Reported Tuberculosis in the United States, 2018

Estimates of Recent Transmission, 2017–2018

Overview

Control of recent TB transmission is a vital component of the national TB elimination strategy. Programs can begin to monitor trends in recent transmission by comparing current state- and county-specific estimates published in this section of the annual report with 2016–2017 and 2015–2016 estimates published in previous editions of this report.

These estimates might also help state and local TB programs to

• use limited public health resources to plan and prioritize TB control activities;
• enhance service delivery by applying sufficient resources;
• identify geographic, demographic, and social disparities in the proportion of cases attributed to recent transmission;
• develop specific prevention strategies and interventions to interrupt transmission; and
• facilitate connections among jurisdictions to share TB control strategies.

Recent transmission can occur in communities regardless of the overall TB incidence in a particular state; even states with a low incidence of TB overall might have counties where extensive recent transmission is occurring.¹ However, these estimates are not intended for comparisons across jurisdictions because TB incidence, demographics, and molecular epidemiology vary considerably among states and counties.

Terminology

Recent Transmission

A TB case is designated as attributed to recent transmission if a plausible source case² can be identified in a person who

1. has the same M. tuberculosis genotype,
2. has an infectious form of TB disease,
3. resides ≤10 miles from the TB case,
4. is aged ≥10 years, and
5. received the TB diagnosis ≤2 years before the TB case.

Recent transmission estimates are mapped as counts to describe the relative numbers of cases attributed to recent TB transmission (Figure 1).

Extensive Recent Transmission

A TB case is designated as attributed to extensive recent transmission when the criteria specified previously for recent transmission are met, and furthermore, the case belongs to a plausible transmission chain of ≥6 cases (i.e., the plausible source case and ≥4 other cases identified ≤3 years before the TB case). Extensive recent transmission estimates are presented as a percentage of all genotyped cases to identify areas and populations disproportionately affected by extensive recent transmission among counties with ≥10 genotyped cases (Figure 2).

Recent Transmission in the United States

Nationally, 1,712 (12.6%) of 13,601 genotyped cases reported during 2017–2018 were attributed to recent transmission (Table 57). Among all genotyped cases, 589 (4.3%) were attributed to extensive recent transmission. These estimates are lower than previous estimates for 2015–2016, in which 13.7% and 5.0% of genotyped cases were attributed to recent transmission and extensive recent transmission, respectively.

Geography

Forty-seven counties or county equivalents had >5% of cases attributed to extensive recent transmission (Table 58). Among these counties, the median number of genotyped cases was 32 (range: 10–888 cases). The median percentage of genotyped cases occurring among U.S.-born patients (40.3%) was significantly higher in these 47 counties, compared with the national proportion of TB cases among U.S.-born persons (29.5%).

Demographic and Social Characteristics

Cases among U.S.-born persons were more frequently attributed to both recent transmission (25.1%) and extensive recent transmission (9.8%) than cases among non-U.S.–born persons (7.6% and 2.2%, respectively) (Table 59). Racial/ethnic disparities in the proportions of cases attributed to recent transmission and extensive recent transmission were identified among American Indians/Alaska Natives, Native Hawaiian/Other Pacific Islander, and non-Hispanic Blacks/African Americans (Table 59).

Additionally, greater proportions of cases attributed to recent transmission and extensive recent transmission were identified among persons who reported drug use and excess alcohol use, compared with persons without these risk factors. Cases were also more frequently attributed to recent transmission and extensive recent transmission among persons experiencing homelessness within the previous year and residents of a correctional facility at the time of diagnosis. However, these 2017–2018 estimates are lower than comparable estimates during 2015–2016 for both recent transmission and extensive recent transmission.

Limitations

These methods for estimating recent transmission can only be applied to culture-confirmed, genotyped cases that are eligible to be evaluated for recent transmission (n = 13,601 cases). Pediatric and other clinically diagnosed cases are likely underrepresented because cases without genotyping results are excluded. This limitation is especially relevant for TB cases among young children, which are most likely to be caused by recent transmission.

The proportions of cases attributed to recent transmission typically are higher in areas with fewer M. tuberculosis genotypes and where prevalent or common genotypes (Table 26) have been predominant for years; among relatively closed populations and remote areas (e.g., parts of Alaska), recent transmission might be overestimated. As evidenced by whole-genome sequencing (WGS), genomic diversity might be greater than what is apparent by using current genotyping methods among cases reported by areas bordering Mexico. Consequently, definitively distinguishing cases attributed to recent transmission from cases caused by reactivation of longstanding, untreated latent TB infection can be difficult when using the methods employed for this report.

Future Applications

Although national surveillance data can only provide aggregated estimates, these methods offer state and local TB control programs opportunities for tracking trends in recent transmission and prioritizing public health activities and interventions. As universal WGS results are further adopted into programmatic practice, characterization and investigation of recent TB transmission will become increasingly precise.