The Asian longhorned tick (ALT) is a newcomer to the U.S., and its presence has already caused significant damage to cattle. Blood-sucking ticks on the body are harmful enough, but the pathogens they transmit can be devastating.
Denise Bonilla, USDA-APHIS entomologist, discussed the circumstances that led to identifying ALT in the U.S.
In 2017, a New Jersey woman visited a local vector control agency and complained about ticks. The woman had ticks all over her clothes, and sheep in a paddock at her farm had a large number of ticks in various life stages. The entomologist at the agency was familiar with ticks but said the tick didn’t sound like what they normally see.
“We found out that it was Haemaphysalis longicornis, or Asian longhorned tick,” said Bonilla. “This is the first time it was found outside one of our quarantine facilities, and it had infested this premises. We wanted to find out if it was just that farm or just one area of New Jersey.”
When entomologists looked more closely at the tick and reviewed archives, they found the same tick was found in 2010 in West Virginia. Today, ALT has been identified in 17 states, and Virginia leads with 38 known infested counties. As of mid-2022, ALT had been identified in Arkansas, Connecticut, Delaware, Georgia, Kentucky, Maryland, Missouri, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, South Carolina, Tennessee, Virginia and West Virginia.
ALT infests wild mammals including deer, raccoons, opossums, fox, rabbits, groundhogs, skunks and mice. “Mice are important for other three-host ticks in the transmission of Lyme disease,” Bonilla said. “There are also infestations on birds including red-tailed hawks, owls and geese. Birds are moving this tick around.” While humans are not a preferred host for ALT, Bonilla said there are reports of larval stage ALTs on children, and there’s evidence that ALT has the ability to carry and spread pathogens to humans, including the bacteria that causes Rocky Mountain spotted fever.
“We see a lot on dogs,” said Bonilla. “Dogs are moving this tick when people travel. We don’t know where it came from, but we do know by looking at the genetics of ticks found throughout the U.S. that there have been at least three introductions into the U.S. We don’t really know where it started or when, but it has happened more than once.”
ALT hatches from an egg, molts into a six-legged larva, then into an eight-legged nymph. The final stage is adult. The adult ALT, which resembles the brown dog tick, takes a blood meal from its host, drops off and seeks another host. One unusual trait of ALT is that females can reproduce parthenogenically – without a male. This eliminates the need for male ticks and results in much faster reproductive rates, with females laying up to 2,000 eggs a day.
Cows are one of the favorite hosts for ALT, and it’s also been found on sheep, goats and pigs. Tick-infested animals don’t appear to be sick for any other reason, just covered with thousands of ticks. Animals infested with ALT can die from exsanguination (loss of blood). But the most serious problem is that ALT harbors and transmits the parasite Theileria orientalis, which causes infectious anemia in cattle. Cattle with Theileriosis present similarly to those with anaplasmosis.
Dr. Kevin Lahmers, veterinary pathologist at Virginia Tech, became involved in diagnosing Theileriosis after exploring unexplained cattle deaths in Virginia.
“We figured out what it was in early 2018 … In 2019 we started finding it in more counties and others in 2020,” he said. “The point is we are identifying more counties partly because we are looking and partly because the prevalence is increasing throughout the region.”
Lahmers said positive identification of T. orientalis in the herd was the first step. Because Theileria has several known genotypes, the next step was identifying the genotype responsible for disease, which is the Ikeda genotype. The disease is not a threat to humans, but it is economically important for cattle. In addition to being transmitted by ALT, Theileriosis is transmissible through shared needles, lice, biting flies and potentially trans-placentally.
“We see acute disease one to eight weeks after infection,” said Lahmers. “During acute disease there’s anemia, lethargy, fever, jaundice and ventral edema.” Typical death loss is 1% to 5%, although Lahmers said losses of 10% are not unusual and losses can be up to 35% in a herd. Most animals show no obvious clinical signs when initially infected, but may be anemic, jaundiced or weak. Infected cows abort calves, and some calves die shortly after birth.
Lahmers said in some herds, all animals are carrying the disease but the owner has no idea animals are positive. Late-term abortions may occur, and Lahmers is investigating whether earlier term abortions may be going undetected.
“Calf mortality is associated with Theileria orientalis Ikeda,” he said. “This is in contrast to Anaplasma marginale (the cause of anaplasmosis), where we typically don’t see calf losses associated with that disease.”
Theileriosis involves acute infection and is often spurred during periods of stress such as with calving, heat and nutritional stress. “Once the animal has controlled the acute infection, the numbers of organisms greatly decrease,” said Lahmers. “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.”
by Sally Colby