Multistate Survey of American Dog Ticks (Dermacentor variabilis) for Rickettsia Species

Dermacentor variabilis , a common human-biting tick found throughout the eastern half and along the west coast of the United States, is a vector of multiple bacterial pathogens. Historically, D. variabilis has been considered a primary vector of Rickettsia rickettsii , the causative agent of Rocky Mountain spotted fever. A total of 883 adult D. variabilis , collected between 2012 and 2017 from various locations in 12 states across the United States, were screened for rickettsial DNA. Tick extracts were evaluated using three real-time PCR assays; an R. rickettsii -specific assay, a Rickettsia bellii -specific assay, and a Rickettsia genus-specific assay. Sequencing of omp A gene amplicons generated using a seminested PCR assay was used to determine the rickettsial species present in positive samples not already identified by species-specific real-time assays. A total of 87 (9.9%) tick extracts contained R. bellii DNA and 203 (23%) contained DNA of other rickettsial species, including 47 (5.3%) with Rickettsia montanensis , 11 (1.2%) with Rickettsia amblyommatis , 2 (0.2%) with Rickettsia rhipicephali , and 3 (0.3%) with Rickettsia parkeri . Only 1 (0.1%) tick extract contained DNA of R. rickettsii . These data support multiple other contemporary studies that indicate infrequent detection of R. rickettsii in D. variabilis in North America.