Scrub typhus: vector competence of Leptotrombidium chiangraiensis chiggers and transmission efficacy and isolation of Orientia tsutsugamushi.

A total of 4,281 rodents, belonging to 20 species, was collected from three villages in Chiangrai Province, northern Thailand, from January 1994 to December 2001. The predominant species were Rattus rattus, R. losea and Bandicota indica, accounting for 96% of the total. More than 135,000 chigger mites were removed from wild rodents of which 1% were identified as Leptotrombidium chiangraiensis, a new species vector of scrub typhus. Five Orientia tsutsugamushi-infected L. chiangraiensis colonies established from chiggers removed from R. rattus and R. losea rodents. The prevalence of O. tsutsugamushi infection in colonies ranged from 7-89%. Vertical and horizontal transmission efficacies of O. tsutsugamushi were stable between generations among colonies of L. chiangraiensis. The two isolates of O. tsutsugamushi obtained from individual adult L. chiangraiensis mites represent the first successful isolation of this bacterium from individual adult mites. Characterization of the O. tsutsugamushi isolates is under way.

Characterization of Orientia tsutsugamushi isolated from wild-caught rodents and chiggers in northern Thailand.

We previously reported Orientia tsutsugamushi detection from tissue samples (kidney, liver, spleen, and whole blood) of 12 wild-caught rodents from Chiangrai Province, northern Thailand. Of the 30 chiggers individually removed from scrub typhus-infected rodents, 2 were found positive for O. tsutsugamushi. We further characterized the O. tsutsugamushi detected from these rodents and chiggers by RFLP using three different enzyme digestions. All 14 O. tsutsugamushi samples (12 from tissue samples and 2 from chiggers) showed different digestion patterns when compared to those of reference strains (Karp, Kato, and Gilliam). Interestingly, nine RFLP profiles were observed from these 14 samples suggesting the presence of high genetic diversity of O. tsutsugamushi in this area. Furthermore, one sample displayed the same RFLP pattern as that of O. tsutsugamushi mild resistant strain previously isolated from scrub typhus patient in Chiangrai. Of the two samples from positive chiggers, only one was found to have a similar RFLP pattern to that of its host rodent. DNA sequencing of the entire 56 kDa genome of these O. tsutsugamushi samples is in progress.