Study On Epidemiological Characteristics And Transmission For Severe Fever With Thrombocytopenia Syndrome

Objectives:(1) We described the distribution of cases of sever fever with thrombocytopenia syndrome(SFTS) to explore the three distribution patterns of endemic diseases, analyzed clinical features to establish disease severity indicators, and carried out pathogen detection to explore pathogen spectrum of SFTS. (2) To explore the risk factors of SFTSV infection by analyzing the exposure history of cases. (3) To explore the transmission and risk factors of SFTSV by analyzing its distribution and infection status among humans, host animals, and vectors combined with genetic evolution analysis.Methods:(1) Described the distribution of SFTS cases diagnosed from 2011 to 2015 using descriptive analysis. (2) With death as the end point of the research, comprehensively analyzed of the risk evaluation indexes on severity of SFTS confirmed cases, using percentile frequency distribution method to establish index severity grade and with mental state Glasgow coma index to formulated SFTS severity rating criteria. (3)Explored the characteristics of the pathogenic spectrum of SFTS cases through the pathogenic detection of SFTS cases. (4) A case-control study based on hospital data was applied to detect the potential risk factors for SFTS in the regions of endemicity of Hubei Province. (5) Made an in-depth investigation of villages to svstematicallv understand population distribution and transmission of SFTSV among humans, host animals, and vectors, in which there were confirmed cases of new infections. (6)Analyzed SFTSV phylogenetic characteristics and transmission among different population using phylogenetic analysis.Results:(1) The description epidemiological analysis results of cases showed SFTS cases had high morbidity from May to July and disseminate from April to October, mainly concentrated in the Dabie mountain area of Huanggang, suizhou. There were more SFTS cases among farmers, mainly engaged in agricultural work in hilly areas. Epidemic of SFTSV infected cases was highly correlated with ticks density curves (r=0.84), and mortality rate increased along with age increase (?2=115.76, P=0.000). (2) SFTS cases characterized fever with platelet and white cell reduced. Median course of SFTS was 15 days and mortality rate was about 5.97%,which was significantly lower than the national average.PLT was relatively low at the early stage of SFTS course and began to increase at around 10th day and recovered at around 15th day. Age, virus load, platelet number AST, LDH, CK, MB(CK-MB) were related to SFTS severity, death cases have a unique cytokine pattern, and the grade determination and grading standards of the severity of the disease were established. (3) Pathogenic characteristics showed that SFTS cases were mainly infected with SFTSV, some negative cases were detected with syndrome hemorrhagic fever virus and brucella, no human granulocytic anaplasmosis was detected, all cases were not detected with two or more pathogens infection. (4) A case-control study found "contact with cattle ticks" was the major risk factor after adjusting for tick exposure (Conditional Logistic Regression OR-MH=8.62,95% CI=1.79-41.51), and outdoor activities and working in a weedy environment and in hillside fields could increase the risk of cattle tick contact and SFTS infection (Conditional Logistic Regression OR-MH=8.82,95% CI= 1.69-46.05). (5) The results of in-depth investigation showed that domesticated animals had highest SFTSV carrier rate (14.00%), followed by ticks (3.11%) and humans(1.67%), which was significantly different (?2=23.67, P<0.05); For domesticated animals specimens, PCR detection indicated that cattle serum specimens had highest positive rate (26.32%), followed by goats(9.09%); For ticks specimens, haemaphysalis longicornis was the dominant species (70.82%), with the highest nucleotides positive rate (4.95%); Ticks from domesticated animals were found having the highest SFTSV nucleotides positive rate (5.13%), followed by those from goats (2.30%) and grass (1.89%); SFTSV antibody positive rate of humans was significantly lower than that of domesticated animals, and SFTSV antibody was not found in wildlife. Among domesticated animals serum specimens, cattle had the highest SFTSV antibody positive rate (73.68%,14/19), followed by goats (59.09%, 13/22); "Tick bites" were a risk factor for SFTSV infection (Conditional Logistic Regression OR=2.45,95%CI,1.01-6.39) and close contact with SFTS infected cases was not found to be a risk factor for natural transmission. SFTSV in humans and ticks in the same environment was found a high consistency on SFTSV genetic evolution (DNA fragment S homology:98.00%). (6) phylogenetic analysis showed fragment S, M, L of SFTSV in Hubei can be found in 3 evolution lineages in China and the lineages were highly homologous. What's more, there were no significantly different in terms of population (cases and ticks), illness status (light cases, sever cases and death cases), time, age, occupation and area (nucleotide homology:95.00%-100.00%, amino acids homology:97.00%-100.00%). Additionally, compared with other provinces, fragment M independently form a lineage.Conclusions:(1) This study illustrated distribution of SFTS cases and explored the characteristics of pathogenic spectrum of SFTS cases in Hubei province. (2) The present study preliminarily demonstrated that SFTS cases'clinical feature and cell factor level were associated with SFTS course and severity. Established the grade determination and evaluation criteria of severity index, which could provide a good reference and useful exploration for the evaluation, judgment and treatment of severe and critical illness cases. (3) Provided systematic evidence on SFTSV population distribution and transmission mode for SFTSV from reservoir hosts (domesticated animals) to vectors (H. longicornis) to humans, and close contact with SFTS confirmed patients was not found to be a risk factor for natural transmission. However, "contact with cattle ticks" and "tick bites" were the main risk factors for exposure of SFTSV. SFTSV gene was highly homologous among different population, illness status, time, age, occupation and area.