Rickettsia rickettsii (Rickettsiales: Rickettsiaceae) Infecting Amblyomma sculptum (Acari: Ixodidae) Ticks and Capybaras in a Brazilian Spotted Fever-Endemic Area of Brazil

Abstract

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.

Epidemiology of capybara-associated Brazilian spotted fever

“…sculptum by R . rickettsii in BSF-endemic areas to be very low, usually <1% [7–10]. Within this scenario, mathematical models have indicated that an A .…”

“…sculptum in most of the BSF-endemic areas of southeastern Brazil [11, 13, 14]. However, it is important to note that the tick Amblyomma dubitatum has also been frequently found infesting capybaras in southeastern Brazil, albeit with no direct role on BSF-epidemiology [7–10, 14, 15].…”

“…Because capybara is considered to be the main host for A . sculptum in such landscapes [10, 15, 20], and at the same time an efficient R . rickettsii amplifying host [13], the increase of BSF incidence in the state of São Paulo during the last three decades has been associated to the afore mentioned anthropogenic modifications [11, 14, 21].…”

Habitat selection in natural and human-modified landscapes by capybaras (Hydrochoerus hydrochaeris), an important host for Amblyomma sculptum ticks

“…The role of vertebrate-amplifying hosts in sustaining R. rickettsii populations has been welldiscussed, with results showing that A. sculptum is unable to sustain the bacterium by itself over consecutive generations [10,23]. In the Brazilian Cerrado, previous research showed that this tick species is more abundant in forested habitats (cerradão and gallery forests) than in open fields or seasonally flooded habitats [24,25].…”

Human-induced changes in habitat preference by capybaras (Hydrochoerus hydrochaeris) and their potential effect on zoonotic disease transmission

“…Capybaras can also act as amplifying hosts of R. rickettsii among A. sculptum populations [16, 17]. Even though the role of capybaras in BSF epidemiology have been well-discussed [16, 17, 18, 19], little is known about the potential effects of human-driven variation in capybara habitat selection to BSF spread and transmission.…”