Risk Factors for Bunyavirus-Associated Severe Fever with Thrombocytopenia Syndrome: A Community-Based Case-Control Study

Direct transmission of severe fever with thrombocytopenia syndrome virus from farm-raised fur animals to workers in Weihai, China

BACKGROUND

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease. SFTS virus (SFTSV) is transmitted by tick bites and contact with the blood or body fluids of SFTS patients. Animal-to-human transmission of SFTS has been reported in Japan, but not in China. In this study, the possible transmission route of two patients who fed and cared for farm-raised fur animals in a mink farm was explored.

METHOD

An epidemiological investigation and a genetic analysis of patients, animals and working environment were carried out.

RESULTS

It was found that two patients had not been bitten by ticks and had no contact with patients infected with SFTS virus, but both of them had skinned the dying animals. 54.55% (12/22) of the farm workers were positive for SFTS virus antibody. By analyzing the large, medium and small segments sequences, the viral sequences from the two patients, animals and environments showed 99.9% homology.

CONCLUSION

It is suspected that the two patients may be directly infected by farm-raised animals, and that the virus may have been transmitted by aerosols when skinning dying animals. Transmission by direct blood contacts or animal bites cannot be ignored.

The ecological and etiological investigation of ticks and rodents in China: results from an ongoing surveillance study in Zhejiang Province

OBJECTIVES

This study aimed to analyze the population density of vector ticks and reservoir hosts rodents, and to investigate the relevant pathogen infection in Zhejiang Province, China.

METHODS

In this surveillance study, the data of ticks density were collected with the tick picking method on animal body surface and the drag-flag method, while the rodent density with the night trapping method. The samples of ticks were examined for the severe fever with thrombocytopenia syndrome virus (SFTSV), and blood serum and organs from rodents were subjected for SFTSV, hantavirus, Leptospira, Orientia tsutsugamushi (O. tsutsugamushi) and Yersinia pestis (Y. pestis) screening in the laboratory.

RESULTS

From 2017 to 2022 in Zhejiang Province, 16,230 parasitic ticks were found in 1848 positive animals, with the density of parasitic ticks of 1.29 ticks per host animal, and a total of 5,201 questing ticks were captured from 1,140,910 meters of vegetation distance with the questing tick density of 0.46 ticks/flag·100 m. Haemaphysalis longicornis (H. longicornis) was the major species. A total of 2,187,739 mousetraps were distributed and 12,705 rodents were trapped, with the density of 0.58 per 100 trap-nights. Rattus norvegicus was the major species. For SFTSV screening, two groups nymphal ticks of H. longicornis were tested to be positive. For the rodents samples, the Leptospira had a positive rate of 12.28% (197/1604), the hantavirus was 1.00% (16/1604), and the O. tsutsugamushi was 0.15% (2/1332). No positive results were found with SFTSV and Y. pestis in the rodents samples.

CONCLUSION

Findings from this study indicated that the ticks and rodents were widely distributed in Zhejiang Province. Particularly, the positive detection of SFTSV, Leptospira, hantavirus and O. tsutsugamushi in ticks or rodents from this area suggested that more attention should be paid to the possibilities of relevant vector-borne diseases occurrence.

Severe fever with thrombocytopenia syndrome incidence could be associated with ecotone between forest and cultivated land in rural settings of central China

Severe fever with thrombocytopenia syndrome (SFTS), an emerging tick-borne disease first reported in rural areas of central China, has become a major public health concern in endemic areas. The epidemic dynamic and ecologic factors of SFTS incidence at a village scale remain unclear. Here we analyzed the epidemiological characteristics of SFTS cases in Shangcheng County, the first reported areas of SFTS in China. A retrospective space-time cluster analysis was conducted to identify the dynamics of hotspot areas, and the negative binomial regression model was conducted to examine potential factors contributing to the incidence of SFTS at the village level. A total of 1,219 SFTS cases were reported in Shangcheng County from 2011 to 2020, with a case fatality rate of 12.0%. The median age of patients was 64 years, and 81.7% of patients were over 50 years old. Women accounted for 60.3% of all cases, and the incidence rate was significantly higher than that of men (Pearson χ² test, P<0.001). Five spatial-temporal clusters were identified, and mostly distributed in the central part of the county. Higher risk of SFTS incidence was shown in villages with higher percentage coverages of forest and tea plantation, and higher goat density. In villages where the ratio of cultivated land area to forest land area was between 0.2 and 1.2, the risk of SFTS incidence increased significantly, with an incidence rate ratio of 1.33 (95% CI: 1.04‒1.72, p = 0.024). Our findings indicated that ecotone between forest and cultivated land might be the most important risk settings for exposure and infection with SFTS virus in endemic areas of central China. Precise identification of risk factors and high-risk areas at a suitable scale is conducive to carrying out targeted measures and improving the surveillance of the disease.

Clinical characteristics and risk factors of 267 patients having severe fever with thrombocytopenia syndrome-new epidemiological characteristics of fever with thrombocytopenia syndrome: Epidemiological characteristics of SFTS

OBJECTIVE

To analyze the epidemiological distribution, clinical characteristics, and prognostic risk factors of patients having severe fever with thrombocytopenia syndrome (SFTS).

METHODS

We enrolled 790 patients with SFTS divided into the ordinary group and the severe group, analyzed the clinical characteristics, and screened the risk factors of severious patients by univariate logistic regression analysis.

RESULTS

Most of the 790 patients (SFTS) are farmers (84.56%). The proportion of patients with fieldwork history was 72.41%, of which 21.27% had a clear history of a tick bite and 98.61% were sporadic cases. The annual peak season is from April to November. 16.33% patients were not accompanied by fever. The incidence of severe thrombocytopenia was 47.59%. They were statistically significant between the 2 groups in indicators such as age, hypertension, coronary heart disease, diabetes mellitus, bunyavirus nucleic acid load and mean platelet count (P < .05). Multivariate non conditional Logistic regression analysis showed that the risk factors of the mild patients deteriorating severe disease were age (OR = 1.985, P  ≤ .003), diabetes mellitus (OR = 1.702, P  ≤ .001), coronary heart disease (OR = 1.381, P ≤ .003), platelet count (OR = 2.592, P  ≤ .001), viral nucleic acid loading (OR = 3.908, P  ≤ .001).

CONCLUSION

The incidence population and seasonal distribution characteristics of patients with SFTS are obvious. The risk factors for poor prognosis of severe patients are old age, multiple basic medical histories, high viral load, a serious decrease of mean platelet count, and delay of treatment time.

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.

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.

Infection with severe fever with thrombocytopenia virus in healthy population: a cohort study in a high endemic region, China

Background

Severe fever with thrombocytopenia (SFTS) caused by SFTS virus (SFTSV) was a tick-borne hemorrhagic fever that posed significant threat to human health in Eastern Asia. The study was designed to measure the seroprevalence of SFTSV antibody in healthy population residing in a high endemic region.

Methods

A cohort study was performed on healthy residents in Shangcheng County in Xinyang City from April to December in 2018, where the highest SFTS incidence in China was reported. Anti-SFTSV IgG was measured by indirect enzyme-linked immunosorbent assay and neutralizing antibody (NAb) was detected by using PRNT50. The logistic regression models were performed to analyze the variables that were associated with seropositive rates.

Results

Totally 886 individuals were recruited. The baseline seroprevalence that was tested before the epidemic season was 11.9% (70/587) for IgG and 6.8% (40/587) for NAb, which was increased to 13.4% (47/350) and 7.7% (27/350) during the epidemic season, and further to 15.8% (80/508) and 9.8% (50/508) post epidemic. The IgG antibody-based seropositivity was significantly related to the patients aged ≥ 70 years old [adjusted odds ratio (OR) = 2.440, 95% confidence interval (CI): 1.334–4.461 compared to the group of < 50 years old, P = 0.004], recent contact with cats (adjusted OR = 2.195, 95% CI: 1.261–3.818, P = 0.005), and working in tea garden (adjusted OR = 1.698, 95% CI: 1.002–2.880, P = 0.049) by applying multivariate logistic regression model. The NAb based seropositivity was similarly related to the patients aged ≥ 70 years old (adjusted OR = 2.691, 95% CI: 1.271–5.695 compared to the group of < 50 years old, P = 0.010), and recent contact with cats (OR = 2.648, 95% CI: 1.419–4.941, P = 0.002). For a cohort of individuals continually sampled with 1-year apart, the anti-SFTSV IgG were maintained at a stable level, while the NAb level reduced.

Conclusions

Subclinical infection might not provide adequate immunity to protect reinfection of SFTSV, thus highlighting the ongoing threats of SFTS in endemic regions, which called for an imperative need for vaccine development. Identification of risk factors might help to target high-risk population for public health education and vaccination in the future.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s40249-021-00918-0.

Seroepidemiological study of severe fever with thrombocytopenia syndrome in animals and humans in Okinawa, Japan

In Okinawa prefecture, Japan, the first case of severe fever with thrombocytopenia syndrome (SFTS) was confirmed in August 2016, and this case remains to be the only reported case of SFTS in Okinawa. The epidemiological investigation indicated that the patient had been infected on the main island of Okinawa, but source and route of infection were unknown. Therefore, to understand the possible source and route of SFTS virus (SFTSV) infection in Okinawa, we performed a seroepidemiological study of SFTSV among animals and dwellers in Okinawa and conducted a questionnaire survey to investigate risk factors for tick bites in Okinawa. Among the 1,035 serum samples from four different animal species, anti-SFTSV antibodies were detected in only 4.2% wild mongoose (Herpestes auropunctatus) serum samples. To our knowledge, this is the first study to report the detection of anti-SFTSV antibodies in wild mongooses. Meanwhile, all 1,104 human inhabitants tested negative for anti-SFTSV antibodies, suggesting that the frequency of SFTSV exposure is low in Okinawa. Logistic regression analysis of the questionnaire results showed that outdoor activity was associated with an increased risk of tick bite among Okinawa residents. Despite the current low frequency of SFTSV infection in animals and humans, endemic circulation of the virus in Okinawa should be carefully monitored in the area for preventing future infections.

Factors and outcomes in Severe Fever with Thrombocytopenia Syndrome (SFTS): A systematic review

BACKGROUND

An emerging infectious zoonosis known as Severe Fever with Thrombocytopenia Syndrome (SFTS) is discovered mainly in Japan, South Korea and China. SFTS virus (SFTSV) which is recently recognised as bunyavirus is borne by ticks such as Haemaphysalis longicornis. It has the capabilities to spread as develop clusters and become a considerable public health threat as this virus could experience rapid evolution via gene mutation. Case fatality rate has been reported up to higher than 30%. The aim of this review is to determine the associated risk factors of SFTS and its outcome.

MATERIALS AND METHODS

Literature search was conducted using online databases PubMed, ScienceDirect, and Scopus. A total of 517 records were identified from searches in PubMed, ScienceDirect, and Scopus. From the final exclusions, a total of 26 studies were included for final analysis.

RESULTS

Associated risk factors to getting SFTS infection include occupation, history of bite from a tick, biological susceptibility, and owning of domestic animal. Fatality rates apart from single case reports range from 15.1% to 50% and are contributed by various factors including delay in hospital admission, high viral load, older age group and presence of comorbid and complication.

CONCLUSION

A seroprevalence study can be conducted amongst the high-risk occupation group such as farmers and agricultural workers, as well as testing cases where viral fever is suspected but available tests for other diseases turns out negative.

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.

Environmental Risk Factors and Geographic Distribution of Severe Fever with Thrombocytopenia Syndrome in Jiangsu Province, China

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging natural focus, tick-borne disease caused by a novel bunyavirus named SFTS virus (SFTSV). The main purpose of this study was to analyze the environmental risk factors and geographic distribution of SFTS natural foci in Jiangsu Province. A retrospective space-time analysis by SaTScan software was used to detect clusters at the town level. The maximum entropy modeling method was applied to construct the ecological niche model and analyze the environmental risk factors, and then to draw the predicted risk map. The performance of the model was assessed using the area under the curve (AUC) and known occurrence locations. During the years 2010-2016, a total of 140 laboratory-confirmed indigenous SFTS cases occurred in Jiangsu Province, with 66 occurrence locations. The reported number of SFTS cases increased year by year and SFTS cases occurred from April to October with a peak between May and August each year. Three clusters detected by space-time scan statistical analysis were connected together and shared the similar ecological environmental characteristic of hilly landscape. Fifteen environmental variables with different percent contribution can influence the ecological niche model in different degrees, whereas slope (suitable range: 0.1-4) and maximum temperature of warmest month (suitable range: 32.8-34.2°C) as the key environmental factors contributed 46.1% and 23.2%, respectively. The model had high accuracy on prediction with the averaged training AUC of 0.926. Within a predicted risk map, potential areas at high risk and 10 previously unidentified endemic regions were recognized. The distribution of SFTS natural foci was under the influence of multidimensional environmental factors. Slope and maximum temperature of warmest month were the key environmental risk factors. These results provide a valuable basis for the selection of prevention and control strategies, and the identification of potential risk areas.

Community-based comprehensive measures to prevent severe fever with thrombocytopenia syndrome, China

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging disease caused by the SFTS virus (SFTSV) of the family Bunyaviridae. Since the virus was first isolated in 2009, it has become widespread in China, with an increasing number of cases year on year. Although the disease has been researched extensively in past years, there are still no effective measures to suppress the epidemic situation. This article reports a pilot study of comprehensive measures, including health education and risk communication, weed removal, livestock management, and tick control, to prevent this emerging disease in an endemic region of China. The density of ticks decreased dramatically month by month after acaricides were sprayed in the areas surrounding recreational and agricultural settings. The number of SFTS cases and villages involved declined in the years after the integrated measures were applied. Comprehensive measures, especially community-based tick control, may be a promising means of preventing SFTS in endemic regions.

Replication capacity and adaptability of a severe fever with thrombocytopenia syndrome virus at different temperatures

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging disease caused by the SFTS virus (SFTSV). Although fever and thrombocytopenia are the typical manifestations of SFTS, a specific SFTS case with no fever was observed in Zhejiang, China. In this report, we aimed to explore the probable reason for the absence of fever by analyzing the genetic characteristics and temperature sensitivity (ts) of the SFTSV strain ZJ2013-06, which was isolated from the specific case. Phylogenetically, different clusters of SFTSV strains circulated in Zhejiang. ZJ2013-06 was farthest from ZJ2014-02, an isolate belonging to a Chinese dominant cluster, and nearest to the coastal strain NB24/CHN/2013. Ts tests, performed on Vero cells at 37°C and 39°C, indicated that ZJ2013-06 had restricted replication at 39°C. Its viral loads were substantially reduced at 39°C compared with that at 37°C (approximately 100-fold reduction) and were significantly lower than that of ZJ2014-02 at 39°C (P < 0.01). By adaptive culture at 39°C, the induced strain ZJ2013-06-P7 was obtained. Owing to a reverse mutation (S1616), ZJ2013-06-P7 lost the ts of the original strain, displaying similar replication processes with NB24/CHN/2013. The results indicated that the amino acid residue 1616 was related to the ts characteristics of ZJ2013-06. Our study revealed that ZJ2013-06 was temperature-sensitive and may be related to the absence of fever in our case.

Impact of an intervention programme on knowledge, attitudes and practices of population regarding severe fever with thrombocytopenia syndrome in endemic areas of Lu'an, China

Knowledge, attitudes and practices (KAP) of the population regarding severe fever with thrombocytopenia syndrome (SFTS) in endemic areas of Lu'an in China were assessed before and after an intervention programme. The pre-intervention phase was conducted using a sample of 425 participants from the 12 selected villages with the highest rates of endemic SFTS infection. A predesigned interview questionnaire was used to assess KAP. Subsequently, an intervention programme was designed and applied in the selected villages. KAP was re-assessed for each population in the selected villages using the same interview questionnaire. Following 2 months of the programme, 339 participants had completed the re-assessed survey. The impact of the intervention programme was evaluated using suitable statistical methods. A significant increase in the KAP and total KAP scores was noted following the intervention programme, whereas the proportion of correct knowledge, the positive attitudes and the effective practices toward SFTS of respondents increased significantly. The intervention programme was effective in improving KAP level of SFTS in populations that were resident in endemic areas.