Eco-epidemiology of Rickettsia amblyommatis and Rickettsia parkeri in naturally infected ticks from South Carolina

Introduction: Rickettsia rickettsii and Rickettsia parkeri are spotted fever group rickettsia (SFGR) species shown to cause disease. An SFGR of particular interest is Rickettsia amblyommatis, one of the most commonly found species infecting 50% or more of questing ticks. Rickettsia amblyommatis’ role in the eco-epidemiology of other SFGRs remains unknown, having direct public health implications. Methods: 2021-2022 state-wide tick surveillance and land-use classification data were used to define the relations between R. amblyommatis and R. parkeri tick positivity among South Carolina-collected ticks. State park questing ticks and animal shelter host-attached ticks were evaluated for R. parkeri and R. amblyommatis in pools using RT-PCR. A Bayesian multivariable logistic regression model for pool testing data was used to assess associations between Rickettsia sp. and land-use classification variables among questing ticks. Spearman correlation was used to evaluate the relationship between tested pathogens. Results: R. amblyommatis and R. parkeri prevalence was 24.8% (23.4-26.3%) and 19.0% (17.6-20.5%) for questing ticks; and 39.5% (37.4-42.0%) and 22.4% (20.3-24.5%) among feeding ticks, respectively. The most common tick species were Amblyomma americanum, Dermacentor variabilis, and Amblyomma maculatum. R. parkeri and R. amblyommatis infection were negatively correlated. Discussion: R. amblyommatis was positively associated with forests, and negatively associated with fields, and wetlands. R. parkeri presence was positively associated with multiple landscapes, but negatively associated with fields. These findings provide novel insight into the eco-epidemiology of tick positivity in South Carolina. Negative correlations between SFGR species, and environmental impact on tick positivity highlight the need for ecological studies evaluating ticks-hosts-pathogen interactions.