Study On Epidemic Characteristics And Risk Factors Of Severe Fever With Thrombocytopenia Syndrome

Severe fever with thrombocytopenia syndrome (SFTS) was a new vector-born viral infectious disease and was emerged in 2009 in China. Clinical manifestations included high fever, thrombocytopenia, leukocytopenia and multiple organ dysfunctions. Most cases were farmers over 50 years old. The case fatality rate was 12-30%. The pathogen was a novel bunyavirus which was first isolated in 2009 by research group directed by professor Yu Xue-jie. The new virus was recorded as SFTS virus (SFTSV) by International Committee on Taxonomy of Viruses now. SFTS is endemic in Shandong Province now. We know very little about the epidemic feature, risk factors, transmission and strategy for prevention and control of SFTS because of its novelty. Only a few studies on SFTS were available in public data. As one of newly emerged infectious diseases in recent years, severe fever with thrombocytopenia syndrome's infection status in the population, latent infection, and the prevalence in all age groups is not clear.Between 2011 and 2014,5352 suspected probable and lab-confirmed cases of SFTS were reported in 23 provinces in China. SFTS was endemic in Shandong Province and the total SFTS cases reported in Shandong Province from 2011-2014 ranked second in China. Only limited studies were carried out in some pandemic areas at municipal or county level. Studies on healthy population, host animals and vectors in China were mainly focused on endemic areas of STFS, while few researches were performed in non-endemic areas. Studies on risk factors of SFTS, specially genetic susceptibility is rare. The dissertation was divided into two parts. One part was focused on the epidemiological characteristics of SFTS in Shandong Province; the other was on the risk factors. It included the descriptive analysis on epidemic features and detecting specific SFTSV antibodies or SFTSV RNA in serum samples from healthy individuals, suspicious host animals and vectors in the first section. To looking for the external risk factors and internal risk factors of SFTS was the main task of the second part. The association between SFTS and HLA-A, HLA-B, HLA-DRB1 was studied for searching the internal risk factors of individuals.This study would be helpful to deeply understand the disease epidemic characteristics and mechanism of SFTS, improve public awareness of protection, provide basic study data for the prevention and control the incidence and prevalence of new infectious diseases.Objectives1.to illustrate the epidemic and clinical characteristics of SFTS patients and to determine risk factors for SFTS;2.to conduct serum epidemiological study on people, host animals and vectors infected by SFTS virus and to analyze people and host animals infection conditions, antibody levels and the relationship among SFTS-vetors-animals.3. to search the external risk factors and internal risk factors of SFTS, and to investigatethe association of HLA locus A, B and DRB1 alleles with the occurrence of SFTS.Methodsl.The regional distribution, time distribution and population distribution of SFTS cases from 2010 to 2014 in Shandong Province were analyzed using descriptive epidemiological methods. The analyzed parameters included the number of cases, incidence rate,and constituent ratio. Statistical analysis was performed with SPSS 18.0.2. According to previous outbreaks reported in Shandong Province,2 counties of high incidence(Daiyue and Laizhou),2 counties of very low incidence (Ningyang and Gaotang)were selected to conduct SFTSV specific antibodies or SFTSV RNA in healthy individuals, suspicious host animals and vectors.3. The objects were divided into patients group and control group and were selected according to the proportion of 1:3 for the case-control study. The HLA locus A, B and DRB1 alleles were detected to find the correlation between HLA locus A, B and DRB1 alleles with SFTS.4^ Using Microsoft Office Excel 2007 and Epi data 3.1 to summarize and organize data, using SPSS 18.0 software for statistical analysis. Using chi-square test for qualitative data, P value less than 0.05 was considered as statistically significant. The frequency difference between the patient group and control group was compared using ?2 test or the Fisher's exact test. The extent of correlation of HLA alleles and haplotypes between the patient group and the control group was indicated by the odds ratio (OR). A corrected P value was calculated with a bonferroni correction. Correct P-values less than 0.05 was considered significant.Results(1)The epidemic feature of SFTS in Shandong Province:In total 1,155 SFTS cases were reported in Shandong Province including 142 death cases from 2010-2014. The case fatality rate was 12.29%. The average incidence rate was 0.2264/100 thousands and the average mortality rate of SFTS was 0.02682/100 thousands from 2010 to 2014. Region distribution:Except for Heze and Jining cities, the other 15 municipalities in Shandong Province all had SFTS case reports. The top five municipalities were Yantai,Weihai,Tai'an,Weifang and Jinan, which accounted for 80.44% of reported SFTS cases in the province from 2010-2014. The reported SFTS cases were distributed in 77 villages of 33 towns in 6 counties of 5 municipalities in 2010 and expanded to 441 villages of 249 towns in 63 counties of 15 municipalities in 2014. The epidemic areas of SFTS tended to spread from the east to the west. Time distribution:SFTS cases showed a significant seasonal distribution each year with a peak in June and July. Population distribution:575 cases were male,514 cases were female with a male to female ratio of 1.12:1. The elderly accounted for 93.94% of SFTS cases, of which 55-69 year-old age group was the highest, accounting for 44.72%, the median age was 60. Deaths were reported overthe age of 35,75.97% of the fatal cases were age 55 to 79; 85.49% of reported cases were farmers. (2)Clinical features:The periods from the onset of symptoms to the time of admission varied from 0-15 days, with the media of five days. All of the 267 lab-confirmed patients got fever, of which 88.30% cases had fever as the first symptom.25.66% of the 267 cases accompanied with lymph node enlargement.85.02% of the cases accompanied with white blood cells or platelets below normal values.70%-80% of cases accompanied with the elevation of alanine aminotransferase (ALT) and aspartate transaminase(AST). (3)Transmission route:In the first two weeks of the onset of the disease,71.69% of the cases had worked in the mountains or farmland.55.25% of cases saw ticks one month before onset of symptoms. Only 26 SFTS patients stated they had the history of being tick bitten recently before onset of symptoms. Breeding animals were common in the families of 155 cases. Ticks were found in the skins of some breeding animals in some patients' households.(2) Infection status of healthy people:The total positive rate to SFTSV antibody was 1.22%(63/5154). SFTSV antibody positive rate of high incidence county was higher than low-incidence counties (1.66% versus 0.80%).The difference was significant(p< 0.05). Statistical analysis indicated that the antibody positive rate was significantly different between people from different regions (x,2=51.16, P<0.01). Positive samples were detected from all eight age groups, of which 70-group has the highest antibody positive rate. Statistical analysis of different age groups showed no significant difference (p> 0.05) among people from four regions (.P>0.05). Although the antibody positive rate differences between the sexes was not statistically significant(0.98% versus 1.82%,?2=2.19, p>0.05), male or female total antibody positive rates were higher in high incidence counties than that in low-incidence counties. Except for children, farmers, students, workers were all detected SFTSV specific antibodies, the antibody positive rates were 1.34%,1.35% and 0.37%, respectively. In these three occupational groups antibodies difference was not statistically significant (?2=3.74, p>0.05).(3) Infection status of host animals and vectors:The total SFTSV antibody positive rate among animals from different regions was 15.65% (359/2294).2294 animal blood samples, SFTSV antibody positive rates of goats,cattle,dogs, pigs and chicken blood samples were 28.45%(103/362), 26.71%(129/483),9.43%(35/371),4.15%(31/747) and 24.11%(61/253), respectively, and specific antibodies were not detected from duck (0/78). SFTSV antibody positive rate among animals from different regions was statistically significant (P<0.05). SFTSV antibody positive rate of each animal in high incidence county was higher than low-incidence counties; the rate difference was statistically significant. A total of 1,558 mosquitoes were divided into different groups and grinded fully for SFTSV nucleic acid detection. The results of mosquitoes were all negative. A total of 876 ticks were divided into 61 groups for SFTSV nucleic acid detection. Two groups of no blood sucking Haemaphysalis longicornis ticks obtained in Laizhou were positive for SFTSV. The minimal infection rates(MIR) was 2.3% for ticks. No ticks captured in other counties were SFTSV RNA positive.(4) Risk factors:1)The 5 risk factors of SFTS were the most important independent factors including feeding dog strolling, contact with the cat one month before the onset of the disease, pull out tick from skin by hands, working in the fields, living with weeds and shrubs flourishing around the houses.2)The frequencies of A*30B*13 and A*30-B*13in the SFTS patient group were lower than that in the control group with no significant difference(Pc<0.05). A*30-B*13-DRB1*07 and A*02-B*15-DRB1*04 may have strong associations with SFTS resistance and susceptibility respectively (Pc=0.0412 and 0.0001, OR=0.43 and 5.07).Conclusions:(1) Shandong Province is one of the major inflicted provinces of severe fever with thrombocytopenia syndrome, and the number of reported cases increased yearly.The cases were mainly occurred in rural population in mountainous areas. SFTS cases were highly sporadic and relatively concentrated, spreading from central mountain area of Shandong to its surrounding counties. Disease incidence among men is slightly higher than it among women, but the difference was not significant. Main cases were the elderly people aged over 40, among which,aged 60-69 were the most cases, and more than 90% of cases were farmers who had high mortality. Severe fever with thrombocytopenia syndrome case numbers varies by seasons, showing a single peak from April to November, with the highest incidence in June and July.(2)Antibody level among high and low-incidence counties varies, but there was not statistically significant. People of different ages, different occupations, and different gender groups from high and low-incidence counties can be infected by SFTSV, but SFTSV infection rate was not significantly different among all these groups. Animal antibody positive rate to SFTSV in high-incidence counties was significantly higher than that of low-incidence counties. Antibody positive rate of sheep, cattle was significantly higher than the rate of dogs and pigs, and the difference was statistically significant.(3) The host HLA class ? polymorphism might play an important role with the occurrence of SFTS. Negative associations were observed with HLA-A*30, HLA-B*13 and Haplotype A*30-B*13, although the associations were not statistically significant. A*30-B*13-DRB1*07 had negative correlation with the occurrence of SFTS; in contrast, haplotype A*02-B*15-DRB1*04 was positively correlated with SFTS.Significance and innovation(1) This study took the lead to established the database of more than 1000 SFTS cases in Shandong Province. It described and analyzed the epidemiological features of SFTS cases from high-incidence counties and population reported in in Shandong Province over the years.(2) For the first time, the study made a comparative study on the infection status of healthy population and host animals with large sample sizes in endemic and non endemic areas, the results will provide a scientific basis for SFTS prevention and control.(3) For the first time, environmental risk factors and genetic disease risk factors were combined together in the study. The association study of HLA alleles with SFTS was firstly carried out to try to disclose the susceptible and protective genes with the occurrence of SFTS. Our study may provide scientific basis for study on pathogenesis and vaccine development.