Risk of Mycobacterium tuberculosis infection and disease among health care workers, Chiang Rai, Thailand

SETTING: A provincial referral hospital in northern Thailand, where a cross-sectional study during 19951996 reported on the occupational risk of Mycobacterium tuberculosis transmission. OBJECTIVE: To describe the effectiveness of prevention strategies for nosocomial tuberculosis (TB). DESIGN: A prospective study among health care workers (HCW) including annual tuberculin skin test (TST) screening and active TB surveillance. Following a comprehensive risk assessment, preventive interventions were implemented targeting HCWs, hospitalised patients, and the hospital environment. RESULTS: The number of pulmonary TB cases diagnosed increased steadily from 102 in 1990 to 356 in 1999. The TST conversion rate was 9.3 (95%CI 3.3-15) per 100 person-years (py) in 1995-1997, but declined steadily to 2.2 (95%CI 0.0-5.1) in 1999. HCWs first screened within 12 months of employment had higher TST conversion rates (adjusted RR = 9.5, 95 %CI 1.8-49.5) compared to those employed for longer than 12 months. The annual rate of active TB per 100000 HCWs was 536 in 1995-1999. CONCLUSION: These HCWs were exposed to active TB patients and were at risk for M. tuberculosis infection, particularly during their first 12 months of employment. Implementation of nosocomial TB control measures in 1996 was followed by declining TST conversion rates, despite increasing exposure to active TB patients.