Theileria Orientalis

The most important impact of the Asian Longhorned Tick (ALT) on cattle herds is its role in the spread of the blood protozoan parasite Theileria orientalis. Theileriosis is a relatively new disease in the U.S. Theileriosis was first recognized in the U.S. in 2018. Japan, Korea, China, Vietnam, New Zealand, and Australia all had theileriosis diagnosed in their cattle herds prior to its identification in the U.S.

Theileria orientalis genotype Ikeda is the Theileria organism of importance in the U.S. There have been other species of Theileria present in the U.S. for years. Chief among these species is Theileria orientalis genotype Chitose. While the other genotypes of Theileria orientalis can infect cattle, they do not cause any clinical signs of disease. Theileria orientalis Ikeda is the first Theileria species in the U.S. recognized to cause clinical disease in cattle.

Theileria is spreading rapidly in the eastern U.S. and will likely continue to spread. The spread of Theileria and ALT are very closely related, and ALT is assumed to be the most important route of transmission from one bovine animal to another.

Both Theileria orientalis Ikeda and the bacterium Anaplasma marginale cause infectious bovine anemia. Both tick-borne organisms are red blood cell parasites that lead to the destruction of the cell, causing anemia in clinically diseased animals. The clinical signs of theileriosis are the same as for anaplasmosis. It is impossible to distinguish between the two diseases based solely on clinical signs, and a laboratory diagnosis is required. Just like anaplasmosis, most of the cattle that test positive are asymptomatic carriers that do not show clinical signs of disease. It is possible that animals may be infected with both anaplasmosis and theileriosis concurrently. Only a small number of animals that are infected with Theileria orientalis Ikeda will ever show clinical signs of disease.

Other resources:
https://extension.missouri.edu/publications/g2113
https://www.aphis.usda.gov/sites/default/files/bovine-theileriosis-infosheet.pdf