M. tuberculosis is transmitted from human-to-human and spread is mainly airborne. The source of infection is usually a patient with pulmonary or laryngeal TB. During coughing, speaking, or sneezing, the patient produces tiny infectious droplets. These particles, called droplet nuclei, are about 1 to 5 microns in diameter. Depending on the environment, they can remain suspended in the air for several hours.
Transmission may occur when these infectious droplets are inhaled. UV light (sunshine or artificial sources) and ventilation reduce the probability of transmission (Chapter 14).
Other modes of transmission are far less common. Inoculation of cutaneous or mucous membranes rarely occurs, although such cases have been observed in laboratory personnel.
Congenital infection (by transplacental transmission or via aspiration or swallowing of infected amniotic fluid at birth) has been reported, but is very rare. Transmission through breast milk does not occur.
The infectiousness of a patient is associated with the quantity of bacilli contained in their sputum. Patients with smear-positive sputum on microscopy are by far the most infectious. Those with smear-negative/culture-positive results are less infectious, but still contribute to TB transmission due to more frequent delays in diagnosis.
Patients infected with M. tuberculosis, but who have not developed active TB (latent tuberculosis infection), are not infectious. Patients with extrapulmonary TB (EPTB) are only infectious in exceptional circumstances.
Children are generally much less infectious than adults. This may be due to weaker cough mechanics, less sputum production and lower bacillary load.
Not everyone who is exposed to an infectious TB patient becomes infected with M. tuberculosis. The probability that TB will be transmitted depends on several factors:
Infectiousness of the source (the most important factor)
• Bacteriological status: smear-positive patients are the most infectious.
• Virulence of the bacilli: some strains are highly transmissible (and/or more likely to cause active TB).
Environment where the exposure occurred
• Outdoor environments or those with good ventilation and sunlight are less likely to lead to transmission. Small rooms or rooms with no ventilation are conditions most likely to lead to transmission.
• The proximity of the person to the patient is also important (e.g. the risk is higher if the person sleeps next to the patient than if they sleep 20 metres away from the patient).
Duration of exposure
People in close and prolonged contact with TB patients are at highest risk of becoming infected with M. tuberculosis. They may be family members, roommates, friends, co-workers or other people who spend several hours a day with the infectious patient.
The best way to stop transmission is to start effective TB treatment as soon as possible. It is estimated that a person with untreated smear-positive TB transmits the bacillus to 10 to 20 people a year (with variations according to living conditions and environment).