Detection of Rickettsia species with seroreactivity to Rickettsia tillamookensis and Rickettsia rickettsii subsp. californica in domestic dogs in Northern California.
Rickettsia species phylotype G022 (G022) and Rickettsia tillamookensis both maintain a low infection prevalence in Ixodes pacificus ticks and share a close phylogenetic history with pathogenic Rickettsia species. The transmission mechanisms and pathogenic potential of I. pacificus-borne Rickettsia are poorly understood. The low prevalence of G022 and R. tillamookensis suggests that vertebrate species are utilized for maintenance, though identification of vertebrate amplifier and reservoir hosts utilized by Rickettsia is limited. To evaluate the molecular occurrence of G022 and R. tillamookensis and the sentinel utility of domestic dogs, a molecular survey was conducted. Buffy coat and blood plasma samples from 175 domestic dogs collected from Humboldt and Sonoma counties in northern California were screened for rickettsial DNA and anti-rickettsial antibodies, respectively. Dogs were screened for antibodies against Rickettsia spp. and tested for seroreactivity against R. tillamookensis, Rickettsia rickettsii subsp. californica, and Rickettsia rhipicephali. Because G022 has not yet been isolated, this Rickettsia species was omitted from the serological survey. Of the 175 dogs tested, three (1.71%; 95% CI: 0.58%-4.90%) were seropositive for Rickettsia spp. by indirect immunofluorescence antibody assay (IFA); two dogs (1.14%; 95% CI: 0.31%-4.10%) demonstrated seroreactivity by enzyme-linked immunosorbent assay (ELISA), suggesting previous exposure to Rickettsia spp. Among dogs from Sonoma County, 2.88% (3/104; 95% CI: 1.0%-8.14%) of plasma samples were seropositive for Rickettsia spp. by IFA, and 1.92% (2/104; 95% CI: 0.53%-6.74%) demonstrated seroreactivity by ELISA, while all the dogs from Humboldt County were seronegative for both methods. Although the ELISA results were not considered species-specific, two plasma samples demonstrated the greatest seroreactivity against R. tillamookensis and R. rickettsii subsp. californica, respectively, suggesting previous exposure to Rickettsia spp. with epitope similarity to R. tillamookensis and R. rickettsii subsp. californica, respectively. No DNA was detected for G022 or R. tillamookensis by quantitative real-time PCR, with a limit of detection of approximately 3.6 target copies per reaction for G022 and 1 genomic copy per 2 μl of template DNA per reaction for R. tillamookensis.