The first clue was a Sunday-night phone call.
Dr. Dan Goehl, large animal veterinarian and owner at Canton Veterinary Clinic, was at home when one of his regular producers called in a panic. A group of yearling heifers had been turned out on grass after breeding. When the producer checked them that evening, eight were dead and several more were missing.
The initial suspicion wasn’t Theileria orientalis.
“We immediately talked about toxins,” Goehl says. “That was kind of my immediate assumption.”
The cattle were brought in from pasture the next morning for examination. That’s when the investigation took an unexpected turn.
They found an extraordinary tick burden.
“There were thousands and thousands of ticks on those animals to the point that some of them could barely hold their ears up,” Goehl recalls.
The finding shifted the investigation toward Theileria, a tick-borne protozoal infection that can cause severe anemia in cattle. The team treated the cattle to reduce the tick burden, collected ticks for testing and submitted cattle for diagnostic testing.
The results confirmed Theileria.
At the time, it was the first recognized case in Iowa for Goehl’s practice area. There had been scattered cases in Missouri, but the severity of this outbreak raised questions about both where the infection came from and how long it had been present.
The Wyoming Connection
The herd consisted of 67 yearling heifers turned out to pasture. Seventeen ultimately died.
The deaths weren’t evenly distributed.
Fifty of the heifers had been brought in from Wyoming earlier that winter. Every heifer death came from that group. None of the native heifers died. That translated to 32% mortality among the Wyoming heifers.
The group also included a bull that died, adding another complication to the pattern.
Goehl describes the outbreak as a potential “perfect storm.” His working hypothesis was that the Wyoming cattle may have been naive to the infection, encountered a higher parasite burden than they had previously experienced and were exposed to an extraordinary number of Asian longhorned ticks.
The case provided an opportunity to investigate something that was even more important than the original deaths: how widespread was Theileria exposure across the operation?
The Investigation Moves to the Pasture
The veterinary team soon realized the problem extended beyond the pasture where the deaths occurred. They went back and tested cattle across the operation, finding Theileria positivity in every pasture they examined. In one pasture, all 40 cattle tested positive. Other pastures had positivity rates of 87%, 95%, 99% and 81%.
A pasture containing calves was particularly revealing. When the calves were initially tested, 46% tested positive. Thirty days later, 97% tested positive.
The team then turned their attention to the pastures themselves, conducting tick drags with white sheets through areas where they suspected ticks might be concentrated. The results helped demonstrate how difficult it could be to identify the source of exposure by looking for ticks alone.
In the pasture associated with the original outbreak, 93% of the ticks they collected were Asian longhorned ticks. Yet in some pastures where cattle tested positive for Theileria, the team did not recover an Asian longhorned tick during their sampling.
“Just because you don’t see the tick doesn’t mean you don’t have it,” Goehl says.
That became one of his strongest take-home messages from the investigation.
The Asian longhorned tick can be difficult to detect because of its small size, particularly in its larval and nymphal stages. The ticks also appeared to cluster in localized areas of pasture. Cattle could encounter a heavily infested “hot spot” without the producer realizing how much exposure had occurred.
The field experience made the scale of the problem more apparent. When cattle were gathered on horseback, riders could pick up thousands of ticks on their chaps after moving through some areas.
Goehl’s conclusion was that the infection likely had been present in the area well before the dramatic mortality event brought it to everyone’s attention.
A Positive Test Isn’t the Whole Diagnosis
The case also highlights a challenge veterinarians face when Theileria is on the differential.
Clinical signs can be relatively nonspecific. Goehl describes affected cattle as off-feed, depressed animals with pale mucous membranes, sometimes with icterus. Packed cell volume can fall below 15%.
The disease can resemble anaplasmosis, another cause of severe anemia, although Goehl says there are differences in how the cattle present. Theileria has also shown up more often in younger cattle in his practice, including feeder calves.
In calves, the clinical picture can look like chronic bovine respiratory disease, making it difficult to determine whether Theileria is contributing to poor performance or mortality.
That same diagnostic challenge came up when Goehl’s practice investigated a group of 107 newly received cattle from southeast Missouri. The calves were experiencing more morbidity and mortality than the practice expected, but there was an obvious competing explanation: respiratory disease is common in newly received cattle.
Then one calf died after appearing to have chronic respiratory disease. When Goehl performed a necropsy, however, the lungs were “beautiful.” There was no obvious lung disease to explain why the calf had continued to struggle.
By that point, Goehl had already seen Theileria in calves in the area, so the team added it to the investigation. They began testing the cattle for both Theileria and anaplasmosis.
The team later compared Theileria-positive cattle with their treatment histories to see whether positivity was associated with cattle that had been treated for respiratory disease. They did not find a clear pattern.
The case highlights the importance of broadening the differential when a calf shows signs of chronic respiratory disease but lacks the expected lung lesions, particularly in an area where Theileria is present. A positive test should then be interpreted alongside the animal’s clinical presentation, degree of anemia, exposure history and other potential causes of disease.
Prevention May Be More Useful Than Treatment
Once cattle develop severe clinical disease, options become limited.
Goehl’s practice has used ivermectin-class products to reduce tick burdens, along with supportive approaches and, in selected severe cases, whole-blood transfusions. The practice has not seen a consistently high success rate with transfusion, which Goehl believes may partly reflect how late treatment is initiated.
“For the cows that we’ve performed transfusions on, we’ve started too late,” he says.
That leaves tick management as one of the most practical tools available.
Pasture burning can be highly effective where it is feasible. Managing brush, broadleaf vegetation and moist areas can also reduce habitats favorable to ticks. Rotational grazing may help by managing vegetation and reducing favorable tick habitats.
Chemical control can be more complicated. Spraying may have limited effectiveness because it is difficult to penetrate the vegetation where ticks are concentrated. Goehl suggests organophosphates may provide better tick control than pyrethrins, although product selection and use need to fit the operation and label.
Biosecurity is another piece of the puzzle.
The Asian longhorned tick is the primary biological transmission concern, but blood-transfer routes such as stable flies, needles, tattoo equipment and other shared equipment can contribute to mechanical transmission.
However, the greater concern may be on the western edge of the tick’s range, where an infected animal could enter a population that has little or no previous exposure.
Preparing for an Endemic Disease
Goehl expects Theileria to become endemic in his region. As exposure becomes more common, he believes the disease may eventually resemble anaplasmosis, becoming another endemic cause of anemia that veterinarians routinely consider.
That doesn’t mean producers should ignore the disease.
Instead, the Iowa case suggests that the biggest risk may come from assuming the problem isn’t present because the evidence isn’t obvious.
“The big take-home to our producers is that just because you don’t see the tick doesn’t mean you don’t have it,” Goehl says.
That means keeping Theileria on the differential when cattle present with unexplained anemia, poor performance or death, particularly in younger or newly introduced cattle. It also means looking beyond the individual animal and asking where the cattle came from, what pastures they have occupied and whether tick exposure could have occurred even when ticks aren’t readily visible.
The disease may already be moving through a herd long before the first dramatic mortality event makes it impossible to ignore.
Goehl’s advice is therefore less about panicking over the next dead animal and more about recognizing a slower-moving change in the disease landscape: improve surveillance, manage tick habitat, reduce opportunities for blood transfer and use diagnostics when the clinical picture doesn’t fit.
The ticks may be small. The lesson from this herd was not.
