For decades, anaplasmosis was the tick-borne disease cattle producers in the South and Midwest learned to watch for every summer. Now a second, newer disease is moving into the same pastures — and it can look almost exactly the same from the chute.
Bovine theileriosis, caused by the protozoan Theileria orientalis Ikeda, was first identified in U.S. cattle in a Virginia cow-calf herd in 2017. Since then it has spread by the Asian longhorned tick, an invasive species now confirmed in more than 25 states. As that tick’s range grows, veterinarians and diagnostic labs are increasingly fielding cases that look like classic anaplasmosis but test negative — or positive for both diseases at once.
When a mature cow goes off feed, turns lethargic and shows signs of anemia, most producers in endemic areas immediately think anaplasmosis. Dr. Craig Payne, University of Missouri director of veterinary extension and continuing education, says the challenge is that “these diseases behave very similar, at least in terms of the clinical signs that you will see,” and “the only way that you’re going to know the difference between the two is through diagnostic tests using the PCR.”
With ALHTs on the rise in parts of the U.S., more producers are running into an old problem with a new twist: cows that look like classic anaplas cases, but don’t respond the way they “should respond to antibiotic.” In many herds, the question is no longer if red blood cell diseases will show up, but which one you’re dealing with — and what, realistically, you can do about it.
Old Enemy, New Rival
Both diseases attack red blood cells, producing the same baseline picture — fever, lethargy, pale or yellowed mucous membranes, reduced milk production and weight loss. Both can cause abortion in pregnant cows, and both leave survivors as lifelong, asymptomatic carriers that quietly maintain the pathogen within a herd.
The differences, where they exist, are subtle:
- Age susceptibility. Anaplasmosis symptoms are rarely seen in cattle younger than 2 years old, while Theileria affects calves and adults alike.
- Behavior. Aggression due to low blood oxygen can be seen in severe anaplasmosis cases; it is not typically seen with Theileria.
- Physical signs. Theileria-affected cattle may show ventral edema and less pronounced spleen enlargement than anaplasmosis cases.
- Timing of abortion. Late-gestation abortion appears more common with Theileria infection.
Payne recommends focusing on what matters chute‑side. “Mucous membranes will be pale or yellow. They may have elevated respiration rate to accommodate for an anemia. They’re feverish because they’re going through an infection, they’re off feed just simply because it’s an infectious process,” he explains.
None of those distinctions are reliable enough to diagnose on sight. Confirming either disease requires laboratory testing, typically PCR, and some diagnostic labs now offer a duplex PCR panel that screens for both pathogens in a single test.
Payne says PCR comes with a higher price tag than cELISA, which can limit whole‑herd testing. But when the clinical picture doesn’t fit, or cattle don’t respond to expected treatments, testing is crucial.
“If a person is dealing with an animal, it looks like they have clinical signs of anaplasmosis, that they’re non‑responsive to treatment with an antimicrobial therapy, that’s another possible indicator that that’s the disease that they’re dealing with is Theileria versus anaplas,” Payne says.
Treatment and Prevention: Very Different Toolboxes
The two diseases diverge sharply from a management standpoint.
Anaplasmosis is caused by a bacterium that is susceptible to tetracycline antibiotics. Oxytetracycline injections are used to treat sick cattle, while chlortetracycline can be delivered through mineral or feed to help control the disease.
An experimental vaccine is available only through veterinarians and not in all states.
“Your CTC is going to be used for control,” Payne notes. “There is an anaplas vaccine — it has to come through a veterinarian — it’s not commercially available.”
Theileriosis offers no such treatment option. Because T. orientalis is a protozoan rather than a bacterium, oxytetracycline has no effect on it. There is currently no approved antibiotics for treatment of the disease in the U.S., leaving supportive care — minimizing stress, ensuring access to feed and water, and blood transfusions for more valuable animals — as the only options.
Regarding vaccine, Payne says Medgene is reported to be releasing a Theileria vaccine in August. The vaccine will be available with veterinary prescription only however efficacy of the vaccine has not been established.
Medgene’s tick vaccine targets multiple tick species, including ALHT, aiming to reduce tick numbers, feeding time and reproduction — it’s for the tick, not the protoza, Payne explains.
Age Patterns and Herd Status: Naïve vs. Endemic
The two diseases behave somewhat differently across age groups.
Payne says with anaplasmosis the biggest concern is in adult cattle older than 2 years of age. In comparison theileriosis can affect cows and calves, and you can see anaplas‑like signs even in younger animals.
“You can see clinical signs in any age with Theileria versus anaplasmosis, usually they don’t see anything in an animal less than 2 years of age,” Payne notes.
Herd status is just as important as age. Both diseases tend to establish a chronic carrier state, and cause the worst losses the first time they sweep through a naïve herd.
Over time, many herds shift from shock to routine management, especially with anaplasmosis. Theileriosis is newer, so producers and veterinarians are still learning what “normal” looks like once it becomes endemic.
Dr. John Currin, Virginia Tech extension veterinarian, says, “Over 90% of the herds that get infected with Theileria never know that they’ve been infected. The vast majority of herds out there today in Virginia that have Theileria don’t know that they have Theileria unless they’ve tested for it.”
He stresses once Theileria is in an area it spreads rapidly.
“When it moves into a naive area where none of the livestock have been affected, it primarily seems to affect adult cows, and particularly late‑pregnant adult cows,” he explains. “We would see a little more spike of problems around late pregnancy.”
On the cow side, there is no in‑utero spread of this organism, so every is born naïve. So as a herd becomes endemic the clinical signs change from adult cows to calves.
“What we see is a very small percentage of 2‑month‑old calves that get severe anemia, and some of them will die from the anemia,” he explains.
Currin stresses producers do not want animals completely clear of either organism if they’re in an endemic area, because that will just set them up to potentially get reinfected and have a problem again.
Vectors and Geography: Old Disease vs. New Kid on the Block
For anaplasmosis, Payne points to the American dog tick as a key biological vector if you want to be tick‑specific.
Biting flies and needle or instrument transmission play a more mechanical role by transferring infected red blood cells from animal to animal.
For theileriosis, the story now heavily involves the Asian longhorn tick. That invasive tick is driving much of the current concern, especially where populations are building and naïve cattle are heavily exposed.
Payne describes high‑mortality situations as likely involving a combination of naïve population and animals that are stressed.
Geographically, anaplasmosis is an “ancient” disease and widely distributed, making precise state‑by‑state numbers difficult. Theileriosis, by contrast, is newer and easier to track in native cattle.
The Role of Your Veterinarian
Because the diseases are complex, rapidly evolving and emotionally charged for producers, Payne repeatedly comes back to one theme: you shouldn’t navigate this alone.
From deciding when to test, to choosing realistic prevention strategies, to managing naïve vs. endemic herds, your veterinarian is critical for both anaplasmosis and theileriosis.
Key Takeaways for Producers
In summary here are six key points for producers to remember:
- Both diseases destroy red blood cells and cause anemia; that’s what drives most of the clinical signs.
- Clinical signs in adult cattle can look almost identical, so you cannot reliably tell them apart just by looking.
- Anaplasmosis has approved antimicrobial tools (tetracyclines) and an experimental vaccine; theileriosis does not — management leans on supportive care and tick control.
- Naïve herds take the hardest hit when either disease first moves through; endemic herds usually see lower losses.
- PCR testing is the only reliable way to separate the two, often using a duplex test for both anaplasmosis and Theileria.
- Your veterinarian is essential for diagnostic decisions, treatment plans and long‑term herd strategy.
For now, anaplasmosis remains the more treatable and better-understood disease of the two. Theileriosis is the one without a treatment safety net — which is exactly why producers and veterinarians in newly affected states are being urged to test rather than guess.
