2.5 Transmission of the tuberculosis bacilli

Patients suffering from microbiologically confirmed pulmonary TB (or in rare instances of Laryngeal TB) constitutes the most important source of infection. This is an airborne infection and occurs most commonly through droplet nuclei that is generated by a patient with untreated pulmonary TB while coughing, sneezing etc. and inhaled via the respiratory route. The chances of getting infected depend upon the duration & frequency of exposure along with the load and virulence of TB bacilli and the immune status of an individual.

During coughing, speaking, or sneezing, the patient, especially if untreated, produces tiny infectious droplets. These particles, called droplet nuclei, are about 1 to 5 microns in diameter—about 1-5/1000 of millimeter. 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.

The infectiousness of a patient is linked to the quantity of bacilli contained in his or her sputum. Patients who are positive on sputum smear 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 (LTBI), cannot transmit TB. 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 four factors:

Infectiousness of the source: This is directly related to the number of tubercle bacilli that a diseased person expels into the air. Smear positive patients have a higher bacillary load and hence are more infectious.
Susceptibility: This refers to the immune status of the exposed individual and determines whether the person will get infected post exposure. For example, PLHIV, Diabetics and malnourished persons are more likely to get infected and progress to active disease due to compromised host immunity.
Exposure related factors: This refers to the proximity, frequency, and duration of exposure. For example, close household contacts of open cases are more likely to get infected.
Environment related factors: These too play an important role and affect transmission. For example, inadequately ventilated settings will lead to increase transmission whereas exposure to sunlight and adequate ventilation will reduce the chances of transmission.

The most effective way to cut the chain of transmission is to promptly diagnose the patient and initiate him or her on an effective anti TB drug regimen. The exact length of time required for a TB patient to become non-infectious after starting TB therapy is not known. However, once an effective TB therapy is started, as long as the patient follows the prescribed treatment regimen, there is considerable evidence showing that the infectiousness can rapidly decline, even after a few days of treatment initiation. 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)