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.
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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)