Investigation of spotted fever group Rickettsia in dogs and ticks in northern California

Spotted Fever Group (SFG) Rickettsiae are important zoonoses affecting both people and dogs. Rocky Mountain spotted fever (RMSF) caused by Rickettsia rickettsii, is the most well-known zoonotic SFG; historically in California cases were identified from Modoc plateau area but lately the annual number of cases rarely exceeds three. More recently, a distinct but related serotype and genotype to R. rickettsii, Rickettsia spp., strain 364D (proposed name R. philipii), was identified from a tick-bite associated eschar from a northern Californian human that brought attention to SFG Rickettsia in northwestern California. Historic cases of RMSF and this recent case of 364D raise the public health question as to whether SFG Rickettsia are more prevalent in northern California than previously reported. Also, dogs may play a role in bringing humans in contact with SFG Rickettsia or serve as sentinels for human disease. The aim of this study was to conduct a large scale survey of Lake and Mendocino County's dog populations for infection with SFG Rickettsia where both 364D and R. rickettsii has been shown to be endemic in the local tick population. Serologic assessment using an indirect immunofluorescence assay was used to evaluate past exposure of dogs to Rickettsia rickettsii. Real-time quantitative Polymerase Chain Reaction (qPCR) was used to molecularly detect the presence of the pathogen in the blood. It targeted the ompA gene that codes for an outer membrane protein A (ompA), present on all SFG Rickettsia. This gene allowed us to target all potential SFG Rickettsia that might potentially infect the local canine population. Any positive SFG Rickettsia identified were further classified by a nested PCR assay targeting an intergenic region RR0155-rpmB and was confirmed by sequencing. From June to September 2010, a total of 242 individual blood samples and 36 ticks were collected from dogs by veterinarians and county shelter personnel in northern California. 45 out of 242 (19%) serum samples tested were positive with reactive titers of 1:64 or greater to R. rickettsii. The highest seropositivity occurred in July when compared to the other three collection months (p0.05). Of all the dog samples tested, two out of 240 (0.83%) had detectable R. rickettsia DNA. In addition, we identified one Dermacentor occidentalis tick with SFG Rickettsia and one Dermacentor variabilis tick with R. rickettsii. This is the first time that R. rickettsii has been detected in a D. variabilis tick in California thereby adding to the known R. rickettsii positive tick species in California. Additionally, we show that R. rickettsia occurs in dogs and ticks in northern California, expanding the known geographic and host species distribution of documented R. rickettsii infections in California. This may be of public health concern given the close proximity of owners to their pets and shared peri-domestic environments. This research shows that even though the incidence of RMSF is low in California the potential risk of infection still exists and residents need to be aware of the risks in order to protect themselves and their pets.