Serological study on scrub typhus and murine typhus among field workers of national forest in South Korea, 2016

Gill Byoungchul, Kang Byung Hak
Division of Bacterial Diseases, Center for Laboratory control of Infectious Diseases, KCDC
Lee Jeongmin
Division of Research Planning, KNIH, KCDC
Lee Yeong Seon
Division of Bacterial Diseases Research, Center for Infectious Diseases Research, KNIH, KCDC
Lim Hyun-Sul, Pack Ji-Hyuk
Department of Preventive Medicine, Dongguk University College of Medicine

Background: Scrub typhus is an acute febrile disease characterized by fever, rash, eschar, pneumonitis, and meningitis. It is caused by infection with Orientia tsutsugamushi, transmitted by trombiculid mites and occurs frequently in the autumn in Korea. The incidence of the disease has increased gradually to 11,105 cases reported in Korea in 2016. Rickettsia typhi, a typhus group rickettsia, is the etiologic agent of murine typhus. Although characterized by a low mortality rate is low (1% of reported cases), in severe cases, murine typhus can result in meningoencephalitis, disseminated vascular lesions. Without specific treatment, 99% of those infected will clear the disease within weeks. In Korea, 18 cases were reported in 2016. This serologic study was conducted to obtain basic data on scrub typhus and murine typhus for high-risk populations.
Methods/Results: This study included National Park Service workers, considered to be a high-risk population for scrub typhus and murine typhus. The eligible participants were 763 (36.8%) out of 2,074 forestry workers. Sera were collected and antibodies specific to O. tsutsugamushi and R. typhi were titrated using indirect immunofluorescence antibody assay (IFA). Single cut-off titers of 1:256 for IgG or 1:16 for IgM were used as references for seroprevalence, and seroreactivity showing any titration to the single cut-off. The seropositivity and seroreactivity of scrub typhus were 4.8% (37/763) and 10.0% (76/763), respectively while murine typhus yielded showed 3.7% (28/763) and 8.9% (68/763), respectively. Moreover, seropositivity of both scrub typhus and murine typhus was 0.5% (4/763).
Conclusion: This is the first serological study of scrub typhus and murine typhus among National Park Service workers as a high-risk population for vector-borne diseases. Further analyses of this population for other vector-borne diseases are also needed to identify the risk-status and to improve the control and prevention of these diseases in high-risk populations. Additional studies or investigations are also needed for figuring out the relationship between population density or infection rate of vectors and patients prevalence.

Keywords: Serological study, Scrub typhus, Murine typhus, Orientia tsutsugamushi, Forestry workers