© 2008 American Thoracic Society
Decline in Prevalence of Latent Tuberculosis Infection: Is the Waning of Tuberculin Reaction a Factor?To the Editor:We read with great interest the two articles by Dr. Bennett and colleagues and Dr. Khan and colleagues on the prevalence of latent tuberculosis infection (LTBI) in the United States (1, 2). Through slightly different approaches, both articles reported very substantial improvement in LTBI prevalence, from 14.4% in 1971–1972 down to about 5% in 1999–2000, in parallel with the rapidly falling incidence of active TB (1, 2). Infection by the tubercle bacillus is often chronic and may last for decades. In the absence of large-scale screening and treatment of LTBI, the reported rate of decline might have been in excess of what would be expected by the replacement of heavily infected older birth cohorts with newer ones. Indeed, the LTBI prevalence of the 25–44 age group in 1971–1972 was 9.26%, substantially higher than 6.21% in the 55–64 age group and 6.47% in the 64–74 age group in the corresponding birth cohort in 1999–2000 (2). From the 1999–2000 survey, it was estimated that only 25.5% of persons with LTBI had been diagnosed, and only 13.2% had been prescribed treatment (1). Substantial annual risk of infection must still have remained, taking into account the ongoing number of incident active TB cases (1, 2). The LTBI prevalence among immigrants was substantially higher than the local-born (1, 2). Thus, immigration would only have worsened the situation.
Diminishing tuberculin reaction with age could have accounted for part of the observed decline in LTBI prevalence in the United States, but its effect might not be too prominent among the large segment of relatively healthy population under 65 (3). Tuberculin skin test (TST) readings are well known to decline with time after bacille Calmette-Guérin (BCG) vaccination (4). A similar decline could possibly follow infection, especially with natural containment and/or clearance of the tubercle bacillus. The possibility of a time-related decline in TST reading after infection is supported by the observation of a much higher risk of developing disease among schoolchildren with TST readings
Department of Health
Grantham Hospital FOOTNOTES Conflict of Interest Statement: Neither author has a financial relationship with a commercial entity that has an interest in the subject of this manuscript. REFERENCES
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