Tuberculosis Flashcards
Which organism causes TB?
Myobacterium Tuberculosis
What are the characteristics of the organism that causes TB
Characteristics of Myobacterium Tuberculosis:
- Obligate aerobe
- Acid and alcohol fast bacilli (staining)
- Non- motile rods
- Long chain fatty acid and glycolipids in cell wall gives structural rigidity
- Slow growing
What are some of the risk factors for getting TB?
- Non-UK born / recent migrant from South Asia or Sub-Saharan Africa
- HIV
- Immunocompromised
- Homeless
- Drug user
- Prisoner
- Young adults
- In close contact with someone who has TB
How is TB transmitted?
Spread by infected droplets
Droplet nuclei are small and suspended in air and can reach the lower airways
Contagious but need prolonged exposure of at least 8 hours/day for up to 6 months
Explain the pathogenesis of TB
- Inhaled aerosols of TB
- Engulfed by alveolar macrophages but unable to phagocytose
- Infected macrophages get into lymph nodes
- Initates develipment of cell mediated immunity
- Either progresses to active primary disease
- Or get a latent infection which will either self cure (95%) or reactivate (post primary TB)
How long does it take for activated macrophages that can kill MTB to develop?
6 weeks
What are some of the symptoms of TB?
- Cough
- Fever
- Weight loss and anorexia
- Night sweats
- Haemoptysis
- Breathlessness if pleural effusion
What is a Ghon focus/ complex?
Focus seen on CXR in someone with primary TB
Harm to lung is minimal, patient may be asymptomatic/ minimally symptomatic
What is post primary TB?
- Reactivation or exogenous re-infection
- Occurs >5 years after primary infection
- 5-10% lifetime risk of development
What is the primary complex in primary TB infection?
Ghon’s focus + the draining lymph node (hilar node)
What are some of the risk factors for reactivation of TB?
- HIV
- Substance abuse
- Prolonged therpay with steroids
- TNF-alpha antagonists
- Organ transplant
- Haematological malignancy
- Severe Kidney Disease
- Diabetes Mellitus
- Silicosis
- Low Body Weight
What is extra-pulmonary TB? In which patients is this seen?
When TB spreads to other places than the lungs:
- Larynx
- Lymph Nodes
- Pleura
- Brain
- Kidneys
- Bones and Joints
Seen more often in: HIV infected/ immunocompromised people and children
What is Miliary TB?
When TB is carried to all parts of the body through the blood stream
RARE
Seen in very young children or very immunosuppressed patients
What do you see in histology of someone infected with TB?
Caseating Granulomta of Lagnhan giant cells
What signs would you find on examination of someone with TB?
- Often no chest signs despite CXR abnormality
- May have crackles in affected area
- Signs of cavitation and fibrosis in extensive disease
How do you investigate pulmonary TB?
- Chest X Ray
- Sputum samples: 3 early morning samples of 5ml minimum volume
- Induce sputum if not producing any, may need alveolar lavage
- Bronchoscopy in patients with dry cough
What are classic findings on pulmonary TB chest X ray?
- Apex of lung often involved
- Ill defined patchy consolidation
- Cavitation
What is the gold standard for TB diagnostics?
TB Culture although this can take a long time!
What test would indicate that TB is infectious?
Smear postive case
Seeing TB bacilli under microscope in Zeihl-Neilson staining
What is the fastest way to identify primary TB?
Nucleic Acid Amplification Test (PCR)
- Gives rapid diagnosis of smear positive cases
- Shows drug resistant mutations
Explain the tuberculin skin test
- Tuberculin injected intradermally and read 48-72 hours later
- If antigen is present it will produce a hypersensitivity reaction
What are the advantages/ disadvantages of the tuberculin skin test?
Advantages:
- cheap
- no lab evidence required
Disadvantages:
- False positives
- False negatives if immunocompromised
- Subjective interpretation
Which blood test can you use to check for latent TB that specifically looks for the antigen to mycobacterium TB ?
Inferon Gamma Assays
Explain how the interferon gamma assay works
- T cell based assay
- Antigen presenting cells present antigens to primed T cells in vitro
- Blood from patient cultured with the antigens to mycobacterium tuberculosis. If previous exposure to TB, T Lymphocytes produce interferon gamma response
