Evaluation of the pathogenic potential of Rickettsia canada and Rickettsia prowazekii organisms in dogs

Abstract

Summary An unusual pattern of seroreactivity to antigens of rickettsial organisms (Rickettsia rickettsii, R rhipicephali, R montana, and R bellii), particularly to R bellii antigen, was detected in 3 dogs during a 2-month period. Thus, studies were initiated to clarify the pathogenic potential of the more distantly related rickettsial organisms (R canada and R prowazekii) in dogs. Because R bellii are nonpathogenic rickettsiae that share numerous common properties with spotted fever-group and typhus-group rickettsiae, and because closely related pathogenic relatives of R bellii have not been identified, we examined the pathogenic potential of these typhus-group rickettsiae by testing stored serum samples, by attempting rickettsial isolation from febrile dogs, and by experimentally inoculating dogs with R canada and R prowazekii. Evaluation of results of a serosurvey of acute and convalescent serum samples from 80 dogs in which Rocky Mountain spotted fever had been considered as a differential diagnosis, but seroconversion to R rickettsii had not been documented, identified 1 dog with a fourfold increase in antibody titer to R rhipicephali and 3 dogs with fourfold increases in antibody titer to 1 or more antigens of typhus-group rickettsial organisms. A study of 15 dogs that were febrile during summer months failed to identify serologic or tissue culture evidence of typhus-group rickettsial infection or typhus-group rickettsemia, but did result in isolation of R rickettsii and Ehrlichia canis, respectively, from 1 dog each. In our final study, after experimentally inoculating 6 dogs with R canada and R prowazekii, all dogs seroconverted to the respective rickettsiae, but rickettsemia or clinical and hematologic evidence of disease was not observed. Collectively, our results did not provide convincing evidence to support a pathogenic role for R canada and R prowazekii organisms in dogs. Our findings supported the conclusion that an unidentified microorganism, which results in production of antibodies against R bellii and antigens of typhus-group rickettsial organisms, contributed to an unexplained febrile illness of dogs in the southeastern United States. In an effort to identify dogs in which serologic evidence supports this conclusion, we recommend the use of selected spotted fever-group, typhus-group, and R bellii antigens for serodiagnostic purposes. We also recommend that diagnosticians attempt tissue culturing to isolate organisms when a rickettsial agent is suspected.