Evaluation of Rickettsia japonica Pathogenesis and Reservoir Potential in Dogs by Experimental Inoculation and Epidemiologic Survey

Abstract

Rickettsia japonica pathogenesis and reservoir potential in dogs were evaluated by both experimental inoculation and epidemiologic survey. In the experimental inoculation study, dogs 1 and 2 were pretreated with an immunosuppressive dose of cyclosporine 14 days before inoculation and became ill after exposure to R. japonica. Dogs exhibited clinical signs, including fever, anorexia, depression, and decreased water consumption, between 36 and 96 h after inoculation, but these signs disappeared spontaneously by 5 days after inoculation. Dogs 3 and 4 were not pretreated with cyclosporine, and no clinical signs were detected in them throughout the 14-day observation period. The control dog was clinically normal and had a normal rectal temperature throughout the study period. We attempted to detect rickettsial DNA from peripheral blood and aspiration samples from kidney and spleen by nested PCR, but all samples examined were negative. The control dog lacked detectable titers to R. japonica antigen on day 14, while positive antibodies to R. japonica were detected in all four experimentally infected dogs, with titers of 1:160 to 1:80. In the epidemiologic survey, 24 (1.8%) of the 1,363 dogs examined throughout Japan had antibodies against R. japonica, with titers of 1:40 or more. However, we observed neither clinical signs at the time of sample collection nor nested PCR results indicative of rickettsial infection in these dogs. In conclusion, dogs in Japan can be exposed to R. japonica, and infected dogs with immunosuppressive conditions can temporarily develop clinical symptoms, including fever, anorexia, depression, and decreased water consumption.