M. tuberculosis is transmitted from human-to-human and is mainly spread by airborne route. The source of infection is a patient with pulmonary or laryngeal tuberculosis (TB) who expectorates bacilli. During coughing, speaking, or sneezing, the patient produces tiny infectious droplets. These particles, called droplet nuclei, are about 1 to 5 microns in diameter—about 1-5/1000 of millimetre. Droplet nuclei can remain suspended in the air for several hours, depending on the environment.
Transmission may occur when these infectious droplets are inhaled. Sunlight, UV light and ventilation are effective in decreasing the ability of the droplets reaching the lung (Chapter 14).
The other modes of transmission are far less common. Cutaneous or mucous inoculation rarely occurs, although such cases have been observed in laboratory personnel. A rare cause of digestive transmission of TB can occur with M. bovis, most commonly through cow’s milk.
The infectiousness of a patient is linked to the quantity of bacilli contained in his sputa. Patients with sputum smear-positive microscopy are by far the most contagious. Those with smear-negative/culture-positive results are less contagious. Patients whose sputum smear microscopy and culture are both negative are usually not contagious.
Patients who are infected with M. tuberculosis, but do not have active disease, cannot transmit TB. Extrapulmonary (EP) forms of TB are only contagious in exceptional circumstances. Children are generally much less contagious 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 three factors:
Contagiousness of the source (the greatest factor):
– Bacteriological status: smear-positive being the most infectious;
– Virulence of the tubercle bacilli: certain strains are very transmissible (and/or more likely to cause active disease).
Environment where the exposure occurred:
– Open air and sunlight are conditions less likely to lead to transmission, whereas small rooms/settings with no ventilation are the conditions most likely to lead to transmission.
– The proximity of the person to the patient is also important (i.e. sleeping next to the patient in the ward versus sleeping 20 meters away).
Duration of exposure:
Close contacts of TB patients are at highest risk of becoming infected with M. tuberculosis. They may be family members, roommates, friends, co-workers or others who spend multiple hours per day with the TB patient while the person is infectious.
The best way to stop transmission is to start giving patients effective TB treatment as soon as possible. The length of time required for a TB patient to become non-infectious after starting TB therapy is not exactly known. However, once an effective TB therapy is started, as long as the patient follows the prescribed treatment regimen, there is considerable evidence showing the infectiousness can rapidly decline, even after a few days1,2,3.
It is estimated that a person with smear-positive TB, undiagnosed and untreated, transmits the bacillus to 10 to 20 people per year (this varies according to lifestyle and environment).