Transmission dynamics of Francisella tularensis subspecies and clades by nymphal Dermacentor variabilis (Acari: Ixodidae).

In the United States, the American dog tick, Dermacentor variabilis (Say) is considered an important biological vector of Francisella tularensis, the etiologic agent of tularemia. In this study, we evaluated the vector efficiency of nymphal D. variabilis infected as larvae with differing clades and subspecies (A1b, A2, and type B) of F. tularensis. In all cases, D. variabilis larvae were able to acquire, maintain, and transstadially transmit F. tularensis. Significant replication of the bacteria also occurred in infected nymphs. Transmission of F. tularensis to Swiss Webster mice was not observed with A1b, and low rates were observed with A2 (8.0%) and type B (13.5%). Negative effects on tick survivorship were also observed for A1b, A2, and type B infections. Our results provide evidence of a high fitness cost and low transmission rates during the immature stages, suggesting that D. variabilis may play a limited role in enzootic maintenance of F. tularensis.