphocytes
what are the possible outcomes of primary complex formation
- progression to primary active disease (5%)
- healing/self-cure
+/- latent infection: before healing occurs in many Ps, some bacilli enter blood and seed other parts of lungs or extra-pulmonary sites via haematogenous spread
what is a Ranke complex
healed Ghon focus that has calcified
name 3 differences between active and latent TB
Active
1- active, multiplying tubercle bacilli in body
2- symptomatic
3- infectious
Latent
1- inactive, contained tubercle bacilli in body
2- no symptoms
3- not infectious
how are active and latent TB differentiated in diagnostic tests
- TST or IFNy tests: usually +ve in both
- sputum smears and cultures: +ve in active, -ve in latent
- CXR: abnormal in active, normal in latent
what are the risk factors for TB reactivation
immunosuppression, eg.
- HIV
- corticosteroids or other immunosuppressants
- diabetes mellitus
- extremes of age
why is post-primary TB often more serious than primary TB
- secondary immune system activation - stronger response means more tissue damage
- bacteria has time to mutate and adapt
what are the symptoms of TB
Pulmonary symptoms:
- cough
- haemoptysis occasionally
- dyspnoea if pleural effusion
General symptoms:
- fever and night sweats
- weight loss and anorexia
- tiredness and malaise
why does post-primary TB more commonly occur in upper lung zones
higher alveolar pO2 relatiev to rest of lung
name 4 negative pulmonary effects of TB infection
- cavity formation: softening and liquefaction of caseous material which is discharged into bronchus… cavity formation… fibrosis
- Haemorrhage from extension of caseous process into vessels of cavity walls… haemoptysis
- Spread to rest of lung: caseous and liquefied material spreads infection through bronchial tree to other lung zones
- Pleural effusion
name common sites of extra-pulmonary TB
- Cervical LNs (scrofula)
- GI or peritoneal (ascitic or adhesive)
- Genito-urinary - slow progression to renal disease and subsequent spreading to lower urinary tract
- Bones and joints, esp. spine (Pott’s disease)
- CNS (tuberculous meningitis)
