Dermacentor Variabilis and Its Associated Rickettsiae in Southeastern Virginia; The Potential Role of Pathogen Spillover From Local Sympatric Tick Species

Hard-bodied ticks are the most medically important group of arthropods in the United States (US). Dermacentor variabilis, the American dog tick, is a common vector in the US with its geographic range expanding across the eastern and western portions of the country. This tick is the historical vector of Rickettsia rickettsii, the causative agent of Rocky Mountain Spotted Fever and has been reported to harbor other spotted fever group rickettsiae anecdotally. In Virginia, spotted fever group rickettsiosis cases range between 300-400 per year based on passive surveillance data which do not often identify the rickettsial agent to species. There are a variety of rickettsial species reported in Virginia which include R. rickettsii, R. amblyommatis, R. montanensis, and R. parkeri. Each rickettsia can be detected in multiple tick species, but are typically transmitted by a single vector; all of which are sympatric with D. variabilis. The purpose of this study was assess current D. variabilis populations in Virginia and their associated rickettsiae. From 2012 to 2018 as part of a long-term active surveillance project, adult D. variabilis were collected using standard flagging methods and their immatures were collected using small mammal trapping. The presence of rickettsiae was detected using real-time PCR and confirmed using Sanger sequencing. Dermacentor variabilis in Virginia were observed to harbor R. montanensis, R. parkeri, and R. amblyommatis, with the most common rickettsial species being R. montanensis. No samples had detectable levels of R. rickettsii infection. The occurrence of pathogen spillover among sympatric tick species appears to be common in D. variabilis under select pressures (i.e. when other tick species are dominant in the same area). Once that pressure is removed, R. montanensis, the common endosymbiont of D. variabilis, becomes the predominant rickettsia detected.