Isolation of Candidatus Rickettsia andeanae (Rickettsiales: Rickettsiaceae) in Embryonic Cells of Naturally Infected Amblyomma maculatum (Ixodida: Ixodidae)

The Gulf Coast tick, Amblyomma maculatum Koch, has become increasingly important in public health for its role as a vector of the recently recognized human pathogen, Rickettsia parkeri. More recently, these ticks were also found to harbor a novel spotted fever group rickettsia, Candidatus Rickettsia andeanae. First identified in Peru, and subsequently reported in ticks collected in the United States, Chile, and Argentina, Ca. R. andeanae remains largely uncharacterized, in part because of the lack of a stable isolate. Although the isolation of Ca. R. andeanae was recently described in DH82, Vero, and Drosophila S2 cells, its stability in these cell lines was not shown. To evaluate Ca. R. andeanae transmission and pathogenicity in vertebrates, as well as further describe biological characteristics of this candidate species to fulfill criteria for its establishment as a new species, availability of a stable isolate is essential. Here we describe the propagation of Ca. R. andeanae by using a primary culture derived from naturally infected A. maculatum embryos. Subsequent passage of the Ca. R. andeanae isolate to ISE6 (Ixodes scapularis embryonic) and Vero (African green monkey kidney epithelial) cell lines demonstrated limited propagation of the rickettsiae. Treatment of the infected primary cells with tetracycline resulted in cultures negative for Ca. R. andeanae by polymerase chain reaction and microscopy. Establishment of an isolate of Ca. R. andeanae will promote further investigation into the significance of this tick-associated rickettsia, including its role in spotted fever and interactions with the sympatric species, R. parkeri in A. maculatum.