17 - Tuberculosis Flashcards
What is TB caused by?
Mycobacterium tuberculosis
- Aerobic
- Acid and alcohol fast bacilli
- Slow growing
How can we stain for tuberculosis?
Sputum smear stained with Ziehl-Nielsen method
Takes 2-6 weeks to grow colonies
How does TB transmit from person to person?
- Infected droplets in the air
- Need quite a prolonged exposure
- Patients sputum is infective until 2 weeks of treatment with anti TB chemotherapy
Where is the most common site of pulmonary TB?
Right lung apex as high pO2 in these areas compared to the rest of the lungs
What is the pathogenesis of TB?
- Alveolar macrophages phagocytose MTB but cannot kill them as cell wall lipids of MTB block fusion of phagosome and lysosome
- Macrophages initiate cell mediated immunity so activated macrophages can come and kill MTB, takes about 6 weeks
- Granulomatous reaction from macrophages
What does TB look like microscopically?
- Granuloma with central caseous necrosis (cheese) surround by epitheliod macrophages, langhans giant cells and lymphocytes
What could cause a latent TB infection to reactivate?
- HIV
- Chemotherapy
- Malnutrition
- Old age
How can we test for latent infection?
IGRA (QuantiFERON)
MTB antigens can make the body produce interferon gamma. Lymphocytes from the patient are cultured with MTB antigens and if T lymphocytes have been exposed before they will produce interferon gamma. The MTB antigen is not present in BCG or atypical mycobacteria so can distinguish latent from BCG vaccine
Tuberculin Skin Test:
Protein from MTB injected intradermally. Skin reaction 48-72 hours later indicates previous TB exposure, type IV hypersensitivity reaction to MTB
What is the likelihood of someone being infected with TB actually developing the active disease?
10% lifetime risk
- 5% develop primary TB at initial infection when primary complex does not heal
- 5% develop post primary TB up to 60 years after initial infection
What are some of the changes a patient may have with post primary TB?
- Cavity formation: liquefaction of caseous material. Fibrous tissue usually around periphery of lesions
- Haemorraghe: extension of caseous process into vessels. leads to haemoptysis
- Spread to rest of lung
- Pleural effusion: seeding of TB into pleura or hypersensitivity
- Miliary TB: rupture of caseous pulomnary focus into blood vessel so widespread dissemination through body
What are some sites of extrapulmonary TB?
- Lymph nodes
- Bones
- Joints
- CNS
- GI tract
- Urinary tract
What are some clinical features of pulmonary TB?
- Gradual onset over weeks or months
- Tiredness
- Malaise
- Weight loss
- Fever
- Sweats
- Cough with haemoptysis
- Can be asymptomatic even when CXR abnormal
- Crackles may be present
- Signs of pleural effusion or fibrosis
What does a CXR of TB look like?
- Patchy solid lesions
- Cavity solid lesions
- Streaky fibrosis
- Flecks of calcification
How do we diagnose active TB?
- Need to stain or culture
- Isolate organisma and determine drug susceptibility as well
How do we treat TB?
- Rifampicin (red urine)
- Isoniazid (INAH)
- Pyrazinamide
- Ethambutol
All 4 drugs for 2 months and then just R and INAH for a further 4 months. Give pyridoxine with INAH to stop peripheral nerve damage
What should you do if you diagnose a patient with TB?
- Isolate and notify public health
- PPE
- Contact trace and vaccinate
What are the differences between a latent and active TB infection?
What are some advantages and disadvantages of the tuberculin skin test?
+ Cheap
+ Lab infrastructure not needed
- False positives in BCG
- False negatives in immunocompromised
- Subjective interpretation
What are some of the adverse effects of TB drugs?
What are some extra-pulmonary signs of miliary TB?
- Headaches as meningeal involvement
- Pericardial and pleural effusions
- Retinal involvement
- Ascites
