Theileriosis in Cows: Symptoms, Tick Spread, and Control

Theileriosis is a tick-borne disease caused by Theileria parasites. In U.S. cattle, the main concern is Theileria orientalis, especially the Ikeda genotype, which can cause bovine infectious anemia. The parasite infects blood cells and can lead to red blood cell destruction, poor oxygen delivery, weakness, and production losses.

Some infected cattle show few signs, while others become seriously ill. Clinical disease may include fever, pale mucous membranes, jaundice, reduced milk production, weight loss, abortion, and death. Cattle that survive may remain infected for life and can continue to serve as carriers within a herd.

This disease has become more important in the United States as the Asian longhorned tick has spread into more regions. That means herd risk depends not only on the individual cow, but also on pasture exposure, animal movement, and local tick pressure.

If you are seeing anemia-like signs in one or more cattle, your vet can help sort out whether theileriosis, anaplasmosis, babesiosis, blood loss, or another disease is most likely.

Theileriosis is caused by infection with Theileria parasites. In the United States, the most important form is Theileria orientalis Ikeda, an emerging blood-borne protozoan associated with clinically important anemia in cattle. Other genotypes such as Chitose and Buffeli may circulate too, but Ikeda is the genotype most strongly linked with severe illness.

The parasite is spread mainly by ticks, especially the Asian longhorned tick (Haemaphysalis longicornis). As infected ticks feed, they can transmit the parasite to cattle. USDA also notes that spread may occur mechanically through blood-contaminated needles or equipment, and Cornell reports possible contribution from biting flies or lice.

Herd-level risk rises when cattle are moved from one region to another, when new animals are introduced without testing, or when pastures and field edges support heavy tick populations. Pregnant heifers, calves, and cattle under stress may be more likely to show clinical disease.

Because recovered cattle may remain carriers for life, a herd can continue to face ongoing exposure even after the first obvious cases have passed.

Diagnosis starts with your vet looking at the whole picture: history, region, tick exposure, herd movement, pregnancy status, and signs of anemia. Theileriosis can look a lot like anaplasmosis or other causes of red blood cell loss, so clinical signs alone are not enough.

The most useful confirmatory test is usually PCR on EDTA whole blood. PCR is considered the most specific option for detecting Theileria orientalis, and additional testing may be used to identify the genotype, such as Ikeda. A blood smear may show piroplasms inside red blood cells, but smear review is less sensitive than PCR and may miss low-level infections.

Your vet may also recommend a CBC, chemistry panel, and sometimes necropsy testing if an animal dies. These tests help measure anemia severity, look for jaundice or other organ effects, and rule in or out coinfections. Cornell notes that coinfections, including Anaplasma marginale, can occur and may complicate the case.

If one cow tests positive, herd-level planning often becomes the next step. That may include testing additional animals, reviewing recent purchases, and building a tick-control and biosecurity plan with your vet.

Prevention focuses on tick control and biosecurity. USDA recommends keeping grasses shorter, removing weeds and brush from grazing areas, limiting access to heavily tick-infested pastures, and using approved tick-control products on animals, vegetation, and equipment according to label directions. Regular tick checks matter too, especially around the ears, under the tail, between the hind legs, and in udder folds.

If your herd is in or near an area where the Asian longhorned tick has been found, work with your vet on a seasonal plan. APHIS notes that these ticks are generally active from March through November, although timing can vary by year and region. A prevention plan may need to include repeated treatments, pasture rotation, and closer monitoring during peak tick activity.

Animal movement is another major risk point. Cornell advises avoiding introduction of the parasite into naive herds by testing cattle before purchase or movement when risk is present. New arrivals should be checked closely for ticks and signs of illness before and after introduction.

There is no vaccine and no FDA-approved treatment for bovine theileriosis in the United States at this time. That makes prevention especially important. If your herd has unexplained anemia, abortions, or weakness, early veterinary testing can help limit losses and guide practical control steps.