Clinical Report: Increased Tuberculosis Transmission Linked to Cross-Border Mobility

Overview

This investigation combined genomic data and detailed interviews to examine a 13-case tuberculosis (TB) cluster spanning Almería and Madrid, Spain. The findings linked cases across regions and revealed possible exposures missed during initial investigations. One patient’s prolonged diagnostic delay was considered a likely contributor to extensive transmission. Targeted sequencing helped identify additional cases where population-wide genomic surveillance was unavailable.

Background

Whole-genome sequencing can help identify related TB cases, but surveillance limited to one geographic area may miss transmission across regions. The authors examined a cluster complicated by movement within Spain, a prolonged diagnostic delay, and exposure settings that were not apparent from initial contact tracing.

Objective

To improve the investigation of TB transmission by integrating genomic data from Almería and Madrid, refining epidemiological interviews, and using targeted sequencing to identify additional cluster cases.

Approach

Researchers combined available sequencing data and stored isolates from the two regions. They compared genetic differences among related strains, conducted detailed follow-up interviews, and used targeted sequencing of strain-marker variants to screen Madrid isolates. Whole-genome sequencing was used to confirm newly identified cluster cases.

Data Highlights

• The investigation identified 13 cases in a cluster spanning Almería and Madrid.

• One patient had symptoms for at least one year before diagnosis and advanced TB at presentation.

• Targeted sequencing screened 164 Madrid isolates and identified four additional candidate cases, subsequently confirmed by whole-genome sequencing.

Key Findings

• Follow-up interviews revealed stays in Madrid and possible exposures at social gatherings that initial investigations had not established.

• Integrating genomic data from both regions connected cases that separate investigations might have treated as unrelated.

• The authors considered the prolonged diagnostic delay in one patient a likely contributor to extensive transmission. The investigation did not measure whether diagnostic delays were more common among migrants generally.

Interpretation

Genomic comparisons were most informative when combined with epidemiological information from across regional boundaries. Targeted sequencing helped extend the search for related cases in Madrid, where comprehensive genomic surveillance was unavailable.

Clinical Implications

The authors propose connecting genomic and epidemiological investigations across relevant regions and using detailed interviews to clarify possible exposures. These are surveillance proposals drawn from one cluster, not a clinical guideline or evidence that all migrant populations have increased TB transmission risk.

Limitations

Madrid did not have population-wide genomic surveillance, so additional related cases may have been missed. Detailed follow-up interviews were unavailable for some Madrid cases, leaving their specific exposure links unresolved. The authors caution that the full extent of the cluster may be greater than identified.

Conclusion

This cluster shows how movement within one country, incomplete regional surveillance, and a prolonged diagnostic delay can complicate TB transmission investigations. Integrating genomic data with detailed epidemiological work helped reveal connections across Almería and Madrid.

Related Resources & Content

The following publication details appear in the supplied paper or its reference list:

• Saleeb SM, Vallejo-Godoy S, Marcos-Abellán A, et al. “Tuberculosis enhanced transmission due to interterritorial mobility and prolonged diagnostic delay: a call for an integrated genomic analysis.” International Journal of Infectious Diseases. 2026;172:109061.

• Walker TM, Ip CLC, Harrell RH, et al. “Whole-genome sequencing to delineate Mycobacterium tuberculosis outbreaks: a retrospective observational study.” The Lancet Infectious Diseases. 2013;13:137–146.

• Golub J, Bur S, Cronin WA, et al. “Delayed tuberculosis diagnosis and tuberculosis transmission.” International Journal of Tuberculosis and Lung Disease. 2006;10:24–30.

• Rodríguez-Grande C, Vallejo-Godoy S, Martínez-Lirola M, et al. “Long-term refined genomic analysis of tuberculosis clusters to distinguish between ongoing transmission, reactivations or diagnostic delays, Almería, Spain, 2003 to 2024.” Eurosurveillance. 2026;31:2500301.

• Gardy JL, Johnston JC, Sui SJH, et al. “Whole-genome sequencing and social-network analysis of a tuberculosis outbreak.” New England Journal of Medicine. 2011;364:730–739.