Background: Scrub typhus (ST) is a disease caused by an obligate intracellular bacterium, Orientia tsutsugamushi,\nan organism that requires a BSL3 laboratory for propagation. The disease is hallmarked by an eschar at the site of\nthe chigger bite, followed by the development of fever, malaise, myalgia, anorexia, and papulomacular rash. Indirect\nimmunofluorescent assay (IFA) is the gold standard for scrub typhus diagnosis, however, the subjectivity of the\nassay, the need for a specialized laboratory and instruments has limited the wide use of the test in resource limited\nareas.\nMethods: A recombinant-protein based enzyme linked immunosorbent assay (ELISA) using the most abundant\nand immunodominant protein for the detection of Orientia specific antibodies in serum has been developed.\nThe performance of the assay was evaluated using prospectively collected acute sera from 248 randomly selected\npatients in Thailand. The ELISA assay was evaluated using two different cutoff values.\nResults: The receiver operating characteristic (ROC) curve generated cutoff values gave slightly better consistency\nwith diagnosis of ST than those cutoff values established by averaging ELISA optical density of known negatives\nat 99% confidence interval. Both cutoff values provided similar statistical parameters when compared with the\ndiagnosis of ST, indicating the validity of both calculations to derive cutoff values. These results suggest that both\nIgG and IgM ELISA performed well to accurately diagnose scrub typhus cases in endemic areas using only acute\nserum samples.\nConclusions: We have successfully developed an ELISA assay for the detection of Orientia-specific antibodies\nin serum that could provide effective screening of acute sera under clinical setup and it is also a useful assay to\nestimate seroprevalence in various endemic areas.
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