A 28-year-old male construction worker from rural Maharashtra presents to a primary health centre with a 3-week history of productive cough, low-grade fever in the evenings, and night sweats. He shares a single-room dwelling with 6 family members, including his 65-year-old mother who had tuberculosis 2 years ago (treated and cured). Sputum smear microscopy shows acid-fast bacilli. His chest X-ray reveals cavitary lesions in the right upper lobe. Which of the following epidemiological factors is MOST significant in understanding the transmission risk in this household?

Explanation

## Epidemiological Significance of Cavitary TB in Household Transmission **Key Point:** Cavitary pulmonary tuberculosis with positive sputum smear microscopy represents the highest infectious form of TB. The patient is a "case of public health importance" — a source case capable of sustained transmission. ### Why This Patient Is Highly Infectious | Feature | Epidemiological Impact | |---------|------------------------| | Cavitary disease | Bacilli directly expelled into airways; high bacterial load | | Sputum smear positive | Infectious to others; transmits via airborne droplet nuclei | | Productive cough | Active expulsion of infectious particles | | Crowded household (6 members) | Prolonged, close contact amplifies transmission risk | **High-Yield:** A smear-positive cavitary case can infect 10–15 susceptible contacts over a year of untreated disease. Household contacts are at highest risk due to prolonged exposure in confined spaces. ### Household Transmission Risk 1. **Close contacts** (family members sharing a room) have 20–30% lifetime risk of developing TB if exposed to an untreated case. 2. **Crowded living** (6 people in one room) increases the concentration of infectious droplet nuclei and duration of exposure. 3. **Vulnerable household members** include the elderly mother (age 65), children, and any immunocompromised individuals. **Clinical Pearl:** The mother's prior TB does NOT confer lasting immunity. Reinfection is possible, especially with household exposure to a smear-positive case. TB immunity is incomplete and wanes over time. **Mnemonic: CAVITARY TB = Communicable, Amplified, Vulnerable contacts, Infectious, Transmissible, Airborne, Rapid spread, Year-long risk** — this patient is a priority for contact tracing and preventive therapy initiation in household members. ### Public Health Action - Isolate patient from household until 2 weeks of effective therapy. - Initiate contact tracing for all 6 household members. - Tuberculin skin test (TST) or interferon-gamma release assay (IGRA) for contacts. - Isoniazid preventive therapy (IPT) for TST-positive contacts without active disease. [cite:Park 26e Ch 8]