A national assessment of the epidemiology of severe fever with thrombocytopenia syndrome, China

Tick-borne viruses: Epidemiology, pathogenesis, and animal models

Tick-borne viruses, capable of infecting animals and humans, are expanding geographically and increasing in prevalence, posing significant global public health threats. This review explores the current epidemiology of human pathogenic tick-borne viruses, emphasizing their diversity and the spectrum of symptomatic manifestations in humans, which range from mild to severe. We highlight how the infrequent and unpredictable nature of viral outbreaks complicates the precise identification and understanding of these viruses in human infections. Furthermore, we describe the utility of animal models that accurately mimic human clinical symptoms, facilitating the development of effective control strategies. Our comprehensive analysis provides crucial insights into disease progression and emphasizes the urgent need for continued research. This work aims to provide insight into knowledge gaps to mitigate the health burden of tick-borne infections and open an avenue for further study to enhance our understanding of these emerging infectious diseases.

Prevalence, Isolation, and Molecular Characterization of Severe Fever with Thrombocytopenia Syndrome Virus in Cattle from the Republic of Korea

Background: Severe fever with thrombocytopenia syndrome (SFTS) is an emerging tick-borne disease caused by Bandavirus dabieense. Initially identified in China, this disease has spread throughout Asian countries via tick bites and animal-to-human transmission. However, reports of the prevalence of SFTS virus (SFTSV) in cattle in Korea are lacking. This study aimed to investigate SFTSV infections in grazing cattle in the Republic of Korea (ROK). Materials and Methods: In total, 845 grazing cattle serum samples were collected over 2 years (2019 and 2020) in the ROK, and viral RNA was extracted using a kit. One-step RT-nested PCR was performed to amplify the S-segment of SFTSV. Positive serum samples were used to isolate SFTSV in Vero E6 cells, and the full sequences were analyzed. A phylogenetic tree was constructed using the maximum-likelihood method with MEGA X. In addition, immunoglobulin G antibodies against SFTSV were investigated using an enzyme-linked immunosorbent assay. Results: Here, 4.0% of serum samples (34/845) were positive for SFTSV S-segments, and one virus isolate was cultured in Vero E6 cells. Phylogenetic analysis based on the partial S-segment classified 4 SFTSV isolates as the B-2 genotype, 9 as the B-3 genotype, 18 as an unclassified B genotype, and 3 as the D genotype. One cultured virus was classified as the B-2 genotype based on SFTSV L-, M-, and S-segments. Antibody detection results showed that 21.1% of serum samples (161/763) were positive for SFTSV. Conclusion: To the best of our knowledge, this is the first study performed to identify the prevalence of SFTSV in grazing cattle in the ROK. Our findings indicate the necessity for more intensive and continuous SFTSV monitoring, not only in cattle but also in other animals, to comprehend the genetic diversity of the virus and its potential eco-epidemiological impact on human health.

Spatial and Temporal Analysis of Severe Fever with Thrombocytopenia Syndrome in Anhui Province from 2011 to 2023

OBJECTIVE

To analyze the spatial autocorrelation and spatiotemporal clustering characteristics of severe fever with thrombocytopenia syndrome(SFTS) in Anhui Province from 2011 to 2023.

METHODS

Data of SFTS in Anhui Province from 2011 to 2023 were collected. Spatial autocorrelation analysis was conducted using GeoDa software, while spatiotemporal scanning was performed using SaTScan 10.0.1 software to identify significant spatiotemporal clusters of SFTS.

RESULTS

From 2011 to 2023, 5720 SFTS cases were reported in Anhui Province, with an average annual incidence rate of 0.7131/100,000. The incidence of SFTS in Anhui Province reached its peak mainly from April to May, with a small peak in October. The spatial autocorrelation results showed that from 2011 to 2023, there was a spatial positive correlation(P < 0.05) in the incidence of SFTS in all counties and districts of Anhui Province. Local autocorrelation high-high clustering areas are mainly located in the south of the Huaihe River. The spatiotemporal scanning results show three main clusters of SFTS in recent years: the first cluster located in the lower reaches of the Yangtze River, the eastern region of Anhui Province; the second cluster primarily focused on the region of the Dabie Mountain range, while the third cluster primarily focused on the region of the Huang Mountain range.

CONCLUSIONS

The incidence of SFTS in Anhui Province in 2011-2023 was spatially clustered.

Antiviral Activity of Selective Estrogen Receptor Modulators against Severe Fever with Thrombocytopenia Syndrome Virus In Vitro and In Vivo

Severe fever with thrombocytopenia syndrome virus (SFTSV), also known as the Dabie Banda virus, is an emerging tick-borne Bunyavirus that causes severe fever with thrombocytopenia syndrome (SFTS). Currently, symptomatic treatment and antiviral therapy with ribavirin and favipiravir are used in clinical management. However, their therapeutical efficacy is hardly satisfactory in patients with high viral load. In this study, we explored the antiviral effects of selective estrogen receptor modulators (SERMs) on SFTSV infection and the antiviral mechanisms of a representative SERM, bazedoxifene acetate (BZA). Our data show that SERMs potently inhibited SFTSV-induced cytopathic effect (CPE), the proliferation of infectious viral particles, and viral RNA replication and that BZA effectively protected mice from lethal viral challenge. The mode of action analysis reveals that BZA exerts antiviral effects during the post-entry stage of SFTSV infection. The transcriptome analysis reveals that GRASLND and CYP1A1 were upregulated, while TMEM45B and TXNIP were downregulated. Our findings suggest that SERMs have the potential to be used in the treatment of SFTSV infection.

Comparison of epidemiological characteristics between hemorrhagic fever with renal syndrome patients and severe fever with thrombocytopenia syndrome patients

Hemorrhagic fever with renal syndrome (HFRS) and severe fever with thrombocytopenia syndrome (SFTS) are both endemic in rural areas and some characteristics are similar between HFRS and SFTS, which usually lead to misdiagnosis. In this study, we summarized and compared some characteristics of HFRS and SFTS which will provide scientific information for differential diagnosis. From 2011 to 2022, a total of 4336 HFRS cases and 737 SFTS cases were reported in Zhejiang Province. Compared to SFTS, there was a higher proportion of males among HFRS cases (72.46% [3142/4336] vs. 50.88% [375/737], p = 0.000). The median age of all 4336 HFRS cases was 49 (39, 59), while the median age of SFTS cases was 66 (57, 74). In addition, the involved counties of HFRS were more than SFTS, but the number of counties affected by SFTS increased from 2011 to 2022. The majority of SFTS cases occurred in summer (from May to July), but besides summer, HFRS cases also showed a peak in winter. Finally, our results showed that the case fatality rate of SFTS was significantly higher than that of HFRS. Although there were some similarities between HFRS and SFTS, our study found several differences between them, such as gender distribution, age distribution, and seasonal distribution, which will provide scientific information for differential diagnosis of HFRS and SFTS. Further studies should be carried out to explore the mechanism of these differences.

Seroprevalence for severe fever with thrombocytopenia syndrome virus among the residents of Miyazaki, Japan: An epidemiological study

INTRODUCTION

Severe fever with thrombocytopenia syndrome (SFTS) is a tick-borne infectious disease caused by the SFTS virus (SFTSV). The Miyazaki Prefecture has the highest number of SFTS cases in Japan and requires countermeasures for prevention. In this study, we aimed to conduct an epidemiological survey in Miyazaki Prefecture to determine the exposure conditions of SFTSV by measuring the seroprevalence among residents of Miyazaki and to evaluate the factors that influence the endemicity of SFTS.

METHODS

The survey was conducted between June 2014 and April 2019 in all 26 municipalities in Miyazaki Prefecture. SFTSV antibodies were detected using an enzyme-linked immunosorbent assay in the blood samples of 6013 residents (3184 men and 2829 women). A questionnaire-based survey of the living environment was also conducted.

RESULTS

Multiple logistic regression analysis revealed that age and occupation were significant factors related to the proportion of participants with an optical density (OD) value > 0.2 and a seroprevalence of 0.9 % (54/6013). Seven seropositive individuals (0.1 %) with an OD value of >0.4 were identified (three men and four women, aged 54-69 years), and all were asymptomatic. One participant had a higher OD than the positive control.

CONCLUSION

Although SFTS is endemic in Miyazaki Prefecture, Japan, its seroprevalence is relatively low. Since some risk areas in Miyazaki prefecture have been identified, it is important to enhance awareness of SFTS in residences and reduce contact with ticks, especially in high-risk areas.

Time series models in prediction of severe fever with thrombocytopenia syndrome cases in Shandong province, China

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease caused by the SFTS virus (SFTSV). Predicting the incidence of this disease in advance is crucial for policymakers to develop prevention and control strategies. In this study, we utilized historical incidence data of SFTS (2013-2020) in Shandong Province, China to establish three univariate prediction models based on two time-series forecasting algorithms Autoregressive Integrated Moving Average (ARIMA) and Prophet, as well as a special type of recurrent neural network Long Short-Term Memory (LSTM) algorithm. We then evaluated and compared the performance of these models. All three models demonstrated good predictive capabilities for SFTS cases, with the predicted results closely aligning with the actual cases. Among the models, the LSTM model exhibited the best fitting and prediction performance. It achieved the lowest values for mean absolute error (MAE), mean square error (MSE), and root mean square error (RMSE). The number of SFTS cases in the subsequent 5 years in this area were also generated using this model. The LSTM model, being simple and practical, provides valuable information and data for assessing the potential risk of SFTS in advance. This information is crucial for the development of early warning systems and the formulation of effective prevention and control measures for SFTS.

Projecting spatiotemporal dynamics of severe fever with thrombocytopenia syndrome in the mainland of China

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease with increasing incidence and geographic extent. The extent to which global climate change affects the incidence of SFTS disease remains obscure. We use an integrated multi-model, multi-scenario framework to assess the impact of global climate change on SFTS disease in China. The spatial distribution of habitat suitability for the tick Haemaphysalis longicornis was predicted by applying a boosted regression tree model under four alternative climate change scenarios (RCP2.6, RCP4.5, RCP6.0, and RCP8.5) for the periods 2030-2039, 2050-2059, and 2080-2089. We incorporate the SFTS cases in the mainland of China from 2010 to 2019 with environmental variables and the projected distribution of H. longicornis into a generalized additive model to explore the current and future spatiotemporal dynamics of SFTS. Our results demonstrate an expanded geographic distribution of H. longicornis toward Northern and Northwestern China, showing a more pronounced change under the RCP8.5 scenario. In contrast, the environmental suitability of H. longicornis is predicted to be reduced in Central and Eastern China. The SFTS incidence in three time periods (2030-2039, 2050-2059, and 2080-2089) is predicted to be increased as compared to the 2010s in the context of various RCPs. A heterogeneous trend across provinces, however, was observed, when an increased incidence in Liaoning and Shandong provinces, while decreased incidence in Henan province is predicted. Notably, we predict possible outbreaks in Xinjiang and Yunnan in the future, where only sporadic cases have been reported previously. These findings highlight the need for tick control and population awareness of SFTS in endemic regions, and enhanced monitoring in potential risk areas.

Multiple piroplasm parasites (Apicomplexa: Piroplasmida) in northeastern populations of the invasive Asian longhorned tick, Haemaphysalis longicornis Neumann (Ixodida: Ixodidae), in the United States

Piroplasms, which include the agents of cattle fever and human and dog babesiosis, are a diverse group of blood parasites of significant veterinary and medical importance. The invasive Asian longhorned tick, Haemaphysalis longicornis, is a known vector of piroplasms in its native range in East Asia and invasive range in Australasia. In the USA, H. longicornis has been associated with Theileria orientalis Ikeda outbreaks that caused cattle mortality. To survey invasive populations of H. longicornis for a broad range of piroplasms, 667 questing H. longicornis collected in 2021 from 3 sites in New Jersey, USA, were tested with generalist piroplasm primers targeting the 18S small subunit rRNA (395–515 bp, depending on species) and the cytochrome b oxidase loci (1009 bp). Sequences matching Theileria cervi type F (1 adult, 5 nymphs), an unidentified Theileria species (in 1 nymph), an undescribed Babesia sensu stricto (‘true’ Babesia, 2 adults, 2 nymphs), a Babesia sp. Coco (also a ‘true Babesia’, 1 adult, 1 nymph), as well as Babesia microti S837 (1 adult, 4 nymphs) were recovered. Babesia microti S837 is closely related to the human pathogen B. microti US-type. Additionally, a 132 bp sequence matching the cytochrome b locus of deer, Odocoileus virginanus, was obtained from 2 partially engorged H. longicornis. The diverse assemblage of piroplasms now associated with H. longicornis in the USA spans 3 clades in the piroplasm phylogeny and raises concerns of transmission amplification of veterinary pathogens as well as spillover of pathogens from wildlife to humans.

Genetic variants of Dabie bandavirus: classification and biological/clinical implications

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease caused by Dabie bandavirus (DBV), a novel Bandavirus in the family Phenuiviridae. The first case of SFTS was reported in China, followed by cases in Japan, South Korea, Taiwan and Vietnam. With clinical manifestations including fever, leukopenia, thrombocytopenia, and gastrointestinal symptoms, SFTS has a fatality rate of approximately 10%. In recent years, an increasing number of viral strains have been isolated and sequenced, and several research groups have attempted to classify the different genotypes of DBV. Additionally, accumulating evidence indicates certain correlations between the genetic makeup and biological/clinical manifestations of the virus. Here, we attempted to evaluate the genetic classification of different groups, align the genotypic nomenclature in different studies, summarize the distribution of different genotypes, and review the biological and clinical implications of DBV genetic variations.

Epidemiological and spatiotemporal analysis of severe fever with thrombocytopenia syndrome in Eastern China, 2011-2021

BACKGROUND

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease, which is caused by severe fever with thrombocytopenia syndrome virus (SFTSV) with high fatality. Recently, the incidence of SFTS increased obviously in Jiangsu Province. However, the systematic and complete analysis of spatiotemporal patterns and clusters coupled with epidemiological characteristics of SFTS have not been reported so far.

METHODS

Data on SFTS cases were collected during 2011-2021. The changing epidemiological characteristics of SFTS were analyzed by adopting descriptive statistical methods. GeoDa 1.18 was applied for spatial autocorrelation analysis, and SaTScan 10.0 was used to identify spatio-temporal clustering of cases. The results were visualized in ArcMap.

RESULTS

The annual incidence of SFTS increased in Jiangsu Province from 2011 to 2021. Most cases (72.4%) occurred during May and August with the obvious peak months. Elderly farmers accounted for most cases, among which both males and females were susceptible. The spatial autocorrelation and spatio-temporal clustering analysis indicated that the distribution of SFTS was not random but clustered in space and time. The most likely cluster was observed in the western region of Jiangsu Province and covered one county (Xuyi county) (Relative risk = 8.18, Log likelihood ratio = 122.645, P < 0.001) located in southwestern Jiangsu Province from January 1, 2017 to December 31, 2021. The Secondary cluster also covered one county (Lishui county) (Relative risk = 7.70, Log likelihood ratio = 94.938, P < 0.001) from January 1, 2017 to December 31, 2021.

CONCLUSIONS

The annual number of SFTS cases showed an increasing tendency in Jiangsu Province from 2011 to 2021. Our study elucidated regions with SFTS clusters by means of ArcGIS in combination with spatial analysis. The results demonstrated solid evidences for the orientation of limited sanitary resources, surveillance in high-risk regions and early warning of epidemic seasons in future prevention and control of SFTS in Jiangsu Province.

Rapid invasion and expansion of the Asian longhorned tick (Haemaphysalis longicornis) into a new area on Long Island, New York, USA

Since its discovery in the United States in 2017, the Asian longhorned tick (Haemaphysalis longicornis) has been detected in most eastern states between Rhode Island and Georgia. Long Island, east of New York City, a recognized high-risk area for tick-borne diseases, is geographically close to New Jersey and New York sites where H. longicornis was originally found. However, extensive tick surveys conducted in 2018 did not identify H. longicornis on Long Island. In stark contrast, our 2022 tick survey suggests that H. longicornis has rapidly invaded and expanded in multiple surveying sites on Long Island (12 out of 17 sites). Overall, the relative abundance of H. longicornis was similar to that of lone star ticks, Amblyomma americanum, a previously recognized tick species abundantly present on Long Island. Interestingly, our survey suggests that H. longicornis has expanded within the Appalachian forest ecological zone of Long Island's north shore compared to the Pine Barrens located on the south shore of Long Island. The rapid invasion and expansion of H. longicornis into an insular environment are different from the historical invasion and expansion of two native tick species, Ixodes scapularis (blacklegged tick or deer tick) and A. americanum, in Long Island. The implications of H. longicornis transmitting or introducing tick-borne pathogens of public health importance remain unknown.

A new model for predicting the outcome and effectiveness of drug therapy in patients with severe fever with thrombocytopenia syndrome: A multicenter Chinese study

BACKGROUND

There are a few models for predicting the outcomes of patients with severe fever with thrombocytopenia syndrome (SFTS) based on single-center data, but clinicians need more reliable models based on multicenter data to predict the clinical outcomes and effectiveness of drug therapy.

METHODOLOGY/PRINCIPAL FINDINGS

This retrospective multicenter study analyzed data from 377 patients with SFTS, including a modeling group and a validation group. In the modeling group, the presence of neurologic symptoms was a strong predictor of mortality (odds ratio: 168). Based on neurologic symptoms and the joint indices score, which included age, gastrointestinal bleeding, and the SFTS virus viral load, patients were divided into double-positive, single-positive, and double-negative groups, which had mortality rates of 79.3%, 6.8%, and 0%, respectively. Validation using data on 216 cases from two other hospitals yielded similar results. A subgroup analysis revealed that ribavirin had a significant effect on mortality in the single-positive group (P = 0.006), but not in the double-positive or double-negative group. In the single-positive group, prompt antibiotic use was associated with reduced mortality (7.2% vs 47.4%, P < 0.001), even in individuals without significant granulocytopenia and infection, and early prophylaxis was associated with reduced mortality (9.0% vs. 22.8%, P = 0.008). The infected group included SFTS patients with pneumonia or sepsis, while the noninfected group included patients with no signs of infection. The white blood cell count and levels of C-reactive protein and procalcitonin differed significantly between the infection and non-infection groups (P = 0.020, P = 0.011, and P = 0.003, respectively), although the absolute difference in the medians were small.

CONCLUSIONS/SIGNIFICANCE

We developed a simple model to predict mortality in patients with SFTS. Our model may help to evaluate the effectiveness of drugs in these patients. In patients with severe SFTS, ribavirin and antibiotics may reduce mortality.

Microhabitat modeling of the invasive Asian longhorned tick (Haemaphysalis longicornis) in New Jersey, USA

The Asian longhorned tick (Haemaphysalis longicornis) is a vector of multiple arboviral and bacterial pathogens in its native East Asia and expanded distribution in Australasia. This species has both bisexual and parthenogenetic populations that can reach high population densities under favorable conditions. Established populations of parthenogenetic H. longicornis were detected in the eastern United States in 2017 and the possible range of this species at the continental level (North America) based on climatic conditions has been modeled. However, little is known about factors influencing the distribution of H. longicornis at geographic scales relevant to local surveillance and control. To examine the importance of local physiogeographic conditions such as geology, soil characteristics, and land cover on the distribution of H. longicornis we employed ecological niche modeling using three machine learning algorithms - Maxent, Random Forest (RF), and Generalized Boosting Method (GBM) to estimate probability of finding H. longicornis in a particular location in New Jersey (USA), based on environmental predictors. The presence of H. longicornis in New Jersey was positively associated with Piedmont physiogeographic province and two soil types - Alfisols and Inceptisols. Soil hydraulic conductivity was the most important predictor explaining H. longicornis habitat suitability, with more permeable sandy soils with higher hydraulic conductivity being less suitable than clay or loam soils. The models were projected over the state of New Jersey creating a probabilistic map of H. longicornis habitat suitability at a high spatial resolution of 90×90 meters. The model's sensitivity was 87% for locations sampled in 2017-2019 adding to the growing evidence of the importance of soil characteristics to the survival of ticks. For the 2020-2022 dataset the model fit was 57%, suggestive of spillover to less optimal habitats or, alternatively, heterogeneity in soil characteristics at the edges of broad physiographic zones. Further modeling should incorporate abundance and life-stage information as well as detailed characterization of the soil at collection sites. Once critical parameters that drive the survival and abundance of H. longicornis are identified they can be used to guide surveillance and control strategies for this invasive species.

Investigation of predictors for invasive pulmonary aspergillosis in patients with severe fever with thrombocytopenia syndrome

Patients with severe fever with thrombocytopenia syndrome (SFTS) had been confirmed to have immune dysfunction and were prone to invasive pulmonary aspergillosis (IPA), which was directly related to the increased mortality. The aim of this study was to investigate the predictors for IPA in SFTS patients, and the results were expected to be helpful for early identification of IPA and initiation of anti-fungal therapy. The study was performed to review laboratory confirmed SFTS patients in two tertiary hospitals in Shandong province (Qilu Hospital of Shandong University and Shandong Public Health Clinical Center) from April 2021 to August 2022. The enrolled patients were further divided into IPA group and non-IPA group. Demographic characteristics, clinical manifestations and laboratory parameters between IPA group and non-IPA group patients were analyzed and compared to identify the independent predictors for IPA by univariate analysis and multivariable logistic regression analysis. Sensitivity and specificity of independent predictors were evaluated by receiver operating characteristic (ROC) curve analysis. In total, 67 SFTS patients were enrolled with an average age of 64.7 (± 8.4) years old. The incidence of IPA was 32.8% (22/67). Mortality of patients in IPA group was 27.3% (6/22), which was significantly higher than that in non-IPA group. Results of univariate analysis showed that uncontrolled diabetes, central nervous system symptoms, platelet < 40 × 109/L, CD4+ T cell < 300/μL and CD8+ T cell < 400/μL were risk factors for development of IPA. These factors were further analyzed by multivariable logistic regression analysis and the results indicated that uncontrolled diabetes, platelet < 40 × 109/L, CD4+ T cell < 300/μL and CD8+ T cell < 400/μL could be recognized as independent predictors for IPA in SFTS patients. In conclusion, IPA is a serious complication for SFTS patients and increases mortality. It is necessary to early identify predictors of IPA for improving survival of SFTS patients.

Clinical Application of Thromboelastography in Patients With Severe Fever With Thrombocytopenia Syndrome

AIM

To investigate the clinical application of thromboelastography (TEG) in severe fever with thrombocytopenia syndrome (SFTS).

METHODS

One hundred and fifty-seven patients with SFTS were included in the study. The participants were distributed into 3 groups; A, B, and C. And 103 patients in group A met the clinical criteria as they exhibited slight liver and kidney dysfunction. Group B consisted of 54 patients with SFTS who were critically ill while group C was a healthy control group with 58 participants.

RESULTS

Patients with SFTS exhibited lower coagulation than the healthy participants. Group B patients exhibited significantly lower coagulation compared to group A. There was no significant difference in platelet count and fibrinogen content between patients in group A and group B, but platelet aggregation function and fibrinogen activity were significantly lower in group B patients.

CONCLUSION

Our results suggest that it is risky to solely rely on platelet count and the fibrinogen in SFTS. Monitoring of TEG and other coagulation indexes should be emphasized.

Ecology of Middle East respiratory syndrome coronavirus, 2012-2020: A machine learning modelling analysis

The ongoing enzootic circulation of the Middle East respiratory syndrome coronavirus (MERS-CoV) in the Middle East and North Africa is increasingly raising the concern about the possibility of its recombination with other human-adapted coronaviruses, particularly the pandemic SARS-CoV-2. We aim to provide an updated picture about ecological niches of MERS-CoV and associated socio-environmental drivers. Based on 356 confirmed MERS cases with animal contact reported to the WHO and 63 records of animal infections collected from the literature as of 30 May 2020, we assessed ecological niches of MERS-CoV using an ensemble model integrating three machine learning algorithms. With a high predictive accuracy (area under receiver operating characteristic curve = 91.66% in test data), the ensemble model estimated that ecologically suitable areas span over the Middle East, South Asia and the whole North Africa, much wider than the range of reported locally infected MERS cases and test-positive animal samples. Ecological suitability for MERS-CoV was significantly associated with high levels of bareland coverage (relative contribution = 30.06%), population density (7.28%), average temperature (6.48%) and camel density (6.20%). Future surveillance and intervention programs should target the high-risk populations and regions informed by updated quantitative analyses.

Epidemiological characteristics of severe fever with thrombocytopenia syndrome and its relationship with meteorological factors in Liaoning Province, China

Background

Severe fever with thrombocytopenia syndrome (SFTS), one kind of tick-borne acute infectious disease, is caused by a novel bunyavirus. The relationship between meteorological factors and infectious diseases is a hot topic of current research. Liaoning Province has reported a high incidence of SFTS in recent years. However, the epidemiological characteristics of SFTS and its relationship with meteorological factors in the province remain largely unexplored.

Methods

Data on reported SFTS cases were collected from 2011 to 2019. Epidemiological characteristics of SFTS were analyzed. Spearman’s correlation test and generalized linear models (GLM) were used to identify the relationship between meteorological factors and the number of SFTS cases.

Results

From 2011 to 2019, the incidence showed an overall upward trend in Liaoning Province, with the highest incidence in 2019 (0.35/100,000). The incidence was slightly higher in males (55.9%, 438/783), and there were more SFTS patients in the 60–69 age group (31.29%, 245/783). Dalian City and Dandong City had the largest number of cases of SFTS (87.99%, 689/783). The median duration from the date of illness onset to the date of diagnosis was 8 days [interquartile range (IQR): 4–13 days]. Spearman correlation analysis and GLM showed that the number of SFTS cases was positively correlated with monthly average rainfall (r_s = 0.750, P < 0.001; β = 0.285, P < 0.001), monthly average relative humidity (r_s = 0.683, P < 0.001; β = 0.096, P < 0.001), monthly average temperature (r_s = 0.822, P < 0.001; β = 0.154, P < 0.001), and monthly average ground temperature (r_s = 0.810, P < 0.001; β = 0.134, P < 0.001), while negatively correlated with monthly average air pressure (r_s = −0.728, P < 0.001; β = −0.145, P < 0.001), and monthly average wind speed (r_s = −0.272, P < 0.05; β = −1.048, P < 0.001). By comparing both correlation coefficients and regression coefficients between the number of SFTS cases (dependent variable) and meteorological factors (independent variables), no significant differences were observed when considering immediate cases and cases with lags of 1 to 5 weeks for dependent variables. Based on the forward and backward stepwise GLM regression, the monthly average air pressure, monthly average temperature, monthly average wind speed, and time sequence were selected as relevant influences on the number of SFTS cases.

Conclusion

The annual incidence of SFTS increased year on year in Liaoning Province. Incidence of SFTS was affected by several meteorological factors, including monthly average air pressure, monthly average temperature, and monthly average wind speed.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05395-4.

Mapping the viruses belonging to the order Bunyavirales in China

BACKGROUND

Viral pathogens belonging to the order Bunyavirales pose a continuous background threat to global health, but the fact remains that they are usually neglected and their distribution is still ambiguously known. We aim to map the geographical distribution of Bunyavirales viruses and assess the environmental suitability and transmission risk of major Bunyavirales viruses in China.

METHODS

We assembled data on all Bunyavirales viruses detected in humans, animals and vectors from multiple sources, to update distribution maps of them across China. In addition, we predicted environmental suitability at the 10 km × 10 km pixel level by applying boosted regression tree models for two important Bunyavirales viruses, including Crimean-Congo hemorrhagic fever virus (CCHFV) and Rift Valley fever virus (RVFV). Based on model-projected risks and air travel volume, the imported risk of RVFV was also estimated from its endemic areas to the cities in China.

RESULTS

Here we mapped all 89 species of Bunyavirales viruses in China from January 1951 to June 2021. Nineteen viruses were shown to infect humans, including ten species first reported as human infections. A total of 447,848 cases infected with Bunyavirales viruses were reported, and hantaviruses, Dabie bandavirus and Crimean-Congo hemorrhagic fever virus (CCHFV) had the severest disease burden. Model-predicted maps showed that Xinjiang and southwestern Yunnan had the highest environmental suitability for CCHFV occurrence, mainly related to Hyalomma asiaticum presence, while southern China had the highest environmental suitability for Rift Valley fever virus (RVFV) transmission all year round, mainly driven by livestock density, mean precipitation in the previous month. We further identified three cities including Guangzhou, Beijing and Shanghai, with the highest imported risk of RVFV potentially from Egypt, South Africa, Saudi Arabia and Kenya.

CONCLUSIONS

A variety of Bunyavirales viruses are widely distributed in China, and the two major neglected Bunyavirales viruses including CCHFV and RVFV, both have the potential for outbreaks in local areas of China. Our study can help to promote the understanding of risk distribution and disease burden of Bunyavirales viruses in China, and the risk maps of CCHFV and RVFV occurrence are crucial to the targeted surveillance and control, especially in seasons and locations at high risk.

Clinical Characteristics and Immune Status of Patients with Severe Fever with Thrombocytopenia Syndrome

Severe fever with thrombocytopenia syndrome (SFTS) is a novel infectious disease caused by bunya virus. The purpose of this study was to investigate the clinical characteristics of SFTS patients and their virus-related immune disorders in vivo. Patients with SFTS admitted to Nanjing Drum Tower Hospital from 2017 to 2020 were retrospectively analyzed, and divided into survival group and death group according to the 28-day survival. Clinical characteristics and laboratory examination results of SFTS patients were recorded, and dynamic changes of immune function and inflammatory factors were statistically analyzed. Prolonged activated prothrombin time (APTT) (p = 0.001), high viral load (p = 0.001), and elevated human leukocyte antigen DR (HLA-DR) level (p = 0.002) were independent prognostic risk factors for SFTS patients. Compared to the survival group, the nonsurvival group was more prone to hemorrhagic and neurological symptoms (p < 0.05). Natural kill (NK) cell count, interleukin-10, interferon-α, and tumor necrosis factor-α scores in the nonsurvival group continued to increase after admission, while CD3+ T, CD4+ T, and CD8+ T cell counts continued to decrease. CD3+ T lymphocyte count was negatively correlated with viral load (R = 0.3883, p < 0.001), CD4+ T lymphocyte count was negatively correlated with viral load (R = 0.28933, p < 0.001), CD8+ T lymphocyte count was negatively correlated with viral load (R = 0.781, p < 0.001), and HLA-DR was positively correlated with viral load (R = 0.489, p < 0.001). High viral load, prolonged APTT time, and elevated HLA-DR level are independent prognostic risk factors for SFTS patients. The T lymphocyte subsets of SFTS patients continue to decrease after infection, and the number of T lymphocyte subsets can reflect the severity of the disease.

The role of selenium in severe fever with thrombocytopenia syndrome: an integrative analysis of surveillance data and clinical data

OBJECTIVES

Selenium deficiency can be associated with increased susceptibility to some viral infections and even more severe diseases. In this study, we aimed to examine whether this association applies to severe fever with thrombocytopenia syndrome (SFTS).

METHOD

An observational study was conducted based on the data of 13,305 human SFTS cases reported in mainland China from 2010 to 2020. The associations among incidence, case fatality rate of SFTS, and crop selenium concentration at the county level were explored. The selenium level in a cohort of patients with SFTS was tested, and its relationship with clinical outcomes was evaluated.

RESULTS

The association between selenium-deficient crops and the incidence rate of SFTS was confirmed by multivariate Poisson analysis, with an estimated incidence rate ratio (IRR, 95% confidence interval [CI]) of 4.549 (4.215-4.916) for moderate selenium-deficient counties and 16.002 (14.706-17.431) for severe selenium-deficient counties. In addition, a higher mortality rate was also observed in severe selenium-deficient counties with an IRR of 1.409 (95% CI: 1.061-1.909). A clinical study on 120 patients with SFTS showed an association between serum selenium deficiency and severe SFTS (odds ratio, OR: 2.94; 95% CI: 1.00-8.67) or fatal SFTS (OR: 7.55; 95% CI: 1.14-50.16).

CONCLUSION

Selenium deficiency is associated with increased susceptibility to SFTS and poor clinical outcomes.

Meteorological factors and tick density affect the dynamics of SFTS in jiangsu province, China

Background

This study aimed to explore whether the transmission routes of severe fever with thrombocytopenia syndrome (SFTS) will be affected by tick density and meteorological factors, and to explore the factors that affect the transmission of SFTS. We used the transmission dynamics model to calculate the transmission rate coefficients of different transmission routes of SFTS, and used the generalized additive model to uncover how meteorological factors and tick density affect the spread of SFTS.

Methods

In this study, the time-varying infection rate coefficients of different transmission routes of SFTS in Jiangsu Province from 2017 to 2020 were calculated based on the previous multi-population multi-route dynamic model (MMDM) of SFTS. The changes in transmission routes were summarized by collecting questionnaires from 537 SFTS cases in 2018–2020 in Jiangsu Province. The incidence rate of SFTS and the infection rate coefficients of different transmission routes were dependent variables, and month, meteorological factors and tick density were independent variables to establish a generalized additive model (GAM). The optimal GAM was selected using the generalized cross-validation score (GCV), and the model was validated by the 2016 data of Zhejiang Province and 2020 data of Jiangsu Province. The validated GAMs were used to predict the incidence and infection rate coefficients of SFTS in Jiangsu province in 2021, and also to predict the effect of extreme weather on SFTS.

Results

The number and proportion of infections by different transmission routes for each year and found that tick-to-human and human-to-human infections decreased yearly, but infections through animal and environmental transmission were gradually increasing. MMDM fitted well with the three-year SFTS incidence data (P<0.05). The best intervention to reduce the incidence of SFTS is to reduce the effective exposure of the population to the surroundings. Based on correlation tests, tick density was positively correlated with air temperature, wind speed, and sunshine duration. The best GAM was a model with tick transmissibility to humans as the dependent variable, without considering lagged effects (GCV = 5.9247E-22, R² = 96%). Reported incidence increased when sunshine duration was higher than 11 h per day and decreased when temperatures were too high (>28°C). Sunshine duration and temperature had the greatest effect on transmission from host animals to humans. The effect of extreme weather conditions on SFTS was short-term, but there was no effect on SFTS after high temperature and sunshine hours.

Conclusions

Different factors affect the infection rate coefficients of different transmission routes. Sunshine duration, relative humidity, temperature and tick density are important factors affecting the occurrence of SFTS. Hurricanes reduce the incidence of SFTS in the short term, but have little effect in the long term. The most effective intervention to reduce the incidence of SFTS is to reduce population exposure to high-risk environments.

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging vector-borne disease caused by SFTS virus. After the first case was detected in China in 2009, SFTS endemic areas have gradually increased, with more than 23 provinces and cities reporting SFTS cases. In order to explore the transmission mechanism of SFTS and explain the impact of meteorological factors and tick density on the transmission routes of SFTS, this study collected SFTS cases data, meteorological data and tick surveillance data in Jiangsu Province from 2017 to 2019 to investigate the study question. The multi-population and multi-route dynamic model established in the previous study was used to calculate the infection rate coefficients of various transmission routes of SFTS in Jiangsu Province, and the generalized additive model was established to further elaborate the influence of SFTS transmission mechanism.

Transovarial Transmission of Heartland Virus by Invasive Asian Longhorned Ticks under Laboratory Conditions

We demonstrated experimental acquisition and transmission of Heartland bandavirus by Haemaphysalis longicornis ticks. Virus was detected in tick salivary gland and midgut tissues. A total of 80% of mice exposed to 1 infected tick seroconverted, suggesting horizontal transmission. H. longicornis ticks can transmit the virus in the transovarial mode.

Epidemiology, clinical characteristics, and treatment of severe fever with thrombocytopenia syndrome

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging tick-borne infectious disease caused by a novel phlebovirus (SFTS virus, SFTSV) in the family Phenuiviridae of the order Bunyavirales. The disease causes a wide spectrum of clinical signs and symptoms, ranging from mild febrile disease accompanied by thrombocytopenia and/or leukocytopenia to hemorrhagic fever, encephalitis, multiple organ failure, and death. SFTS was first identified in China and was subsequently reported in South Korea and Japan. The case-fatality rate ranges from 2.7% to 45.7%. Older age has been consistently shown to be the most important predictor of adverse disease outcomes. Older age exacerbates disease mainly through dysregulation of host immune cells and uncontrolled inflammatory responses. Tick-to-human transmission is the primary route of human infection with SFTSV, and Haemaphysalis longicornis is the primary tick vector of SFTSV. Despite its high case-fatality rate, vaccines and antiviral therapies for SFTS are not currently available. The therapeutic efficacies of several antiviral agents against SFTSV are currently being evaluated. Ribavirin was initially identified as a potential antiviral therapy for SFTS but was subsequently found to inefficiently improve disease outcomes, especially among patients with high viral loads. Favipiravir (T705) decreased both time to clinical improvement and mortality when administered early in patients with low viral loads. Anti-inflammatory agents including corticosteroids have been proposed to play therapeutic roles. However, the efficacy of other therapeutic modalities, such as convalescent plasma, is not yet clear.

Factors Associated With Severe Fever With Thrombocytopenia Syndrome in Endemic Areas of China

Objective

To explore the influence of climatic, environmental and socioeconomic factors on SFTS occurrence in Shandong Province, China.

Methods

We used generalized additive model to estimate the association between SFTS cases and climatic factors, environmental factors and socioeconomic factors, including annual average temperature, precipitation, land cover, normalized difference vegetation index, altitude, population density, meat production, milk production, and gross domestic product (GDP).

Results

There were a total of 4,830 cases reported in 100 (70.9%) counties and districts in Shandong Province from 2010 to 2020. The results showed that the annual average temperature, precipitation, forest and grassland coverage rate, altitude and meat production (square root transform) had a reversed “V” relationship with SFTS occurrence, with the inflection points around 12.5–13.0°C in temperature, around 650 mm in precipitation, around 0.3 in forest and grassland coverage rate, around 300 m in altitude, and around 200–300 tons in meat production (square root transform), respectively. SFTS occurrence had a “V” relationship with milk production (square root transform) and GDP (square root transform), with the inflection points around 100–200 tons in milk production (square root transform), and around 150,000–200,000 yuan in GDP (square root transform), respectively.

Conclusions

Climatic, environmental, and socioeconomic factors contributed to the heterogeneous distribution of SFTS in Shandong Province, and the influence of these factors on SFTS occurrence was nonlinear.

Vectors, Hosts, and the Possible Risk Factors Associated with Severe Fever with Thrombocytopenia Syndrome

Severe fever with thrombocytopenia syndrome (SFTS) is a disease caused by infection with the SFTS virus (SFTSV). SFTS has become a crucial public health concern because of the heavy burden, lack of vaccines, effective therapies, and high-fatality rate. Evidence suggests that SFTSV circulates between ticks and animals in nature and is transmitted to humans by tick bites. In particular, ticks have been implicated as vectors of SFTSV, where domestic or wild animals may play as the amplifying hosts. Many studies have identified antigens and antibodies against SFTSV in various animals such as sheep, goats, cattle, and rodents. Besides, person-to-person transmission through contact with blood or mucous of an infected person has also been reported. In this study, we reviewed the literature and summarized the vectors and hosts associated with SFTS and the possible risk factors.

Estimating severe fever with thrombocytopenia syndrome transmission using machine learning methods in South Korea

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging tick-borne infectious disease in China, Japan, and Korea. This study aimed to estimate the monthly SFTS occurrence and the monthly number of SFTS cases in the geographical area in Korea using epidemiological data including demographic, geographic, and meteorological factors. Important features were chosen through univariate feature selection. Two models using machine learning methods were analyzed: the classification model in machine learning (CMML) and regression model in machine learning (RMML). We developed a novel model incorporating the CMML results into RMML, defined as modified-RMML. Feature importance was computed to assess the contribution of estimating the number of SFTS cases using modified-RMML. Aspect to the accuracy of the novel model, the performance of modified-RMML was improved by reducing the MSE for the test data as 12.6–52.2%, compared to the RMML using five machine learning methods. During the period of increasing the SFTS cases from May to October, the modified-RMML could give more accurate estimation. Computing the feature importance, it is clearly observed that climate factors such as average maximum temperature, precipitation as well as mountain visitors, and the estimation of SFTS occurrence obtained from CMML had high Gini importance. The novel model incorporating CMML and RMML models improves the accuracy of the estimation of SFTS cases. Using the model, climate factors, including temperature, relative humidity, and mountain visitors play important roles in transmitting SFTS in Korea. Our findings highlighted that the guidelines for mountain visitors to prevent SFTS transmissions should be addressed. Moreover, it provides important insights for establishing control interventions that predict early identification of SFTS cases.

Evaluating the spatial and temporal patterns of the severe fever thrombocytopenia syndrome in Republic of Korea

Severe fever with thrombocytopenia syndrome (SFTS) is a new infectious disease with a high mortality rate and increased incidence in Republic of Korea since the first case was reported in 2013. The average mortality rate varies by region and year but remains high in Asia. This study aimed to evaluate the spatial and temporal patterns of SFTS cases reported to the national Disease Control and Prevention Agency (KDCA). We analysed the spatial and temporal distribution of SFTS and observed changes in areas vulnerable to the disease. We analysed data concerning 1086 confirmed SFTS patients from 2013 to 2019 categorized according to the 247 district level administrative units. To better understand the epidemiology of SFTS, we carried out spatiotemporal analyses on a yearly basis and also calculated and mapped spatial clusters of domestic SFTS by global (regional) and local Moran's indices. To observe the annual changes in SFTS incidence rate, scan statistics for each city and district were calculated. The incidence rate showed significant clustering in specific regions, which reoccurred annually in some regions. In Republic of Korea, SFTS clusters have been expanding into the southern regions, with annual clusters concentrated between May and October. This pattern allows prediction of SFTS occurrences through spatiotemporal analysis, which makes it possible to guide measures of disease prevention.

Modelling the transmission dynamics of severe fever with thrombocytopenia syndrome in Jiangsu Province, China

BACKGROUND

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease that is regionally distributed in Asia, with high fatality. Constructing the transmission model of SFTS could help provide clues for disease control and fill the gap in research on SFTS models.

METHODS

We built an SFTS transmission dynamics model based on the susceptible-exposed-infectious-asymptomatic-recovered (SEIAR) model and the epidemiological characteristics of SFTS in Jiangsu Province. This model was used to evaluate the effect by cutting off different transmission routes and taking different interventions into account, to offer clues for disease prevention and control.

RESULTS

The transmission model fits the reported data well with a minimum R2 value of 0.29 and a maximum value of 0.80, P < 0.05. Meanwhile, cutting off the environmental transmission route had the greatest effect on the prevention and control of SFTS, while isolation and shortening the course of the disease did not have much effect.

CONCLUSIONS

The model we have built can be used to simulate the transmission of SFTS to help inform disease control. It is noteworthy that cutting off the environment-to-humans transmission route in the model had the greatest effect on SFTS prevention and control.

Mapping ticks and tick-borne pathogens in China

Understanding ecological niches of major tick species and prevalent tick-borne pathogens is crucial for efficient surveillance and control of tick-borne diseases. Here we provide an up-to-date review on the spatial distributions of ticks and tick-borne pathogens in China. We map at the county level 124 tick species, 103 tick-borne agents, and human cases infected with 29 species (subspecies) of tick-borne pathogens that were reported in China during 1950-2018. Haemaphysalis longicornis is found to harbor the highest variety of tick-borne agents, followed by Ixodes persulcatus, Dermacentor nutalli and Rhipicephalus microplus. Using a machine learning algorithm, we assess ecoclimatic and socioenvironmental drivers for the distributions of 19 predominant vector ticks and two tick-borne pathogens associated with the highest disease burden. The model-predicted suitable habitats for the 19 tick species are 14‒476% larger in size than the geographic areas where these species were detected, indicating severe under-detection. Tick species harboring pathogens of imminent threats to public health should be prioritized for more active field surveillance.

Clinical Progress and Risk Factors for Death from Severe Fever with Thrombocytopenia Syndrome: A Multihospital Retrospective Investigation in Anhui, China

Knowledge of the clinical progress of severe fever with thrombocytopenia syndrome (SFTS) and the associated predictors of mortality is important for providing appropriate treatment in severe cases. A multihospital retrospective study was conducted in three SFTS-endemic cities, in 2018. Of the 208 SFTS-confirmed cases, there were 189 survivors and 19 deaths. The median age was 64 years; 104 (50.0%) patients were men, and 188 (90.4%) were farmers. Furthermore, 203 (97.6%) patients reported fever and 70 (33.7%) reported fatigue. Most fatal cases had complications including multiple-organ failure, central nervous syndrome (CNS) abnormalities, and disseminated intravascular coagulation. During the fever phase, alanine transaminase, aspartate aminotransferase (AST), blood urea nitrogen (BUN), creatinine, D-dimer, glucose, hydroxybutyrate dehydrogenase, lactate dehydrogenase (LDH), procalcitonin, prothrombin time, and uric acid levels were higher in fatal than in nonfatal cases (P < 0.05). Creatine kinase (CK), CK-MB (CKMB), AST, and LDH levels were significantly lower in nonfatal than in fatal cases (P < 0.05). Central nervous syndrome abnormalities (odds ratio [OR] = 20.9, 95% CI: 4.3, 100), body temperature ≥ 38.5°C (OR = 23.2, 95% CI: 3.4, 158), BUN levels ≥ 6.4 mmol/L (OR = 9.9, 95% CI: 2.2, 44), CKMB levels ≥ 100 U/L (OR = 33.2, 95% CI: 5.8, 192), and LDH levels ≥ 1,000 U/L (OR = 8.3, 95% CI: 1.9, 37) were predictors of mortality. Our findings reveal that the presence of specific complications and laboratory parameters may serve as predictors of mortality and aid in early identification of severe SFTS cases in clinical practice.

Factors associated with spatial distribution of severe fever with thrombocytopenia syndrome

Severe fever with thrombocytopenia syndrome (SFTS) was firstly identified in mainland China in 2009 and the geographic distribution has expanded in recent years. In this study, we constructed ecological niche models (ENM) of SFTS with meteorological factors, environmental factor, and density of domestic animals using MaxEnt. We found four significant associated factors including altitude, yearly average temperature, yearly accumulated precipitation, and yearly average relative humidity which accounted for 94.1% percent contribution. SFTS occurrence probability was high when altitude was between -100 m and 100 m, and the probability was nearly 0 when altitude was beyond 3000 m. Response curves of SFTS to the yearly average temperature, yearly accumulated precipitation, and yearly average relative humidity were all reversed V-shape. SFTS occurrence probability was high where the yearly average temperature, yearly accumulated precipitation, and yearly relative humidity were 12.5-17.5 °C, 700-2250 mm and 63-82%, respectively. ENMs predicted that the potential high-risk areas were mainly distributed in eastern areas and central areas of China. But there were some predicted potential high-risk areas where no SFTS case was reported up to date. More researches should be done to make clear whether SFTS case had occurred in these areas.

Baseline mapping of severe fever with thrombocytopenia syndrome virology, epidemiology and vaccine research and development

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a newly emergent tick-borne bunyavirus first discovered in 2009 in China. SFTSV is a growing public health problem that may become more prominent owing to multiple competent tick-vectors and the expansion of human populations in areas where the vectors are found. Although tick-vectors of SFTSV are found in a wide geographic area, SFTS cases have only been reported from China, South Korea, Vietnam, and Japan. Patients with SFTS often present with high fever, leukopenia, and thrombocytopenia, and in some cases, symptoms can progress to severe outcomes, including hemorrhagic disease. Reported SFTSV case fatality rates range from ~5 to >30% depending on the region surveyed, with more severe disease reported in older individuals. Currently, treatment options for this viral infection remain mostly supportive as there are no licensed vaccines available and research is in the discovery stage. Animal models for SFTSV appear to recapitulate many facets of human disease, although none of the models mirror all clinical manifestations. There are insufficient data available on basic immunologic responses, the immune correlate(s) of protection, and the determinants of severe disease by SFTSV and related viruses. Many aspects of SFTSV virology and epidemiology are not fully understood, including a detailed understanding of the annual numbers of cases and the vertebrate host of the virus, so additional research on this disease is essential towards the development of vaccines and therapeutics.

Severe fever with thrombocytopenia syndrome virus: a highly lethal bunyavirus

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a novel bunyavirus. Since 2007, SFTS disease has been reported in China with high fatality rate up to 30%, which drew high attention from Centre for Disease Control and Prevention and government. SFTSV is endemic in the centra l and eastern China, Korea and Japan. There also have been similar cases reported in Vietnam. The number of SFTSV infection cases has a steady growth in these years. As SFTSV could transmitted from person to person, it will expose the public to infectious risk. In 2018 annual review of the Blueprint list of priority diseases, World Health Organisation has listed SFTSV infection as prioritised diseases for research and development in emergency contexts. However, the pathogenesis of SFTSV remains largely unclear. Currently, there are no specific therapeutics or vaccines to combat infections of SFTSV. This review discusses recent findings of epidemiology, transmission pathway, pathogenesis and treatments of SFTS disease.

Mapping the global potential transmission hotspots for severe fever with thrombocytopenia syndrome by machine learning methods

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease with increasing spread. Currently SFTS transmission has expanded beyond Asian countries, however, with definitive global extents and risk patterns remained obscure. Here we established an exhaustive database that included globally reported locations of human SFTS cases and the competent vector, Haemaphysalis longicornis (H. longicornis), as well as the explanatory environmental variables, based on which, the potential geographic range of H. longicornis and risk areas for SFTS were mapped by applying two machine learning methods. Ten predictors were identified contributing to global distribution for H. longicornis with relative contribution ≥1%. Outside contemporary known distribution, we predict high receptivity to H. longicornis across two continents, including northeastern USA, New Zealand, parts of Australia, and several Pacific islands. Eight key drivers of SFTS cases occurrence were identified, including elevation, predicted probability of H. longicornis presence, two temperature-related factors, two precipitation-related factors, the richness of mammals and percentage coverage of water bodies. The globally model-predicted risk map of human SFTS occurrence was created and validated effective for discriminating the actual affected and unaffected areas (median predictive probability 0.74 vs. 0.04, P < 0.001) in three countries with reported cases outside China. The high-risk areas (probability ≥50%) were predicted mainly in east-central China, most parts of the Korean peninsula and southern Japan, and northern New Zealand. Our findings highlight areas where an intensive vigilance for potential SFTS spread or invasion events should be advocated, owing to their high receptibility to H. longicornis distribution.

Correlation between thrombocytopenia and host response in severe fever with thrombocytopenia syndrome

Severe Fever with Thrombocytopenia Syndrome (SFTS) is an emerging infectious disease caused by a novel bunyavirus, SFTS virus (SFTSV), with fatal outcome developed in approximately 17% of the cases. Thrombocytopenia is a hallmark feature of SFTS, and associated with a higher risk of fatal outcome, however, the pathophysiological involvement of platelet in the clinical outcome of SFTS remained under-investigated. In the current study, by retrospectively analyzing 1538 confirmed SFTS patients, we observed that thrombocytopenia was associated with enhanced activation of the cytokine network and the vascular endothelium, also with a disturbed coagulation response. The platelet phenotypes were also extensively altered in the process of thrombocytopenia development of SFTS patients. More importantly, all these disturbed host responses were related to the severity of thrombocytopenia, thus were considered to play in a synergistic way to influence the disease outcome. Moreover, the clinical effect of platelet transfusion was assessed by comparing two groups of patients with or without receiving this therapy. As a result, we observed no therapy effect in altering frequencies of fatal outcome, clinical bleeding development, or dynamic change of platelet count during the hospitalization. It’s suggested that platelet supplementation alone acted a minor role in improving disease outcome, therefore new therapeutic intervention to regulate host response should be proposed. The current results revealed some evidence of interrelationship between platelet count and clinical outcome of SFTS disease from the perspective of activation of the cytokine network, the vascular endothelium, and the coagulation/fibrinolysis system. These evaluations might help to attain a better understanding of the pathogenesis and therapy choice in SFTS.

Thrombocytopenia in SFTSV is a multifactor-process involving a combination of platelet size or morphology alterations, fibrinolysis activation and coagulation abnormalities, increased inflammatory response and endothelial injury. Platelet supplementation alone shows minor role in improving disease, therefore new therapeutic intervention to regulate host response should be proposed.

Severe Fever with Thrombocytopenia Syndrome: Japan under Threat from Life-threatening Emerging Tick-borne Disease

Japan, like many other parts of the world, is under threat from newly emerging, potentially fatal diseases. Severe fever with thrombocytopenia syndrome (SFTS), first clinically identified in 2009, is an emerging tick-borne hemorrhagic viral disease, currently limited in distribution to East Asia. Relatively little is known about the disease with an initial Case Fatality Rate ranging from 5% to 40%. It primarily affects the elderly living in rural areas, which is particularly troublesome given Japan’s rapidly aging population. Control efforts are severely hampered by lack of specific knowledge of the disease and its means of transmission, coupled with the absence of both a vaccine and an effective treatment regime, although some antiviral drugs and blood transfusions are successful in treating the disease. Despite both the causative virus and vector ticks being commonly found throughout Japan, the disease shows a very specific, limited geographical distribution for as yet unknown reasons.

Severe fever with thrombocytopenia syndrome: a systematic review and meta-analysis of epidemiology, clinical signs, routine laboratory diagnosis, risk factors, and outcomes

Background

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease with the high case-fatality rate, and lack of vaccines. We aimed to systematically analysed the epidemiological characteristics, clinical signs, routine laboratory diagnosis, risk factors, and outcomes.

Methods

Documents on SFTS were collected by searching the Chinese National Knowledge Infrastructure, Wan Fang Data, PubMed, Embase, and Web of Science databases from 2011 to 2018. Meta-analysis was performed by using Review Manager and Stata software.

Results

Twenty-five articles involving 4143 cases were included. Diarrhea (odds ratio (OR) =1.60, 95% confidence interval (CI): 1.06 to 2.42, P = 0.02), and vomiting (OR = 1.56, 95% CI: 1.01 to 2.39, P = 0.04) on admission were associated with the fatal outcomes of SFTS. Compared to patients with mild symptoms, patients with severe symptoms had significantly elevated levels of lactic acid dehydrogenase (standard mean difference (SMD) =1.27, 95% CI: 0.59 to 1.94), alanine aminotransferase (SMD = 0.55, 95% CI: 0.24 to 0.85), aspirate aminotransferase (SMD = 1.01, 95% CI: 0.69 to 1.32), and creatine kinase (SMD = 1.04, 95% CI: 0.74 to 1.33) but had reduced platelet counts (SMD = -0.87, 95% CI: − 1.16 to − 0.58) and albumin levels (SMD = -1.00, 95% CI: − 1.32 to − 0.68). The risk factors for poor prognosis included age (mean difference (MD) =6.88, 95% CI: 5.41 to 8.35) and farming (OR = 2.01, 95% CI: 1.06 to 3.80). For the risk factors of contracting SFTS, the incidence of SFTS related to tick bites was 24% [95% CI: 0.18 to 0.31]. The pooled case-fatality rate of SFTS patients was 18% [95% CI: 0.16 to 0.21].

Conclusions

China is the country with the highest incidence of SFTS. May to July was the peak of the epidemic, and farmers were a high-risk group. The risk factor for SFTS included age (poor prognosis) and tick bites (contracting SFTS). Patients with severe diarrhea and vomiting symptoms on admission should be noted. Clinicians could use routine laboratory parameters and clinical symptoms as references for clinically suspected cases, classification of SFTS, and timely treatment, especially in basic hospitals.

Emerging tick-borne pathogens of public health importance: a mini-review

Ticks are the most important vectors of human pathogens, leading to increased public health burdens worldwide. Tick-borne pathogens include viruses (e.g. tick-borne encephalitis and Powassan); bacteria, such as the causative agents of Lyme disease, spotted fever rickettsiosis and human anaplasmosis; and malaria-like protozoan parasites causing babesiosis. Tick-borne diseases are emerging due to the geographical expansion of their tick vectors, especially in the northern hemisphere. Two examples of this phenomenon are Ixodes scapularis and Amblyomma americanum, which have expanded their ranges in the USA in recent decades and are responsible for the continuous emergence of Lyme disease and human ehrlichiosis, respectively. This phenomenon is also occurring worldwide and is reflected by the increasing number of tick-borne encephalitis and haemorrhagic fever cases in Europe and Asia. In this review, we provide a concise synopsis of the most medically important tick-borne pathogen worldwide, with a particular emphasis on emerging public health threats.

Epidemiological and genetic investigation of a cluster of cases of severe fever with thrombocytopenia syndrome bunyavirus

Background

To analyze and discuss the transmission route of a cluster of cases of severe fever with thrombocytopenia syndrome bunyavirus (SFTSV).

Method

We performed an epidemiological investigation and a genetic analysis of patients with severe fever with thrombocytopenia syndrome (SFTS) caused by SFTSV, their close contacts and the surrounding population.

Results

We found that all patients had contact with the blood of the first patient. The comparison of gene sequences in the three isolated SFTSV strains showed that the strains were closely related. Six close contacts and nine individuals in the surrounding population were positive for SFTSV IgM antibody.

Conclusion

We suspect that the cluster outbreak was transmitted via blood and that the natural reservoir host of SFTSV exists in the patients’ environment.

Four Year Surveillance of the Vector Hard Ticks for SFTS, Ganghwa-do, Republic of Korea

The seasonal abundance of hard ticks that transmit severe fever with thrombocytopenia syndrome virus was monitored with a collection trap method every April to November during 2015–2018 and with a flagging method every July and August during 2015–2018 in Ganghwa-do (island) of Incheon Metropolitan City, Republic of Korea. This monitoring was performed in a copse, a short grass field, coniferous forest and broad-leaved forest. A total of 17,457 ticks (8,277 larvae, 4,137 nymphs, 3,389 females, and 1,654 males) of the ixodid ticks comprising 3 species (Haemaphysalis longicornis, H. flava, and Ixodes nipponensis) were collected with collection traps. Of the identified ticks, H. longicornis was the most frequently collected ticks (except larval ticks) (94.26%, 8,653/9,180 ticks (nymphs and adults)), followed by H. flava (5.71%, 524/9,180) and Ix. nipponensis (less than 0.04%, 3/9,180). The ticks collected with collecting traps were pooled and assayed for the presence of SFTS virus with negative results. In addition, for monitoring the prevalence of hard ticks, a total of 7,461 ticks (5,529 larvae, 1,272 nymphs, 469 females, and 191 males) of the ixodid ticks comprising 3 species (H. longicornis, H. flava, and Ix. nipponensis) were collected with flagging method. H. longicornis was the highest collected ticks (except larval ticks) (99.53%, 1,908/1,917 ticks (nymphs and adults)), followed by H. flava (1.15%, 22/1,917).

De Novo RNA-seq and Functional Annotation of Haemaphysalis longicornis

PURPOSE

Haemaphysalis longicornis (Neumann) is a hematophagous tick widely distributed in northern China. It not only causes enormous economic loss to animal husbandry, but also as a vector and reservoir of various zoonotic pathogens, it spreads natural focal diseases, such as severe fever with thrombocytopenia syndrome, seriously threatening human health. Lack of transcriptomic and genomic data from H. longicornis limits the study of this important medical vector.

METHODS

The engorged female H. longicornis from Gansu, China, was used for RNA extraction, de novo RNA-seq, functional annotation, and ORF prediction.

RESULTS

As a result, 53.09 million clean reads (98.88%) with a GC content of 54.29% were obtained. A total of 65,916 Unigenes were assembled, of which 34.59% (23,330) were successfully annotated. Of these Unigenes, 22,587 (34.27%) were annotated to species by NCBI non-redundant protein (nr). Ixodes scapularis, Limulus polyphemus, Parasteatoda tepidariorum, Stegodyphus mimosarum, and Metaseiulus occidentalis were the top BLAST hit species, accounting for 47.23%, 9.58%, 4.11%, 3.50%, and 2.69%, respectively. A total of 29,182 ORFs were predicted, and 35 complete ORFs for functional genes were identified, including ORFs involved in digestion (14), stress responses (8), anticoagulation (3), reproduction (3), antimicrobial (2), drug resistance (2), movement (2), autophagy (1), and immunity (1), respectively. The Unigene ORFs encoding cathepsin and heat shock proteins were further analyzed phylogenetically.

CONCLUSION

De novo RNA-seq and functional annotation of H. longicornis were successfully completed for the first time, providing a molecular data resource for further research on blood-sucking, pathogen transmission mechanisms, and effective prevention and control strategies.

Seroprevalance of antibodies specific for severe fever with thrombocytopenia syndrome virus and the discovery of asymptomatic infections in Henan Province, China

Background

Severe fever with thrombocytopenia syndrome (SFTS) is a severe emerging disease caused by SFTS virus (SFTSV), and the geographical distribution of SFTS has been increasing throughout China in recent years. To assess SFTSV-specific antibody seroprevalence, a cross-sectional study was conducted for healthy people in high SFTS endemic areas of Henan province in 2016.

Methods

This study used a stratified random sampling method to select 14 natural villages as the investigation sites. From April to May 2016, participants completed a questionnaire survey and serum samples were collected. All serum samples were subjected to ELISA to detect SFTSV-specific IgM and IgG. All IgM-positive samples were further tested by real-time RT-PCR, and isolation of virus from serum was attempted. Any participant who was IgM-positive was followed up with a month later to confirm health status.

Results

In total, 1463 healthy people participated in this study. The average seropositive rates for SFTSV-specific IgG and IgM were 10.46% (153/1463) and 0.82% (12/1463), respectively. IgM was detected in 12 individuals, and SFTSV RNA was detected in six of them. Virus was isolated from five of the six SFTSV RNA-positive individuals, and phylogenetic analyses revealed that all five isolates belonged to SFTSV group A. No IgM-positive participants exhibited any symptoms or other signs of illness at the one-month follow up.

Conclusions

This study identified a relatively high incidence of SFTSV-specific antibody seropositivity in healthy people in Xinyang city. Moreover, our data provide the first evidence for asymptomatic SFTSV infections, which may have significant implications for SFTS outbreak control.

Severe fever with thrombocytopenia syndrome (SFTS) is a severe emerging infectious disease caused by SFTS virus (SFTSV) that was first discovered in rural areas of China. Henan province has had the largest number of SFTS cases in China every year since the disease was discovered, however, seropositivity for SFTSV-specific antibodies in healthy people in this region is still not clear. To address this issue, a cross-sectional survey was performed in high endemic areas from April to May 2016. The results showed that SFTSV seroprevalence was relatively high and possibly increasing. Notably, SFTSV RNA, as well as virus itself, was isolated from specimens obtained from healthy people. This study confirmed there are asymptomatic SFTSV infections in humans, and it is the first to report SFTSV isolation from healthy people.

Arginine deficiency is involved in thrombocytopenia and immunosuppression in severe fever with thrombocytopenia syndrome

Severe fever with thrombocytopenia syndrome (SFTS) caused by a recently identified bunyavirus, SFTSV, is an emerging infectious disease with extensive geographical distribution and high mortality. Progressive viral replication and severe thrombocytopenia are key features of SFTSV infection and fatal outcome, whereas the underlying mechanisms are unknown. We revealed arginine deficiency in SFTS cases by performing metabolomics analysis on two independent patient cohorts, suggesting that arginine metabolism by nitric oxide synthase and arginase is a key pathway in SFTSV infection and consequential death. Arginine deficiency was associated with decreased intraplatelet nitric oxide (Plt-NO) concentration, platelet activation, and thrombocytopenia. An expansion of arginase-expressing granulocytic myeloid-derived suppressor cells was observed, which was related to T cell CD3-ζ chain down-regulation and virus clearance disturbance, implicating a role of arginase activity and arginine depletion in the impaired anti-SFTSV T cell function. Moreover, a comprehensive measurement of arginine bioavailability, global arginine bioavailability ratio, was shown to be a good prognostic marker for fatal prediction in early infection. A randomized controlled trial demonstrated that arginine administration was correlated with enhanced Plt-NO concentration, suppressed platelet activation, and elevated CD3-ζ chain expression and eventually associated with an accelerated virus clearance and thrombocytopenia recovery. Together, our findings revealed the arginine catabolism pathway-associated regulation of platelet homeostasis and T cell dysregulation after SFTSV infection, which not only provided a functional mechanism underlying SFTS pathogenesis but also offered an alternative therapy choice for SFTS.

Severe fever and thrombocytopenia syndrome virus infection: Considerations for vaccine evaluation of a rare disease

Infection caused by the severe fever and thrombocytopenia syndrome virus (SFTSV) causes a hemorrhagic illness with a mortality between 20% and 40%. Initially recognized in 2009 in China, cases have additionally been documented in Japan and Korea although retrospective studies have documented seroprevalence since 1996. Although case rates have increased due to increased awareness and more widely available diagnostics, SFTSV infection remains rare with the highest rates documented in Korea for Jeju Province (3.5 cases per 100,000 population) and the Inje-gun region (66.2 cases per 100,000). Because of the very low incidence of infection, a placebo-controlled study with 1:1 randomization to evaluate an SFTSV vaccine would require a sample size that is 25% greater than the region of study. We discuss alternatives to licensure. Vaccine effectiveness may be assessed through a registry, comparing rates of infection over time between vaccine recipients versus regional populations. Modeled data can be updated based on actual case rates and population changes over the years of follow-up. Using one model, statistically significant differences are seen after 10 years in Inje-gun and 15 years of follow-up in Jeju. This approach may be applicable to other uncommon infectious diseases for which a standard study design is difficult.

Prevalence of severe fever with thrombocytopenia syndrome virus in animals in Henan Province, China

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease caused by severe fever with thrombocytopenia syndrome virus (SFTSV). SFTSV has been found in humans, ticks and animals, and SFTS has high mortality and increasing prevalence in East Asia. In the study, the samples (heart, liver, lung, kidney, spleen, brain tissue and serum) were collected from 374 domestic animals and 241 wild animals in Pingqiao District and Xinxian County of Xinyang in Henan Province, China. 275 (44.72%, 275/615) animals were positive for anti-SFTSV antibodies, the anti-SFTSV antibodies positive ratios of domestic and wild animals were 43.58% (163/374) and 46.47% (112/241), respectively. There was no significant difference in domestic and wild animals, but significant differences were detected among different species of animals (χ² = 112.59, P < 0.0001). Among 615 animals, 105 (17.07%, 105/615) animals were positive for SFTSV RNA, and only one SFTSV strain was isolated from heart tissue of a yellow weasel. The phylogenetic analysis shows that the sequence from animals belonged to the same group with viral sequences obtained from humans. The animals maybe play a reservoir host in maintaining the life cycle of SFTSV in nature.

Spatial epidemiological determinants of severe fever with thrombocytopenia syndrome in Miyazaki, Japan: a GWLR modeling study

Background

Cases of severe fever with thrombocytopenia syndrome (SFTS) have increasingly been observed in Miyazaki, southwest Japan. It is critical to identify and elucidate the risk factors of infection at community level. In the present study, we aimed to identify areas with a high risk of SFTS virus infection using a geospatial dataset of SFTS cases in Miyazaki.

Methods

Using 10 × 10-km mesh data and a geographically weighted logistic regression (GWLR) model, we examined the statistical associations between environmental variables and spatial variation in the risk of SFTS. We collected geospatial and population census data as well as forest and agriculture mesh information. Altitude and farmland were selected as two specific variables for predicting the presence of SFTS cases in a given mesh area.

Results

Using GWLR, the area under the receiver operating characteristic curve (AUC) was estimated at 73.9%, outperforming the classical logistic regression model (72.4%). The sensitivity and specificity of the GWLR model were estimated at 90.9 and 58.7%, respectively. We identified altitude (odds ratio (OR) = 0.996, 95% confidence interval (CI): 0.994–0.999 per one-meter elevation) and farmland (OR = 0.999, 95% CI: 0.998–1.000 per % increase) as useful negative predictors of SFTS cases in Miyazaki.

Conclusions

Our study findings revealed that the risk of SFTS is high in geographic areas where farmland area begins to diminish and at mid-level altitudes. Our findings can help to improve the efficiency of ecological and animal surveillance in high-risk areas. Using finer geographic resolution, such surveillance can help raise awareness among local residents in areas with a high risk of SFTS.

Electronic supplementary material

The online version of this article (10.1186/s12879-019-4111-3) contains supplementary material, which is available to authorized users.

A Cluster of Bunyavirus-Associated Severe Fever With Thrombocytopenia Syndrome Cases in a Coastal Plain Area in China, 2015: Identification of a Previously Unidentified Endemic Region for Severe Fever With Thrombocytopenia Bunyavirus

Background

Severe fever with thrombocytopenia syndrome (SFTS) is a typical tick-borne, natural focal disease. The natural foci of SFTS were considered to exist in hilly and mountainous areas before 2015. A cluster of 3 patients exposed to a patient with a fulminant disease consistent with SFTS occurred from July to August 2015 in Dongtai County, which is characterized by alluvial plains; this prompted investigation.

Methods

The epidemiological, clinical, and laboratory features of 4 patients in the cluster were analyzed. Serum samples from the indigenous healthy population and native domesticated animals were collected to conduct laboratory tests, along with small wild animals and ticks.

Results

In 3 secondary case patients, high fever, thrombocytopenia and leukopenia developed within 8-13 days after contact with blood or bloody secretions from the index patient; SFTS was then diagnosed by means of reverse-transcription polymerase chain reaction. Genomic sequencing and analysis of S and L segments of 2 viral strains isolated from 2 secondary case patients showed that they shared 99.8%-99.9% homology in nucleotide sequence. The seroprevalences among indigenous healthy population, native livestock, native poultry, and small wild animals was 0.74%, 17.54%, 6.67%, and 1.12%, respectively. Three questing ticks, 61 feeding ticks, and 178 small wild animals were collected in August 2015. Survey on tick density and seasonal fluctuation in 2016 showed that ticks were active from March to October. All ticks were identified as Haemaphysalis longicornis. Severe fever with thrombocytopenia bunyavirus (SFTSV)-specific RNA was detected in the ticks collected in 2016, and the minimum SFTSV infection rate in these ticks was 0.54% (1 of 185).Wild mammals and ticks collected in August 2015 tested negative for SFTSV-specific RNA.

Conclusions

Aside from hilly or mountainous area, a coastal plain was identified as the natural foci of SFTSV in Dongtai County, China. The involvement of migration in the evolution of SFTSV might lead to a transregional transmission event of SFTSV.

Modeling the Asian Longhorned Tick (Acari: Ixodidae) Suitable Habitat in North America

Asian longhorned or bush tick (Haemaphysalis longicornis Neumann) is a vector species of considerable medical and veterinary importance within its native range in East Asia, and in introduced areas of Australia and Oceania. Recently, this tick species was detected in several regions of the United States. This study aimed at modeling areas suitable for H. longicornis in North America using maximum entropy distribution modeling or Maxent. Occurrence records of H. longicornis within its present range were obtained from published literature. The Maxent model contained a small number of a priori climatic and ecological variables. Annual temperature, precipitation, and ecological zones were found the most important in creating sensitive and specific model (success rate = 91.8%) that had a good fit to the existing data. The model predicted suitable H. longicornis habitat in most of eastern North America from southern Canada to the Gulf Coast, and in a small temperate area on the West Coast. Coastal areas were among the highest ranked suitable habitat. Another highly suitable region was identified in mid-western and southern United States where Heartland virus transmission takes place. This finding is of concern, since H. longicornis ticks vector a closely related virus in East Asia. Delineation of areas suitable for H. longicornis can facilitate detection, prepare public health authorities, and inform the general public about this potentially pestiferous and medically important species.