Chronic disease caused by Mycobacterium tuberculosis (generally)

Aetiology

• M. tb is the main causative organism in humans – rod-shaped gram-positive bacillus, acid fast
• M. bovis can jump the species barrier from cattle to humans

Risk factors

• Immigrants
• Recent contacts
• Social deprivation
• Immunosuppression - virulent MTB or atypical infection

Pathophysiology

1. Pathogen transmitted via aerosol route and reaches alveoli
2. Pathogen phagocytosed in alveoli and carried to hilar lymph nodes → immune activation
3. This leads to a granulomatous response in the nodes, with caseous necrosis occurring in the granulomas
   1. Type IV hypersensitivity - granulomas with necrosis
4. The infection is either cleared completely or will become latent (LTBI)
5. In a few cases the infection is overwhelming and spreads throughout the body, setting up many foci of infection (e.g. miliary TB) - extra-pulmonary TB

Clinical features

Pulmonary features

• 90% present with pulmonary features only
• Cough +/- haemoptysis
• Dyspnoea