Background Human rickettsioses are worldwide zoonoses and it is not easy to differentiate them from other infectiousdiseases because of their atypical manifestation.In recent years the number of patients with fever of unknown causesfrom Hongta District CDC,Yuxi city of Yunnan Province has been increasing significantly in the summer.Diagnosis ofscrub typhus was made by local clinicians.In order to ascertain the disease,we undertook a laboratory investigation forsuch patients from August 18 to 26,2005.Methods Active surveillance was conducted by Hongta District CDC Yuxi city of Yunnan Province from 2002 to 2004and basic data were obtained from cases confirmed according to clinical definitions.Average incidences and town-levelincidences were calculated during the study periods.Blood samples were analyzed by PCR and serological test.Basedon the groEL gene sequences a paired general outer primers(Gro-1 and Gro-2)targeting typhus,spotted fever as well asscrub typhus and two paired inner primers(SF1,SR2 and TF1,TR2)for typhus together with spotted fever and scrubtyphus,respectively,were designed to perform a multiplex-nested PCR.Serological assay was carried out by indirectimmunofluorescence assay with 7 different rickettsial antigens,i.e.,R.mossori,R.sibirica,R.conorii,O.tsutsugamushi,B.quintana,B.henselae and Coxilella burnetii phase Ⅱ Ag.Results Epidemiological surveillance showed that from 2002 to 2004,the average incidences of the scrub typhus orscrub typhus with murine typhus were 222.1/10~5,204.3/10~5 and 109.6/10~5,respectively.Of 13 blood samples takenduring acute stage of illness,6 showed the amplified products for scrub typhus and the sequenced products showed100%,99%,99%,99%,99%,99% similarity to O.tsutsugamushi Karp but they shared the same deduced amino acidsequences,which indicated 100% identity with the heat shock protein of the O.tsutsugamushi Karp strain.Five yieldedPCR products for murine typhus and their corresponding nucleotide sequences exhibited 100%,100%,99%,99% and99% similarity to R.mossori Wilmington and the analyses of predicted amino acid sequences indicated 100%,100%,98%,98% and 98% identity with the heat shock protein of R.mossori Wilmington strain.Of the 8 PCR positive patients,3showed a co-infection of scrub typhus with murine typhus.All the 13 serum samples from febrile patients were positiveagainst O.tsutsugamushi and 8 of them were positive against R.mossori.All of the 8 paired specimens had four-foldelevation of antibody against O.tsutsugamushi,and seroconversion for typhus was demonstrated in 3 paired serumsamples.Another finding in the study was that a high seropositive prevalence(76.9%)of Q fever was detected.Conclusion It’s confirmed that co-prevalence of scrub typhus with murine typhus are occurring in Yuxi city of Yunnanprovince,China.Other rickettsial diseases also need to be investigated in these areas.Chin Med J 2007;120(15):1314-1318 Background Human rickettsioses are worldwide zoonoses and it is not easy to differentiate them from other infectiousdiseases because of their atypical manifestation.In recent years the number of patients with fever of unknown causesfrom Hongta District CDC, Yuxi city of Yunnan Province has been increased significantly in the summer. Diagnosis of scrub typhus was made by local clinicians. In order to ascertain the disease, we undertook a laboratory investigation forsuch patients from August 18 to 26, 2005. Methods Active surveillance was conducted by Hongta District CDC Yuxi city of Yunnan Province from 2002 to 2004and basic data were obtained from cases confirmed according to clinical diagnosis .s.Blood samples were analyzed by PCR and serological test. Based on the groEL gene sequences a paired general outer primers (Gro-1 and Gro-2) targeting typhus, spotted fever as well asscrub typhus and two paired inner prim ers (SF1, SR2 and TF1, TR2) for typhus together with spotted fever and scrubtyphus, respectively, were designed to perform a multiplex-nested PCR. Serological assay was carried out by indirectimmunofluorescence assay with 7 different rickettsial antigens, ie, R. mossori , R.sibirica, R.conorii, O.tsutsugamushi, B.quintana, B.henselae and Coxilella burnetii phase II Ag.Results Epidemiological surveillance showed that from 2002 to 2004, the average incidences of the scrub typhus orscrub typhus with murine typhus were 222.1 / 10 ~ 5, 204.3 / 10 ~ 5 and 109.6 / 10 ~ 5, respectively.Of 13 blood samples takenduring acute stage of illness, 6 showed the amplified products for scrub typhus and the sequenced products showed 100%, 99%, 99%, 99 %, 99%, 99% similarity to O. tsutsugamushi Karp but they shared the same deduced amino acid sequence, which shows 100% identity with the heat shock protein of the O. tsutsugamushi Karp strain. Family yieldedPC products for murine typhus and their corresponding nucleotide sequences exhibits 10 0%, 100%, 99%, 99% and 99% similarity to R. mossori Wilmington and the analyzes of predicted amino acid sequences indicated 100%, 100%, 98%, 98% and 98% identity with the heat shock protein of R. mossori Wilmington strain. Of the 8 PCR positive patients, 3 showed a co-infection of scrub typhus with murine typhus. All the 13 serum samples from febrile patients were positive for indigestion O. tsutsugamushi and 8 of them were positive against R. mossori. All of the 8 paired specimens had four-foldelevation of antibody against O. tsutsugamushi, and seroconversion for typhus was demonstrated in 3 paired serum samples. Another finding in the study was that a high seropositive prevalence (76.9%) of Q fever was detected. Confidence It’s confirmed that co-prevalence of scrub typhus with murine typhus are occurring in Yuxi city of Yunnanprovince, China.Other rickettsial diseases need to be investigated in these areas. Chin Med J 2007; 120 (15): 1314-1318