| Summary: | <i>Theileria orientalis</i> Ikeda, an emerging U.S. bovine hemoparasite, causes anemia, abortion, ill-thrift, and occasionally death. While <i>Haemaphysalis longicornis</i> is the primary vector, it is possible that other U.S. ticks are capable of parasite transmission and may contribute to disease spread. <i>Dermacentor variabilis</i> is highly prevalent in the U.S., exhibits a similar geographical distribution to <i>T. orientalis</i>, and is a competent vector of the related parasite, <i>Theileria equi</i>. Herein, we conducted controlled acquisition and transmission studies using splenectomized calves to assess whether <i>D. variabilis</i> can transstadially transmit <i>T. orientalis</i>. <i>D. variabilis</i> nymphs were applied to an infected, splenectomized calf for parasite acquisition and subsequently incubated to molt into adults. Freshly molted adults were applied to two splenectomized <i>T. orientalis</i>-naïve calves to investigate parasite transmission. Calves were monitored for 59 days, and no evidence of parasite transmission was detected using PCR for the <i>T. orientalis</i> Ikeda major piroplasm surface protein gene, blood smear cytology, complete blood counts, or physical examination. Salivary glands from a subset of <i>D. variabilis</i> adults were assessed for <i>T. orientalis</i> using PCR, and the parasite was not detected. These findings support the conclusion that <i>D. variabilis</i> is not capable of transstadial transmission of the U.S. <i>T. orientalis</i> Ikeda isolate.
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