# Nasopharyngeal Swabs for Orientia tsutsugamushi Detection in Doxycycline Treatment: A Prospective Cohort Study in Hainan, China

## TL;DR

Nasopharyngeal swabs can detect scrub typhus bacteria shortly after starting doxycycline treatment, offering a noninvasive option for follow-up when blood testing becomes less reliable.

## Contribution

Nasopharyngeal swabs are proposed as a supplementary noninvasive specimen for short-term follow-up of scrub typhus during doxycycline treatment.

## Key findings

- Before treatment, 75% of blood samples and 25% of nasopharyngeal swabs tested positive for O. tsutsugamushi DNA.
- At 24 hours after doxycycline, blood samples were all negative, but 15% of nasopharyngeal swabs remained positive.
- Nasopharyngeal swab positives showed high genetic homology with blood samples, suggesting consistent strain detection.

## Abstract

Scrub typhus, caused by Orientia tsutsugamushi, remains a neglected cause of acute febrile illness. Molecular testing of blood supports early diagnosis, yet once doxycycline is started, blood qPCR positivity can drop rapidly, complicating short-term follow-up and relapse surveillance. We compared detection across multiple clinical specimens and evaluated nasopharyngeal swabs (NPSs) as noninvasive supplementary specimens during treatment initiation. In a prospective single-center cohort from Hainan, China, we enrolled 20 patients with scrub typhus. Blood, NPS, urine, and stool were collected before doxycycline administration 24 h after the first dose and on day 5. qPCR was performed for the analysis of Orientia tsutsugamushi. qPCR-positive specimens were subjected to nested PCR targeting TSA56, and nested PCR-positive amplicons were Sanger sequenced for genotyping. Before treatment, O. tsutsugamushi DNA was detected in 15/20 blood samples (75.00%) and 5/20 NPS samples (25.00%), but 0/20 urine samples (0%) and 0/20 stool samples (0%). At 24 h after treatment, detection in blood was 0/20 (0%) while NPS samples were positive in 3/20 (15.00%). All specimens were negative by day 5 after treatment. Across sequenced NPS positives (n = 3), Karp 2/3 (66.77%) and Gilliam 1/3 (33.33%) predominated. In paired blood–NPS positives, inter-specimen homology was high (percentage nucleotide identity 100% for Karp and 100% for Gilliam). NPS is not sensitive enough for primary diagnosis; however, within the first 24 h after doxycycline it offers a practical, noninvasive supplementary specimen to support short-term follow-up and community-based sampling when venipuncture or transport are constrained. Larger, multi-center studies are warranted to refine sampling windows and diagnostic performance.