Etiology and Risk Factors of Tuberculosis
Tuberculosis is caused by Mycobacterium tuberculosis transmitted person-to-person via airborne droplet nuclei, with HIV infection representing the single most powerful risk factor for progression from latent infection to active disease.
Etiology
TB is caused by inhalation of airborne droplet nuclei containing M. tuberculosis expelled from persons with infectious pulmonary TB 1, 2. The highest transmission risk occurs from:
- Sputum smear-positive patients (especially those with cavitary disease on chest radiograph)
- Patients with frequent cough
- Approximately 17% of transmission occurs even from smear-negative patients 1
Transmission is amplified by:
- Confined spaces with poor ventilation (homes, hospitals, correctional facilities, homeless shelters) 3, 1
- Prolonged exposure duration and frequency 1
- High density of infectious droplet nuclei in ambient air 1
- Outdoor transmission is negligible due to rapid dispersal and UV inactivation by sunlight 1
Risk Factors for Infection
Epidemiologic Risk Groups (Recent Infection)
Highest risk populations for recent M. tuberculosis infection include 3:
- Close contacts of infectious pulmonary TB cases
- Recent immigrants from high TB-burden countries (rates approach country of origin for first several years) 3
- Homeless persons
- Injection drug users
- Persons residing/working in congregate settings (hospitals, shelters, prisons, nursing homes, AIDS facilities) 3
- Healthcare workers, particularly those traveling from low- to high-incidence regions 4
Age-specific vulnerabilities:
Risk Factors for Progression to Active Disease
Quantified Risk by Factor (Cases per 1,000 Person-Years)
The most critical clinical risk factors with quantified progression rates are 3:
Highest Risk:
- HIV infection: 35-162 cases/1,000 person-years (the single most powerful risk factor) 3
- HIV + injection drug use: 76 cases/1,000 person-years 3
- Silicosis: 68 cases/1,000 person-years (30-fold increased risk) 3
High Risk:
- Recent TB infection (<1 year): 12.9 cases/1,000 person-years 3
- Injection drug use (HIV-negative): 10 cases/1,000 person-years 3
- Radiographic fibrotic lesions (prior untreated TB): 2.0-13.6 cases/1,000 person-years 3
Moderate Risk:
- Underweight >15% below standard: 2.6 cases/1,000 person-years (2-3 fold increased risk) 3
- Chronic renal failure/hemodialysis: 10-25 times general population risk 3
- Diabetes mellitus: 2-4 fold increased risk (higher with insulin-dependent or poorly controlled diabetes) 3
Additional Clinical Risk Factors
Immunosuppression:
- Corticosteroids ≥15 mg/day prednisone equivalent for 2-4 weeks represents the threshold for TB reactivation risk 3
- TNF-blocking agents 2
- Other immunosuppressive medications 3
Medical Conditions:
- Malignancies (lung cancer, lymphoma, leukemia, head/neck carcinoma) 3
- Gastrectomy with malabsorption 3
- Jejunoileal bypass 3
- Solid organ transplantation (renal, cardiac) 3
Behavioral/Social Factors:
- Smoking: 4.43-fold increased risk 5
- Alcohol use (difficult to isolate as independent factor due to confounders) 3
- Low household income 5
- Illiteracy (2.15-fold increased risk) 5
- Overcrowding/inadequate ventilation (houses with ≤1 window: 1.81-fold increased risk) 5
Environmental Risk Factors
- Indoor air pollution and second-hand smoke exposure 4
- Inadequate ventilation in living/working spaces 5
- Climate change and air pollution (emerging factors) 6
Protective Factors
- BCG vaccination reduces risk by two-thirds (AOR = 0.34) 5
- Being overweight >5% above standard weight (0.7 cases/1,000 person-years) 3
Critical Clinical Pitfalls
Do not overlook:
- TB screening in patients starting immunosuppressive therapy, particularly TNF-blockers or corticosteroids ≥15 mg/day 3, 2
- Recent immigrants remain at elevated risk for several years after arrival, not just immediately 3
- Smear-negative TB patients still transmit disease—maintain infection control precautions 1
- Healthcare workers require ongoing surveillance, not just baseline screening 4
- Children <5 years require aggressive evaluation due to disseminated disease risk 3