Annual risk of tuberculosis infection in a rural population of South India and its relationship with prevalence of smear positive pulmonary tuberculosis.

ABSTRACT

Background: We conducted a tuberculin survey to estimate the annual risk of tuberculous infection (ARTI) among children in a sub-division of rural Bangalore district. A TB disease survey was conducted in the same area around the same time and has already been published. DOTS strategy is being implemented in the study area since 2002. Methods: The tuberculin survey was conducted during 2010-2011 among 3838, 5-9-year-old children attending 147 schools selected by simple random sampling. Children were tuberculin tested with 2TU PPD RT23 with Tween 80 and maximum diameter of induration was measured between 48-96 hours. ARTI was computed from prevalence of infection estimated by mirror-image technique. Prevalence of smear positive pulmonary TB estimated during the disease prevalence survey in 2008-10 was used to find out its relationship with ARTI. Results: Using the observed mode of tuberculin reaction sizes at 19 mm, among surveyed children, prevalence of infection was estimated at 7.3% (CI 6.5-8.1); ARTI was computed at 1.05%. Considering the mean age of children, estimated ARTI most closely approximated to the year 2008. Every one per cent ARTI was found to correspond to a prevalence of 103 sputum smear positive patients of PTB, which was similar to the ratio of 106 found in the same study area during 1960s. Conclusion: There has been no change in the relationship between ARTI and prevalence of smear positive pulmonary TB from the pre-DOTS era and thus in the number of children infected by each adult point prevalent case of smear positive pulmonary TB each year suggesting the need for early case detection and treatment.