Part 2: Dealing with Theileria
When Virginia Tech veterinary pathologist Dr. Kevin Lahmers became involved in determining the cause of death in Virginia cattle, he didn’t realize it was the same problem other countries, including Australia and New Zealand, had been dealing with.
Since the identification of the tick-borne disease Theileria orientalis, Ikeda genotype, in cattle, researchers created a predicted distribution map for the Asian longhorned tick (ALT) – the carrier of the infective protozoan. The map covers most of eastern U.S.
Lahmers said when ALT are in an area, they’re present in large numbers – hundreds of thousands. “There are multiple life stages of the tick in the same animal,” he said. “We can see larvae, nymph and adult in the same year.”
In Virginia, the ticks are seen more in higher elevations. However, some of those data are likely due to lower stocking density in tidewater regions.
ALT presence peaks in April and again in September and October. This correlates with spring and fall calving and it’s also when the most illness and deaths occur. “This also follows when the nymph and the adults are out and active,” Lahmers said.
To help determine the range and presence of ALT, researchers began to monitor sale barns in Virginia. In 2018 and 2019, about 2% of sale barn cattle tested were positive for the Ikeda genotype. “That jumped in 2020 to between 10% and 25%, and in 2022, closer to 50% were positive,” said Lahmers. “It isn’t just that we are doing more surveillance – it’s becoming more prevalent and more widespread.”
The estimated cost per positive animal based on treatment, abortion, production loss and potential mortality is $760. A veterinarian in central Virginia shared data on mortality, morbidity and abortions, and Lahmers came up with a potential estimated client loss of more than $250,000 over the last three years.
“This is not a theoretical problem,” he said. “This is actually happening in the Mid-Atlantic right now.”
Affected animals are typically registered, purebred animals with high genetic value in well-maintained herds that practice good biosecurity. In Virginia, one of the biggest risks is bringing in breeding stock, especially breeding bulls. Lahmers said such animals are a genetic investment, so the loss of a bull is a hit to genetic progress.
“If the bull is naïve and gets infected during breeding season, there will be decreased libido,” said Lahmers. “One of the things we see is the first few cows are bred, then there’s a month to a month and a half gap, then the remainder of cows are bred. Or if you’re pulling the bull, a few [cows] are bred and the rest are open.”
Some farmers are considering bringing Ikeda-positive animals into endemic regions so they don’t run the risk of losing those animals when they eventually become exposed.
PCR-based tests can detect Theileria. Pooled testing is available in fringe areas or areas with low prevalence to reduce the per-animal testing cost. “When we find a clinically positive animal, we ask to test more animals in that herd,” said Lahmers. “We have found that once the first clinical animal is found, it’s most likely that 80% to 100% of the herd is positive but the majority didn’t show clinical signs.”
While there is currently no approved treatment, controlling ticks can help prevent infection.
The majority of infected cattle have limited clinical signs and not all disease in a positive animal is the result of Theileria. “There are no obvious long-term health or production effects of persistent infection,” said Lahmers. “Once infected animals are protected from subsequent clinical disease, they can become infected with other genotypes but they don’t typically have clinical disease.”
At this point, the best management steps for Theileria include tick control, testing for detection to determine what’s present in a herd and supportive care for anemic animals.
“Stick with single-use needles, which is important for Anaplasma, Theileria and bovine leucosis,” said Lahmers. “Repeater syringes for vaccines are nice, but there’s potential for transmission. We have seen outbreaks associated with herd vaccination that happen four weeks after the entire herd was vaccinated.”
Lahmers said there are tick-positive regions that do not yet have Ikeda, and regions that have Ikeda but don’t yet have ALT. “We need to think of endemic areas where we have the tick and have Ikeda versus areas where we don’t have the tick and potentially don’t have Ikeda,” he said. “We want to prevent disease in the ‘free’ zones. Test animals, try to prevent the spread of the tick, try to prevent bringing in Ikeda positive animals. If you’re in the endemic zone, bringing in naïve animals [those that have not been exposed] puts them at risk in that they would potentially become infected and either die or be sick and recover.”
Efforts to manage Theileria include continued surveillance along with watching trends and expanded range in collaboration with USDA-APHIS. Researchers are also looking at potential interventions and percent positives within herds based on housing including drylot, pasture or confinement.
Lahmers said there is no risk of contracting Theileria from a carcass – it isn’t a human disease. Unless someone is mechanically moving blood or blood products into an open wound on another animal, it is not a risk to other animals.
Theileria testing and management depends on location, and state regulations vary. “Clinically ill, positive animals in New York are quarantined,” said Lahmers. “That may impact decisions. Surveillance would be for if you are in a fringe area where you have the tick in the region or have indication of Ikeda in the region, you might want to know whether your herd is positive or negative so you can interpret open cattle.”
He added that registered seedstock herds may want to consider testing and using Theileria status as a marketing tool.
“At this point in Virginia, some folks are trying to find Theileria-positive breeding stock so they don’t have to deal with the potential loss or decreased libido associated with a naïve animal getting infected,” said Lahmers.
by Sally Colby
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