Veterinarian View | November 2025

Cattle, Ticks, and Theileriosis

contributed article by Dr. Lisa Kellihan, Associate Veterinarian, Collison Embryo and Veterinary Services, Rockwell City, Iowa

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Considering that it has now spread to 24 states, you may have heard of the Asian longhorned tick (ALHT, Haemaphysalis longicornis) by now. The ALHT is native to eastern Asia and is already an introduced pest to Australia, New Zealand, and several western Pacific Islands. At some point, likely in or before 2010, the tick made its way to the U.S. and was first detected in New Jersey in 2017.

Since then, it has steadily moved westward as far as eastern Oklahoma at a rate of about three states per year. This is concerning to cattle producers and veterinarians for two main reasons: the ALHT can cause production and death losses on its own due to massive infestations and it is carrying with it the causative agent of Bovine theileriosis, Theileria orientalis Ikeda.

The Tick

Asian longhorned ticks are light brown in color and are typically very small. Adult females are smaller than a sesame seed prior to feeding but swell to the size of a pea once full with a blood meal. ALHT can be found anywhere in the environment, but may prefer tall grasses and wooded areas.

They are a three-host tick but have no obligate species – all stages feed on just about anything and feed side-by-side with other tick species. ALHT larvae, nymphs, and adults can all be found feeding on a wide variety of hosts, including birds, pet dogs and cats, horses, livestock, and wildlife. The tick has also been found on people. However, there are currently no known risks to human health in the U.S.

Unlike other tick species, the ALHT female can reproduce without a male. Additionally, she can lay 1,000 to 2,000 eggs at a time. As a result, a single female can establish a new ALHT population in a new location. This also leads to large infestations on individual animals, causing growth and production losses due to stress and even death due to excessive blood loss in severe cases.

As mentioned previously, disease spread is another threat. The ALHT is the main vector for transmission of the blood parasite, Theileria orientalis Ikeda. This pathogen is mainly a problem in cattle and spreads to susceptible animals during tick feeding through the tick’s saliva.

Infected cattle can also spread the parasite to uninfected ALHT that feed on it. The parasite then replicates and matures in the digestive tract of the tick after it has been attached to the host for 48 to 72 hours, making the tick infective to its next susceptible host. Once infected, ALHT can remain infected on a pasture for up to two years under favorable conditions.

Additionally, once Theileria is established in an ALHT population, it can persist for generations. This is because ticks can get infected at any stage of development and remain infected through each stage. However, they only transmit Theileria when they feed on cattle as adults. Individual cattle may have large infestations of ALHT, but it only takes a few to transmit disease.

The Disease

There are 11 major genotypes of Theileria orientalis present in the U.S. To date, only the Ikeda genotype has been shown to cause severe illness in animals. T. orientalis Ikeda is a protozoan parasite that invades red and white blood cells of cattle, leading to the disease bovine theileriosis.

This disease is characterized by anemia and immune dysfunction. Signs and symptoms can look similar to anaplasmosis but, unlike anaplasmosis, theileriosis affects calves and young cattle in addition to adults. More specifically, signs of theileriosis can include lethargy, weakness, fever, loss of appetite, pale or yellow mucous membranes, abortions, difficulty breathing, and sudden death in severe cases. Infected cattle may begin to show signs of illness within three weeks of exposure to infected ALHT, and illness results in death in up to 5 percent of cases, especially in calves and pregnant heifers.

There is currently no approved treatment for theileriosis in the U.S., so supportive care is all that can be provided while the disease runs its course. Since T. orientalis Ikeda is a protozoan, not a bacterium, antibiotics are ineffective. Cattle that recover from theileria infections usually become lifetime carriers. This means that they can have relapses of disease if they become stressed at any time after their initial recovery.

Prevention and Control

Since there’s no approved treatment for theileriosis in the U.S., tick control and herd management are crucial to reduce the spread of T. orientalis Ikeda and protect cattle. This can be challenging because the ALHT reproduces so rapidly and because this tick spends most of its life on the ground, rather than on its hosts. However, slowing disease spread can be achieved using several strategies.

1. Reduce tick habitat by keeping grasses cut short and removing weeds and brush from grazing areas. Burning pastures in the spring before they green up will also significantly reduce the number of ticks in the environment. Leaving pastures ungrazed will not control ticks as they can survive for about a year without feeding and will happily feed on any wildlife that utilize the pasture.

2. Reduce tick exposure by keeping cattle out of wooded areas, restricting grazing in heavily tick-infested pastures, and providing chemical control for ALHT from March into November, when these ticks are most active. Combining the use of pesticide-impregnated ear tags, backrubbers, pour-ons, and injectable pesticides can be effective if used appropriately.

Permethrins, organophosphates, and avermectins should be effective against the ALHT but should be used responsibly as this would be considered extra-label use and to avoid promoting the development of resistance. Work closely with your veterinarian to create a suitable tick control program for your herd.

3. Prevent introduction of ALHT and/or theileriosis by exercising caution when importing cattle, especially from regions where either parasite has been diagnosed. This was an important lesson learned in Kansas, where feeder cattle brought into the state from the eastern U.S. were apparently free of the ALHT, but were infected with theileriosis, leading to incidences of the disease within the state. Imported cattle should be isolated for at least three weeks and treated for ticks prior to herd introduction.

4. Prevent unintentional spread of T. orientalis Ikeda by observing good biosecurity. Avoid re-using needles and sharing equipment between animals as these practices have been shown to transmit disease.

5. Finally, identify new infestations of ALHT by regularly monitoring herds for unusual tick species, especially in areas where the ALHT is known to be present. Focus on cattle that appear unthrifty, have patchy hair, or are experiencing poor weight gain or weight loss. ALHT tend to prefer certain areas of the body including the head and neck, inside the ears, under the tail, in the armpits and groin, and in udder skin folds.

As the ALHT and T. orientalis Ikeda continue to spread farther west, producers should be vigilant in monitoring for ticks and signs of theileriosis. If you suspect that you have identified ALHT in your herd or observe signs of anemia, lethargy, jaundice, abortion, or difficulty breathing in your livestock, contact your herd veterinarian to collect appropriate samples for diagnostics and consultation.

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Dr. Lisa Kellihan is an associate veterinarian with Collison Veterinary Services in Rockwell City, Iowa.