Presence of B. thailandensis and B. thailandensis expressing B. pseudomallei-like capsular polysaccharide in Thailand, and their associations with serological response to B. pseudomallei

Abstract

BackgroundBurkholderia pseudomallei is an environmental Gram-negative bacillus and the cause of melioidosis. B. thailandensis, some strains of which express a B. pseudomallei-like capsular polysaccharide (BTCV), is also commonly found in the environment in Southeast Asia but is considered non-pathogenic. The aim of the study was to determine the distribution of B. thailandensis and its capsular variant in Thailand and investigate whether its presence is associated with a serological response to B. pseudomallei.Methodology/principal findingsWe evaluated the presence of B. pseudomallei and B. thailandensis in 61 rice fields in Northeast (n = 21), East (n = 19) and Central (n = 21) Thailand. We found BTCV in rice fields in East and Central but not Northeast Thailand. Fourteen fields were culture positive for B. pseudomallei alone, 8 for B. thailandensis alone, 11 for both B. pseudomallei and B. thailandensis, 6 for both B. thailandensis and BTCV, and 5 for B. pseudomallei, B. thailandensis and BTCV. Serological testing using the indirect hemagglutination assay (IHA) of 96 farmers who worked in the study fields demonstrated that farmers who worked in B. pseudomallei-positive fields had higher IHA titers than those who worked in B. pseudomallei-negative fields (median 1:40 [range: <1:10–1:640] vs. <1:10 [range: <1:10–1:320], p = 0.002). In a multivariable ordered logistic regression model, IHA titers were significantly associated with the presence of B. pseudomallei (aOR = 3.7; 95% CI 1.8–7.8, p = 0.001) but were not associated with presence of B. thailandensis (p = 0.32) or BTCV (p = 0.32). One sequence type (696) was identified for the 27 BTCV isolates tested.Conclusions/significanceThis is the first report of BTCV in Thailand. The presence of B. pseudomallei and B. thailandensis in the same field was not uncommon. Our findings suggest that IHA positivity of healthy rice farmers in Thailand is associated with the presence of B. pseudomallei in rice fields rather than B. thailandensis or BTCV.

Presence of Burkholderia pseudomallei in the ‘Granary of Myanmar’

“…Twelve of seventeen B. thailandensis -positive samples were found at different study sites, but in several cases both B. thailandensis and B. pseudomallei were detected in the same site. This is consistent with findings from a Thai study [56].…”

Human Immune Responses to Melioidosis and Cross-Reactivity to Low-Virulence Burkholderia Species, Thailand1

“…In addition, a recent epidemiologic study showed high prevalence of environmental B. pseudomallei in rice paddy fields across multiple regions of Thailand, especially in the east (6). Subsequent serologic responses against B. pseudomallei in healthy rice farmers are associated with exposure to environmental B. pseudomallei rather than B. thailandensis and BTCV (20); the cause could be a higher prevalence of B. pseudomallei. It is impossible to distinguish between the serologic responses to B. pseudomallei and BTCV because CPS components are highly cross-reactive (9).…”

“…Healthy persons who live in melioidosis-endemic areas, on the other hand, primarily demonstrate cellular immune cross-reactivity. We recommend further investigation of human immune responses in healthy persons where the less pathogenic strains are prevalent, such as in the central and eastern parts of Thailand (20). It is possible that exposure to the less virulent B. thailandensis and BTCV generates immune cross-reactivity, which could confer some protection against melioidosis.…”

“…We prepared antigens in accordance with published methods, unless otherwise stated (17,18). For IHA, we used crude culture filtrate antigens from pooled isolates with the following specifications: B. pseudomallei Thai clinical isolates 199a and 207a (19), B. thailandensis Thai environmental isolates E264 (ATCC700388) and STBCC006 (20), and BTCV Thai environmental E555 and USA clinical isolate CDC3015869. We prepared B. thailandensis and BTCV antigens following a traditional IHA antigen preparation as described previously (17).…”

Antibodies in Melioidosis: The Role of the Indirect Hemagglutination Assay in Evaluating Patients and Exposed Populations

“…Nevertheless, the existence of cross-reactive antibody responses has not been clearly established, since previous studies have not been able to demonstrate cross-reactive IHA titers between Bp and Bt in healthy controls. 23 , 24 We are presently exploring the contribution of cross-reactivity to antibody responses in our laboratory.…”