The Asian longhorned tick (ALHT) is not your typical tick. No longer a problem confined to a handful of Eastern states, the invasive tick has spread to 27 states since its first U.S. detection in New Jersey in 2017. As it moves west and south into new cattle country, producers and veterinarians who have never dealt with the tick — or the disease it carries, theileriosis — are facing a steep learning curve.
Unfed ALHTs range from a light reddish tan to a dark red/brown. While the adult female grows to the size of a pea when full of blood, other stages of the tick are very small — about the size of a sesame seed. Once blood fed, adult females are a grey green with yellowish markings, similar to native ticks. Male ticks are rare.
APHIS reports it only takes a single tick to create a population in a new location. ALHTs need warm-blooded hosts to feed and survive. They have been found on various species of domestic animals — such as sheep, goats, dogs, cats, horses, cattle and chickens — and wildlife. The tick has also been found on people.
Kansas State University Extension entomologist Cassandra Olds warns cattle producers ALHT and theileriosis are a challenge that is both wideespread and a long-term concern.
“While New World screwworm, has received a lot of attention, we have great tools to deal with the outbreaks and eventually re-eliminate the pest,” she explains. “For ALHT and bovine theileriois, it will be a problem that we have to deal with indefinitely.”
Olds and Virginia Tech Veterinarian John Currin have witnessed the tick and tracked the disease for years. Here are the 10 things they say every producer needs to understand about ALHT:
1. ALHT Can Reproduce Without a Mate.
Unlike most tick species, the ALHT can reproduce asexually. A female can lay up to 2,000 eggs at a time without ever encountering a male. That reproductive shortcut is a major reason populations can explode in a single season once the tick establishes in an area.
Biologically, the ALHT is a three-host tick, meaning each life stage — larva, nymph and adult — feeds on a different host.
2. Double Threat: The Tick And The Parasite It Carries Is
ALHTs carry Theileria orientalis, a protozoan parasite that infects red and white blood cells, causing anemia and, in severe cases, death. Bovine theileriosis is the disease caused by Theileria orientalis.
Heavy tick infestations alone can also stress an animal enough to reduce performance — USDA APHIS has noted that a dairy cow can see a 25% drop in milk production simply from the burden of feeding ticks, independent of any disease they transmit.
Olds says because theileriosis can spread without the tick being present through biting insects like lice and shared needle use, this isn’t just a problem where ALHT is already established.
3. Once An Animal Is Infected, It Carries Theileria For Life.
There is no cure. An infected animal becomes a chronic carrier and the parasite can flare back up any time that animal faces significant stress — shipping, weather, weaning or calving. That’s why high-stress groups deserve extra attention long after the initial outbreak has passed.
4. The Disease Looks Completely Different In Calves Than In Adult Cattle.
Currin says in animals over about a year old, theileriosis tends to mimic anaplasmosis. In calves under a year, it more closely resembles pneumonia. In both cases, the giveaway is that the animal won’t respond to antibiotics no matter how many doses it receives — because there is no direct treatment for theileriosis, only supportive care.
5. Watch For An Animal Going Off Feed — Don’t Wait For Anemia.
By the time an animal shows visible anemia, the disease has often progressed significantly. Earlier warning signs include loss of appetite, severe lethargy, a dejected or “droopy-headed” appearance, fever, coughing or labored breathing, and rapid weight loss. Sick animals also tend to be unusually withdrawn rather than aggressive, a contrast to the behavior often seen with anaplasmosis.
6. Mortality Is Highest During The Initial Outbreak.
Olds reports mortality rates due to Theileria range from 5% to 20%, with the highest mortality occurring when the pathogen is first introduced to a herd. Once an area becomes endemic and most animals are exposed as calves with maternal protection still active, severe outbreaks become far less common.
7. In Endemic Areas A Positive Test Means Almost Nothing.
This is one of the most important lessons from areas that have lived with the tick the longest. Currin, who has treated cattle through Virginia’s Theileria outbreak since 2017, put it this way: “The tick epidemiology is — I’ll use a scientific term — weird.”
Currin reports more than 90% of herds in his part of Virginia are infected, many without producers ever knowing it. His key takeaways for producers in newly affected areas:
- 99% of herds will experience minimal problems.
- Most infected animals show no significant clinical signs.
- The disease primarily causes anemia.
- There’s no direct treatment available.
- Positive tests are now almost meaningless in endemic areas.
8. As A Region Becomes Endemic, The Disease Shifts From Cows To Calves.
Currin has watched this play out in Virginia: Theileria initially hit adult pregnant cows hardest when the disease was new to the area, but now more commonly affects 2-month-old calves in spring-calving herds. The reason is straightforward — there is no in-utero transmission of Theileria, so “every calf starts naive,” Currin says.
Roughly 80% of calves in infected herds become positive by three months of age, though most never show severe clinical signs.
9. Feedlot Performance Impact Appears Minimal.
Contrary to early fears, Currin says research has found no significant difference in weight gain between Theileria-positive and Theileria-negative cattle in a feedlot setting. Olds adds death loss can still occur from outbreaks.
Currin says his primary concern instead lies with cow-calf operations bringing naive animals into already-infected areas — not with finishing cattle that test positive.
10. This Isn’t A Fight You Can Win By Keeping The Tick Out — It’s One You Manage.
Veterinarians and researchers across affected regions agree on this point: Producers cannot realistically prevent the ALHT and Theileria from eventually reaching their operation. The goal instead is knowing your herd’s status, watching high-risk animals — young, pregnant, recently weaned or recently shipped — more closely, and having a working relationship with a veterinarian who can respond quickly when an outbreak hits.
Olds notes the Theileria can spread through tick bites and through shared veterinary needles. She says her team is currently researching the role of biting flies in transmission. She stresses the importance of maintaining animal health and reducing stress to prevent disease outbreaks.
“Keep animals happy and healthy. Stress does weird things to immune responses,” she adds.
According to Olds, these strategies can help control AHLT:
- Burning pastures in regions where this is permitted
- Keeping grass short and removing woody encroachment
- Careful use of insecticides
- Pasture rotation
There are currently no labeled treatments for the disease or a definitive cure. Typically, treatment focuses on secondary issues and providing basic care. Olds suggests producers avoid mass injecting ivermectin, as it can cause resistance to other pest populations like biting flies and native ticks and harm beneficial insects like dung beetles..
Medgene has developed a tick vaccine that is designed to work on multiple tick species, including ALHT. Veterinarians can purchase directly from Medgene. The vaccine requires two initial doses, followed by yearly booster shots.
Olds says the goal is to achieve “endemic stability” — when most animals are exposed early and develop lifelong immunity with minimal disease symptoms.
She emphasizes community-level management over eradication.
“The Asian longhorned tick and Theileria are not going away, and it is not a producer’s fault if the disease appears,” she summarizes. “Many herds are likely already infected without obvious illness.”
Instead of trying to keep it out forever — an unrealistic goal — she encourages producers to know their herd’s status through testing, plan how they move cattle in and out, stay vigilant with high-risk groups and continue doing business while adapting to this new endemic reality.
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