What's Driving the Decline in Tuberculosis in Arkansas? A Molecular Epidemiologic Analysis of Tuberculosis Trends in a Rural, Low-Incidence Population, 1997–2003

American Journal of Epidemiology, Sep 2007

Incident cases of tuberculosis may result from a recently acquired Mycobacterium tuberculosis infection or from the reactivation of a latent infection acquired in the remote past. The authors used molecular fingerprinting data to estimate the relative contributions of recent and remotely acquired infection to the yearly incidence of tuberculosis in Arkansas, a state with a largely rural population where the incidence of tuberculosis declined from 7.9 cases per 100,000 population to 4.7 cases per 100,000 between 1997 and 2003. The authors used a time-restricted definition of clustering in addition to the standard definition in order to increase the specificity of the clustering measure for recent transmission. The greatest overall declines were seen in non-Hispanic Blacks (from 13.8 cases per 100,000 in 1997 to 6.5 cases per 100,000 in 2003) and persons aged 65 years or more (from 19.9 cases per 100,000 in 1997 to 8.5 cases per 100,000 in 2003). In both groups, the incidence of nonclustered cases declined more dramatically than the incidence of clustered cases. This suggests that the decline in rates resulted primarily from declining rates of disease due to reactivation of past infections. Declines in the overall incidence of tuberculosis in a population may not necessarily result from declines in active transmission.

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What's Driving the Decline in Tuberculosis in Arkansas? A Molecular Epidemiologic Analysis of Tuberculosis Trends in a Rural, Low-Incidence Population, 1997–2003

Anne Marie France 2 M. Donald Cave 0 1 Joseph H. Bates 3 4 Betsy Foxman 2 Toby Chu 0 1 Zhenhua Yang ) 2 0 Central Arkansas Veterans' Healthcare Center , Little Rock, AR 1 Department of Neurobiology and Developmental Sciences, College of Medicine, University of Arkansas for Medical Sciences , Little Rock, AR 2 Department of Epidemiology, School of Public Health, University of Michigan , Ann Arbor, MI 3 Division of Health, Arkansas Department of Health and Human Services , Little Rock, AR 4 Departments of Internal Medicine and Microbiology, College of Public Health, University of Arkansas for Medical Sciences , Little Rock, AR Incident cases of tuberculosis may result from a recently acquired Mycobacterium tuberculosis infection or from the reactivation of a latent infection acquired in the remote past. The authors used molecular fingerprinting data to estimate the relative contributions of recent and remotely acquired infection to the yearly incidence of tuberculosis in Arkansas, a state with a largely rural population where the incidence of tuberculosis declined from 7.9 cases per 100,000 population to 4.7 cases per 100,000 between 1997 and 2003. The authors used a time-restricted definition of clustering in addition to the standard definition in order to increase the specificity of the clustering measure for recent transmission. The greatest overall declines were seen in non-Hispanic Blacks (from 13.8 cases per 100,000 in 1997 to 6.5 cases per 100,000 in 2003) and persons aged 65 years or more (from 19.9 cases per 100,000 in 1997 to 8.5 cases per 100,000 in 2003). In both groups, the incidence of nonclustered cases declined more dramatically than the incidence of clustered cases. This suggests that the decline in rates resulted primarily from declining rates of disease due to reactivation of past infections. Declines in the overall incidence of tuberculosis in a population may not necessarily result from declines in active transmission. Arkansas; cohort effect; DNA fingerprinting; epidemiology, molecular; infection control; Mycobacterium tuberculosis; rural health; tuberculosis Abbreviations: MSA, Metropolitan Statistical Area; RFLP, restriction fragment length polymorphism; TB, tuberculosis. - racial/ethnic groups and between US- and foreign-born persons persist (2). In order to best focus the limited resources available on the elimination of TB in the United States, it is important to understand what factors have driven the decline and how those factors vary across subpopulations. Because of the complex natural history of TB, incident cases in a population may be due to infections that were acquired recently and therefore represent evidence of active chains of transmission, or they may be due to the reactivation of latent infections acquired years or even decades earlier (3). Clinically, it is difficult to distinguish between recently acquired and reactivated disease (4), but the frequency of each type in the population has important implications for infection control. DNA genotyping of Mycobacterium tuberculosis isolates provides a tool for drawing inferences about the transmission history of a clinical isolate. Cases that produce isolates with identical or highly similar DNA genotyping patterns, identified as clusters, reflect a common chain of transmission (5) and are considered to be caused by the same strain. Clusters occurring within a short time period are considered to reflect active transmission followed by rapid progression to clinical disease. By contrast, cases involving unique isolates are considered to result from reactivation of a latent infection acquired in the past. A number of investigators have used clustering analyses to estimate the proportion of disease resulting from recent transmission (69), and a few have used it to obtain insight into trends in TB incidence over time. However, these latter studies have focused on urban populations (10, 11). In rural populations, the dynamics of TB transmission probably differ considerably from those of urban settings; thus, a clustering analysis of TB trends over time in a rural setting would be particularly revealing. In Arkansas, the reported incidence of TB declined from 7.9 cases per 100,000 population in 1997 to 4.7 cases per 100,000 in 2003 (1218). It is uncertain whether or not this decline can be attributed to a decrease in recent transmission. Using DNA genotyping data on M. tuberculosis isolates collected in Arkansas between 1996 and 2003, we estimated the relative contributions of changes in the incidence of recently acquired disease, due to active transmission in the population, and reactivation disease, due to the reactivation of infections acquired in the past, to the overall changes in TB incidence in this population between 1997 and 2003. MATERIALS AND METHODS Arkansas demographic characteristics We characterized the Arkansas population using demographic information obtained from 2000 US Census data (19). National and sta (...truncated)


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Anne Marie France, M. Donald Cave, Joseph H. Bates, Betsy Foxman, Toby Chu, Zhenhua Yang. What's Driving the Decline in Tuberculosis in Arkansas? A Molecular Epidemiologic Analysis of Tuberculosis Trends in a Rural, Low-Incidence Population, 1997–2003, American Journal of Epidemiology, 2007, pp. 662-671, 166/6, DOI: 10.1093/aje/kwm135