Surveillance for Metastriate Ticks and Spotted Fever Group Rickettsioses in Southern Illinois

Spotted fever group rickettsioses (SFGR) are a growing public health concern in the United States. The increase in SFGR cases, coupled with the geographical expansion of tick species, including Amblyomma americanum and A. maculatum, necessitates a better understanding of Rickettsia species prevalence in newly established areas. Amblyomma maculatum is concerning because of its role as a Rickettsia parkeri vector. This study investigated the prevalence of Rickettsia species in ticks collected across 16 counties in southern Illinois, a region experiencing increasing reported SFGR cases. We collected A. maculatum, A. americanum, and Dermacentor variabilis and tested them by real-time polymerase chain reaction for R. amblyommatis, R. parkeri, and R. rickettsii and sequencing of the gltA gene for all Rickettsia species. Amblyomma maculatum was found in a wider range of counties than previously reported, with a prevalence of R. parkeri (16.4%) and Candidatus R. andeanae (28.7%). R. montanensis and R. bellii were detected in D. variabilis; A. americanum exhibited prevalences of R. amblyommatis of 48.5% in adults and 48.4% in nymphs. A positive correlation was observed between the density of R. amblyommatis-infected A. americanum and SFGR incidence rates. The range expansion of A. maculatum has introduced R. parkeri and Candidatus R. andeanae to southern Illinois, increasing the diversity of SFGR-causing agents. Although current evidence supports a role for A. americanum as driver of SFGR incidence in this region, continued surveillance is crucial to monitor the impact of A. maculatum expansion and potential shifts in Rickettsia spp. transmission dynamics.