Theileria orientalis 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 23 states, including most recently Iowa and Michigan in 2025. The disease is carried by a new invasive tick, the Asian Longhorned Tick (ALHT), whose native range includes eastern China, Japan, the Russian Far East, and Korea. The ALHT and Theileria orientalis have also been previously found in SE Asia, Australia, New Zealand, Africa and South America. The initial finding of the ALHT in North America was in New Jersey in 2017, although a reexamination of tick specimens from 2010 showed that ALHT was present at least from 2010.

This photograph depicted a dorsal view of an adult female, Haemaphysalis longicornis tick, commonly known as the longhorned tick. This specimen was meandering about, climbing a blade of grass. H. longicornis ticks are able to reproduce in an asexual manner. Males are rare.

James Gathany, CDC

Theileria orientalis is spreading rapidly in the eastern U.S. and will likely continue to spread into areas where temperatures and humidity are favorable. Several habitat suitability models suggest that populations of this tick species could potentially inhabit most of the eastern half of the United States and areas on the Pacific Coastline. The spread of Theileria orientalis and ALHT are very closely related, and ALHT is assumed to be the most important route of transmission from one bovine animal to another.

The first case of ALHT bite on a human in the United States was reported in 2018. The ALHT also has been found on white-tailed deer, raccoons, opossums, and numerous other mammals and birds. To date, the United States host list for the ALHT includes dogs, cats, cattle, goats, sheep, chickens, pigs, horses, white-tailed deer, elk, black bears, opossums, grey foxes, red foxes, coyotes, eastern cottontails, deer mice, groundhogs, raccoons, striped skunks, gray squirrels, eastern chipmunks, red-tailed hawks, great horned owls, barred owls, Canada geese, brown boobys, gray catbirds, blue jays, Carolina wrens, northern cardinals, house wrens, song sparrows, and humans. To date, direct or indirect impacts of ALHT have not been detected in wildlife.

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 the disease is related to the severity of anemia. Other Theileria species have been detected in wildlife, but Theileria orientalis has not been detected in white-tailed deer yet.

Controlling ALHT Populations

There are no vaccines or antibiotics in the U.S. that will prevent or control this disease, so it is important to control ALHT populations because of the significant role they play in transmission. Tennessee reported 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.

This is one more reason to be tick aware as wildlife professionals are afield.