Molecular and antigenic comparison of Ehrlichia canis isolates from dogs, ticks, and a human in Venezuela

We previously culture isolated a strain of Ehrlichia canis, the causative a gent of canine ehrlichiosis, from a human in Venezuela. In the present stud y, we examined whether dogs and ticks are infected with E. canis in Venezue la and, if so, whether this is the same strain as the human isolate. PCR an alysis using E. canis-specific primers revealed that 17 of the 55 dog blood samples (31%) and all three pools of four Rhipicephalus sanguineus ticks e ach were positive. An ehrlichial agent (Venezuelan dog Ehrlichia [VDE]) was isolated and propagated in cell culture from one dog sample and was furthe r analyzed to determine its molecular and antigenic characteristics. The 16 S rRNA 1,408-bp sequence of the new VDE isolate was identical to that of th e previously reported Venezuelan human Ehrlichia isolate (VHE) and was clos ely related (99.9%) to that of E. canis Oklahoma. The 5' (333-bp) and 3' (6 53-bp) sequences of the variable regions of the 16S rRNA genes from six add itional E. canis-positive dog blood specimens and from three pooled-tick sp ecimens were also identical to those of VHE. Western blot analysis of serum samples from three dogs infected with VDE by using several ehrlichial anti gens revealed that the antigenic profile of the VDE was similar to the prof iles of VHE and E. canis Oklahoma. Identical 16S rRNA gene sequences among ehrlichial organisms from dogs, ticks, and a human in the same geographic r egion in Venezuela and similar antigenic profiles between the dog and human isolates suggest that dogs serve as a reservoir of human E. canis infectio n and that R. sanguineus, which occasionally bites humans residing or trave ling in this region, serves as a vector. This is the first report of cultur e isolation and antigenic characterization of an ehrlichial agent from a do g in South America, as well as the first molecular characterization of E. c anis directly from naturally infected ticks.