BackgroundBrazilian spotted fever (BSF), caused by the bacterium Rickettsia rickettsii, has been associated with the transmission by the tick Amblyomma sculptum, and one of its main hosts, the capybara (Hydrochoerus hydrochaeris).MethodsDuring 2015–2019, we captured capybaras and ticks in seven highly anthropic areas of São Paulo state (three endemic and four nonendemic for BSF) and in two natural areas of the Pantanal biome, all with established populations of capybaras.ResultsThe BSF-endemic areas were characterized by much higher tick burdens on both capybaras and in the environment, when compared to the BSF-nonendemic areas. Only two tick species (A. sculptum and Amblyomma dubitatum) were found in the anthropic areas; however, with a great predominance of A. sculptum (≈90% of all ticks) in the endemic areas, in contrast to a slight predominance of A. dubitatum (≈60%) in the nonendemic areas. Tick species richness was higher in the natural areas, where six species were found, albeit with a predominance of A. sculptum (≈95% of all ticks) and environmental tick burdens much lower than in the anthropic areas. The BSF-endemic areas were characterized by overgrowth populations of A. sculptum that were sustained chiefly by capybaras, and decreased populations of A. dubitatum. In contrast, the BSF-nonendemic areas with landscape similar to the endemic areas differed by having lower tick burdens and a slight predominance of A. dubitatum over A.sculptum, both sustained chiefly by capybaras. While multiple medium- to large-sized mammals have been incriminated as important hosts for A. sculptum in the natural areas, the capybara was the only important host for this tick in the anthropic areas.ConclusionsThe uneven distribution of R. rickettsii infection among A. sculptum populations in highly anthropic areas of São Paulo state could be related to the tick population size and its proportion to sympatric A. dubitatum populations.
The bacterium Rickettsia rickettsii is the agent of Brazilian spotted fever (BSF), a highly fatal disease that is transmitted in Brazil mainly by the tick Amblyomma sculptum Berlese, which uses capybaras (Hydrochoerus hydrochaeris Linnaeus) as major hosts. In 2015, we captured nine capybaras in a BSF-endemic area of southeastern Brazil. From each capybara, we collected blood sera that were tested through the immunofluorescence assay using Rickettsia spp. antigens, and A. sculptum ticks, processed for isolation of R. rickettsii through guinea pig inoculation. All capybaras (100%) were seroreactive to Rickettsia spp., with highest titers to R. rickettsii. A total of 166 A. sculptum ticks were macerated and inoculated into nine guinea pigs, from which only one presented high fever and seroconverted to R. rickettsii. Blood from this febrile animal was inoculated into additional guinea pigs (guinea pig passages), which also became febrile and seroconverted, or when euthanized during the fever period, their internal organs (spleen, lung) were shown to contain R. rickettsii DNA. The present rickettsial isolate has been maintained cryopreserved as infected guinea pig organs. There was at least one R. rickettsii-infected tick among the 166 macerated ticks, giving a minimal infection rate of 0.6% (1/166). This infection rate is within the range of previous studies, which reported that only 0.05% to at most 1.28% A. sculptum ticks were infected by R. rickettsii in other BSF-endemic areas. These low infection rates support the low incidence of BSF, despite of A. sculptum being the most frequent human-biting tick in southeastern Brazil.
Brazilian spotted fever (BSf), caused by Rickettsia rickettsii, is the most lethal tick-borne disease in the western hemisphere. in Brazil, Amblyomma sculptum ticks are the main vector. capybaras (Hydrochoerus hydrochaeris), the largest living rodents of the world (adults weighing up to 100 Kg), have been recognized as amplifying hosts of R. rickettsii for A. sculptum in BSf-endemic areas; i.e., once primarily infected, capybaras develop bacteremia for a few days, when feeding ticks acquire rickettsial infection. We conducted experimental infections of five capybaras with an A. sculptum-derived strain of R. rickettsii and performed clinical and bacteremia evaluation during primary and subsequent infections. Bacteremia was detected in all capybaras during primary infection, but not in subsequent infections. All animals seroconverted to R. rickettsii (titres range: 64-32,768), and remained seropositive throughout the study. primary infection resulted in clinical spotted fever illness in four capybaras, of which two had a fatal outcome. Subsequent infections in seropositive capybaras resulted in no clinical signs. capybaras developed a sustained immune response that prevented a second bacteremia. this condition may imply a high reproduction rate of capybaras in BSf-endemic areas, in order to continuously generate capybaras susceptible to bacteremia during primary infection. The bacterium Rickettsia rickettsii is the etiological agent of Rocky Mountain spotted fever, also known in Brazil as Brazilian spotted fever (BSF), a disease that has been registered in different American countries including Canada, United States, Mexico, Costa Rica, Panama, Colombia and Argentina 1,2. This bacterium is transmitted by different tick species throughout Americas [i.e., Dermacentor variabilis, Dermacentor andersoni, Rhipicephalus sanguineus sensu lato (s.l.), Amblyomma cajennense species complex, and Amblyomma aureolatum] 2. In Brazil, in the southeastern region, Amblyomma sculptum (a member of A. cajennense species complex) is the main incriminated vector, for which capybaras (Hydrochoerus hydrochaeris) and horses act as primary hosts for all parasitic stages 3,4. BSF is the most lethal tick-borne disease in Brazil with increasing numbers of cases and deaths. Between 2007 and 2015, 17,117 suspected cases of spotted fever (including other spotted fever group rickettsioses) were reported and 1,245 were confirmed as SFG rickettsioses in 12 Brazilian states from all regions 1. Moreover, case-fatality rates have attained values of 30% or higher, which could be associated with low index of suspicion and misdiagnosis by health-care professionals and exposure to particular eco-epidemiological risk factors 1 .
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