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Background  Brazilian spotted fever (BSF), caused by the bacterium  Rickettsia rickettsii,  is the deadliest spotted fever of the world. In most of the BSF-endemic areas, capybaras ( Hydrochoerus hydrochaeris ) are the principal host for the tick  Amblyomma cajennense,  which is the main vector of BSF.    Methods  In 2012, a BSF case was confirmed in a child that was bitten by ticks in a residential park area inhabited by  A. cajennense- infested capybaras in Itú municipality, southeastern Brazil. Host questing  A. cajennense  adult ticks were collected in the residential park and brought alive to the laboratory, where they were macerated and intraperitoneally inoculated into guinea pigs. A tick-inoculated guinea pig that presented high fever was euthanized and its internal organs were macerated and inoculated into additional guinea pigs (guinea pig passage). Tissue samples from guinea pig passages were also used to inoculate Vero cells through the shell vial technique. Infected cells were used for molecular characterization of the rickettsial isolate through PCR and DNA sequencing of fragments of three rickettsial genes ( gltA ,  ompA , and  ompB ). Blood serum samples were collected from 172 capybaras that inhabited the residential park. Sera were tested through the immunofluorescence assay using  R. rickettsii  antigen.    Results  A tick-inoculated guinea pig presented high fever accompanied by scrotal reactions (edema and marked redness). These signs were reproduced by consecutive guinea pig passages.  Rickettsia  was successfully isolated in Vero cells that were inoculated with brain homogenate derived from a 3 rd  passage-febrile guinea pig. Molecular characterization of this rickettsial isolate (designated as strain ITU) yielded DNA sequences that were all 100% identical to corresponding sequences of  R. rickettsii  in Genbank. A total of 83 (48.3%) out of 172 capybaras were seroreactive to  R. rickettsii,  with endpoint titers ranging from 64 to 8192.    Conclusions  A viable isolate of  R. rickettsii  was obtained from the tick  A. cajennense,  comprising the first viable  R. rickettsi  isolate from this tick species during the last 60 years. Nearly half of the capybara population of the residential park was seroreactive to  R. rickettsii,  corroborating the findings that the local  A. cajennense  population was infected by  R. rickettsii.

Rickettsial infection in Amblyomma cajennense ticks and capybaras (Hydrochoerus hydrochaeris) in a Brazilian spotted fever-endemic area