Following rodent surveys in a rice-growing area of central Thailand where we found Bandicota savilei, B. indica, and Rattus rattus infected with Rickettsia tsutsugamushi, we performed a study of pathogenesis of R. tsutsugamushi in laboratory-reared B. savilei. Eight animals were injected with saline and 19 animals were injected with 4.0 x 10(6) mouse 50% lethal dose units of a strain of R. tsutsugamushi isolated from a human in central Thailand. Animals were evaluated at intervals for IgG and IgM antibodies to R. tsutsugamushi by an indirect immunoperoxidase assay, the presence of the pathogen in liver and spleen by murine inoculation, and the pathology of representative tissues by gross and microscopic examination. The infected animals began to show internal evidence of mild illness 7-14 days after inoculation, and exhibited no changes in behavior. Total white blood cell counts decreased on day seven (including lymphocytes and polymorphonuclear leukocytes), followed by an almost equal increase on day 14. Gross pathology noted at necropsy was limited to slight liver and spleen enlargement accompanied by low numbers of abscesses and fibrinous tags present in the abdominal cavity. In addition to the gross morphologic changes, histopathologic lesions noted were all mild, consisting of vasculitis of the lung, activation of the mononuclear phagocyte system, abdominal mesothelial cell hyperplasia, and peritonitis. Rickettsiae were isolated from liver and spleen on days 0, 7, and 14, but not thereafter. Specific antibody response was first observed on day 14, peaked on day 21, and it decreased to levels observed in uninfected animals between days 120 and 180.(ABSTRACT TRUNCATED AT 250 WORDS)