title: Age is a critical risk factor for severe fever with thrombocytopenia syndrome.
abstract: BACKGROUND
  Severe Fever with Thrombocytopenia Syndrome (SFTS) is an emerging infectious disease in East Asia. SFTS is a tick borne hemorrhagic fever caused by SFTSV, a new bunyavirus named after the syndrome. We investigated the epidemiology of SFTS in Laizhou County, Shandong Province, China.
  
  
  METHODS
  We collected serum specimens of all patients who were clinically diagnosed as suspected SFTS cases in 2010 and 2011 in Laizhou County. The patients' serum specimens were tested for SFTSV by real time fluorescence quantitative PCR (RT-qPCR). We collected 1,060 serum specimens from healthy human volunteers by random sampling in Laizhou County in 2011. Healthy persons' serum specimens were tested for specific SFTSV IgG antibody by ELISA.
  
  
  RESULTS
  71 SFTS cases were diagnosed in Laizhou County in 2010 and 2011, which resulted in the incidence rate of 4.1/100,000 annually. The patients ranged from 15 years old to 87 years old and the median age of the patients were 59 years old. The incidence rate of SFTS was significantly higher in patients over 40 years old and fatal cases only occurred in patients over 50 years old. 3.3% (35/1,060) of healthy people were positive to SFTSV IgG antibody. The SFTSV antibody positive rate was not significantly different among people at different age groups.
  
  
  CONCLUSIONS
  Our results revealed that seroprevalence of SFTSV in healthy people in Laizhou County was not significantly different among age groups, but SFTS patients were mainly elderly people, suggesting that age is the critical risk factor or determinant for SFTS morbidity and mortality.
authors.affiliation: School of Public Health, Shandong University, Jinan, Shandong Province, China; Department of Viral Infectious Diseases Control and Prevention, Shandong Center for Disease Control and Prevention, Shandong Provincial Key Laboratory of Communicable Disease Control and Prevention, Jinan, Shandong Province, China.
authors.forename: Shujun
authors.initials: S
authors.lastname: Ding
authors.affiliation: Key Laboratory for Medical Virology, MOH, National Institute for Viral Disease Control and Prevention, China CDC, Beijing, China; Tianjin International Travel Health Care Center, Tianjin, China.
authors.forename: Guoyu
authors.initials: G
authors.lastname: Niu
authors.affiliation: Department for communicable Diseases Control and Prevention, Laizhou City Center for Disease Control and Prevention, Laizhou, Shandong Province, China.
authors.forename: Xuehua
authors.initials: X
authors.lastname: Xu
authors.affiliation: Department for communicable Diseases Control and Prevention, Laizhou City Center for Disease Control and Prevention, Laizhou, Shandong Province, China.
authors.forename: Jinping
authors.initials: J
authors.lastname: Li
authors.affiliation: Department of Viral Infectious Diseases Control and Prevention, Shandong Center for Disease Control and Prevention, Shandong Provincial Key Laboratory of Communicable Disease Control and Prevention, Jinan, Shandong Province, China.
authors.forename: Xiaomei
authors.initials: X
authors.lastname: Zhang
authors.affiliation: Department for communicable Diseases Control and Prevention, Laizhou City Center for Disease Control and Prevention, Laizhou, Shandong Province, China.
authors.forename: Haiying
authors.initials: H
authors.lastname: Yin
authors.affiliation: Department for communicable Diseases Control and Prevention, Laizhou City Center for Disease Control and Prevention, Laizhou, Shandong Province, China.
authors.forename: Naijie
authors.initials: N
authors.lastname: Zhang
authors.affiliation: Department of Viral Infectious Diseases Control and Prevention, Shandong Center for Disease Control and Prevention, Shandong Provincial Key Laboratory of Communicable Disease Control and Prevention, Jinan, Shandong Province, China.
authors.forename: Xiaolin
authors.initials: X
authors.lastname: Jiang
authors.affiliation: Key Laboratory for Medical Virology, MOH, National Institute for Viral Disease Control and Prevention, China CDC, Beijing, China.
authors.forename: Shiwen
authors.initials: S
authors.lastname: Wang
authors.affiliation: Key Laboratory for Medical Virology, MOH, National Institute for Viral Disease Control and Prevention, China CDC, Beijing, China.
authors.forename: Mifang
authors.initials: M
authors.lastname: Liang
authors.affiliation: Department of Viral Infectious Diseases Control and Prevention, Shandong Center for Disease Control and Prevention, Shandong Provincial Key Laboratory of Communicable Disease Control and Prevention, Jinan, Shandong Province, China.
authors.forename: Xianjun
authors.initials: X
authors.lastname: Wang
authors.affiliation: School of Public Health, Shandong University, Jinan, Shandong Province, China; Department of Pathology, University of Texas Medical Branch, Galveston, Texas, United States of America.
authors.forename: Xue-jie
authors.initials: XJ
authors.lastname: Yu
mesh_terms: D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D002044:Bunyaviridae Infections / Q000097:blood / Q000401:mortality* / Q000821:virology; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D008297:Male; D008875:Middle Aged; D016856:Phlebovirus / Q000235:genetics / Q000276:immunology*; D012307:Risk Factors; D016019:Survival Analysis; D055815:Young Adult