Epidemiology of capybara-associated Brazilian spotted fever

Abstract

Background: Brazilian 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).

Methods: During 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.

Results: The 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.

Conclusions: The 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.

Fig 1. Areas where capybaras and ticks were sampled in the state of São Paulo (SP) (highly anthropic, low diversity areas), and in the states of Mato Grosso (MT) and Mato Grosso do Sul (MS) (low anthropic, high diversity areas).

Map source was obtained from the “Instituto Brasileiro de Geografia e Estatística”(IBGE) website (www.ibge.gov.br) and the final figure was constructed with the use of CorelDraw Graphics Suite 2017.

Fig 2. Boxplot representing the serological endpoint titers for six Rickettsia species of capybaras from Brazilian spotted fever (BSF)-endemic areas and BSF-nonendemic areas.

Different lower case letters mean statistically different (P<0.05) endpoint titers between Rickettsia species in endemic or nonendemic areas.

Fig 3. Boxplot representing the mean abundance of total tick infestations of capybaras from Brazilian spotted fever (BSF)-endemic areas, BSF-nonendemic areas, and natural areas.

Fig 4. Dotplot representing the number of Amblyomma sculptum and Amblyomma dubitatum ticks per capybara among 3 Brazilian spotted fever (BSF)-endemic areas, and 4 BSF-nonendemic areas.

Fig 5. Boxplot representing the mean abundance of Amblyomma sculptum and Amblyomma dubitatum infestations of capybaras from Brazilian spotted fever (BSF)-endemic areas and BSF-nonendemic areas.

Different lower case letters mean statistically different (P<0.05) mean abundance values of either A. sculptum or A. dubitatum between endemic and nonendemic areas.

Fig 6. Density of host-questing larvae, nymphs and adult ticks (Amblyomma sculptum and Amblyomma dubitatum) collected during all seasons of the study period (total) or during the larval (autumn), nymphal (winter) or adult (summer) seasons during 2015–2019 in Brazilian spotted fever (BSF)-endemic areas, BSF-nonendemic areas, and natural areas.