Tick-borne rickettsioses are an expanding threat to public health and veterinary health in western North America. The emergence of Rocky Mountain spotted fever (RMSF) in northern Mexico and Arizona in the last 20 years exemplifies the changing risk, with outbreaks carrying an unusually high human case fatality rate, ranging from 10% to over 30%. These outbreaks are associated two species of the brown dog tick, Rhipicephalus sanguineus s.l., which can spread Rickettsia rickettsii (the cause of RMSF) to dogs, humans, and potentially wildlife species. It is unknown whether risk factors for disease transmission differ between the two species, which are known as the tropical and temperate lineage. Known risk factors for RMSF in the region include a high density of domestic dogs, particularly free roaming dogs. However, this alone is not adequate to explain the sporadic but rapid emergence of RMSF, and the reasons why RMSF emerged in western North America when it did remain unknown. In addition, though R. rickettsii has been identified in wildlife species, a sylvatic or wildlife cycle has not been identified in this region. The focus of the three chapters of this dissertation were to investigate the factors influencing transmission of R. rickettsii and other tick-borne pathogens that may be associated with RMSF in southern California and northern Mexico. In Chapter 1, the effect of hot temperatures on the host preference of two lineages (tropical lineage and temperate lineage) of brown dog tick were assessed in the laboratory. In Chapter 2, to investigate the potential role of lagomorphs and their ectoparasites in the maintenance and transmission of rickettsial pathogens, a field study was studied in northern Baja California, testing rabbits, hares, and their ectoparasites for exposure to and infection with rickettsial organisms. Finally, a cross sectional study of tick-borne pathogen exposure and infection in sheltered dogs southern California and northern Baja California was conducted to determine the potential for pathogen transmission by brown dog ticks in this population.
Cases of rickettsial disease and rates of humans being bitten by brown dog ticks have been observed to increase during hot weather events in Europe. To investigate whether weather events may influence biting of humans in western North America, ticks of each lineage were tested to determine whether they chose to move towards a human or a dog at room temperature (23.5 °C) and hot temperature (38 °C). Adult tropical lineage ticks were 2.5 times more likely (95% CI: 1.71-3.65) to choose humans at 38 °C than at 23.5 °C, without a change in risk of choosing a dog, demonstrating a reversal in preference at hot temperatures. Temperate lineage adults were significantly less likely to choose the dog at 38 °C than 23.5 °C (RR 0.34, 95% CI 0.20-0.57) but the risk of ticks choosing humans was unchanged. Hot weather events may increase the risk of human exposure to bites by the brown dog tick, especially where the tropical lineage is present.
The role of lagomorphs in the transmission of RMSF has been hypothesized but not proven, and they are understudied for their role in rickettsial maintenance and amplification in western North America. Rabbits and hares were trapped in two locations and tested for infection with and exposure to rickettsial organisms, and ticks and fleas. Rabbits from Ensenada had significantly higher odds of being seropositive than rabbits from Mexicali (p = 0.04), and 88% of Dermacentor parumapertus ticks collected in that location (95% CI: 0.77-0.95) were infected with Rickettsia bellii, a rickettsial organism not known to be pathogenic. A single tissue sample from a hare in Ensenada was also positive for R. bellii. These findings indicate that rickettsial species that are not believed to cause disease may be affecting immunity and susceptibility in lagomorph populations.
Dogs may serve as sentinels for zoonotic pathogens and dogs in shelters may be at especially high risk of exposure. A cross sectional study of dogs from seven shelters in four locations (San Diego County, Imperial County in southern California and Mexicali and Ensenada in northern Baja California) showed that dogs in all four locations had exposure to pathogens carried by brown ticks (Anaplasma, Ehrlichia, and Rickettsia species). In Tijuana, two dogs from different shelters were infected at the time of sampling with R. rickettsii. Infection with E. canis and A. platys ranged across shelters from 0 to 27% and 0 to 33%, respectively. Dogs in all four locations demonstrated exposure to all three pathogens, though Rickettsia and Ehrlichia seropositivity was highest in Mexicali (81% and 49%, respectively) and Anaplasma seropositivity was highest in Tijuana (45%). Findings from this research suggest that sheltered dogs may serve as valuable sentinels for RMSF in communities, and that while the risk of RMSF in southern California may be lower, the potential for transmission by brown dog ticks exists in both San Diego and Imperial counties.