Lecture 17: Tuberculosis Flashcards
What are the risk factors of TB?
- Non-UK born/recent migrants (South asia/sub-saharan africa)
- HIV and other immunocompromised conditions
- homeless/prison (socioeconomic conditions that lead to overcrowding)
- drug users/smoking
- malignancy
- diabetes mellitus
- chronic renal failure
What is TB caused by?
Mycobacterium tuberculosis (MTB)
- aerobic
- acid and alcohol fast bacilli (retain bright red stain from Ziehl-Neelson stain after acid-alcohol mix applied)
- can be demonstrated on smears (sputum smear)
How is TB transmitted?
Infected droplets
When does infectivity of infected sputum become minimal?
After 2 weeks of commencing effective treatment (patient discharged after 2 weeks)
-but treatment must continue for the full duration to eradicate the disease
What is the pathogenesis of TB?
- alveolar macrophages phagocytose MTB deposited in alveoli but are unable to kill them
- these macrophages initiate the cell mediated immunity which lead to the emergence of activated macrophages with enhanced ability to kill MTB (this takes around 6 weeks)
- ingestion of MTB by macrophages causes granulomatous reaction (caseous necrosis surrounded by epitheloid macrophages, langerhans giant cells, lymphocytes)
What is primary infection of TB?
-on first exposure to MTB
-deposition of MTB in alveoli is followed by a sub-pleural focus of tubercles called the primary focus/Gohn’s focus
-MTB drains from primary focus into the hilar lymph nodes
Most primary infections will heal without calcification of the primary complex, but before healing occurs MTB will enter the bloodstream so it spreads to other organs
-cell mediated immunity contains the infection
-primary complex heals but a small number of organisms remain viable
What is the Primary complex?
Primary focus + draining hilar lymph nodes together
What is latent TB?
MTB persists in the host without causing disease
- potential for reactivation is present
- reactivation usually occurs when the patients immune mechanisms wane/fail
How can you test for latent TB?
- positive ‘QuantiFERON’ test
- positive TB skin test
What is primary TB?
When primary complex doesn’t heal and progresses to form active TB- usually self limiting, but can go on to form miliary TB
What is post primary TB?
Reactivation of latent TB
What happens in post primary pulmonary TB?
Most often seen in upper lung zones as these have higher pO2
- cavity formation: softening and liquifaction of the caseous material which is discharged into the bronchus results in cavity formation (fibrous tissue forms around the periphery of such lesions)
- haemorrhage resulting from extension of the caseous process into vessels in cavity walls: causes haemoptysis
- spread to involve the rest of the lungs
- seeding of MTB into pleura or hypersensitivity can result in pleural effusion
What is miliary TB?
Rupture of caseous pulmonary focus into a blood vessel which may result in widespread dissemination of bacilli throughout the body
What is extra-pulmonary TB?
Reactivation of latent TB in sites other than the lungs, resulting in active TB at these sites
e.g. lymphnodes/bones/joints/CNS/GI tract/urinary tract
What are the clinical features of pulmonary TB?
Onset is gradual (weeks/months)
- tiredness
- malaise
- weight loss
- fever
- sweats
- cough (dry/productive)
- haemoptysis
May be no clinical sings on examination even when CXR is abnormal, crackles may be present
CXR: pulmonary shadowing, patchy solid lesions, cavitated solid lesions, streaky fibrosis, flecks of calcification
