Positive Serologic Test Results for Burkholderia pseudomallei in Asymptomatic Persons

Abstract

1We caution against the use of a poorly validated serologic test to provide evidence of melioidosis in asymptomatic persons. The authors use the indirect fluorescent antibody (IFA) assay, which has not been used in disease-endemic countries for many years, and no validation of this assay is presented. Although we may assume that the test is broadly similar to the previous descriptions of the IFA assay, 2 the bacterial strains used (or if purified antigen was used, the nature of the antigen) are not defined. The authors cite earlier work by Ashdown and others who described a high specificity of the IFA assay, but in this report, only 11 of 89 patients had culture-confirmed melioidosis, with the remaining 78 diagnosed with subclinical melioidosis on the basis of serologic test results. 3 The clinical significance of asymptomatic persons with positive serologic results is unclear; whether such people have truly been infected and harbor latent infection is not known. Nevertheless, as the authors note, it is apparent that overt melioidosis does not develop in most of such patients. Furthermore, there may have been confounding by exposure to the antigenically similar but less virulent B. thailandensis that is present in Thailand but not in Australia. The higher prevalence of positive indirect hemagglutination (IHA) test results using B. pseudomallei antigen in Thai patients compared with those from northern Australia, with a broadly similar incidence of disease, may suggest that exposure to B. thailandensis may result in detectable antibodies against B. pseudomallei . 4,5 Studies of cross-reactivity between the IHA using B. thailandensis as the antigen in patients with cultureconfirmed melioidosis have not been consistent. 6,7 We suggest that the quantification of risk in groups exposed to mud and pooled surface water should be based on surveillance of culture-confirmed melioidosis in the population at risk. The authors do not offer any data on the incidence of melioidosis in occupationally exposed groups in the same area, such as during previous military exercises or in rice farmers. Approximately 20 cases of melioidosis per year are reported at the Chonburi Hospital approximately 80 km from the study site (with an unknown denominator), and a low rate of melioidosis has been documented in a hospital survey in the central region of Thailand compared with the northeast Issan region. 8,9 In summary, the clinical significance of seropositive asymptomatic persons in this study is doubtful, given the use of a poorly characterized serologic test, possible confounding by exposure to B. thailandensis , the lack of a non-exposed control group, and the lack of a clinical picture characteristic of melioidosis in all but one of these patients.