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Severe fever with thrombocytopenia syndrome (SFTS) was first identified as an emerging tick-borne infectious disease caused by the SFTS virus (SFTSV) in China and has also been found to be endemic to Japan and South Korea, indicating that SFTS is of great concern in East Asia. The aim of the present study was to determine the seroprevalence of SFTSV antibodies in humans and animals in SFTS-endemic regions of Japan. One of 694 (0.14%) healthy persons over 50 years of age and 20 of 107 (18.7%) wild and domestic animals in Ehime prefecture of western Japan were determined to be seropositive for SFTSV antibodies by virus neutralization test and ELISA, respectively. The seropositive person, a healthy 74-year-old woman, was a resident of the southwest part of Ehime prefecture engaged in citriculture and field work. This woman's sample exhibited neutralizing activity against SFTSV although she had neither a clear experience with tick bites nor SFTS-like clinical illness. These findings indicate that most people living in the endemic regions are not infected with SFTSV and suggest that most of the SFTS patients reported so far do not reflect the tip of an iceberg of people infected with SFTSV, but at the same time, that SFTSV infection does not always induce severe SFTS-associated symptoms. These findings also suggested that SFTSV has been maintained in nature within animal species and ticks., (Copyright © 2018 Japanese Society of Chemotherapy and The Japanese Association for Infectious Diseases. Published by Elsevier Ltd. All rights reserved.)

Background: Severe fever with thrombocytopenia syndrome (SFTS) is a tick-borne acute infectious disease caused by the SFTS virus (SFTSV). SFTS has been reported in China, South Korea, and Japan as a novel Bunyavirus. Although several molecular epidemiology and phylogenetic studies have been performed, the information obtained was limited, because the analyses included no or only a small number of SFTSV strains from Japan., Methods: The nucleotide sequences of 75 SFTSV samples in Japan were newly determined directly from the patients' serum samples. In addition, the sequences of 7 strains isolated in vitro were determined and compared with those in the patients' serum samples. More than 90 strains that were identified in China, 1 strain in South Korea, and 50 strains in Japan were phylogenetically analyzed., Results: The viruses were clustered into 2 clades, which were consistent with the geographic distribution. Three strains identified in Japan were clustered in the Chinese clade, and 4 strains identified in China and 26 in South Korea were clustered in the Japanese clade., Conclusions: Two clades of SFTSV may have evolved separately over time. On rare occasions, the viruses were transmitted overseas to the region in which viruses of the other clade were prevalent., (© The Author 2015. Published by Oxford University Press on behalf of the Infectious Diseases Society of America. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com.)

Severe fever with thrombocytopenia syndrome (SFTS) is characterized by sudden onset of fever, leukopenia, thrombocytopenia, and gastrointestinal tract symptoms and approximately 12% of patients die from disseminated intravascular coagulation and/or multiple organ failures. Agent of the disease is a novel bunyavirus SFTS virus, and is transmitted by bite of a possible vector tick, Haemaphysalis longicornis, and through direct contact with virus-containing patient body fluids, or through unknown routes. SFTS case reports have been limited in China, and more than two thousand cases were reported in 2011 and 2012. In late 2012, a woman living in Yamaguchi prefecture in Japan showed symptoms reminiscent of those of SFTS and died 6 days after the onset of symptoms. Virus was isolated from her acute serum in Vero cells and a next generation-sequencing identified it as SFTS virus. SFTS viral genome and proteins were detected in the patient's serum. Based on the first demonstration of SFTS in Japan, a retrospective study started. Until March of 2013, totally 8 patients were diagnosed as having SFTS and the most early case was in 2005. Phylogenetic analysis of virus sequences revealed that Japanese isolates form an independent branch distinct from Chinese isolates, indicating that SFTS has been present not only in China but also in Japan.