Vector competence and transovarial transmission of Rickettsia rickettsii in Rickettsia bellii-infected and -uninfected Amblyomma dubitatum ticks.

Brazilian spotted fever (BSF) is endemic in southeastern Brazil, where Amblyomma sculptum is the main vector incriminated in the transmission of R. rickettsii to humans. In many BSF-endemic areas capybaras are the main hosts for A. sculptum and also efficient amplifiers of R. rickettsii for this tick species. Capybaras are also primary hosts for Amblyomma dubitatum, a tick species frequently found infected by Rickettsia bellii but with no records of natural infection by R. rickettsii. The present study aimed to evaluate transovarial transmission and vector competence for R. rickettsii in A. dubitatum naturally infected and non-infected with R. bellii. For this purpose, two A. dubitatum colonies were used, one naturally infected by R. bellii, another with no R. bellii infection. Ticks from both colonies were divided into three experimental groups: group GL (ticks fed on R. rickettsii-inoculated guinea pigs in the larval stage), group GN (ticks fed on R. rickettsii-inoculated animals in the nymphal stage) and group GC (ticks never exposed to R. rickettsii). DNA samples from eight to 20 unfed nymphs and adults fed on each guinea pig, and from all engorged females at the end of oviposition, and from their respective eggs (one pool of 20-50 eggs/female) and larvae (10 larvae/female), were subjected to species-specific qPCR protocols for either R. rickettsii or R. bellii detection. Finally, larvae originating from R. rickettsii-infected females were fed on uninfected guinea pigs. In both A. dubitatum colonies, nymphs from group GL and adults from groups GL and GN were able to transmit R. rickettsii to susceptible animals, confirming vectorial competence and transstadial perpetuation of the pathogen. Further, infection by R. rickettsii was detected in eggs and larvae from 18% to 75% of GN and GL R. bellii-uninfected females, and from 25% to 44% of GN and GL R. bellii-infected females, respectively, confirming the transovarial transmission of the pathogen, regardless of R. bellii infection. Finally, larvae originating from engorged females infected with R. rickettsii were able to transmit the pathogen to susceptible animals, confirming the vectorial competence of ticks infected by transovarial transmission.