Tuberculosis Flashcards
What organisms cause TB?
Mycobacterium tuberculosis
Mycobacterium bovis
Describe Mycobacterium tuberculosis as an organism
Obligate aerobe, acid fast bacillus, non motile, gram +ve, rich glycolipid cell wall, slow growing.
Resistant so can survive in aerosols over long distances
Acid fast so can survive in gastric secretions
Waxy so resistant to antibiotics
How is TB transmitted?
Person to person by infectious droplets. Needs to be a prolonged exposure - at least 8 hours a day for 6 months
Describe some risk factors for catching TB
- Immunosuppression - HIV, diabetes, steroid treatment
- Drug abuse
- Alcoholism
- Preexisting lung disease
- Poverty
- Residents of congregate settings eg prisons, hospitals, homeless shelters
Describe the pathophysiology for when TB enters the lungs…
- Alveolar macrophages phagocytose the TB bacteria but cannot eliminate them
- Cord factor (glycolipid in cell wall) inhibits fusion of phagosome to prevent lysis
- TB remains in the macrophage and divides so as not to provoke humoral response
- Phagocytosed TB can only be destroyed by activated lymphocytes
What would you see on histology if a patient had TB?
- Caseating granulomas
- Polynuclear Langhan’s giant cells, central necrosis, lymphocytes and epithelioid histiocytes
How does active infection of TB occur?
- Occurs when containment by the immune system (T cells/macrophages) is inadequate
- Arises from primary infection or reactivation of latent disease
- 5% of cases progress to active disease from a primary infection
How does latent TB occur?
- Granuloma formation prevents bacteria growth and spread aka the immune system contains it
- Patient is asymptomatic and non infectious. 1/3 of the worlds population have latent TB.
What is post primary TB and why does it occur?
- Reactivation of latent TB (5/10% lifetime risk)
Due to: New infection, HIV, organ transplantation, immunosuppression, silicosis, IVDU, malnutrition, high risk settings, haemodialysis
What are the symptoms of latent TB?
NONE - not even on scans
What are symptoms of active TB?
Constitutional - fever, weight loss, night sweats, fatigue, lymphadenopathy
Pulmonary - dyspnoea, productive cough of over 3 weeks, haemoptysis, pleurisy, pleural effusion
What are some extrapulmonary symptoms of TB?
Tuberculosis lymphadenitis - Painless enlargement of cervical/supraclavicular lymph nodes.
GI TB - Most disease is ileocaecal. Colicky abdominal pain, bowel obstruction due to bowel wall thickening, inflammatory adhesions or strictures.
Spinal TB - Local pain and bony tenderness for weeks/months. Slow progression.
CNS TB - Meningitis, confusion
Genitourinary TB - Dysuria, frequency, loin pain, haematuria, sterile pyuria
Cardiac TB - Pericarditis, pericardial effusion, rare to get myocardial involvement
Skin - Lupus vulgaris, scrofuloderma
What is miliary TB?
Haematogenous dissemination of TB.
Leads to millet appearance of the lung as discrete foci of granulomatous tissue forms.
Needs prompt treatment!!!
What are the diagnostic tests for latent TB?
- Tuberculin skin testing - Mantoux test. Intradermal infection of PPD tuberculin. Size of skin induration is used to determine positivity depending on vaccination history and immune status (>5mm if risk factors, >15mm if no risk factors)
- Interferon gamma release assays - More specific than TST if history of BCG vaccination. (blood test)
- Neither test can diagnose or exclude active disease (false negative 25%)
- If immunosuppressed, sensitivity of testing decreases
What are the diagnostic tests for active TB?
- CXR - Fibronodular/linear opacities in the upper lobe, cavitation, calcification, miliary disease, effusion, lymphadenopathy
- Sputum smear - Can be induced with nebulised saline. Stained f (Ziehl Neelson) or acid fast bacilli.
- Sputum culture - More sensitive, culture takes 1-3 weeks. Can assess drug sensitivity.
- Nucleic acid amplification test - Direct detection of TB in sputum by RNA/DNA amplification. Rapid diagnosis <8 hrs. Can detect pulmonary disease
