| BackgroundScrub typhus is an acute febrile illness among humans that is caused by infectionwith the bacterium Orientia tsutsugamushi (O. tsutsugamushi) following the bite ofinfected mite vectors. Scrub typhus occurs over a wide area of eastern Asia to westernpacific region,and is a public health in Asia. After an incubation period that rangesfrom6to21days (usually,10-12days), the onset of disease is characterized by fever,headache, myalgia, cough, rash, and eschar. The diagnosis of scrub typhus is based onthe patient's history of exposure, clinical features and results of serologic test.Before1985, scrub typhus was known to be endemic south of the Yangtze River(below latitude31°N) in China. It had emerged in northern China since the diseasewas first reported in Shandong province in1986. Outbreaks of scrub typhus have nowbeen reported in several northern provinces, such as Tianjin, Hebei, Henan andJiangsu. But outbreaks of scrub typhus had not been reported in Anhui provincebefore2008. On October272008, many patients with unexplained fever werereported in Fuyang city, after diagnosis for measles, rubella, influenza and infectiousmononucleosis were excluded, they were diagnosed as scrub typhus finaly.ObjectivesIn order to identify case-patients to provide them with proper treatment, to analyzeepidemiological characteristic and risk factors, furthermore to identify the hostanimals, the chigger mite of scrub typhus and the genotypes of the prevalentO. tsutsugamushi in Fuyang city Anhui province, we carried out this investigation. Materials and MethodsEpidemiological Analysis on scrub typhus in China and Fuyang cityWe collected the data from National Disease Reporting Management System toanalyze epidemic trend of scrub typhus, and described the distribution of scrub typhusaccording to time, place and person in China.Case definition.A suspected case was defined as onset of fever (≥37.5℃), plus rash or swollenlymph nodes. A probable case was defined as onset of fever (≥37.5℃) pluscharacteristic eschar or lesion. A confirmed case was a suspected or probable casewith any of following: an IgM titer of≥1:32by Indirect ImmunofluorescenceAssay(IFA)；nucleic acid was positive by PCR or O. tsutsugamushi was isolatedthrough animal inoculation or cell culture.Risk factorsWe compared exposures of61probable case-patients with those of randomlyselected control-persons which were1:3frequency matched to case-families by age,sex and occupation.Investigations of the hostIn November2009, live rodents were caught with rat cages in10sites in the fieldsof Fuyang city. The sera of rodents were collected for antibody test and organs as apool such as livers, spleens and kidneys were dissected and inoculated directly intothe peritoneal cavities of three laboratory mice to isolate O. tsutsugamushi.Investigations of chigger miteChigger mites were collected from the ears of captured rodents. Some chiggermites as a pool were homogenized, then the homogenate was detected by nested PCR and inoculated into the peritoneal cavities of three mice to verify whether the mitescarried O. tsutsugamushi naturally. About200chigger mites were sent to Departmentof Infectious Disease, Chinese Central for Disease Control and Prevention for test.The Detection of antibodies to O. tsutsugamushiAll the sera from patients were kept in a-70℃refrigerator before detection. IFAwas used to detect and type the antibodies to O. tsutsugamushi in the sera.PCR and Sequence analysisDNA was extracted from blood clots by using a blood DNA extraction kit (Qiagen,Valencia, CA). The O. tsutsugamushi56-kDa protein gene was amplified by nestedPCR. The nucleotide sequences of O． tsutsugamushi isolates in Fuyang werecompared with those of other O. tsutsugamushi．Alignment of the nucleotide sequenceof O．tsutsugamushi was carried out using MEGA3.1．ResultsEpidemiological Analysis on scrub typhus in China,2009From2006to2010, the reported incidence rate of scrub typhus in China were0.09/100000,0.1/100000,0.19/100000,0.24/100000,0.31/100000respectively. In2009,3235patients were reported, and these patients mainly distributed in Yunnan,Guangdong, Anhui, Sichuan and Fujian province. The peak of onset was during Juneto September in southern parts of Changjiang River but during October and Neverberin northern parts of Changjiang River.48.72%of patients were female, persons aged70-74years had the highest attack rate (0.47/100000), followed by persons aged60-64years (0.44/100000), and persons aged65-69years (0.42/100000).Epidemiological Analysis on scrub typhus in Fuyang city,2008-2009The reported incidence rate of scrub typhus in Fuyang city in2008and2009were 2.07/100000and1.88/100000respectively. The peak of onset was during October andNevember.45.14%(79/175) in2008and47.17%(75/159) in2009were female,persons aged65-69years (7.26/100000) in2008and persons aged65-69years(7.19/100000) in2009had the highest attack rate respectively.Clinical manifestationsAmong97scrub typhus patients investigated in2009, the major symptomsincluded fever(100%), headache(82.5%), rash (77.3%), characteristic eschar (74.2%)and shiver (73.2%).Risk factors62.3%(38/61) of patients compared with52.7%(96/182) controls had labor history in cornfields (OR=3.05,95%CI1.08-8.61);29.5%(18/61) of patients compared with22.0%(40/182)controls had labor history in sweet potato fields (OR=5.65,95%CI1.44-22.22).Reservoir hostIn2009,99.4%(168/169) of mice caught outdoors were Apodemus agrarius and85.2%(115/135) caught indoors were house mouse in Fuyang city.2.2%(1/46)Apodemus agrarius was detected nucleic acid positive by PCR.83.3%(35/42) ofApodemus agrarius were infected with chiggers.Vector miteA total of1200chiggers were collected from35apodemus agrarius.1000chiggerswere divided into4groups for detection of nucleic acid of O. tsutsugamushi and1group was positive. All of200chiggers were identified as Leptorombidium scutellare.Sequence analysis98%(98/104) of patients had IgM antibodies to O. tsutsugamushi by IFA. Onestrain of O. tsutsugamushi was isolated from a patient.34gene segments that encodes the56-kDa antigen of O. tsutsugamushi were extracted from132patient specimen and3PCR products were sequenced by TA cloning. DNA sequencing revealed that thesequences from3patients were identical. This sequence was99%similar to theKawasaki genotypes on the nucleotide level.ConclusionThe reported scrub typhus patients were gradully increasing in recent years, withsummer type of scrub typhus in south of the Changjiang River and autumn-winter onein north of the Changjiang River.The epidemic focus of scrub typhus in Fuyang city is classified as autumn-wintetype. The main animal host and vector mite are apodemus agrarius andleptorombidium scutellare respectively. The genotype of isolated O. tsutsugamushiwas indentical with Kawasaki one.Working in corn and sweet potato fields is risk factor for infection of scrub typhus. |
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