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COLUMBIA, Mo. – University of Missouri Director of Veterinary Extension Craig Payne urges cattle producers to watch for signs of disease associated with Theileria orientalis (ikeda), a protozoal organism recently detected in Missouri.
This organism, which primarily affects cattle, infects red blood cells. The immune system attacks the infected cells, resulting in anemia. The disease is primarily spread by the invasive Asian longhorned tick.
It was first diagnosed in a cow-calf herd in Virginia in 2017. The first Missouri case was detected in Howell County in June 2023. Payne says that by the end of June 2024, there were 10 counties in Missouri where cattle tested positive. As of Oct. 1, 2025, that number is up to 48.
Payne notes the Missouri Department of Agriculture has an updated map showing the counties where the disease has been confirmed.
With mild infections, cattle may show elevated temperature, depression and pale mucous membranes. With severe infections, they can show severe depression, and the mucous membranes around eyes and the vulva appear jaundiced with a yellow tinge. Pregnant animals may abort, and animals will lose body condition.
Most cattle that become infected never show symptoms, and death loss rates are typically less than 5%. However, Payne notes that some herds experience much higher rates of illness and death loss than is reported in the literature.
Symptoms sometimes confused with anaplasmosis
The symptoms are similar to those of anaplasmosis, says Payne. A key difference is that anaplasmosis symptoms are rarely seen in cattle less than 2 years old, but symptoms associated with Theileria can be seen in both calves and adults.
Disease is transmitted several ways
The main route of transmission is through the Asian longhorned tick, an invasive species found in at least 23 states. The tick acquires the protozoa while feeding on an infected animal and later transmits it to other animals through its saliva. Other insects, such as lice, biting flies and other tick species may be involved in transmission as well.
Blood-contaminated equipment can also transfer the organism from infected to uninfected animals. This would include needles and dehorning, castration and tagging equipment.
Once infected, symptoms appear in 1-8 weeks.
Carriers for life
Infected animals will become lifelong carriers of the organism but are unlikely to show symptoms of disease again. Payne recommends discussing with your herd veterinarian whether it’s best to keep or cull these carriers.
Management: Treatment, prevention and control
There are no antibiotics available in the U.S. labeled to treat or control Theileria. Tetracyclines, which are commonly used to treat and control anaplasmosis in cattle, do not appear to be effective against this disease. Recommendations for managing clinical cases include minimizing stress and providing supportive care.
There are currently no vaccines available to prevent the disease, says Payne. The best option is to control Asian longhorned tick populations. Payne recommends a Virginia Cooperative Extension publication, Managing the Asian Longhorned Tick: Checklist for Best Management Practices for Cattle Producers.
Payne also recommends quarantining and treating new animals for ticks. In herds where the disease is already present, changing needles between animals and disinfecting equipment that may be blood-contaminated is good standard practice.
Finally, seek guidance from your veterinarian, who can recommend the best strategy for preventing or controlling the disease in your herd, says Payne.
For more information, the MU Extension publication Theileria orientalis: An Emerging Cattle Disease in Missouri is available free online.
Map of Theileria cases. The Missouri Department of Agriculture has confirmed Theileria orientalis in 48 Missouri counties as of Oct. 1, 2025. The first case in Missouri was confirmed in Howell County in 2023, says University of Missouri Director of Veterinary Extension Craig Payne. Map courtesy of Missouri Department of Agriculture.