In studies to assess the possible involvement of an arthropod vector in the transmission of epizootic bovine abortion (EBA), a disease of range cattle in California, the argasid tick Ornithodoros coriaceus Koch was selected for blood-feeding susceptible heifers. Twelve test heifers, each between the 3rd and 5th months of gestation, were exposed to the feeding of from 100 to 626 O. coriaceus captured from EBA-enzootic areas. Of these, 5 aborted 93 to 117 days after tick exposure, and each fetus possessed pathologic lesions judged to be identical to those of EBA fetuses aborted after naturally acquired exposure. It is suggested that the premature calving experienced by 4 test heifers is also associated with exposure to tick feeding and, possibly, to EBA as well. The 5 control heifers calved normally at term. These results are interpreted as evidence that O. coriaceus exposed test heifers to the etiologic agent or substance of EBA. We view these results as the first good evidence that the EBA agent, whatever it may eventually prove to be, is arthropod-borne in nature. The bionomies of O. coriaceus are, as discussed, consistent with the tick's involvement as a vector of EBA in California.

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