Theileriosis is an intracellular protozoon which infects red and white blood cells of wild and domestic animals in the Bovidae family worldwide.  There are several types of T. orientalis in the United States, but only ikeda strain is known to cause severe illness in animals. There is no known risk to human health.

The Asian Longhorned Tick (Haemaphysalis longicornis) is the primary vector for transmission of T. orientalis ikeda. Spread of the disease is most effective through the bite of the tick and less commonly through equipment or other vectors (i.e. needles, lice, biting flies, and rarely transplacental). 

Found in Asia, New Zealand, Australia Asian Longhorned Ticks (ALHT) are three-host ticks, with larvae, nymphs, and adults feeding on a wide range of wild and domestic species, including birds, white-tailed deer, companion animals, livestock, equines, and humans. Once introduced into suitable habitats, ALHT populations can increase rapidly, partly due to exhibition of multiple genetic types including being parthenogenetic (asexual reproduction) and bisexual.

T. orientalis ikeda can cause anemia, jaundice, weakness, and sometimes death in 1-5% of infected cattle but the presence of clinical signs can be as high as 50%. Cattle may exhibit ventral edema, weight loss, and abortions; younger calves show more severe signs than mature cows/bulls. In dairies, clinical disease if any is associated with time on pasture and there is little production loss in most cattle. 

The majority of infected cattle have limited clinical signs and not all clinical diseases in a positive animal are the result of Theileria infections.  There are higher calf morbidity and mortality rates associated with times of stress – weaning, heat stress, nutritional. After resolution of symptoms, animals can remain infected for life and be a continued source of infection if ticks are present. Once infected, animals are protected from subsequent infections.

Currently, there is no approved treatment or vaccine available in the United States. The best method of prevention is tick control and good management practices for your herd. Minimize stress by using low stress handling methods and providing proper nutrition and access to water. Utilize external parasite control; choose a pyrethroid based product labeled for tick control. Avermectin pour-on (full-dose) may be best intervention strategy. Finally, regularly inspect cattle for ticks – ears, under tail, axillary regions, flanks, udder skin folds. 

Diagnostic testing is required to confirm the presence of the parasite. PCR is the most specific test.

References: USDA, Dr. Kevin Lahmers Virginia-Maryland College of Veterinary Medicine, Merck Veterinary Manual, WOAH