Rickettsia parkeri Infection in Domestic Dogs, Southern Louisiana, USA, 2011

The association between companion animals and tick-borne rickettsial disease has long been recognized and can be essential to the emergence of rickettsioses. We tested whole blood from dogs in temporary shelters by using PCR for rickettsial infections. Of 93 dogs, 12 (13%) were positive for Rickettsia parkeri, an emerging tick-borne rickettsiosis.

None of the 12 dogs with PCR-positive tests were infested with ticks at the time of sampling. Six female dogs and 6 male dogs had detectable levels of R. parkeri DNA in their blood. Nine of the 12 dogs were adults; 3 were <6 months of age. Many of the dogs in the study were classifi ed as mixed breed because breed could not be objectively determined for most of the animals. All animals appeared to be in good health; no overt pathology was noted at the time of blood collection.
Although molecular detection of rickettsial DNA within the blood of vertebrates indicates infection, rickettsial cultures from the positive samples would confi rm patent rickettsemia. Most of the samples in our study were insuffi cient in volume to attempt culture after heartworm testing and DNA extraction. Of the 12 samples with PCR results positive for rickettsial DNA, only 3 were of suffi cient volume to attempt culture, and all of those attempts proved unsuccessful. It would also have been benefi cial to determine if dogs that were positive for R. parkeri harbored ticks that were also positive for R. parkeri. However, it is common practice for animal control centers to treat dogs for ectoparasites on admission; thus, no ticks were present on the dogs in our study at the time of sampling. The presence of rickettsial DNA in the blood of dogs, in the absence of ectoparasites, supports the hypothesis that domestic canines may serve as reservoirs of rickettsial diseases, now specifi cally including the emerging pathogen R. parkeri.

Conclusions
We examined the potential role of domestic dogs in transmission of SFG Rickettsia. R. rickettsii, the causative agent of Rocky Mountain spotted fever, is known to cause clinical disease in dogs, and it is associated with signs and symptoms that are similar to human disease, including cutaneous petechiae and ecchymoses, anorexia, depression, weight loss, and dehydration (8). The role of dogs as vehicles for Rickettsia-infected ticks to encounter susceptible humans has also been proposed (1). The possibility of dogs as reservoirs of rickettsial disease has previously been investigated in studies evaluating R. felis rickettsemia and seropositivity for R. parkeri (9,10); however, strong crossreaction among antibodies precludes fi nding of defi nitive results from serologic testing. The current study suggests that domestic dogs may become rickettsemic with R. parkeri infection, but further investigation of the duration of rickettsemia and monitoring for clinical disease that may be associated with infection is required.
It is also vital to determine the potential for dogs to serve as infectious sources of R. parkeri for feeding ticks. Dogs infected with R. rickettsii, for example, have proven relatively ineffi cient at transmitting rickettsiae to naive ticks and therefore may not play a large role in maintenance or amplifi cation of the R. rickettsii transmission cycle (11). Conversely, domestic dogs have recently been shown to be competent reservoirs for the causative agent of Mediterranean spotted fever, R. conorii, a species closely related to R. parkeri (12). The prevalence identifi ed in this study establishes an important fi rst step in the examination of the domestic dog for reservoir competency of R. parkeri.
Since the fi rst reported case of R. parkeri rickettsiosis in 2004, >20 additional cases have been identifi ed in humans (13), and to date no viable vertebrate reservoirs for the pathogen have been identifi ed. Although the current study consists of a relatively small survey, the results are considerable because of the recognized importance of domestic dogs as potential reservoirs for transmissible pathogens (14). In addition, the presence of R. parkeri has not previously been described in Louisiana; thus, this report expands the known distribution of R. parkeri. The results of the current study clearly establish dog infection by R. parkeri; however, a role for dogs in the natural cycle of this pathogen, and the arthropod vectors involved in transmission, requires further investigation.