Skip directly to site content Skip directly to page options Skip directly to A-Z link Skip directly to A-Z link Skip directly to A-Z link

Disclaimer: Early release articles are not considered as final versions. Any changes will be reflected in the online version in the month the article is officially released.

Volume 32, Number 6—June 2026

Dispatch

Repeated Extraneous Introductions of Cholera, Thailand, 2007–2025

Kazuhisa OkadaComments to Author , Amonrattana Roobthaisong, Warawan Wongboot, Pawinee Doung-ngern, Wichan Bhunyakitikorn, Pilailuk A. Okada, Thanee Wongchai, Witaya Swaddiwudhipong, Tetsuya Iida, and Shigeyuki Hamada
Author affiliation: The University of Osaka Thailand–Japan Research Collaboration Center on Emerging and Re-emerging Infections, Mueang, Thailand (K. Okada, A. Roobthaisong); The University of Osaka Research Institute for Microbial Diseases, Suita, Japan (K. Okada, T. Iida, S. Hamada); Ministry of Public Health, Muang (W. Wongboot, P. Doung-ngern, W. Bhunyakitikorn, P.A. Okada); Maesot General Hospital, Maesot, Thailand (T. Wongchai, W. Swaddiwudhipong)

Main Article

Figure 1

Maximum-likelihood phylogenetic trees of Vibrio cholerae O1 isolates reconstructed for study of repeated extraneous introductions of cholera, Thailand, 2007–2025. A) Phylogenetic relationships of 157 Thailand isolates and 11 representative O1 reference strains. Seven Thailand isolates (orange) belong to the El Tor Sister group lineage. The 7th pandemic El Tor lineage is divided into 5 distinct clusters (a–e), color-coded by isolation period: red-purple (cluster a, 2007–2008), blue-purple (cluster b, 2009–2010), blue (cluster c, 2011–2012), green (cluster d, 2015–2016), and red (cluster e, 2024–2025). B) Detailed phylogeny of the 7th pandemic El Tor lineage incorporating 150 Thailand isolates and 1,425 global genomes. Yellow labels explicitly identify the global reference strains (e.g., N16961, CIRS101) used in the analysis to provide genomic context for the Thailand clades. Light blue shaded region highlights clade A; light pink shaded region highlights clade B. Both were selected for molecular clock analysis. Outbreak-associated Thailand clades (TH1–TH4) are nested within these broader genomic groups. Both trees were reconstructed using maximum-likelihood estimation based on high-quality core genome single-nucleotide polymorphisms and visualized using Interactive Tree of Life (https://itol.embl.de). Dots on the branches indicate bootstrap values >80%.

Figure 1. Maximum-likelihood phylogenetic trees of Vibrio cholerae O1 isolates reconstructed for study of repeated extraneous introductions of cholera, Thailand, 2007–2025. A) Phylogenetic relationships of 157 Thailand isolates and 11 representative O1 reference strains. Seven Thailand isolates (orange) belong to the El Tor Sister group lineage. The 7th pandemic El Tor lineage is divided into 5 distinct clusters (a–e), color-coded by isolation period: red-purple (cluster a, 2007–2008), blue-purple (cluster b, 2009–2010), blue (cluster c, 2011–2012), green (cluster d, 2015–2016), and red (cluster e, 2024–2025). B) Detailed phylogeny of the 7th pandemic El Tor lineage incorporating 150 Thailand isolates and 1,425 global genomes. Yellow labels explicitly identify the global reference strains (e.g., N16961, CIRS101) used in the analysis to provide genomic context for the Thailand clades. Light blue shaded region highlights clade A; light pink shaded region highlights clade B. Both were selected for molecular clock analysis. Outbreak-associated Thailand clades (TH1–TH4) are nested within these broader genomic groups. Both trees were reconstructed using maximum-likelihood estimation based on high-quality core genome single-nucleotide polymorphisms and visualized using Interactive Tree of Life (https://itol.embl.de). Dots on the branches indicate bootstrap values >80%.

Main Article

Page created: May 06, 2026
Page updated: May 20, 2026
Page reviewed: May 20, 2026
The conclusions, findings, and opinions expressed by authors contributing to this journal do not necessarily reflect the official position of the U.S. Department of Health and Human Services, the Public Health Service, the Centers for Disease Control and Prevention, or the authors' affiliated institutions. Use of trade names is for identification only and does not imply endorsement by any of the groups named above.
file_external