| Scrub typhus, caused by the bite of chiggers infected with Orientia tsutsugamushi, was known to occur mainly in south of Yangtse River. It has expanded to north of the Yangtse River in 1986. It has been unknown that the natural foci of Ot infection in Inner Mongolia and Xinjiang located in northernmost of China. The objectives of the study were to determine the presence of Ot and to understand its molecular characteristics.Indirect immunofluorescent antibody assay was carried out to investigate the natural infection with Ot in humans of Inner Mongolia and Xinjiang. The positive rates were 11.11% and 11.56 % respectively. Unconditional multivariate logistic regression analyses were performed with Ot infection as the dependent variable, and revealed that sex and study sites were associated with Ot infection (p<0.05).A nested polymerase chain reaction was applied to detect Ot in rodents. The overall prevalence rates were 3.57% and 2.50% respectively in Inner Mongolia and Xinjiang. In Inner Mongolia, Apodemus agrarius was the dominant species. 108 chiggers were collected from rodents in September, 2006. The total chigger index was 0.49. And the chiggers were consist of Leptotrombidium intermedium and Walchia. L. intermedium was the dominant species in September.The nucleotide sequences of 509-bp Ot-Sta56 gene from the above positive specimens as well as strains from Guangdong, Shandong and Liaoning provinces were analyzed. Three samples belonged to Karp types, one sample was Oishi type, the others (N65, N88) varied from known Ot agents. Furthermore, the full length of Sta56 gene was amplified and analyzed to identify the variation. The sequences of sample N65 were different from other samples and known strains in GenBank. The phylogenetic analysis revealed that N65 was a noval variation.These findings indicate a threat of scrub typhus in northernmost of China, and provide the foundation for the control and prevention of the disease.
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