We collected 2,540 environmental samples, 370 (280 soil, 80 water, 10 environmental water tank scrapes) from Texas and 2,170 (1,650 soil, 520 water) from throughout Puerto Rico. From the collected samples, we detected B. thailandensis DNA in 10 complex broth samples, 4 from Texas and 6 from Puerto Rico (Appendix). Culturing (10) yielded B. thailandensis isolates from 5 samples, 1 from Texas and 4 from Puerto Rico. In addition, we isolated B. thailandensis from a soil sample collected in Mississippi in July 2022 during a melioidosis cluster investigation (11). Further, in 2021, we identified B. thailandensis infection in a 4th case-patient in the United States (Oklahoma) (Table). The patient was suspected to have aspirated water after a motor vehicle rollover into water; he died because of multiple complications (Appendix).

Figure

Figure. Whole-genome maximum-likelihood phylogeny of global isolates in study of Burkholderia thailandensis from the environment in the United States (Table). Tree was constructed with 1,000 bootstrap replicates...

We used whole-genome analysis of those 7 isolates (National Center for Biotechnology Information BioProject nos. PRJNA575701, PRJNA818328, PRJNA908850) to place them within a larger phylogeographic context, including other B. thailandensis isolates from the United States and other global locations (Table; Figure). Environmental B. thailandensis isolates from Texas and Mississippi grouped in the same clade with clinical isolates from Texas and Louisiana and 2 environmental isolates from Asia. The 2021 clinical isolate from Oklahoma was most closely related to the isolate from the 2003 clinical case in Texas. Environmental isolates from Texas and Mississippi differed by more (4,639 single-nucleotide polymorphisms [SNPs]) than environmental isolates from Thailand and Australia (2,671 SNPs); B. pseudomallei isolates found in Australia and Asia are more diverse than isolates in the Americas (10). We observed little diversity among the 4 B. thailandensis isolates from Puerto Rico; total diversity was 62 SNPs, and distance between any 2 isolates was 28–36 SNPs.

Among the isolates identified in our study, in silico multilocus sequence type analysis (https://pubmlst.org/organisms/burkholderia-pseudomallei) revealed novel ace allele 106 in the 4 isolates from Puerto Rico and the clinical isolate from Oklahoma, assigning all 5 to novel sequence type (ST) 1772. Novel gltB allele 175 was identified in the isolate from Texas, which was assigned to novel ST1785. The isolate from Mississippi, which had a unique combination of alleles, was assigned to novel ST2019.

Our study confirms B. thailandensis endemicity in the environment in the United States, albeit of rare occurrence and low abundance, requiring extensive sampling to detect; we found B. thailandensis at only 3.7% of collection sites in Puerto Rico and 8% in Texas. However, the pathogen could be present in other unsampled areas in the southern United States and Puerto Rico. Substantial culturing was required to isolate bacteria from PCR-positive samples, suggesting low abundance or its presence being outcompeted by other bacteria. B. thailandensis abundance might vary seasonally or on the basis of precipitation levels.

We detected B. thailandensis in Texas and Puerto Rico only from water samples, although they comprised only 24% of total (water and soil) samples collected at positive sites; all soil samples were negative for B. thailandensis. In contrast, in Thailand, B. thailandensis is most commonly isolated from soil (12). All 4 clinical cases from the United States were associated with traumatic injuries (1,4), 3 involving water (1), demonstrating the public health risk for disease from traumatic injuries related to contaminated water. This risk is especially relevant in Puerto Rico where B. thailandensis was detected within neighborhoods of the largest city, San Juan. Puerto Rico and the southeastern United States are prone to hurricane-induced flooding, which could increase the risk for infection by both B. thailandensis and B. pseudomallei (13).

Although samples were collected from 3 municipalities in northeastern Puerto Rico during a 1-year period, we found little phylogenetic diversity among the isolates, suggesting B. thailandensis may be widespread but rare in the environment in Puerto Rico and the result of a single introduction, as previously suggested for B. pseudomallei in Puerto Rico (10). We found evidence of possible local adaptation in Puerto Rico, which supports this hypothesis. We identified 113 genes unique to B. thailandensis isolates from Puerto Rico (Appendix), many of them potentially colocated in genomic islands, a pattern similar to one previously observed among B. pseudomallei isolates from Puerto Rico (10). Of note, 2 genes common to all B. thailandensis from Puerto Rico were present in some B. pseudomallei isolates from Puerto Rico but absent from all other global B. pseudomallei genomes (Appendix). In contrast, thousands of SNPs were found among B. thailandensis strains in the continental United States (Arkansas, Louisiana, Mississippi, Oklahoma, and 2003 clinical and 2019 environmental isolates from Texas). This finding suggests a long-term but cryptic presence of B. thailandensis in the southern United States, perhaps in water. It is unknown how long B. thailandensis can persist in water, but B. pseudomallei can survive in water for >16 years without nutrients (14).

Our study provides valuable information regarding B. thailandensis occurrence and the potential of water to serve as a reservoir and source of infection for this opportunistic pathogen in the southern United States and Puerto Rico, especially following flooding events. Because likely autochthonous melioidosis cases also have been reported from Texas (15), Puerto Rico (10), and Mississippi (11), clinicians should be aware of the potential of misidentifying B. thailandensis as B. pseudomallei because of their morphologic and biochemical similarities.