Theileria orientalis (Ikeda genotype) is a protozoal organism that is relatively new in the United States. This organism primarily affects cattle and was first discovered in a cow-calf herd in Virginia in 2017. It has since been detected in many other states, with the first case in Missouri reported in June 2023.
Theileria orientalis causes disease in cattle by infecting red blood cells. The immune system recognizes the infected cells as damaged and eliminates them, creating anemia. The severity of disease is related to the severity of the anemia. In mild cases, cattle may exhibit symptoms such as elevated temperature, depression, and pale mucous membranes. In severe cases, cattle will exhibit jaundice (a yellow tinge to the mucous membranes around the eyes and vulva), and severe depression. Pregnant animals may abort. Some death loss may occur, usually in less than 5% of animals. T. orientalis infection has been consistently associated with decreased weight gains in growing cattle.
For those familiar with the bovine anaplasmosis, caused by Anaplasma marginale, the symptoms are similar. However, a key difference is that symptoms associated with anaplasmosis are seldom seen in cattle less than 2 years of age whereas symptoms associated with Theileria are seen in both calves and adults.
Transmission
Arthropod vectors
Transmission of the protozoan from an infected to an uninfected animal primarily occurs through the Asian longhorned tick (Haemaphysalis longicornis), an invasive species that has been detected in 23 states as of June 2025. The tick acquires the organism when it consumes a blood meal from an infected animal then transmits the organism through its saliva to other animals within the herd or nearby herds. Other arthropods such as biting flies, mosquitoes, lice, and different tick species have also been implicated as potential vectors, but their relative importance is uncertain.
Mechanical transmission
With mechanical transmission, the organism is transferred from infected to uninfected animals through blood-contaminated equipment. This would include items such as needles, dehorning, castration and tagging equipment. While this is a well-demonstrated mode of anaplasmosis transmission, it appears to be less efficient for Theileria.
Vertical transmission
It is estimated that up to 10% of calves born to infected cows will be infected themselves. However, the impact this has on the epidemiology of the disease is likely minimal.
Stages of disease
Incubation
The incubation period of a disease is the time from pathogen exposure to the appearance of clinical disease. The incubation period for this disease ranges from one to eight weeks and is determined by the initial infectious dose, with higher doses resulting in shorter incubation periods.
Clinical disease
Clinical disease occurs once the level of infection reaches a point where significant red blood cell destruction occurs. As previously mentioned, symptoms may include elevated temperature, depression, pale mucous membranes, jaundice, abortion, and death loss and depends on the severity of the anemia. Mortality rates of 1% to 5%, and occasionally much higher, have been reported. Young calves or females in late gestation or early lactation are more likely to be clinically affected but it is important to note, that many infected cattle never show symptoms at all. It has been suggested that once the first clinical case is seen in a herd, a large percentage of the animals may already be infected.
Carrier state
Following the peak of the infection, animals enter an asymptomatic carrier state and maintain a low-grade infection for the rest of their life. The benefit of the carrier state is that an animal rarely develops clinical disease again. However, they do serve as a reservoir of the organism within a herd so culling carrier animals may be warranted if disease prevalence is low.
Diagnosis
Symptoms of Theileria and anaplasmosis are similar, but there are a few differences may help distinguish the two:
- Cattle affected by Theileria may show ventral edema.
- Clinical signs associated with Theileria can occur in cattle of any age, while clinical signs associated with anaplasmosis is more commonly observed in older animals.
- Abortion in late gestation appears to be more common with Theileria.
- Cattle with Theileria do not exhibit the aggressive behavior which sometimes seen with anaplasmosis.
- Splenic enlargement is less pronounced with Theileria.
Clinical signs or necropsy findings alone are not sufficient to confirm the presence of this disease within a herd. Therefore, additional tests are necessary.
In acute cases, Theileria organisms may be visible within red blood cells on a blood smear and may appear linear, circular, or pear-shaped; however, the organisms can be difficult to find, particularly in cattle that have passed the acute phase of infection. The diagnostic test of choice is polymerase chain reaction (PCR), which detects the organism’s genetic material. Some laboratories offer a PCR test exclusively for Theileria orientalis (Ikeda genotype) whereas others offer a duplex PCR which tests for both Theileria and anaplasmosis. The herd veterinarian can provide guidance on the best testing strategy for your operation.
Treatment
There are currently no approved treatments in the U.S. for Theileria. As Theileria is a protozoan and not a bacterium, antibiotic treatment is generally not effective.
Questions often arise about the use of injectable oxytetracycline as a treatment. Some veterinarians report possible improvement when it is administered early in the disease, but research has not supported its effectiveness against Theileria orientalis.
Additional recommendations for managing clinical cases focus on providing supportive care, including minimizing stress, avoiding unnecessary handling, and ensuring access to feed and water. In some cases, separating clinically ill animals to reduce their need to compete for feed and water may be beneficial. Blood transfusions may also be an option for more valuable animals.
Prevention and control
There are no vaccines or antibiotics in the U.S. that will prevent or control this disease, so it is important to control Asian longhorned tick populations because of the significant role they play in transmission. Virginia Cooperative Extension has created a publication that describes tick management practices for cattle producers. A recent study in Tennessee found that maintaining a closed herd (not introducing new cattle), monthly brush hogging of pastures, and topical application of permethrin-based products were associated with significant reductions of longhorned tick populations on a cow-calf farm. The researchers advocate an integrated pest management (IPM) approach to longhorned tick control, as exclusive reliance on chemical control can contribute to faster development of resistance.
Other management practices to consider may include quarantining and testing of new animals for longhorned ticks and/or T. orientalis infection, eliminating carrier animals from a herd if disease prevalence is low, changing needles between animals, and disinfecting equipment that may be blood contaminated between animals. Again, the herd veterinarian can help to develop the best prevention and control strategies for your operation.
References
- Butler, R. A. & Trout Fryxell, R. T. 2023. Management of Haemaphysalis longicornis (Acari: Ixodidae) on a cow-calf farm in East Tennessee, USA. Journal of Medical Entomology.
- Emery, D. 2020. Prophylaxis and treatment of Theileria orientalis. North Sydney, New South Wales, Australia: Meat and Livestock Australia.
- Emery, D., Zhang, S., Loo, C. & Shirley, C. 2021. A longitudinal study of infection with genotypes of Theileria orientalis in calves and introduced cattle at Dorrigo, New South Wales, and the effect on weight gains. Veterinary Parasitology, 296, 109487.
- Ierardi, R. A., Chance, S. M., Morris, C., Nunnelley, J., Odemuyiwa, S. O., Royal, A. B., Schultz, L., Shen, Z., Young, J., & Raghavan, R. K. 2025. Active surveillance for Theileria orientalis and the invasive Asian longhorned tick (Haemaphysalis longicornis) in three Missouri beef herds. PLoS One, 20(4), e0319327.
- Lahmers, Kevin. 2021. Update on Theileria orientalis and Haemaphysalis longicornis (video). Retrieved Aug. 30, 2023.
- Lawrence, K. E., Gibson, M., Hickson, R. E., Gedeye, K., Hoogenboom, A., Fermin, L., Dragnova, I. & Pomroy, W. E. 2018. Experimental infection of Friesian bulls with Theileria orientalis (Ikeda) and effects on the haematocrit, live weight, rectal temperature and activity. Veterinary Parasitology: Regional Studies and Reports, 14, 85–93.
- Lawrence, K. E., Lawrence, B. L., Hickson, R. E., Hewitt, C. A., Gedye, K. R., Fermin, L. M. & Pomroy, W. E. 2019. Associations between Theileria orientalis Ikeda type infection and the growth rates and haematocrit of suckled beef calves in the North Island of New Zealand. New Zealand Veterinary Journal, 67, 66–73.
- United States Department of Agriculture. 2021. Emerging Risk Notice: Theileria orientalis Ikeda (PDF). Retrieved Sept. 9, 2023.