ABSTRACT. Cunha N.C., Lemos E.R.S., Rozental T., Teixeira R.C., Cordeiro M.D., Lisbôa R.S., Favacho A.R., Barreira J.D., Rezende J. & Fonseca A.H. Rickettsiae of the Spotted Fever group in dogs, horses and ticks: an epidemiological study in an endemic region of the State of Rio de Janeiro, Brazil. [Rickettsias do grupo da febre maculosa em cães, equinos e carrapatos: um estudo epidemiológico em região endêmica do estado do Rio de Janeiro, Brasil.] Revista Brasileira de Medicina Veterinária, 36(3):294-300, 2014. Departamento de Epidemiologia e Saúde Pública, Instituto de Veterinária, Universidade Federal Rural do Rio de Janeiro, BR 465, Km 7, Seropédica, RJ 23890-000, Brasil. E-mail: firstname.lastname@example.org Spotted fever is a disease of which Rickettsia rickettsii is the most pathogenic agent. Its transmission is by tick bites and the infected ticks can act as vectors, reservoirs or amplifiers. The purpose of this paper is to assess the potential of dogs and horses as sentinels for brazilian spotted fever (BSF) emergence and become acquainted with the tick species in a municipal region of Resende, Rio de Janeiro State, Brazil, where five BSF cases in man were registered. Dog and horse blood samples were collected from rural and periurban properties to assess IgG anti-Rickettsia rickettsii, using the indirect immunofluorescence assay (IFA). First, an analysis was conducted to detect association between IFA results and answers obtained from a questionnaire. Afterwards, a multivariate investigation was undertaken that presented significant statistical differences. Ticks were collected directly from dogs and horses for taxonomic identification. Out of the 107 canine serum samples, 30 (28.0%) were reactive, with titers varying from 1:64 to 1:4096, and 77 (72.0%) were not reactive. Of 96 animals in the serum analysis of horses, 9 (9.4%) were reactive, all with titers of 1:64, and 87 (90.6%) were non-reactive. The tick species collected from dogs were Rhipicephalus sanguineus, Amblyomma sculptum, Amblyomma aureolatum, Amblyomma ovale, Rhipicephalus microplus and nymphs of Amblyomma sp., R. sanguineus and R. microplus. Adult ticks gathered from horses were A. cajennense, R. microplus and Dermacentor nitens, in addition to nymphs of Amblyomma sp., R. microplus and D. nitens. The results suggest that: (i) the habit of dogs entering forests and living in rural environments positively influenced the presence of anti-rickettsiae of the spotted fever group serum antibodies, (ii) horses were not good sentinels for this study area and (iii) R. sanguineus as well as A. cajennense ticks were the most prevalent ixodidae fauna of the region.