Cattle producers are being urged to watch for signs of an emerging disease caused by “Theileria orientalis,” a protozoal organism transmitted by the Asian longhorned tick. While the invasive tick species is now found in 19 states, only 9 states have so far confirmed the presence of Theileria orientalis and the resulting disease known as “theileriosis.”
Asian longhorned ticks originate from East Asia and are believed to have entered the US as early as 2010. However, it wasn’t until 2017 that the death of seven cattle in Virginia led to the discovery of the new threat the ticks carried.
Theileriosis is characterized by anemia, fever, jaundice, respiratory problems, and weakness in cattle. In some cases, cows become so depleted that they spontaneously abort fetuses; in other cases, cattle die. Research has estimated one particular form of Theileria, known as the Ikeda genotype, causes mortality rates between 1% and 5%. However, variation is wide, according to Dr. Kevin Lahmers, associate lab director of Virginia Tech’s Animal Laboratory Services and a professor with the Virginia-Maryland College of Veterinary Medicine. “There are herds that have zero percent mortality. And we have some that have had 25%.”
Scientists have also been alarmed at how fast the species is spreading. Between 2019 and April 2023, the number of states that had detected the Asian longhorned tick rose from 11 to 19. They include Arkansas, Connecticut, Delaware, Georgia, Indiana, Kentucky, Maryland, Massachusetts, Missouri, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, South Carolina, Tennessee and West Virginia, as well as Virginia.
One reason for their rapid expansion is believed to be the pests’ willingness to feed on just about any animal. While they prefer large mammals, they have also been known to feed on smaller mammals like groundhogs and raccoons, as well as chickens and even migratory birds.
Scientists also credit the ticks’ rapid spread to their asexual method of reproduction, meaning they can reproduce without a mate. In fact, no males of the species have ever been collected in the US. No other tick in North America has this ability. Up to 3,000 eggs can be laid from one adult female Asian Longhorned tick, and it is common to find hundreds of the pests on one animal. There have even been reports of heavily infested cattle dying from exsanguination by the ticks.
In areas where the Asian longhorned tick has been found, they will likely be a long-term management problem, according to Risa Pesapane, assistant professor of veterinary preventive medicine at Ohio State. Because of their ability to hide in vegetation, Asian longhorned ticks also can escape pesticides that kill only when coming into direct contact with a pest. “Where the habitat is ideal, and anecdotally it seems that unmowed pastures are an ideal location, there’s little stopping them from generating these huge numbers.”
Despite the rapid spread of the ticks, veterinarians say there is not reason for panic as most cattle do survive theileriosos. Unfortunately, those cattle will be lifelong carriers of the organism, though, and it may make sense for some producers to cull those chronic carriers if disease prevalence is low.
While ticks are the main form of transmission, blood contaminated equipment such as needles and dehorning and castration equipment can also spread the Theileria orientalis organism. It’s also possible that other insects, such as lice, biting flies, and other tick species may contribute to its transmission.
So far, the Asian longhorned tick is not considered a risk to human health as they mostly prefer large livestock and wildlife. However, in some parts of the country, the pests have also tested positive for Lyme disease as well as Anaplasma phagocytophilium, a pathogen that can cause the disease “anaplasmosis” in both humans and animals.
While theileriosos symptoms can resemble anaplasmosis, antibiotics used to treat and control anaplasmosis in cattle do not appear to be effective against theileriosos. No vaccine is currently available to prevent the disease, so the best option to control it is to manage tick populations. Virginia Tech, which has been at the forefront of studying the disease, has a tick management checklist HERE. (Sources: Virginia Tech, Ohio State, Texas A&M)
