OAHN Companion Animal Project: Rickettsia rickettsii seroprevalence in dogs in an area of suspected Rocky Mountain Spotted Fever emergence (in progress)

Project Lead: Dr. Scott Weese

Collaborators: 

Project Proposal

Rocky Mountain spotted fever (RMSF) is a tickborne disease caused by the bacterium Rickettsia rickettsii. It is considered one of the (if not the) most impactful tickborne diseases in North America based on a combination of incidence and severity. It can affect humans and dogs, causing disease of varying severity, with reported mortality rates of 1-10%. While the main vector, Dermacentor variabilis, is widely distributed in Ontario, RMSF is not currently considered an endemic disease because the bacterium has not been known to be established in ticks or other potential reservoirs in the province. Yet, RMSF was diagnosed in 8 Ontario dogs (with one fatality) in the summer of 2025, all linked to Long Point, Ontario. Subsequently, 2 human cases were identified that were also linked to Long Point. Tick dragging identified R. rickettsii in ticks in Long Point in the summer of 2025, confirming its presence in that location. This was unexpected and raises concern about human and animal risks in 2026 and beyond if this disease has become established in the province.

Much remains to be determined about RMSF in Ontario, and multifaceted study is required to identify the risks to humans and animals. Part of this is understanding whether there is broader R. rickettsii exposure in Ontario dogs, which can be both an indicator of animal exposure and a sentinel for human exposure risk. Factors associated with seropositivity are also important to assess to help identify further research and communication needs.

This study will investigate the seroprevalence for RMSF in dogs, primarily in the area around Long Point, Ontario, as these dogs likely represent the greatest risk group. If seropositive dogs are found in this study, that will provide impetus for similar investigation across a broader area in Ontario. Local veterinary clinics have been recruited to assist with sample collection. The study population will consist of clinically healthy dogs over 1 year of age that have not travelled to the United States (an endemic region). Blood will be collected and serum will be submitted to the Animal Health Laboratory for R. rickettsii serology. Dog owners will complete a short online questionnaire that will capture information about the dog, geographic exposures (e.g. specific Long Point area contacts, park visits), health status, previous diagnosis of tickborne diseases and use of tick preventive medicines (including timeframe and product). The target sample size is 150 dogs. Since this is the first study of an emerging disease, we have no foundational prevalence information on which to base a sample size calculation. With 150 dogs, if the true prevalence is 5% (a reasonable estimate in an emerging situation like this), this sample size would give us a precision of 3.5% (i.e. 95% confidence interval of 1.5-8.5%). This prevalence estimate is lower than in endemic areas in the US (e.g. 10-40% in some regions) because of the suspicion that this is a recent emergence and the main risk of exposure was in 2025, as compared to established regions where there is greater risk of cumulative exposure over a dog’s life.  If the observed prevalence is 0% (0/150), this sample size would provide 95% confidence that the prevalence is <=2.4%.  These are reasonable estimates and ranges for a first study of an emerging disease.

Merck Canada has provided in kind funding in the form of a summer student for the summer of 2026 to help coordinate the project and for a linked evidence synthesis project.