Targeting surface cell antigen 2 increases sensitivity of Rickettsia typhi detection

Area of Science:

• Medical Diagnostics
• Infectious Diseases
• Molecular Biology

Background:

• Murine typhus, caused by Rickettsia typhi, is a flea-borne illness often diagnosed late due to low sensitivity of current molecular assays.
• The standard qPCR assay targets the ompB gene, which has limitations in detecting low concentrations of bacterial DNA in patient blood.

Purpose of the Study:

• To enhance the sensitivity of Rickettsia typhi detection using qPCR by targeting a novel gene, sca2, which contains repetitive sequences.
• To compare the diagnostic accuracy, sensitivity, specificity, and bacterial load measurement of the sca2-based qPCR assay against the standard ompB-based assay.

Main Methods:

• Compared diagnostic performance of sca2 and ompB qPCR assays using buffy coat samples from 88 patients in Laos.
• Validated R. typhi confirmation through bacterial culture, IFA, and ompB PCR.
• Included samples from patients with confirmed murine typhus, negative cases, and those tested via Rapid Diagnostic Tests (RDTs).

Main Results:

• The sca2 qPCR assay demonstrated significantly higher sensitivity (59.09%) compared to the ompB assay (36.36%), while both maintained 100% specificity.
• Bacterial DNA load measured by sca2 qPCR was substantially higher (approx. 2.933 log units) than by ompB qPCR.
• The sca2 assay detected higher median DNA copy numbers (16,500 copies/μL) versus the ompB assay (19.25 copies/μL).

Conclusions:

• qPCR targeting the sca2 gene offers improved sensitivity for detecting Rickettsia typhi, particularly in cases with low bacterial DNA concentrations.
• The sca2 assay represents a promising advancement for the diagnosis of murine typhus, potentially reducing diagnostic delays.