# Effects of exposure to Rickettsia bellii on reservoir competence of guinea pigs for Rickettsia rickettsii

## TL;DR

Exposure to Rickettsia bellii in guinea pigs reduced their ability to spread Rickettsia rickettsii, a finding that could explain the uneven spread of the pathogen in tick populations.

## Contribution

This study demonstrates for the first time that prior exposure to Rickettsia bellii reduces reservoir competence for Rickettsia rickettsii in guinea pigs.

## Key findings

- Guinea pigs exposed to Rickettsia bellii had a 21% infection rate in ticks, compared to 54% in unexposed groups.
- Only one guinea pig in the Rickettsia bellii group died, while most in the unexposed groups died.
- Prior Rickettsia bellii exposure significantly lowers Rickettsia rickettsii transmission in ticks.

## Abstract

Amblyomma sculptum is the main vector implicated in the transmission of Rickettsia rickettsii in southeastern Brazil, where capybaras are known to be the main amplifying hosts of this pathogen. Interestingly, many areas inhabited by large populations of capybaras and A. sculptum remain free of R. rickettsii, suggesting that other drivers may be involved in the pathogen perpetuation. A. sculptum frequently occurs in sympatry with Amblyomma dubitatum ticks, sharing the same hosts with them. Considering that A. dubitatum ticks are frequently infected with Rickettsia bellii, a possible interference of R. bellii in the reservoir competence of capybaras for R. rickettsii could explain the heterogeneous distribution of R. rickettsii in A. sculptum populations in southeastern Brazil.

The present study aimed to evaluate the effect of primary exposure to R. bellii-infected A. dubitatum on the reservoir competence for R. rickettsii using guinea pigs as experimental models.

Three guinea pigs were infested with R. bellii-infected A. dubitatum (Group GB), four guinea pigs with uninfected A. dubitatum (Group GD), and three guinea pigs were not exposed to A. dubitatum (Group GC). After infestation with A. dubitatum, all guinea pigs were exposed to one single R. rickettsii-infected A. sculptum female and then were infested with uninfected A. sculptum larvae. After ecdysis, nymphs were tested for detection of spotted fever group rickettsiae.

Five out of six guinea pigs from GD and GC groups died, while one out of three guinea pigs in the GB group had a fatal outcome. R. rickettsii infection rate among ticks fed on animals from GB group was 21% (17/80), significantly lower than the rate of 54% (60/111) recorded in ticks fed on animals from GD and GC groups.

Prior exposure of guinea pigs to R. bellii-infected A. dubitatum ticks reduced their reservoir competence for R. rickettsii.