Presence of Environmental <i>Burkholderia pseudomallei</i> and Burden of Melioidosis in Thailand

Burkholderia pseudomallei is a soil-dwelling bacterium and the cause of melioidosis. Currently knowledge of the distribution of environmental B. pseudomallei and of the factors determining it is limited. In Thailand B. thailandensis, a closely related species to B. pseudomallei, is common, though the implications for B. pseudomallei distribution is unknown. Due to the difficulties in diagnosis of melioidosis and lack of resources, the distribution and burden of human melioidosis is likely under-reported. Melioidosis is a notifiable disease in Thailand. It has been estimated that more than 2,000 deaths are caused by melioidosis in Thailand, but only around 10 melioidosis deaths were officially reported to the notifiable diseases surveillance system (Report 506) each year. This thesis describes two large epidemiological studies: an environmental survey; and a cross-sectional retrospective, multicenter surveillance study using data from hospital databases nationwide. The environmental survey examined the distribution of environmental B. pseudomallei and B. thailandensis, ecological factors, and the seropositivity of farmers working in the fields. The retrospective surveillance study determined the incidence and mortality of human melioidosis diagnosed in large public hospitals throughout Thailand. I found that B. pseudomallei is widely distributed in East and Northeast Thailand, and unevenly distributed in Central Thailand. B. pseudomallei thrives in rice fields that are nutrient depleted. Presence of B. pseudomallei and B. thailandensis in the same field is not uncommon. In addition, B. thailandensis expressing B. pseudomallei-like capsular polysaccharide (BTCV) was isolated from soil for the first time in Thailand. Background seropositivity against B. pseudomallei of healthy rice farmers in Thailand is associated with presence of B. pseudomallei in rice fields rather than B. thailandensis or BTCV. Finally, melioidosis is endemic and is an important cause of death in Thailand, but is rarely officially reported to the Thai Ministry of Public Health. Data from the national notifiable disease-surveillance system in resource-limited settings should be verified by integrating information from readily available databases