# Seasonal change of Burkholderia pseudomallei in paddy field water strongly correlates with ambient temperature: A study in north-central Vietnam

## TL;DR

The study found that higher temperatures in summer increase the presence of a deadly bacterium in paddy field water, leading to more disease cases.

## Contribution

This is the first study to show a strong correlation between ambient temperature and the presence of Burkholderia pseudomallei in paddy field water.

## Key findings

- B. pseudomallei presence in paddy field water strongly correlates with ambient temperature.
- The positive culture rate of B. pseudomallei ranged from 5% in winter to 82% in summer.
- A novel B. pseudomallei genotype (ST 1994) was found persisting throughout the year.

## Abstract

Melioidosis, a fatal infectious disease caused by the environmental bacterium Burkholderia pseudomallei, is known to be associated with the rainy season. Although several attempts have been made to explain this phenomenon, data on the association between the presence of B. pseudomallei in environmental reservoirs and weather variables remain limited. This study focused on water samples collected from a paddy field in north-central Vietnam to investigate this association. A total of 800 samples were collected over eight different time points in 2018. Using a two-step enrichment method, 349 samples (43.6%) were positive by a B. pseudomallei-specific real-time PCR assay targeting the TTSS1 gene and by B. pseudomallei isolation on Ashdown agar. The positive culture rate of B. pseudomallei ranged from 5% in the winter to 82% in the summer. Quantitative culture method directly detected B. pseudomallei colonies only from samples collected in the summer, with an overall mean count of 93.1 CFU/ml (n = 13; range from 5 to 750). The positive culture rate of B. pseudomallei showed a strong positive correlation with the average ambient temperature when binned from three days up to a month before the sampling dates, with the strongest correlation observed at the 19-day bin data (rs = 0.905; p = 0.002). Two clusters of melioidosis cases were identified in the summer, one after tropical depressions and another after rice harvesting activities. Diverse B. pseudomallei genotypes were found within this small-scale paddy field, with a novel sequence type (ST) 1994 persisting throughout the year. In conclusion, our study demonstrated that increased ambient temperature significantly contributed to the higher occurrence and bacterial load of B. pseudomallei in surface water, leading to more melioidosis cases if combined with severe rainfalls or occupational agricultural exposure. These findings should be considered in the context of global warming and climate change.

Burkholderia pseudomallei is an environmental bacterium that can cause melioidosis, a potentially fatal infectious disease affecting both humans and animals, particularly in many tropical and subtropical regions. While melioidosis cases typically peak during the rainy season, the association between the presence of this bacterium in environmental reservoirs, climatic factors, and disease incidence remains poorly understood. This study provides the first evidence of a significant correlation between ambient temperature and the positive culture rate of B. pseudomallei in paddy field water. The findings suggest that the increase in melioidosis cases during the rainy season can be attributed to a higher occurrence and bacterial load of B. pseudomallei in surface water during the summer, especially after heavy rainfalls and occupational agricultural exposure. Our findings are important to consider in the context of global warming and climate change, which may shift the global distribution of B. pseudomallei and the epidemiology of melioidosis, possibly expanding into areas in subtropical regions where melioidosis is currently suspected to be less common.