Mode of Transmission for Tuberculosis
The answer is A. Airedroplet (airborne transmission via droplet nuclei). Tuberculosis is transmitted almost exclusively through the airborne route when infectious droplet nuclei are expelled into the air by coughing, talking, singing, or sneezing—not through direct contact with contaminated surfaces. 1
Primary Transmission Mechanism
M. tuberculosis is nearly always transmitted through an airborne route, with infecting organisms carried in droplet nuclei (small particles 1-5 micrometers in diameter) that remain suspended in air and can be inhaled deep into the lungs. 1 This is fundamentally different from surface contact transmission.
Why Airborne, Not Surface Contact:
Droplet nuclei remain airborne for extended periods in enclosed spaces with poor ventilation, allowing transmission even without direct contact with the source patient. 1
Direct contact with infected surfaces is NOT a recognized mode of TB transmission in clinical practice. 1 The CDC explicitly states that "the normal mode of transmission is person to person by the airborne route." 1
When TB bacilli are expelled outdoors, they are rapidly dispersed and rendered nonviable by sunlight, making outdoor transmission very limited. 1
Key Transmission Factors in Travel Settings
For your business traveler scenario, several factors would have increased transmission risk:
Environmental Characteristics:
- Confined spaces with poor ventilation (hotels, conference rooms, aircraft) pose major transmission risk. 1
- Indoor environments allow droplet nuclei to remain suspended in finite air volumes unless effective ventilation exists. 1
- Aircraft transmission has been documented on flights >8 hours, with highest risk for those seated within 2 rows of an infectious passenger. 1
Source Patient Characteristics:
- Sputum smear-positive patients (visible acid-fast bacilli on microscopy) have the highest transmission potential. 1
- Cavitary pulmonary disease on chest radiograph increases infectiousness due to higher organism burden and more frequent cough. 1
- Even smear-negative patients account for approximately 17% of TB transmission, though with lower potential than smear-positive cases. 1
Common Pitfall to Avoid
Do not confuse TB with infections that spread via fomites (contaminated surfaces). While historical debates in the early 1900s considered dust-borne transmission from dried sputum, modern evidence firmly establishes that airborne droplet nuclei—not surface contact—are responsible for TB transmission in clinical practice. 2, 3, 4 The only exception is the extremely rare scenario of direct inoculation into wounds (relevant only for healthcare workers during procedures), which is not applicable to typical travel-related acquisition. 5