Isolation of Rickettsia slovaca from the American dog tick, Dermacentor variabilis

                Rickettsia slovaca is the causative agent of tick-borne lymphadenopathy (TIBOLA) and Dermacentor-borne necrosis erythema lymphadenopathy (DEBONEL).  The primary known vectors are D. marginatus and D. reticulatus.  Rickettsia slovaca has not been found in new world ticks, nor has TIBOLA or DEBONEL been reported as a disease in humans in the Americas.  Here we describe the detection of Rickettsia identical to R. slovaca in the North American dog tick, D. variabilis, the isolation of the agent in a cell culture, and transmission to both uninfected ticks and naïve animals.

A colony of D. variabilis was established from adult ticks collected in North Carolina and Virginia and maintained on naïve, pathogen-free NZW rabbits. Detection of seroconversion in rabbits after feeding of laboratory bred D. variabilis prompted PCR testing of colony ticks. DNA of a SFG Rickettsia was identified in larvae, nymphs and adults with a high prevalence. Further analysis of 17kDa, gltA, rpoB, ompA, ompB, and sca4 genes revealed 100% identity to R. slovaca sequences available in the GenBank, whereas similarities to R. montanensis and R. rickettsii ranged 92-98% and  97-99% respectively. The rickettsial agent was successfully isolated from D. variabilis nymphs in Vero cell culture and maintained through multiple passages before being frozen in our collection. Sequences of culture-derived samples were identical to those found in ticks. Exposure of naïve guinea pigs to this agent via either needle inoculation or tick bite resulted in reproducible clinical illness and seroconversion. Uninfected D. variabilis larvae acquired the agent from infected animals.

This study describes the first isolation of Rickettsia slovaca from Dermacentor variabilis ticks. It also provides evidence of the ability of the American dog tick – Dermacentor variabilis to maintain Rickettsia genetically identical to European strains of R. slovaca via multiple transmission routes.