Retrospective discovery of Theileria orientalis Ikeda in Haemaphysalis longicornis Neumann ticks on a cow-calf farm

Authors: BUTLER, REBECCA - Centers For Disease Control And Prevention (CDC) - United States; MULLER, LISA - University Of Tennessee; Poh, Karen; Aguilar, Mitzi; HOKKANEN-HARMON, KYRA - University Of Tennessee; CHANDLER, JENNIFER - University Of Tennessee; GROVE, DANIEL - University Of Tennessee; TROUT FRYXELL, REBECCA - University Of Tennessee

Submitted to: Transboundary and Emerging Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/10/2026
Publication Date: N/A
Citation: N/A

Interpretive Summary: The parasite Theileria orientalis Ikeda, which can cause serious illness in cattle, was recently found in the United States in the invasive Asian longhorned tick (Haemaphysalis longicornis). This parasite has already damaged cattle industries in other parts of the world, and its spread in the U.S. raises concerns for livestock health and wellbeing. On a cattle farm in Tennessee, the parasite was detected in ticks collected from cattle, cats, raccoons, and opossums, showing that multiple animals can help maintain ticks (and possibly infected ticks) in the environment. While the parasite was not found in the blood of these wild animals, the presence of infected ticks suggests they may still play a role in sustaining tick populations. Controlling ticks on these hosts and improving farm practices, such as using and changing gloves and changing needles between animals, could reduce the risk of spreading this parasite and protect cattle health and cattle economies.

Technical Abstract: Background Theileria orientalis Ikeda is a protozoan parasite that was recently detected in Haemaphysalis longicornis Neumann ticks and bovine serum samples in the United States (US). This parasite is known to cause theileriosis in bovine hosts and has negatively impacted cattle industries internationally. The transmission of this pathogen at the livestock-wildlife interface in the US is not fully understood. Case presentation Theileria orientalis Ikeda was reported by producers on a cow-calf farm in eastern Tennessee. Retrospective analysis of field- and host-collected H. longicornis resulted in the detection of T. orientalis Ikeda in nymphal H. longicornis ticks collected from cattle (Bos taurus), domestic cat (Felis catus), raccoon (Procyon lotor), and Virginia opossum (Didelphis virginiana); notably, the protozoan was not amplified from the blood of these hosts. Conclusions Multiple hosts (cattle, raccoon, Virginia opossum, and domestic cat) were responsible for supporting T. orientalis Ikeda infected H. longicornis ticks on the farm. Targeting these hosts with acaricidal treatments could be important for reducing tick abundance and pathogen transmission. Additionally, biosecurity practices such as glove and syringe and needle changes between handling of bovine hosts is important for preventing accidental mechanical transmission for Theileria parasites.