The present study evaluated the rickettsial infection among dogs living in the rural and urban areas of Monte Negro, state of Rondônia, western Brazilian Amazon. Canine sera were tested by the indirect immunofluorescence assay (IFA) using six rickettsial antigens: Rickettsia bellii, Rickettsia amblyommii, Rickettsia rhipicephali, Rickettsia rickettsii, Rickettsia parkeri, and Rickettsia felis. While the first three Rickettsia species are known to occur in the study site, the latter three species are known to occur in southeastern Brazil. For each serum, end point titer reacting with each Rickettsia antigen was determined. Serum showing for a Rickettsia species titer at least fourfold higher than that observed for any other Ricketttsia species was considered homologous to the first Rickettsia species or to a very closely related genotype. A total of 164 rural and 153 urban dogs were tested. Overall, 19 (11.6%) and 6 (3.9%) dogs from rural and urban areas, respectively, reacted positively to at least one Rickettsia species. In the rural area, three sera showed titers to R. parkeri at least four-fold higher than any of the other five antigens. These sera were considered to be homologous to R. parkeri or a very closely related genotype. Using the same criteria, two rural sera were considered homologous to R. amblyommii, two other rural sera to R. rhipicephali, and one urban serum to R. parkeri. Because dogs living in the rural area of Monte Negro are commonly infested by the same tick species infesting humans, they indeed serve as sentinels for human rickettsial diseases. Thus, humans living in Monte Negro are likely to be infected by at least three Rickettsia species: R. parkeri, R. amblyommii, and R. rhipicephali. While R. parkeri is a known human pathogen, further studies are required to verify the potential role of R. amblyommii and R. rhipicephali as human pathogens.

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