Molecular epidemiology of Melioidosis in Southern Thailand

Abstract

Melioidosis, caused by Burkholderia pseudomallei, is a severe bacterial disease prevalent in Thailand, particularly in the Northeast region, with high mortality rates. However, its incidence in southern Thailand has been less documented. This study focused on improving melioidosis surveillance in southern provinces, Songkhla and Phatthalung, where the disease was believed to be underreported. Over seven years, from 2014 to 2020, 473 cases were diagnosed, with a median age of 54, 60% of the patients were adults over 50 years of age, and 71.2% were male. Approximately 40% of the treated patients died, with a median time from admission to death of five days. Approximately, 60% of patients had at least one clinical risk factors: 40% had diabetes. Bacteremia and pneumonia were common manifestations. The number of monthly cases was associated with the amount of rainfall. The average annual incidence was 2.87 cases per 100,000 population, lower than the Northeast, but with comparable mortality rates, confirming the endemicity of melioidosis in southern Thailand. We also investigated the epidemiology and environmental presence of B. pseudomallei in southern Thailand. Multi-locus sequence typing (MLST) of 263 clinical isolates from 156 patients identified 72 distinct sequence types (STs), with some matching those from environmental samples collected by Finkelstein in southern Thailand during 1964-1967. Certain STs, including ST288, 84, 54, 289 and 64 were frequently found among patients. Additionally, we observed strain diversity with multiple STs in 13 of 59 patients, indicating exposure to various genotypes in environment. Environmental surveys in Songkhla Province revealed B. pseudomallei in soil and water samples, with a quarter of soil locations and water sources testing positive. Positive soil samples were prevalent in undisturbed area, animal farming and non-agricultural zones, while positive water samples were from waterfalls and streams. Rice paddies showed a lower positivity rate. These findings suggest a significant proportion of recent melioidosis cases stem from B. pseudomallei strains persisting in the environment for decades. Further characterization of recent environmental isolates is needed to understand the contemporary landscape of B. pseudomallei infections in southern Thailand, aiding regional threat assessment.