Revisiting the Role of Opossums (Marsupialia) as Sentinel for Brazilian Spotted Fever Caused by Rickettsia rickettsii

In southeastern Brazil, Rickettsia rickettsii, the agent of Brazilian spotted fever (BSF), is mainly transmitted to humans by Amblyomma sculptum ticks, whose populations are chiefly sustained by capybaras, Hydrochoerus hydrochaeris. As capybaras are sentinel hosts for BSF, serological analysis of capybaras is the most feasible way to determine whether an A. sculptum population is infected with R. rickettsii or not. Because the immature stages of A. sculptum commonly feed on opossums (Didelphis spp.), which are susceptible to R. rickettsii infection, this study aimed to validate the use of opossums (Didelphis albiventris) as sentinel hosts for active surveillance of the BSF. Between 2017 and 2020, a total of 159 opossums were captured in nine areas of Piracicaba, a BSF-endemic municipality. Ticks and blood were collected from opossums. The ticks were identified at the species level, and the blood was used for serological analysis to detect antibody end-point titers to R. rickettsii and Rickettsia bellii. Area 1a, which had a recent history of BSF, was used as the "positive control" area, whereas area 1b (devoid of capybaras) was used as the "negative control" area. Based on the results from these two areas, the remaining seven areas were categorized according to their risk of BSF based on serological results and the presence and abundance of ticks on opossums. The results indicated that two, one, and four areas were at high, low, and intermediate risk of BSF, respectively. This study validated the use of opossums as alternative sentinels for the active surveillance of BSF.