Conclusions
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.
Acknowledgments
We thank the Mississippi State Health Department, the Oklahoma Department of Health, animal health technicians from the US Department of Agriculture Animal and Plant Health Inspection Service, Veterinary Services in Puerto Rico, and undergraduate researchers at Northern Arizona University.
Funding for this project was provided by the Centers for Disease Control and Prevention through award nos. 75D30118C00594 and 75D30119P05696.
Emerging Infectious Diseases. 2023;29(3):618-621. © 2023 Centers for Disease Control and Prevention (CDC)
