A U.S. isolate of Theileria orientalis ikeda is not transstadially transmitted to cattle by Rhipicephalus microplus

Authors: ONZERE, CYNTHIA - Washington State University; Herndon, David; HASSAN, AMANY - Washington State University; Oyen, Kennan; Poh, Karen; Scoles, Glen; Fry, Lindsay

Submitted to: Pathogens
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2023
Publication Date: 4/5/2023
Citation: Onzere, C.K., Herndon, D.R., Hassan, A., Oyen, K.J., Poh, K.C., Scoles, G.A., Fry, L.M. 2023. A U.S. isolate of Theileria orientalis ikeda is not transstadially transmitted to cattle by Rhipicephalus microplus. Pathogens. 12(4). Article 559. https://doi.org/10.3390/pathogens12040559.
DOI: https://doi.org/10.3390/pathogens12040559

Interpretive Summary: Since 2017, the potentially virulent Theileria orientalis Ikeda strain has been detected in seven U.S. states and is now endemic in Virginia. As this parasite is primarily transmitted by ticks, establishing knowledge of the vector status for ticks native to the U.S. is a key step in pathogen surveillance and disease monitoring and prevention. In this experiment, we tested the ability of a native U.S. tick species, Rhipicephalus microplus, to acquire and transmit the U.S. T. orientalis Ikeda isolate. R. microplus larvae were fed on a splenectomized, T. orientalis Ikeda positive calf for parasite acquisition and removed as freshly molted adults and applied to two, T. orientalis-naïve, splenectomized calves for parasite transmission. T. orientalis was not detected by PCR in the salivary glands of acquisition fed ticks, larval progeny of the acquisition-fed adults, or by PCR and blood smear in the blood of transmission-fed calves. These results suggest that R. microplus is not a competent vector of the U.S. T. orientalis Ikeda isolate.

Technical Abstract: Theileria orientalis Ikeda has caused an epidemic of bovine anemia and abortion across several U.S. states. This apicomplexan hemoparasite is transmitted by Haemaphysalis longicornis ticks; however, it is unknown if other North American ticks are competent vectors. Since the disease movement is largely determined by the host tick range(s), the prediction of the T. orientalis spread among U.S. cattle populations requires determination of additional competent tick vectors. Although Rhipicephalus microplus has mostly been eradicated from the U.S., outbreaks in populations occur frequently, and the U.S. remains at risk for reintroduction. Since R. microplus is a vector of Theileria equi and T. orientalis DNA has been detected in R. microplus, the goal of this study was to determine whether R. microplus is a competent vector of T. orientalis. Larval R. microplus were applied to a splenectomized, T. orientalis Ikeda-infected calf for parasite acquisition, removed as molted adults, and applied to two T. orientalis naïve, splenectomized calves for transmission. After 60 days, the naïve calves remained negative for T. orientalis by PCR and cytology. Additionally, T. orientalis was not detected in the salivary glands or larval progeny of acquisition-fed adults. These data suggest that R. microplus is not a competent vector of the U.S. T. orientalis Ikeda isolate.