2018 State and City TB Report
Estimates of Recent Transmission
Figure 1. Illustration of the plausible-source case method
Control of TB transmission is accomplished through prompt detection and treatment of patients with infectious TB disease, administrative controls designed to reduce exposure to persons with infectious TB, and contact investigations to identify exposed persons at risk for LTBI (i.e., candidates for latent TB infection (LTBI) testing and treatment). Many clinical, epidemiologic, and social factors can make these activities challenging to implement successfully. National estimates of recent transmission for cases reported during 2017–2018 provide a starting place within programs to understand better the populations affected by recent TB transmission and monitor trends over time in TB control. However, findings from local investigations of clusters and outbreaks, using whole-genome sequencing (WGS)pdf icon results when available, are preferable because estimates of recent transmission rely on surveillance data and are limited by the molecular resolution of genotyping.
Figure 2. Illustration demonstrating how cases are categorized as attributed to extensive or limited recent transmission
Using national molecular surveillance data, CDC attributes a TB case to recent transmission if a plausible source case1 can be identified in a person who has the same M. tuberculosis genotype, has an infectious form of TB disease, is at least 10 years of age, resides within 10 miles of the given TB case, and was diagnosed within 2 years before the given TB case. For example, the given case in the center of “Figure 1. Illustration of the plausible-source case method” has two plausible source cases (represented with arrows). The plausible source cases have the same genotype (matching colors), are at least 10 years old (larger), have infectious forms of TB (lungs), and are proximal geographically and in time (dotted circle).
Figure 13: Percentage of TB Cases Attributed to Recent Transmission, United States, 2017–2018
Each TB case attributed to recent transmission is further categorized as attributed to extensive recent transmission if the case belongs to a plausible transmission chain of six or more cases with at least five preceding cases in the chain within the previous three years. Otherwise, the case is categorized as attributed to limited recent transmission. For example, case 1 in “Figure 2. Illustration demonstrating how cases are categorized as attributed to extensive or limited recent transmission” is a plausible source case, but not attributed to recent transmission. Cases 2–5 are attributed to limited recent transmission, and cases 6–8 are attributed to extensive recent transmission.
For 2017–2018, 3 states and 2 cities with 1–51 TB cases reported in 2018 had percentages of TB cases attributed to recent transmission that were above the national estimate (12.6%); these percentages should be interpreted with caution because small denominators can create unstable estimates (Figure 13image icon, Footnotes). Nine states and one city with 52–139 reported cases, and 4 states and 4 cities with 140 or more reported cases, were above 12.6%. The relative amounts of limited and extensive recent transmission varied substantially. Because not only TB incidence but also demographics and molecular epidemiology vary considerably among states, counties, and cities, these data should not be compared across jurisdictions.
Additional information on estimates of recent transmission and their limitations can be found in CDC’s Reported Tuberculosis in the United States, 2018.
1France AM, Grant J, Kammerer JS, Navin TR. A field-validated approach using surveillance and genotyping data to estimate tuberculosis attributable to recent transmission in the United States. Am J Epidemiol 2015; 182: 799–807.
Figure 13. Percentage of TB Cases Attributed to Recent Transmission, United States, 2017–2018*
*This horizontal bar graph shows the percentages of TB cases attributed to recent transmission by individual states and cities in the United States during 2017–2018. The fraction in each parenthesis reports the number of cases attributed to recent transmission out of all genotyped cases that are eligible to be evaluated for recent transmission (i.e., complete data for the plausible-source case method’s algorithm). States and cities are categorized into three groups based on numbers of TB cases reported in 2018 and are ordered by increasing percentage of cases attributed to recent transmission. Dark and light bars show qualitatively the relative proportions of genotyped cases that were attributed to extensive and limited recent transmission, respectively. The total length of the bar represents all recent transmission (i.e., the percentage sums of extensive recent transmission and limited recent transmission equal all recent transmission). During 2017–2018, 12.6% of genotyped cases were attributed to recent transmission in the United States. To understand better the populations affected by recent TB transmission and monitor trends over time in its control, findings from local investigation of clusters and outbreaks, using WGS results when available, are preferable because estimates of recent transmission rely on surveillance data and are limited by the molecular resolution of genotyping. Because not only TB incidence but also demographics and molecular epidemiology vary considerably among states, counties, and cities, these data should not be compared across jurisdictions.
Data sources: National Tuberculosis Surveillance System and National Tuberculosis Genotyping Service as of June 6, 2019.
