The First Report outside the China: Seroprevalence of Severe Fever with Thrombocytopenia Syndrome in Healthy Individuals in Incheon, South Korea

Background: Severe fever with thrombocytopenia syndrome (SFTS) is a newly emerging infectious disease caused by a novel phlebovirus SFTS virus (SFTSV). The major clinical signs and symptoms are fever, nausea, anorexia, thrombocytopenia, leukopenia, lymphadenopathy and hemorrhagic tendency. SFTS was first reported in China, followed by South Korea and Japan. In South Korea, the first patient was reported in 2012 and 35 persons were diagnosed with SFTS in 2013. We conducted a sero-surveillance study of SFTSV of healthy people on Mui Island.

Methods: Mui Island, located in Incheon, western South Korea (126.3876–126.4472E, 37.3648-37.4128N), has an area of 10.27 km² and a population of 835 registered persons. The majority of residents are engaged in both agriculture and coastal fishery. Blood samples were collected during November 2014 from 203 healthy volunteers (female 127, male 76, 30 to 97 years old, median 67). The IgG and IgM antibody against SFTSV was detected using indirect immunofluorescence assay.

Results: We found 12 plasma samples (male 6, female 6) positive to IgG against SFTSV (1:32 to 1:1,024). Thus, the seroprevalence of SFTSV in the investigated population was 5.9% (12/203, 95% CI, 3.4%–10.1%). Seropositive rate was higher in age group >65 than that ≤65 (9.6% vs. 1.1%, OR, 9.2; 95% CI, 1.17–72.69; p=0.014). The difference of seropositive rate was not significant according to sex, outdoor activity, and duration of residence. We found one participant who was positive to both S and M segment of SFTSV.

Conclusion: This is the first report of SFTSV seroprevalence outside China. We found  a 5.9% (12/203, 95% CI, 3.4%–10.1%) seroprevalence of SFTSV among healthy humans on Mui Island, Incheon, South Korea, higher than that of healthy humans reported in China (0.8%-5.51%). Seroprevalence among older (>65 years) participants was significantly higher than that of younger. It is suggested that there are asymptomatic or mild cases of SFTS and reported mortality rates, ranging from 6.3% to 30.0%, have been exaggerated because none of seropositive participants had any typical symptom of SFTS. This study also suggests that SFTSV readily infects humans in rural areas in South Korea. Further serological study is needed in South Korea.