Unraveling a tick-borne disease


Part 1: Identifying the problem

Dr. Kevin Lahmers, veterinary pathologist at Virginia Tech, became involved in solving a mystery disease in cattle after a string of unexplained cattle deaths that occurred in Virginia in 2017 and 2018.

The mystery began when six dead cows were identified in a pond in Albemarle Co., VA. “Eventually, we found out there was Theileria in the herd,” said Lahmers. “Following that and working with others we identified Theileria orientalis, Ikeda genotype.”

T. orientalis is a tickborne protozoan that infects red and white blood cells in several species, including cattle. Clinical signs are similar to those of anaplasmosis, another tick-borne disease.

“We figured out what it was in early 2018 and identified it in two additional counties – one immediately adjacent to Albemarle County and a second county geographically distinct from those first two,” said Lahmers. “In 2019 we started finding more counties and additional counties in 2020. We are identifying more counties partly because we are looking and partly because the prevalence is increasing throughout the region.”

Theileria is not a threat to humans, but is economically important for cattle in the Pacific Rim, Asia, Australia, New Zealand and now the U.S. It’s transmitted by the bites of Asian longhorned tick, lice and biting flies and through shared needles.

“We see acute disease which usually occurs one to eight weeks after infection,” said Lahmers, explaining the progression of Theileria in cattle. “During acute disease, we see anemia, lethargy, fever, jaundice and ventral edema.” Death loss is 1% to 5%, although Lahmers said losses of 10% are not unusual and herd losses can be up to 35%.

A complicating factor is that in some herds, all animals are carrying the disease but the owner is unaware the animals are positive.

“We also see late-term abortions,” said Lahmers, adding that researchers are investigating the possibility of more earlier term abortions that go undetected. “Calf mortality is associated with Theileria orientalis Ikeda. This is in contrast to Anaplasma marginale (the organism that causes anaplasmosis), where we don’t typically see calf losses associated with that disease.”

Lahmers discussed acute vs. chronic Theileria cases. “We see disease associated with acute infection and it often occurs during periods of stress,” he said. “We see it associated with calving – also with heat and nutritional stress. Once the animal has controlled the acute infection, the numbers of organisms greatly decreases. They decrease by 1,000-fold, but the animal is persistently infected for life. That is good if you’re doing surveillance in that you can detect all animals that have ever been infected with Theileria orientalis, but it’s bad in that all animals ever infected … have the potential to be a reservoir for infection of other animals.”

Cattle often show no clinical signs, but may be weak and/or anemic and may abort. Unexpected death is also a sign of disease in a herd. Blood smears may allow detection during acute infection, manifested by more organisms in the blood than would be seen during the chronic phase. Lahmers said a good quality smear and some degree of expertise is required to identify organisms and differentiate Anaplasma from Theileria.

“Virginia Tech developed a duplex PCR test has been developed to look at Anaplasma marginale and Theileria orientalis because they are clinically similar,” said Lahmers, adding that about a dozen labs can conduct this test. “We want to help producers and veterinarians figure out when a sample from a clinically ill animal should be sent for testing. We have a challenge – once animals are infected, they’re infected for life, so if you test for Theileria in an endemic area, it’s quite possible you will have positive animals. But that doesn’t mean Theileria is the cause of clinical disease.”

Treatment options for Theileria are limited. Tetracyclines might work but animals need to be treated before they show clinical signs of illness. The current recommendation is to limit animal handling and minimize stress, provide good nutrition and B vitamins and provide plenty of access to water. Lahmers said some producers are placing brood cows in pens to eliminate competition for food and water.

Disease surveillance in Virginia is being conducted with the Department of Agriculture & Consumer Services for clinical submissions and active sampling at livestock auctions, and in State Department of Corrections herds.

T. orientalis Ikeda has been identified in nine states: Virginia, West Virginia, Kentucky, Tennessee, North Carolina, Pennsylvania, Maryland, New York and Kansas.

“We’ve started working with other states and are open to additional collaborations,” said Lahmers. “As we identify the disease further and further out, more people will be involved.”

by Sally Colby

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