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CM1.3-5 | CM1.3-5 | Determinants and Natural History of Disease — SDL Guide (Part 3)

Applying the Framework — Case-Based Reasoning

The epidemiological triad, natural history, and levels of prevention are most powerful when used together to reason about a real disease. Consider pulmonary tuberculosis as the synthesis case.

Epidemiological triad analysis:
- Agent: Mycobacterium tuberculosis — a biological agent with high infectivity in sputum-smear-positive cases; airborne transmission.
- Host: susceptibility highest in young children, elderly, and immunocompromised individuals (HIV, malnutrition, diabetes); genetic factors (HLA type) modulate risk; prior BCG vaccination offers partial protection.
- Environment: overcrowding and poor ventilation (physical environment) are the dominant environmental determinants; poverty and migration (socioeconomic environment) drive exposure risk and delay in diagnosis.

Natural history stages:
- Prepathogenesis: most exposed individuals mount an effective immune response and remain latently infected without progressing to active disease.
- Early pathogenesis: tuberculin skin test converts; granuloma forms; the patient is sub-clinical but infectious risk exists under immunosuppression.
- Advanced pathogenesis: cough, haemoptysis, fever, weight loss appear; cavitary lesion forms; sputum smear positive.
- Outcomes: complete cure with DOTS, residual bronchiectasis (disability), or death from respiratory failure or disseminated TB.

Preventive interventions by level:
- Primordial: slum clearance, improved housing ventilation standards, poverty reduction programmes.
- Primary — health promotion: nutritional supplementation, HIV control (reduces TB risk in co-infected individuals).
- Primary — specific protection: BCG vaccination of neonates (protects against severe childhood forms — miliary TB, TB meningitis).
- Secondary — early diagnosis: contact tracing, sputum screening of high-risk groups, CBNAAT testing.
- Secondary — disability limitation: DOTS therapy to complete cure and prevent drug resistance.
- Tertiary — rehabilitation: pulmonary rehabilitation for bronchiectasis; vocational support.

This case-based synthesis demonstrates that no single-level intervention can control TB in India — the programme requires simultaneous action at all levels, coordinated through the National Tuberculosis Elimination Programme (NTEP, formerly RNTCP).