Asian longhorned tick. (Kevin Lahmers, Virginia Tech)

The cause of the seven cows’ deaths wasn’t immediately clear.

All of the animals died between late summer and fall of 2017 in Albemarle County, the prosperous, semi-agricultural county that surrounds the city of Charlottesville. Another cow on the property, sick but alive, was found with signs of anemia, weakness and respiratory distress.

Fearing the deaths were a sign of a new foreign animal disease, the U.S. Department of Agriculture came to Albemarle to investigate and found that six other cattle on the farm were infected with Theileria (pronounced tie-lee-ree-uh) orientalis, a fairly benign disease known to infect livestock in the U.S. But a later study done at Virginia Tech that looked at the form the Theileria took made a startling discovery: rather than the more common strain scientists were accustomed to seeing in North America, the disease infecting the Albemarle cattle was a virulent form known as the Ikeda subtype.

Ikeda had never before been detected in the Western Hemisphere. And investigators weren’t happy to see it. Studies from New Zealand have found that about 5% of cattle infected with this subtype die; those that survive often grow more slowly, produce less milk and are more likely to miscarry their calves. In Australia, this results in an estimated $20 million worth of cattle losses annually; in Korea and Japan, that number may be as high as $100 million.

So how did this form of Theileria end up in Virginia? No one was quite sure, and that uncertainty gave rise to worry. Albemarle isn’t particularly known for its cows, but it adjoins both Augusta County, the state’s largest cattle producer, and Rockingham County, the state’s largest cow milk producer.

If the disease spread, it could cause significant damage throughout the Shenandoah milk and beef fields and beyond. And cattle are big business in Virginia: in 2017, state cattle and calf sales amounted to almost $6.8 billion, while cow milk sales were just over $3.3 billion.

Then, on May 10, 2018, state field veterinarian Bruce Bowman brought a tick that had been found on the same property where the seven cattle had died to the office of Kevin Lahmers, a veterinary anatomic pathologist and associate professor at Virginia Tech. Lahmers, and later the USDA, identified the sample as an Asian longhorned tick, an “aggressive biter” that is the primary known carrier of Theileria in Asia. The discovery was a breakthrough.

“In other countries, this is the vector for this disease,” said Lahmers. “And so it’s highly likely that it’s the same here in the U.S.”

The May 10 tick was the first identified in Virginia in a wave of sightings that began in New Jersey in 2017. Since then, the longhorned tick has been confirmed in 11 states, with Virginia reporting its presence in 24 counties — more than any other state. And it has likely spread farther than that, said state veterinarian Charles Broaddus, although the Virginia Department of Agriculture and Consumer Services can’t definitively say that’s the case.

“It’s here, it’s everywhere,” said Broaddus.

In fact, the longhorned tick had already been “here,” although investigators hadn’t known it. Searches through state sample collections found that the tick had been in the United States since at least August 2010, when it was found on a white-tailed deer in Tyler County, West Virginia, and then misidentified.

Researchers haven’t determined how the tick arrived in the U.S., but now, said Lahmers, “the range is expanding.” That expansion is helped along by the species’ ability to reproduce asexually: females can lay fertile eggs without males and give rise to what one USDA presentation called “explosive mini populations.”

“We’re seeing it on cattle in numbers we haven’t before,” Lahmers said. “We think it’s reached a critical mass.”

While the longhorned tick can transmit diseases that affect humans — the most concerning being severe fever with thrombocytopenia syndrome, an emerging disease that has killed 12 to 30 percent of the people who have contracted it in East Asia — the only disease it has so far been found to carry in the U.S. is the Theileria orientalis Ikeda found in Virginia cattle.

North Carolina has linked longhorned ticks to the deaths of five cows in Surry County, just south of the Virginia border, by exsanguination, or severe loss of blood, but those cases haven’t involved any disease transmission. The tick has also showed up without incident on humans, horses, goats, sheep, cats, dogs, elk, coyote and fox.

In Virginia, though, not only are the ticks present, but so is the disease they carry. Four other cases have found longhorned ticks on properties where cows died of Theileria, establishing the closest link between the two in the U.S. to date. (While it’s likely that longhorned ticks transmitted the disease to the Albemarle cattle, the time gap between those cows’ deaths and the discovery of the pest means that the connection in that case can only be speculative.)

But, said Lahmers, “people don’t know about the disease, and so they don’t test for it.”

The Virginia Cattlemen’s Association did not respond to a request for comment.

Still, awareness may increase along with the longhorned tick’s footprint. As the continues to spread, one study found that the virulent Ikeda form of Theileria will likely expand throughout the country. Some researchers have attributed the tick’s rise to climate change, which is driving up temperatures and creating a more hospitable habitat for them, although Broaddus said that he hadn’t heard of any major increases in ticks in Virginia.

In an effort to better control Theileria’s spread, Virginia Tech has developed a rapid diagnostic test to distinguish between it and other, more common diseases. Surveillance continues in and around the 24 Virginia counties that have reported the longhorned tick’s unwelcome presence as researchers work to confirm the link between the two.

“This is people’s livelihoods that we’re talking about,” said Lahmers. “We’re trying very hard to get something that’s useful and help producers in the state and in the region.”