D3305: Molecular and niche modeling approaches to identify potential amplifying hosts for an emerging tick-borne pathogen Rickettsia 364D, the causative agent of Pacific Coast Tick Fever

Pacific Coast tick fever (PCTF) is a recently described and understudied tick-borne, zoonotic disease in California caused by a spotted fever group rickettsia (SFGR) currently designated Rickettsia 364D and vectored by the Pacific Coast tick, Dermacentor occidentalis. Knowledge regarding the transmission cycle and geographic distribution of Rickettsia 364D is limited (Lane et al. 1981, Philip et al. 1981, Padgett et al. 2016, Paddock et al. 2018). We conducted molecular and spatial investigations to determine the potential vertebrate hosts involved in the sylvatic transmission cycle of this pathogen in California. Host pathogen testing identified five individuals from three species (O. beecheyi, L. californicus, and S. audubonii) as PCR-positive for Rickettsia 364D. Additionally, we performed MaxEnt species niche modeling using existing tick occurrence records with Rickettsia 364D infection status to compare the distribution of R364D-infected D. occidentalis to the distribution of all D. occidentalis (R364D-infected and uninfected). The models showed the predicted niche of infected D. occidentalis is a subset of the overall predicted niche of D. occidentalis. These findings support the hypothesis that an amplifying host supports higher infection prevalence in the infected tick regions compared to other areas of the state. Comparative model analyses showed that the distribution of black tailed jackrabbits (Lepus californicus) is strongly correlated (R=0.76) with the distribution of Rickettsia 364D-infected ticks. These results suggest that lagomorphs warrant further examination as potential amplifying hosts, as do other small mammals present throughout the known geographic distribution of Rickettsia 364-infected D. occidentalis ticks.