We appreciate the interest of Currie and others in our report of a case of melioidosis and cases with positive serologic test results for melioidosis among a group of US Marines deployed to Thailand.1 We concur with a number of their remarks and acknowledged most of these as limitations in our article, but there are a few comments that we would like to specifically address.
Cheng and others question the validity of our specific serologic test, the immunofluorescent antibody (IFA) assay. The test we report was performed by Focus Diagnostics using the method of Ashdown and a strain obtained from the American Type Culture Collection. Cheng and others also question the findings of Ashdown and others2 by stating that they used only 11 culture-confirmed cases for determining specificity. Ashdown and others 140 longitudinal serum samples from 42 culture-confirmed patients to calculate the cited specificity, whereas Cheng and others refer to a separate analysis of prospective samples in the same reference. They also state that the IFA assay has not been used in disease-endemic countries for many years. Although the indirect hemagglutination (IHA) test remains the most widely used test despite its poor sensitivity and specificity, investigators from some disease-endemic regions, such as Malaysia, have cited the routine use of the more technically demanding IFA assay since 1995.3–5 Other authors have concluded that that IFA assay is at least as good as the IHA test for melioidosis diagnosis. 6,7
We agree that the clinical significance of positive serologic test results for asymptomatic persons with melioidosis is unknown and acknowledged this in our original paper, although we would emphasize that this uncertainty does not exclude its potential significance. Cheng and others note the relative infrequency of confirmed melioidosis at a regional hospital in the same general vicinity the US Marines visited, but it is obvious to us that the group had plausible exposure to melioidosis; the group traveled as a unit, and one of them developed culture-confirmed disease.
It is unlikely that all our test results represented false-positive results because one member of the exposure group was a culture-confirmed case and because the prevalence of false-positive test results is generally low in previously non-exposed populations. We appreciate that the possibility of cross-reacting antibodies to B. thailandensis and other limitations of the study do not enable us to provide a definitive answer on the precise incidence of melioidosis. However, our study does suggest that exposure during short-term exercises similar to those described in our report may occur, and occasionally result in clinically significant disease. We hope that our report will stimulate future prospective studies of melioidosis associated with travel to disease-endemic countries.
Kronmann KC, Truett AA, Hale BR, Crum-Cianflone NF, 2009. Melioidosis after brief exposure: a serologic survey in US Marines. Am J Trop Med Hyg 80 :182–184.
Ashdown LR, Johnson RW, Koehler JM, Cooney CA, 1989. Enzyme-linked immunosorbent assay for the diagnosis of clinical and subclinical melioidosis. J Infect Dis 160 :253–260.
Vadivelu J, Puthucheary SD, 2000. Diagnostic and prognostic value of an immunofluorescent assay for melioidosis. Am J Trop Med Hyg 62 :297–300.
Khupulsup K, Petchclai B, 1986. Application of indirect hemagglutination test and indirect fluorescent antibody test for IgM antibody for diagnosis of melioidosis in Thailand. Am J Trop Med Hyg 35 :366–369.
Mathai E, Jesudason MV, Anbarasu A, 2003. Indirect immunofluorescent antibody test for the rapid diagnosis of melioidosis. Indian J Med Res 118 :68–70.