2017 State and City TB Report
Estimates of Recent Transmission
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 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 2016-2017 provide a starting place within programs to understand better the populations affected by recent TB transmission and monitor trends over time in its control. However, findings from local investigation of clusters and outbreaks, using whole-genome sequencing (WGS) results when available, are preferable because estimates of recent transmission rely on surveillance data and are
limited by the molecular resolution of genotyping.
Using national molecular surveillance data, CDC attributes a TB case to recent transmission if a plausible source case 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 the figure to the right 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
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 the figure on the left 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 2016-2017, 4 states and 2 cities with 1-53 TB cases reported in 2017 had percentages of TB cases attributed to recent transmission that were above the national estimate (13.1%); these percentages should be interpreted with caution because small denominators can create unstable estimates (Figure 13). Five states and 3 cities with 54-140 reported cases, and 5 states and 4 cities with 141 or more reported cases, were above 13.1%. 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, 2017” at: www.cdc.gov/tb/statistics/reports/2017/default.htm.
4 France 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.