Vector competence of Rhipicephalus sanguineus and Dermacentor variabilis for American isolates of Babesia gibsoni

To determine the identity of the tick vector of enzootic B. gibsoni in California, USA, 2 common ixodid ticks were allowed to engorge upon B. gibsoni-infected dogs. Sporozoites were observed in the salivary glands of prefed nymphs of R. sanguineus that fed as larvae on B. gibsoni-infected dogs. A higher proportion (31%) of nymphal ticks that prefed on an uninfected dog for 48 h contained sporozoites in their salivary glands than did ticks which had fed for 24 h (13%). Sporozoites were not observed in the salivary glands of prefed R. sanguineus nymphs which were derived from the eggs of adult females that fed on an infected dog, in adults that were fed as nymphs on an infected dog, or in the nymphal and adult uninfected controls. D. variabilis ticks appeared not to become infected. Although attempts to transmit B. gibsoni to susceptible, splenectomized dogs were unsuccessful, R. sanguineus would appear to be the most likely tick vector to maintain this piroplasm in California.