Molecular Assay Detects Acute Scrub Typhus

A molecular assay has been developed and evaluated in a clinical setting for the diagnosis of endemic scrub typhus.

A loop-mediated isothermal polymerase chain reaction (PCR) assay (LAMP) has been tested for the diagnosis of acute scrub typhus in low-resource, primary health care settings to guide clinical therapy.

A prospective fever study, including 161 patients from scrub typhus-endemic was carried out in Chiang Rai, northern Thailand by scientists at the Mahidol University (Bangkok, Thailand). The scrub typhus infection criteria was a positive cell culture isolate; an admission immunoglobulin (Ig) M titer equal or greater than 1:12,800 using the “gold standard” indirect immunofluorescence assay (IFA); a four-fold rising IFA IgM titer and/or a positive result in at least two out of three PCR assays.

The molecular assays were a nested 56 kDa PCR, a 47 kDa real time PCR assay, a highly sensitive PCR primer set based on the GroEl gene, and the LAMP test. The LAMP test was performed using the Loopamp kit (Eiken Chemical Co. Ltd., Tokyo, Japan). All PCR assays, the including LAMP demonstrated high specificity ranging from 96% to 99%, with sensitivities varying from 40% to 56%, similar to the antibody based rapid test, which had a sensitivity of 47% and a specificity of 95%. After two days of fever the results of the admission assays were as follows: LAMP assay had a positivity rate of 25%, higher than that of the GroEl and 47 kD real-time assays which were both at 17%, and the immunochromatographic test (PanBio; Brisbane, Australia) was lowest with 8%.

The authors concluded that the diagnostic accuracy of the LAMP assay for scrub typhus was similar to real-time and nested conventional PCR assays, but superior to the antibody-based rapid test in the early disease course. This is where the new LAMP methodology has potential as it is inexpensive, simple to perform and requires only a water bath or simple heating block instead of a thermocycler. The combination of DNA- and antibody-based detection methods increased sensitivity with minimal reduction of specificity, and expanded the timeframe of adequate diagnostic coverage throughout the acute phase of scrub typhus. This disease is caused by the rickettsial organism Orientia tsutsugamushi and transmitted by mites. The study was published in the September 2011 issue of the journal Public Library of Science Neglected Tropical Diseases.

Related Links:
Mahidol University
Eiken Chemical Co. Ltd.
PanBio