Date Published: August 16, 2019
Publisher: Public Library of Science
Author(s): Mi-Seon Bang, Choon-Mee Kim, Jung Wook Park, Jae Keun Chung, Dong-Min Kim, Na Ra Yun, Kalimuthusamy Natarajaseenivasan.
This study investigated the prevalence of Orientia tsutsugamushi, Anaplasma phagocytophilum, and Leptospira interrogans in wild rodents through molecular detection using organ samples and through serological assay using blood samples of mice collected from two distinct sites in Gwangju Metropolitan City, Republic of Korea (ROK). A total of 47 wild rodents, identified as Apodemus agrarius (A. agrarius), were captured from June to August 2016. The seroprevalence of antibodies against bacterial pathogens in A. agrarius sera was analyzed; 17.4% (8/46) were identified as O. tsutsugamushi through indirect immunofluorescence assay and 2.2% (1/46) were identified as Leptospira species through passive hemagglutination assay. Using polymerase chain reaction, the spleen, kidney and blood samples were investigated for the presence of O. tsutsugamushi, A. phagocytophilum, and L. interrogans. Out of the 47 A. agrarius, 19.1% (9/47) were positive for A. phagocytophilum and 6.4% (3/47) were positive for L. interrogans, while none were positive for O. tsutsugamushi. Four out of 46 (8.7%) blood samples, six out of 45 (13.3%) spleen samples, and one out of 47 (2.1%) kidney samples were positive for A. phagocytophilum. Three out of 47 (6.4%) kidney samples were positive for L. interrogans. The sequencing results of PCR positive samples demonstrated > 99% similarity with A. phagocytophilum and L. interrogans sequences. A. phagocytophilum was mostly detected in the spleen, whereas L. interrogans was mostly detected in the kidneys. Notably, A. phagocytophilum and L. interrogans were detected in A. agrarius living in close proximity to humans in the metropolitan suburban areas. The results of this study indicate that rodent-borne bacteria may be present in wild rodents in the metropolitan suburban areas of ROK.
Rodents are known carriers of zoonotic pathogenic agents that are usually transmitted to humans through direct or indirect contact .
Scrub typhus, which is caused by O. tsutsugamushi, is transmitted to humans by the bites of chigger mites (Trombiculidae) . The incidence of scrub typhus is influenced by multiple factors, such as the behavior and population density of Trombiculidae and wild rodents. Human activity may also influence the infection rates [17, 18]. A. agrarius is a dominant wild rodent, which harbors chigger mites carrying O. tsutsugamushi . In this study, O. tsutsugamushi was not detected in 47 A. agrarius through PCR. Eight out of 46 (17.4%) sera were found to be seropositive through IFA with a cutoff titer of 1:16 for IgG. The gold standard for serological detection of scrub typhus is IFA  however, it lacks standardization and the use of variable cutoff titers has shown lab-wise variability in the results . The usage varied significantly across different geographical areas where scrub typhus is endemic and the cutoff ranged from 1:10 to 1:400 . Our IFA and PCR results did not corroborate because the time of wild rodents being infected was unclear. Kim, et al.  observed that IgG antibody titer increases abruptly over the first 2 weeks and reaches its peak at about 4 weeks in scrub typhus patients. The molecular techniques, such as PCR provide the highest sensitivity and specificity for the detection of scrub typhus, especially in the early period of infection, due to the specificity of primers and low detection limits [23–27]. Thus, although the antibody reaction was positive, molecular detection was not achieved in the positive mice. It may be preferable to perform both molecular and serological assays for precisely detecting the pathogens.