What is the contagiousness of tuberculosis (TB)?

Tuberculosis Contagiousness

Tuberculosis is highly contagious through airborne transmission, with infectiousness directly correlating with the number of organisms expelled into the air, particularly in untreated pulmonary or laryngeal TB cases with positive sputum smears and cavitary disease. 1, 2

Factors Determining TB Contagiousness

The infectiousness of TB varies significantly based on several key factors:

Location of Disease

• Pulmonary and laryngeal TB: Most infectious forms 1, 2
• Extrapulmonary TB: Usually not infectious unless there is:
  • Concomitant pulmonary disease
  • Disease located in respiratory tract or oral cavity
  • Open abscess/lesion with high organism concentration and extensive drainage 1, 2

Clinical Factors Increasing Contagiousness

• Untreated disease: Patients not receiving therapy are most infectious 1
• Cough presence: Active, forceful coughing significantly increases transmission risk 1, 2
• Positive AFB sputum smear: Indicates higher bacterial load and greater infectiousness 1
• Uncovered coughing: Failure to cover mouth/nose when coughing 1
• Cavitation on chest radiograph: Associated with higher bacterial burden 1, 2
• Inadequate or short duration of chemotherapy: Improper treatment fails to reduce infectiousness 1
• Cough-inducing procedures: Procedures like sputum induction can aerosolize bacteria 1, 2

Special Patient Populations

Children with TB

• Generally less infectious than adults but can still transmit TB 1, 2
• May be infectious if they:
  • Are not on therapy or just started therapy
  • Are receiving inadequate therapy
  • Have laryngeal or extensive pulmonary involvement
  • Have pronounced cough
  • Have positive sputum AFB smears
  • Have cavitary TB 1, 2

HIV Co-infection

• HIV coinfection does not appear to affect the infectiousness of TB patients 1, 2
• TB patients with AIDS, if smear positive, have similar infectiousness to TB patients without AIDS 1

Effect of Treatment on Contagiousness

• Effective therapy significantly reduces infectiousness by:

  • Reducing coughing
  • Decreasing sputum production
  • Lowering the number of organisms in sputum 1, 2

• Time to non-infectiousness varies between patients 1, 2

• Drug-resistant TB: Patients with unrecognized or inadequately treated drug-resistant TB may remain infectious for weeks or months 1, 2

Practical Approach to Assessing Contagiousness

A patient should be considered infectious if they:

• Have pulmonary or laryngeal TB
• Are coughing or undergoing cough-inducing procedures
• Have positive AFB sputum smears
• Are not on chemotherapy, have just started therapy, or show poor clinical/bacteriologic response 2

A patient can be considered likely non-infectious if they:

• Have drug-susceptible TB
• Are on adequate chemotherapy
• Show significant clinical and bacteriologic improvement 2

Important Caveats

• Unsuspected cases: Infection is most likely to result from exposure to persons with undiagnosed pulmonary TB 1
• Family transmission: When TB is diagnosed in children, the source case is often a family member who should be evaluated 1, 2
• Asymptomatic carriers: People with TB can be asymptomatic but still contagious, requiring a high index of suspicion 3

Proper isolation, ventilation, and respiratory protection measures are essential when managing potentially infectious TB patients, especially in healthcare settings 1, 4.