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Abstract. Extensive sampling of small mammals was conducted in eight provinces of Thailand between September 9, 1992 and April 29, 2001. A total of 3,498 specimens representing 22 species were collected. Eighty-eight percent (3,089 of 3,498) of the animals were collected from a region in Chiangrai Province, which is commonly recognized as endemic for human scrub typhus. Blood and tissue samples from each animal were tested for the presence of Orientia tsutsugamushi, the etiologic agent of scrub typhus. The predominant species collected were Rattus rattus (53%, n ‫ס‬ 1,863), R. losea (18%, n ‫ס‬ 638), Bandicota indica (16%, n ‫ס‬ 564), and R. exulans (4%, n ‫ס‬ 146). Orientia tsutsugamushi was detected in 10 of the 22 species of mammals that included R. bukit (25% infected, 1 of 4), R. rattus (23%, 419 of 1,855), R. argentiventer (22%, 5 of 23), R. berdmorei (22%, 2 of 9), R. losea (13%, 82 of 638), B. indica (

Experiments were conducted to investigate the potential for transmission of Orientia tsutsugamushi, the aetiological agent for scrub typhus, when naturally infected mite larvae were co-feeding with uninfected larvae. Larvae from colonies of Leptotrombidium deliense and L. imphalum infected with O. tsutsugamushi were used. Transmission of O. tsutsugamushi to previously uninfected L. deliense and Blankaartia acuscutellaris co-fed with infected L. deliense was shown to occur. The overall minimum rate of acquisition was 1.6% (4/258) for L. deliense and 2.5% (3/119) for B. acuscutellaris. When individual infected L. deliense were co-fed with B. acuscutellaris acquisition of O. tsutsugamushi was not detected. However, when 4 and 8 infected larvae were co-fed with B. acuscutellaris acquisition of O. tsutsugamushi was detected. Transmission of O. tsutsugamushi was not observed when uninfected L. deliense were co-fed with infected L. imphalum. This novel transmission route may explain the occurrence of rickettsiae in genera other than Leptotrombidium spp, which are considered to be the main vectors of O. tsutsugamushi.