Mode of Transmission of Tuberculosis
Tuberculosis is transmitted almost exclusively through the airborne route via infectious droplet nuclei (1-5 µm in diameter) that are expelled when a person with pulmonary TB coughs, talks, sings, sneezes, or even breathes normally. 1, 2
Primary Transmission Mechanism
- Mycobacterium tuberculosis spreads person-to-person through airborne droplet nuclei, not through direct contact, fomites, or other routes 1, 2
- These microscopic particles are small enough (1-5 µm) to remain suspended in air for extended periods and reach the alveoli when inhaled 3
- The infectious particles are generated through respiratory activities including coughing, talking, singing, and normal tidal breathing 3, 2
Key Factors Determining Transmission Risk
Source Case Characteristics (Infectiousness)
- Sputum smear-positive patients pose the highest transmission risk, with 30-40% of close contacts showing evidence of infection 2
- Cavitary pulmonary disease on chest radiograph strongly predicts infectiousness because these patients are more likely to be smear-positive and have frequent cough 1, 2
- Sputum smear-negative patients can still transmit TB but account for only approximately 17% of transmission 2
- Higher concentration of organisms in sputum and frequency/strength of cough increase transmission probability 2
Environmental Factors
- Indoor, poorly ventilated spaces present the highest transmission risk because droplet nuclei remain suspended in finite air volumes 2
- Confined settings with little ventilation (homes, ships, trains, office buildings, healthcare facilities) have demonstrated major transmission risk 2
- Outdoor transmission risk is very limited because tubercle bacilli are rapidly dispersed and rendered nonviable by sunlight 2
- Duration and frequency of exposure directly correlate with infection risk 2
Air Travel Considerations
- Transmission risk exists on long flights (≥8 hours) with highest risk for passengers and crew near the infectious person 2
- However, the overall public health importance of airplane transmission remains negligible 2
Effect of Treatment on Infectiousness
- Patients with drug-susceptible TB rapidly become noninfectious after starting effective multi-drug chemotherapy 2
- This principle allows for home-based treatment after brief hospitalization for therapy initiation 2
Important Clinical Caveats
- While airborne transmission is the primary route, extra-pulmonary TB (including gastrointestinal TB) can result in shedding in feces and urine, with MTBC detected in wastewater at concentrations up to 5.5 × 10⁵ copies/L 4
- Aerosol generation during wastewater treatment could theoretically pose transmission risks to exposed workers, though this remains a secondary concern compared to direct airborne transmission 4
- Drug-resistant TB (MDR-TB, XDR-TB) requires the same infection control precautions as drug-susceptible disease because person-to-person spread has been repeatedly demonstrated 2