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)
what is miliary TB
haematogenous spread of Mtb causing widespread infection, either during primary active TB or during reactivations
lungs always involved as well as multiple other organs
describe the treatment regimen for TB
- Rifampicin
- Isoniazid
- Pyrazinamide
- Ethambutol
All 4 for 2 mths then 1st 2 for 4 mths
why might compliance to treatment be poor
- length of treatment - 6 mths
- drug side effects, e.g. hepatotoxicity
Need to do DOT/VOT
what is the tuberculin sensitivity test and what does it show
what are the limitations
Tuberculin (mycobacterial protein) is injected intra-dermally. Presence of skin reaction (induration due to hypersensitivity reaction) 48-72hrs later at site indicates previous TB exposure.
limitations:
- false positives (BCG, non-TB)
- false negatives (immunocompromsied)
what is the interferon gamma releasing assay
what are the limitations
P lymphocytes cultured with Mtb Ag… if previous exposure, T lymphocytes produce interferon gamma in response.
No cross-reaction with BCG but cannot distinguish between latent and active TB and similar probs with sensitivity and specificity
