Serosurveillance for scrub typhus and murine typhus in forestry workers in South Korea

Lee Sunjin, Hwang Seon Do, Kang Byung Hak, Kim Jae Ok
Division of Bacterial Diseases, Center for Laboratory Control of Infectious Diseases, KCDC
Lee Kwan, Park Ji-Hyuk
Department of Preventive Medicine, Dongguk University College of Medicine

Scrub typhus is an acute febrile disease characterized by fever, rash, eschar, pneumonitis, and meningitis. It is caused by infection of Orientia tsutsugamushi, transmitted by trombiculid mites, and occurs frequently in the autumn in Korea. The incidence of the reported disease has increased gradually to 10,528 cases in Korea in 2017. Rickettsia typhi, a typhus group rickettsia, is the etiologic agent of murine typhus. Although murine typhus is characterized by a low mortality rate (1% of reported cases), in severe cases, it can result in meningoencephalitis and disseminated vascular lesions. Without specific treatment, 99% of those infected will be clear of the disease within weeks. In Korea, 18 cases were reported in 2017. This serologic study was conducted to obtain basic data regarding scrub typhus and murine typhus in high-risk populations.
This study included forestry workers, who are considered to be a high-risk population for scrub typhus and murine typhus. Sera were collected, and antibodies specific to O. tsutsugamushi and R. typhi were titrated using an indirect immunofluorescence antibody assay (IFA). Single cut-off titers of 1:256 for immunoglobulin(Ig)G or 1:16 for IgM were used as references for seroprevalence, and seroreactivity was determined as showing any titration to the single cut-off. The seropositivity and seroreactivity of scrub typhus were 22.2% (156/704) and 30.7% (216/704), respectively. The seropositivity of murine typhus was 21.3% (150/704). Moreover, the seropositivity of both scrub typhus and murine typhus was 5.7% (40/704).
This is the first serological study of scrub typhus and murine typhus in forestry workers, who are considered 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 determining the relationship between population density or infection rate of vectors and patients' prevalence.

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