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

Felipe S Krawczak,João F Soares,Jonas Moraes-Filho,Marcelo B Labruna,Fernanda A Nieri-Bastos,Fernanda P Nunes

doi:10.1186/1756-3305-7-7

Abstract

BackgroundBrazilian 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.MethodsIn 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.ResultsA 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 3rd 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.ConclusionsA 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.

Highlights

Brazilian spotted fever (BSF), caused by the bacterium Rickettsia rickettsii, is the deadliest spotted fever of the world
We investigated the circulation of R. rickettsii among capybaras and ticks in one area where a laboratory-confirmed case of BSF had just occurred
A viable isolate of R. rickettsii was obtained from the tick A. cajennense collected from a BSF-endemic area

Summary

Introduction

Brazilian spotted fever (BSF), caused by the bacterium Rickettsia rickettsii, is the deadliest spotted fever of the world. Brazilian spotted fever (BSF) is the deadliest spotted fever of the world It has been reported in southeastern Brazil, where 591 laboratory-confirmed cases were reported from 1989 to 2008, resulting in 186 deaths (31.5% fatality rate) [1]. In contrast to the high fatality rates of BSF in southeastern Brazil, RMSF has been reported at 5-10% fatality rates in the United States [2] The reasons for such a contrast are still speculative; two studies have showed genetic differences between R. rickettsii strains from South and North Americas [3,4], highlighting the possibility that more virulent strains circulate in South America, as recently suggested by different authors [4,5,6]

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