Severe Fever with Thrombocytopenia Syndrome, a Viral Hemorrhagic Fever, Endemic to Japan: Achievements in and Directions for Medical Research.

Pathophysiology of severe fever with thrombocytopenia syndrome and development of specific antiviral therapy.

Severe fever with thrombocytopenia syndrome: epidemiology, pathophysiology, and development of specific treatment and prevention measures

Severe fever with thrombocytopenia syndrome (SFTS) is a novel viral infectious disease with high case fatality rate (CFR). SFTS is caused by a novel phlebovirus, SFTS virus (SFTSV). SFTS, a tick-borne viral infection, was reported to be endemic to central and northeastern China and is also endemic to South Korea and western Japan. Patients with SFTS show symptoms of fever, general fatigue, and gastrointestinal symptoms such as diarrhea, whereas those severely ill with SFTS usually show gastrointestinal hemorrhage and deteriorated consciousness. The CFR of SFTS patients ranges from 5 to 40%. Because the time from the discovery of SFTS is short, pathological studies on SFTS remain limited. However, the existing pathological studies revealed that SFTSV replicated mainly in the lymphoreticular organs such as lymph nodes, spleen, and liver. Furthermore, SFTSV antigen-positive cells showed multi-organ infiltration, including to central nervous cells, kidneys, and lungs. Virus infection-related hemophagocytic syndrome associated with cytokine storm is also seen in most severe SFTS patients, suggesting that the mechanisms behind the high CFR may be multi-organ failure induced by hemophagocytic syndrome and coagulopathy due to disseminated intravascular coagulation causing bleeding tendency. To reduce the morbidity and mortality of SFTS, understanding of the pathophysiology of SFTS is required.

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease, caused by severe fever with thrombocytopenia syndrome virus (SFTSV), which is primarily transmitted via tick bites. Clusters of SFTS caused by human-to-human transmission have been reported both at home and abroad, mainly focused on the transmission or exposure modes. However, the correlation between SFTS clusters and viral genotypes has not been investigated. This study mainly reported two clusters of SFTS in Xinyang City, Henan Province, from 2022 to 2023, discussed the possible route of person-to-person transmission of SFTSV infection and analyzed the association between SFTS clusters and virus genotypes. We found that two groups of SFTSV in two clusters were clustered separately into different genotypes through viral sequence analysis of 4 confirmed patients. We also performed phylogenetic analysis, after including SFTSV sequences obtained from SFTS clusters deposited in the GenBank. Three SFTSV genotypes have been reported among cases of human-to-human transmission, suggesting that the occurrence of SFTS clusters may not be related to SFTSV genotypes. This study provided genetic evidence for revealing the chain of human-to-human transmission of SFTS clusters, indicating that contact with patients' blood is an important transmission route of SFTSV. The findings laid the foundation for preventing and controlling human-to-human transmission of SFTS.

Objective To detect the serum viral load, lymphocyte subsets, serum enzymes and blood cell counts of severe fever with thrombocytopenia syndrome (SFTS) patients, and to use logistic regression analysis and receiver operating characteristic (ROC) curve to establish a model to analyze the severity of SFTS. Methods A case-control study of 24 SFTS cases admitted between May 2011 and July 2012 was conducted at the First Affiliated Hospital of Nanjing Medical University. All SFTS cases were defined according to Fever with Thrombocytopenia Syndrome Prevention and Control Guidelines (2010 edition) issued by the Ministry of Health of the People′s Republic of China. According to their disease severity, the patients were divided into two groups, the non-severe group (16 cases) and the severe group (8 cases). In addition, 32 healthy volunteers were also enrolled in this study. Flow cytometry was adopted to detect the CD3+, CD4+, CD8+ lymphocytes and CD3-CD16+CD56+ natural killer cells (NK cells) in the peripheral blood of SFTS patients, and cytometric beads array (CBA) was used to detect Th1/Th2/Th17 cytokines. The serum viral load in patients with severe fever with thrombocytopenia syndrome virus (SFTSV) infection was detected by fluorescent quantitative PCR technology. Besides, white blood cells, platelets and serum enzymes were measured. The multi-index conjunctive model of logistic regression analysis and receiver operating characteristics (ROC) curve were used to analyze the predictive values of indexes on severity of SFTS. Results The ROC analysis of single index found that SFTSV RNA, CD3, CD4, CD8, CD56, AST, LDH, CK, IL-6 and IL-10 have good predictability on severity of SFTS in the early course of the disease; the area under the curve (AUC) were 0.83, 0.84, 0.90, 0.75, 0.94, 0.73, 0.78, 0.87, 0.74 and 0.77 respectively, and the cut-off values of the SFTSV load, CD3+, CD4+, NK cells and CK were 6.19 log10 copies/ml, 57.51%, 19.47%, 15.71% and 696.45 U/L respectively. Using the step-by-step method logistic regression analysis to build a model and analyze the ROC curve of prediction probability, it was found that the predictability of joint indexes of SFTSV RNA/CD3/CD4/CD8/CD56, SFTSV RNA/AST/lactate dehydrogenase (LDH) /CK, or SFTSV RNA/IL-6/IL-10 increases in a certain degree. The areas under the ROC curve were 0.95 (95%CI: 0.00-1.00), 0.87 (95%CI: 0.75-0.99), and 0.83 (95%CI: 0.70-0.97), the sensitivities were 93.3%, 86.70% and 77.30%, and the specificities were 94.4%, 83.30% and 83.30% respectively. Conclusions The level of serum SFTSV is highly related to the severity of the disease. The high levels of serum SFTSV load, LDH, CK, IL-6, and IL-10, together with significant reduction of CD3+cells and CD4+ cells, may indicate poor prognosis of the SFTS patients. Based on the logistic regression model by multiple indexes, the severity of SFTS can be better predicted. (Chin J Lab Med, 2015, 38: 49-54) Key words: Runyaviridae infections; Killer cells, natural; Cytokines; Viral load; Severity of illness index; ROC curve

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging hemorrhagic fever with a high case fatality rate caused by SFTS virus (SFTSV). Effective vaccines and specific therapies for SFTS are urgently sought, and investigation into virus-host cell interactions is expected to contribute to the development of antiviral strategies. In this study, we have developed a pseudotype vesicular stomatitis virus (VSV) bearing the unmodified Gn/Gc glycoproteins (GPs) of SFTSV (SFTSVpv). We have analyzed the host cell entry of this pseudotype virus and native SFTSV. Both SFTSVpv and SFTSV exhibited high infectivity in various mammalian cell lines. The use of lysosomotropic agents indicated that virus entry occurred via pH-dependent endocytosis. SFTSVpv and SFTSV infectivity was neutralized by serial dilutions of convalescent-phase patient sera. Entry of SFTSVpv and growth of SFTSV were increased in Raji cells expressing not only the C-type lectin dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN) but also DC-SIGN-related (DC-SIGNR) and liver and lymph node sinusoidal endothelial cell C-type lectin (LSECtin). 25-Hydroxycholesterol (25HC), a soluble oxysterol metabolite, inhibited the cell entry of SFTSVpv and the membrane fusion of SFTSV. These results indicate that pH-dependent endocytosis of SFTSVpv and SFTSV is enhanced by attachment to certain C-type lectins. SFTSVpv is an appropriate model for the investigation of SFTSV-GP-mediated cell entry and virus neutralization at lower biosafety levels. Furthermore, 25HC may represent a potential antiviral agent against SFTS. SFTSV is a recently discovered bunyavirus associated with SFTS, a viral hemorrhagic fever with a high case fatality rate endemic to China, South Korea, and Japan. Because little is known about the characteristics of the envelope protein and entry mechanisms of SFTSV, further studies will be required for the development of a vaccine or effective therapies. In this study, we investigated the mechanism of SFTSV cell entry using SFTSVpv and the native virus. SFTSV can grow in nonsusceptible cell lines in the presence of certain C-type lectins. Moreover, 25HC, an oxysterol metabolite, may represent a potential therapeutic inhibitor of SFTSV infection.

Severe fever with thrombocytopenia syndrome (SFTS), caused by Dabie bandavirus, generally called SFTS virus (SFTSV), is an emerging zoonosis in East Asia. In Japan, 50-100 cases of SFTS have been reported each year since the first case was reported in 2013. SFTS is a tick-borne infectious disease, and SFTSV has been isolated from ticks in China and South Korea. Haemaphysalis longicornis and Amblyomma testudinarium are considered the primary vectors in Japan. However, the other tick species seldom feeding on humans might also play an important role in maintaining the virus in nature. In this study, we collected ticks on vegetation around the location where two SFTS patients were estimated to have been infected in Miyazaki Prefecture, Japan, isolated live SFTSV, and performed a phylogenetic analysis. A total of 257 ticks were collected, and SFTSV RNA was detected in 19.5% (9/46) of tick pools. A total of 10 infectious SFTSVs were successfully isolated from A. testudinarium, Haemaphysalis flava, Haemaphysalis formosensis, Haemaphysalis hystricis, and Haemaphysalis megaspinosa. Furthermore, the whole viral sequences isolated from ticks were highly homologous to sequences isolated from SFTS patients in the same sampling area in the past. These results suggest that SFTSVs are maintained in these tick species in the sampling area and sporadically transmitted to humans. Surveillance of SFTSV in ticks provides important information about the risk of incidental transmission to humans.

Seven years have passed since the discovery of a novel infectious disease, severe fever with thrombocytopenia syndrome (SFTS) caused by a novel Phlebovirus, SFTS virus (SFTSV), in PR China. It was also confirmed that SFTS was endemic to Japan through an identification of a woman, who died of SFTSV infection in Yamaguchi prefecture in late 2012. Approximately 6 years have passed since the discovery of SFTS-endemicity in Japan. At present, SFTS is endemic to PR China, South Korea and western Japan. SFTSV is maintained between several species of ticks such as Haemaphysalis longicornis and wild and domestic animals in nature. Therefore, we cannot escape from the risk of being infected with SFTSV. Based on the similarity in the characteristics of the clinical symptoms including the high case fatality rate, mode of infection to humans, pathology and virology between SFTS and Crimean-Congo hemorrhagic fever (CCHF), SFTS should be classified as viral hemorrhagic fever. Although the time from the discovery of SFTS is still short, there have been many scientific reports on the epidemiological, clinical, and/or pathological, and virological studies on SFTS. Favipiravir was reported to show an efficacy in the prevention and treatment of SFTSV infections in an animal model. A clinical study to evaluate the efficacy of favipiravir in the treatment of SFTS patients has been initiated in Japan. Specific and effective treatment with antiviral drugs for and preventive measures of SFTS with vaccination shoued be developed through scientific, clinical, and basic research.

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease caused by a new virus (SFTS virus) reported to be endemic to central and northeastern parts of China. SFTS virus, which is classified into the genus Phlebovirus (the Bunyaviridae family), is suspected to be a tick-borne virus owing to evidence in two species of ticks: Haemaphysalis longicornis and Rhipicephalus microplus. SFTS virus is detected among many species of domestic animals in China. The clinical symptoms of SFTS include fever, thrombocytopenia, leucocytopenia, gastrointestinal symptoms, neural symptoms, bleeding tendency. The fatality rate of SFTS is 6-30%. Person-to-person transmission of SFTS virus is possible through blood contact. Clinical and epidemiological studies of SFTS, the cases of SFTS outside China, person-to-person transmission of SFTS virus, evolutionary and molecular analysis of the emergent SFTS virus, and risk assessment of human infection with a novel phlebovirus are considered in this review.

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a lethal threat. Increasing Severe fever with thrombocytopenia syndrome (SFTS) risk in Asia and the United States stems from the spread of natural host, Haemaphysalis longicornis. Rapid and accurate SFTSV molecular diagnosis is crucial for treatment decisions, reducing fatality risk. Blood samples from 17 suspected SFTS patients at Chuncheon Sacred Heart Hospital (September-December 2022) were collected. SFTSV was diagnosed using two reverse transcription-quantitative polymerase chain reaction (RT-qPCR) assays from Gangwon Institute of Health and Environment (RT-qPCR/GIHE) and Asan Medical Center (RT-qPCR/AMC). To address RT-qPCR disparities, amplicon-based MinION sequencing traced SFTSV genomic sequences in clinical samples. In two samples (N39 and N50), SFTSV was detected in both RT-qPCR/GIHE and RT-qPCR/AMC. Among 11 samples, RT-qPCR/AMC exclusively detected SFTSV. In four samples, both assays yielded negative results. Amplicon-based MinION sequencing enabled nearly whole-genome sequencing of SFTSV in samples N39 and N50. Among 11 discordant samples, five contained significant SFTSV reads, aligning with the RT-qPCR/AMC findings. However, another six samples showed insufficient viral reads in accordance with the negativity observed in RT-qPCR/GIHE. The phylogenetic pattern of SFTSV demonstrated N39 formed a genetic lineage with genotype A in all segments. SFTSV N50 grouped with the B-1 sub-genotype for L segment and B-2 sub-genotype for the M and S segments, indicating genetic reassortment. The study demonstrates the robust sensitivity of amplicon-based MinION sequencing for the direct detection of SFTSV in clinical samples containing ultralow copies of viral genomes. Next-generation sequencing holds potential in resolving SFTSV diagnosis discrepancies, enhancing understanding of diagnostic capacity, and risk assessment for emerging SFTSV.

Severe fever with thrombocytopenia syndrome (SFTS) caused by SFTS virus (SFTSV), a novel phlebovirus, was reported to be endemic to central and northeastern China, South Korea, and Japan. SFTS is a tick-borne viral infection with high morbidity and mortality. SFTSV is circulating in nature in the SFTS-endemic areas. SFTSV is maintained in nature between animals and some species of ticks. Humans are usually infected with SFTSV by virus-positive ticks, especially Haemaphysalis longicornis and Amblyomma testudinarium. The mechanism of SFTSV maintenance in nature is crucial to control SFTSV infections in humans. Antibody positive rates to SFTSV in some species of animals such as goats, deer, old aged bovine, and wild boar were reported to be relatively high. Virus genomes were detected in some species of ticks such as H. longicornis, H. flava, H. concinna, A. testudinarium, and Ixodes nipponensis. Animal species, which were studied for SFTSV antibody and SFTSV genome prevalence, are still limited. Based on the data reported so far suggest that SFTSV is maintained in nature through intensive transmission between animals such as goats, deer, and wild boars and Haemaphysalis, Ambryomma, and Ixodes ticks. Further studies are needed to clarify the SFTSV maintenance mechanism in nature in detail.

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a novel emerging virus that has been identified in China, South Korea, and Japan, and it induces thrombocytopenia and leukocytopenia in humans with a high case fatality rate. SFTSV is pathogenic to humans, while immunocompetent adult mice and golden Syrian hamsters infected with SFTSV never show apparent symptoms. However, mice deficient for the gene encoding the α chain of the alpha- and beta-interferon receptor (Ifnar1-/- mice) and golden Syrian hamsters deficient for the gene encoding signal transducer and activator of transcription 2 (Stat2-/- hamsters) are highly susceptible to SFTSV infection, with infection resulting in death. The nonstructural protein (NSs) of SFTSV has been reported to inhibit the type I IFN response through sequestration of human STAT proteins. Here, we demonstrated that SFTSV induces lethal acute disease in STAT2-deficient mice but not in STAT1-deficient mice. Furthermore, we discovered that NSs cannot inhibit type I IFN signaling in murine cells due to an inability to bind to murine STAT2. Taken together, our results imply that the dysfunction of NSs in antagonizing murine STAT2 can lead to inefficient replication and the loss of pathogenesis of SFTSV in mice.IMPORTANCE Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease caused by SFTSV, which has been reported in China, South Korea, and Japan. Here, we revealed that mice lacking STAT2, which is an important factor for antiviral innate immunity, are highly susceptible to SFTSV infection. We also show that SFTSV NSs cannot exert its anti-innate immunity activity in mice due to the inability of the protein to bind to murine STAT2. Our findings suggest that the dysfunction of SFTSV NSs as an IFN antagonist in murine cells confers a loss of pathogenicity of SFTSV in mice.

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging viral infectious disease caused by a novel Bandavirus in the family Phenuiviridae. The SFTS virus (SFTSV) is transmitted to various hosts, including humans, through tick bites, leading to high fever, thrombocytopenia, and leukopenia, with a high case fatality rate (up to 30%) due to multiple organ dysfunction. Therefore, early diagnosis is crucial for effective treatment and preventing disease transmission. In this study, we aimed to develop and characterize monoclonal antibodies targeting the SFTSV nucleocapsid protein (NP). We generated recombinant NP to screen antibodies against SFTSV. Using phage display technology, we identified candidate single-chain variable fragment (scFv) sequences capable of detecting SFTSV NP. Five human IgG antibodies and six chimeric mouse antibodies exhibited strong binding ability to the recombinant NP. Furthermore, their specificity and selectivity were evaluated against NPs from different subtypes and other viral species. A sandwich enzyme–linked immunosorbent assay (ELISA) was performed to determine optimal antibody pairings for SFTSV detection. The mP01A05/hP01C09, mP01A05/hP01B10, and mP02E04/hP01A05 antibody pairs demonstrated high efficacy in diagnosing SFTSV infections. These findings provide valuable antibody resources and establish an effective platform for the diagnosis of SFTS.Key points• Monoclonal antibodies targeting SFTSV NP were developed using phage display technology.• Candidate antibodies showed strong binding ability and high specificity to SFTSV NP.• Optimized antibody pairs enabled effective SFTSV detection via sandwich ELISA.

Severe fever with thrombocytopenia syndrome (SFTS) caused by SFTS virus (SFTSV), a novel phlebovirus, was reported to be endemic to central and northeastern China and is also endemic to South Korea and Japan. SFTS is a tick-borne viral infection with high morbidity and mortality. SFTSV is circulating in nature. Therefore, peoples living in the endemic regions face the risk of SFTSV infection, requiring us to study the virology of SFTSV infection and develop specific treatments and preventive measures against SFTS. There has been advancement in animal experimental models’ development for the study on SFTSV infections. So far, interferon alpha-receptor knockout (IFNAR-KO) mice and signal transducer and activator of transcription 2-knockout hamsters are confirmed to be lethal, when infected with SFTSV at relatively high infectious dose. Furthermore, the efficacy of some drugs including favipiravir and VSV-based SFTSV vaccine candidate in treatment and protection in these animal models, respectively, was evaluated. Further studies are necessary to better understand the characteristics of SFTSV and SFTSV infection and development of treatment and preventive measures against SFTS, necessitating further studies on the animal experimental models’ development.

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging tick-borne infectious disease caused by SFTS virus (SFTSV), which is a novel bunyavirus. SFTSV was first isolated from patients who presented with fever, thrombocytopenia, leukocytopenia, and multiorgan dysfunction in China. Subsequently, it was found to be widely distributed in Southeast Asia (Korea, Japan, and Vietnam). SFTSV can be transmitted not only from ticks but also from domestic animals, companion animals, and humans. Because the case fatality rate of SFTS is high (6–30%), development of specific and effective treatment for SFTS is required. Studies of potential antiviral drugs for SFTS-specific therapy have been conducted on existing or newly discovered agents in vitro and in vivo, with ribavirin and favipiravir being the most promising candidates. While animal experiments and retrospective studies have demonstrated the limited efficacy of ribavirin, it was also speculated that ribavirin would be effective in patients with a viral load <1 × 106 copies/mL. Favipiravir showed higher efficacy than ribavirin against SFTSV in in vitro assays and greater efficacy in animal models, even administrated 3 days after the virus inoculation. Although clinical trials evaluating the efficacy of favipiravir in SFTS patients in Japan are underway, this has yet to be confirmed. Other drugs, including hexachlorophene, calcium channel blockers, 2′-fluoro-2′-deoxycytidine, caffeic acid, amodiaquine, and interferons, have also been evaluated for their inhibitory efficacy against SFTSV. Among them, calcium channel blockers are promising because in addition to their efficacy in vitro and in vivo, retrospective clinical data have indicated that nifedipine, one of the calcium channel blockers, reduced the case fatality rate by >5-fold. Although further research is necessary to develop SFTS-specific therapy, considerable progress has been achieved in this area. Here we summarize and discuss recent advances in antiviral drugs against SFTSV.