Of the four most economically important tickborne diseases of cattle in the world, two have been identified in Virginia, occasionally as co-infections: anaplasmosis and theileriosis. The latter is caused by the emerging infectious agent, the Theileria orientalis complex, in particular the Ikeda and Chitose genotypes. These organisms are carried by the ixodid tick, Haemaphysalis longicornis, recently identified in the United States. Our work has been focused on initially identifying the protozoal organisms, crafting assays to aid in the identification of these organisms in clinically affected animals, and briefly examining the rate of co-occurrence of theileriosis and anaplasmosis. This is important, as Anaplasma marginale - the most common etiologic agent of anaplasmosis in cattle in Virginia - is treatable with a safe, effective, FDA-approved compound, whereas there is no currently approved treatment for theileriosis. Finally, we seek to contextualize theilerosis as a cause of infectious bovine anemia (IBA) and its expected economic impact on the cattle industry in Virginia. / Doctor of Philosophy / Theileriosis is a disease that infects cattle, caused by the blood parasite, Theileria orientalis, specifically two distinct genotypes of T. orientalis, Ikeda and Chitose. Theileriosis is transmitted to cattle by the Asian longhorned tick, Haemaphysalis longicornis, which was recently identified in the United States. Globally, theileriosis is one of four major tickborne diseases of cattle with significant economic importance, so the discovery of this parasite in the state of Virginia is of special importance to the cattle industry in Virginia. My work has revolved around making the initial discovery of T. orientalis Ikeda in the United States, and developing tests for cattle producers and veterinarians to use to help diagnose theileriosis in sick animals. Another tickborne disease of cattle, anaplasmosis, is caused by a bacterium, A. marginale. These two organisms have different biology, are transmitted by different ticks, and are treated differently, but cause identical clinical disease in cattle. In addition to identifying T. orientalis, we have developed a single test that can determine if sick cattle have T. orientalis or A. marginale – this is important, because the antibiotic used to treat A. marginale does not work to treat T. orientalis. In fact, there is no treatment for T. orientalis available in the United States. In addition to developing diagnostic assays, I seek to put into pathobiological, ecological, and economic context the importance of theileriosis on the cattle industry in Virginia.