Study On The Transmission Of Severe Fever With Thrombocytopenia Syndrome Virus In Different Tick Species

Background Severe fever with thrombocytopenia syndrome(SFTSV)is a new infectious disease caused by Severe fever with thrombocytopenia syndrome virus(SFTSV),also known as new bunyavirus,which first occurred in rural areas of China in recent years.The virus was identified as a novel bunyavirus in the genus of Banyangvirus,family Phenuiviridae,order Bunyavirales.The age of onset of the disease is between 25 and 80 years old,with an average age of 61 years.The latest research shown that the case fatality rate is as high as 16.2%(95% CIRV 14.6% Mel 17.8%).The main areas of SFTS occurred are concentrated in China,Japan,South Korea and other Asia countries,and the cases have also been reported in the United States.The cases of the SFTS was reported in present at 25 provinces in China.(The high case fatality rate and increasingly wide geographical distribution of the disease make it an urgent public health problem in the world.)As a newly discovered virus,the pattern of SFTSV transmission in nature is unclear.Since most of patients has a history of tick bitting before the onset of the disease,and specific SFTSV gene segment has been detected from the ticks collected from animal and wide fild,Ticks are considered to be the main transmission vector of SFTSV.At present,it is only confirmed that Haemaphysalis longicornis is as a transsimition vector of SFTSV.The other species of tick may be a transsimition vector because of detected the SFTSV in Rhipicephalus microplus,Rhipicephalus sanguinensis,Haemaphysalis concinna,Haemaphysalis flava and Ixodes nipponensis.Therefore,it is of great significance to determine the other species of tick as transsimintion vector of SFTSV.Research purpose(1)To explore the distribution of common tick species in the epidemic area of fever with thrombocytopenia syndrome,and determine the potential transmission vector of SFTSV.(2)To explore which ticks served as an efficient vector capable of transstadial and transovarial transmitting SFTSV by experimental study.(3)To understand the SFTSV infection of ticks in natural environment.MethodThe distribution description of common ticks and SFTS cases in SFTS epidemic area was obtained by literature search,and the tick species to be studied were determined by Arc Gis 10.0 software.Our laboratory had established colonies of H.longicornis,I.sinensis,I.persulcatus and D.silvarum ticks,kept at 26 ℃,RH(95±5)%,L: D(12h:12h),which were used for the subsequent experimental transmission study.Adult ticks were infected by artificial microinjection,and larvae and nymphs were infected by biting SFTSV positive mice.Real-time quantitative PCR was used to detect SFTSV of infected ticks at different developmental stages.Real-time quantitative PCR and indirect IFA(Indirect Fluorescence Assay)were used to detect the infection of SFTSV in mice or rabbits bitten by infected ticks.In this study,the development time of each tick species in the two groups were statistically tested by Mann-Whitney U test with SPSS 17.0 statistical analysis software.ResultBased on literature review,H.longicornis,R.microplus,D.silvarum,I.persulcatus and I.sinensis ticks were determined to be the top 5 tick species that bites human beings in China.H.Longicornis ticks are mainly distributed in north and central of China.R.microplus ticks are distributed in southwest and central of China.D.silvarum ticks are most common in northeast and northwest of China.I.persulcatus ticks are mainly distributed in the north of China.I.sinensis ticks are mostly distributed in northeast and southeast coastal areas of China.These five ticks are common and widely distributed in SFTS endemic regions.The R.microplus ticks is the main ectoparasite of cattle and considered to be the most important external parasite impacting the cattle industry in the world.There are currently no reports of biting human beings by R.microplus ticks.Tick colonies have been established in our laboratory,which included H.longicornis,I.sinensis,I.persulcatus and D.silvarum ticks.SFTSV nucleartide segments were positive in the adult infected ticks for as long as 21 days in H.longicornis ticks,18 days in I.sinensis ticks,9 days in I.persalcatus ticks,and 6 days in D.silvarum ticks.SFTSV RNA were positive in all the 15 pools of eggs laid by infected I.sinensis ticks.10 of 25 pools of larvae hatched from the infected I.sinensis ticks were positive for SFTSV RNA.None of the egg and larvae pools from I.persalcatus and D.silvarum ticks was positive.SFTSV can be transmitted horizontally to normal mice by infected I.sinensis ticks at 7 days after feeding,but not for I.persalcatus or D. silvarum ticks.I.sinensis ticks served as an efficient vector capable of transstadial transmitting SFTSV,but neither in I.persulcatus nor in D.silvarum ticks.For the transstadial transmission study from larvae to nymph,the engorged larval pools of I.persulcatus ticks shown a positive rate of 55%(11/20),and the molted nymphs was negative(0/20).The positive rate was 100%(10/10)for engorged larval pools and 81.25%(13/16)for molted nymphs in D.silvarum ticks.For the transstadial transmission study from nymph to adult,engorged nymphs of each tick species were all found to be positive for SFTSV,and none of the adult ticks derived from the nymphs were infected with SFTSV.During the transstadial transmission experiment,all uninfected animals bitten by nymphs or adults were negative.These results indicated no transstadial transmission of SFTSV for either I.persulcatus or D.silvarum ticks.The SFTSV were negative in I.persulcatus ticks collected from Liaoning province and D.silvarum ticks collected from Inner Mongolia.The positive rate of SFTSV was 4.1% in H.longicornis in Henan province.The positive rate of SFTSV in H.longicornis ticks collected from sheep in Shandong,Liaoning and Shaanxi province was 14.8%,74.3% and 5.12%.The SFTSV was negative in R.microplus ticks collected from dog of Zhejiang province.ConclusionH.longicornis,I.sinensis,I.persalcatus,R.microplus,D.silvarum ticks are widely distributed in SFTS epidemic areas.Feeding ticks under laboratory conditions shortened the time of their life cycle.It took an average of 134 days for I.sinensis ticks,142 days for I.persalcatus ticks,81 days for D.silvarum ticks and 120 days for H.longicornis ticks.The present study shown that I.sinensis ticks except for H.longicornis ticks,also served as an efficient vector capable of transstadial transmitting SFTSV,therefore posing as a potential threat in causing the circulation of SFTSV.In contrast,D.silvarum and I.persalcatus ticks might not serve as an efficient vector of transmitting SFTSV.SignificanceSevere fever with thrombocytopenia syndrome has become an important public health problem worldwide,which is a continuous threat to human health.In this study,starting with the transmission vector,the potential transmission vector of SFTSV was found through epidemiological studies,and the ability of different ticks to transmit SFTSV during transovarial transmission and transstadial transmission experiment studies.It was confirmed for the first time that I.sinensis ticks is a potential vector of SFTSV.It was proved that D.silvarum and I.persalcatus ticks could not transmit SFTSV,they may not be the vectors of SFTSV.This study provides new ideas and strategies for the prevention and control of severe fever with thrombocytopenia syndrome.