The work of Australian veterinary parasitologist Dr. Matthew Playford is helping to raise awareness of the Asian longhorned tick (ALT) among U.S. cattle producers. While many consumers have learned about the meat allergy (alpha-gal) that can result after a bite from an ALT, Playford focuses on Theileriosis, a serious bloodborne cattle disease caused by Theileria orientalis and transmitted by the ALT.
Playford recalled learning about Japan’s problems with Theleriosis while studying there, and noted that the ALT shows regionality according to climate conditions. Northern areas with colder weather had no ALTs, while central Japan has primarily female ALTs (which don’t require males for reproduction). In more southern areas, both male and female ALTs are found. Notable is that in Hokkaido, Japan, other ticks serve as vectors for Theileriosis. Playford said this could happen in the U.S. with the pathogen as well.
The ALT made its way to Australia about 120 years ago, said Playford. “They were brought to Australia probably from Japan, but the link isn’t established by genetic analysis. It’s simply due to the fact that our ticks are almost all females – the parthenogenetic race. The ticks established in Australia and spread into various areas,” he said. They then spread to New Zealand, taking Theileriosis with them.
Playford described the three strains of T. orientalis in Australia, including Ikeda, Chitose and Buffeli. Buffeli is the most common strain and has been present in Australia for as long as the ALT has been there. The Buffeli strain has potential to be used as a live vaccine strain, but development is still in early stages.
By late 2011, the Ikeda strain had spread widely throughout Australia. Playford said ALT prefers moist, warm conditions and will not establish permanent populations in drier areas, so cattle in arid areas remained uninfected.
In one infected area of Australia, farmers sold their beef cattle and converted to dairy operations. “They inadvertently brought in ticks and the Ikeda strain,” said Playford. “Within a few weeks, cattle, particularly those that were at the point of calving, started having abortions, were recumbent and showed weakness, anemia and death. Over the next six months there were 80 abortions, 30 cattle died and 100 head had chronic illness including metritis and mastitis.”
The cost of Theileriosis in Australia is estimated at about $18 million. “Even though we still have occasional outbreaks, the disease appears to have stabilized and has not gotten worse since 2015,” said Playford.
Playford said the ALT is fairly easy to identify by its physical characteristics, adding that it’s important in Australia to distinguish it from other ticks. “It’s often on cattle without being noticed,” he said. “The unfed stages are very small – only a few millimeters across, the size of a pinhead. Ticks will appear on cattle very shortly after they have been treated. Because it’s a three-host tick, cattle will become reinfested and (ticks) are happy to hop onto dogs, people or other wildlife.”
ALTs overwinter as larval nymphs, and most outbreaks in Australia occur in spring. “The other critical aspect of Theileria infection is it is transstadial – it will be transferred from the larva to nymph to the adult tick,” said Playford. It is not transferred via eggs, however.
Since its introduction to Australia, the ALT has spread throughout the eastern seaboard, from north to south, and moves inland according to weather conditions. It’s also found in western Australia.
Playford described the lifecycle of the ALT, which differs from more familiar ticks. Adults are small, about 3 mm when they climb onto cattle, and grow up to 8 mm – 1 cm. “Then they drop to the ground and lay 3,000 eggs,” he said. “Larvae can survive a long time on the grass – they don’t need to climb onto cattle. Once ticks get onto an infected animal, they become infected themselves. Then they rest on the ground until they find another host, which be wildlife. Then larvae turn into nymphs and climb onto cattle, and this is often the stage that causes the most damage to livestock. They climb on in large numbers and give big doses of Theileria, particularly to young cattle, and cause disease.”
In addition to being spread by ticks, Theileriosis can occur through mechanical transfer such as blood draws or injections with needles previously used on infected animals. Playford said this is an issue when vaccinating cattle or treating cattle with endectocides for tick prevention. Recent research shows potential for Theileria transmission by sucking lice, but Playford said lice are not a biological vector and cannot complete the lifecycle of Theileria – they’re just a mechanical means of transmitting infection.
Diagnosing Theileriosis is through polymerase chain reaction (PCR), a molecular method that can identify different types. “In every population of Theileria, the amount of Ikeda, the most pathogenic strain, varies a lot,” said Playford. “The cost of the test is $40 per test.” The disease can also be diagnosed with a blood smear to examine changes in red blood cells that are different from the changes seen with other tick-borne diseases such as anaplasmosis and babesiosis. Infected cattle also show typical clinical signs.
The color of vulval mucosa is a telltale sign of clinical disease. Rather than normal pink, the vulval mucosa is pale, and there’s a clear link between the color of the mucosa and packed cell volume (hematocrit). “In a normal cow, red blood cells (RBCs) make up about 35% to 40% of circulating blood volume,” said Playford. “As RBCs are destroyed due to Theileria infection, packed cell volume decreases. By the time the animal is down to about 10% RBCs, they are very pale.”
When the packed cell volume reaches 10%, death risk is high. Animals should be confined, allowed to rest and not moved. “It’s common in Australia and New Zealand to treat these animals with a blood transfusion,” Playford said. “Nothing treats the Theileria anyway; they really just need intensive care.”
by Sally Colby