– Dr. Michelle Arnold – DVM, MPH UK Ruminant Extension Veterinarian
Figure 2: Asian longhorned ticks on the ear of a cow that died due to anemia from the massive tick infestation (Photo courtesy of the UKVDL).
In late June 2022, the UKVDL received a yearling Hereford bull for necropsy with a history of “symptoms of pneumonia.” At necropsy, the sclera (white of the eye), mucus membranes, and fat were yellow. Serologic (blood) testing for Anaplasma sp. was negative and PCR testing for Anaplasma marginale was also negative. A sample of spleen submitted to the Virginia Tech Animal Laboratory Services (ViTALS) was positive for Theileria orientalis. Further genotyping confirmed the genotype as Ikeda. This was the first known case of “bovine theileriosis” diagnosed in Kentucky, a tickborne disease caused by the protozoon blood parasite Theileria orientalis Ikeda. Theileria sporozoites (the infective stage) are primarily transmitted to susceptible cattle through the bite of an infected Asian Longhorned Tick (ALHT). In 2022, ALHT had been identified in 16 states, including Kentucky, and the list has now grown to 22 states and Washington DC (Figure 1). While cattle deaths in KY due to theileriosis have been limited in number, they continue to occur, especially as diagnostic capabilities improve. So, the question becomes, what have we learned about this disease and the tick responsible for spreading it since its arrival in 2022?
Figure 1: ALHT in the US as of 8/1/2024. A minimum observation period of 1 year is necessary to determine if this tick species can survive and thrive throughout the various local and seasonal changes. The critical environmental factors involved in establishment of the ALHT are climate, habitat suitability and host availability.
The ALHT, scientifically known as Haemaphysalis longicornis, requires warm-blooded animals such as humans, wildlife (white-tailed deer, raccoons, birds), and domestic animals to feed on for survival. Only parthenogenetic strains of ALHT exist in the USA, meaning male ticks are not required for reproduction. In the US, all ALH ticks are female, and each can produce 1,000-2,000 female offspring, allowing the tick population to rapidly explode. This means an individual animal could host hundreds to thousands of ticks and a severe infestation can kill the animal from excessive blood loss (Figure 2). The ALHT is a “three-host” tick species, meaning they seek a new individual animal on which to feed for each life stage. The tick lifecycle consists of four consecutive stages – eggs hatch into larvae, larvae feed on blood (from Host #1), fall off and molt to nymphs, nymphs feed on blood (from Host #2), fall off and molt to adults, and adults feed on blood (from Host #3) then lay eggs. All tick stages live at the base of pasture plants and “quest” (search) for a host by climbing up plant stems and attaching to a passing animal. Blood-feeding lasts anywhere from 5 to 14 days, longer with older life stages. In the United States, host-seeking nymphs are most active in the spring, adults in the summer, and larval stages in the fall but all stages may be observed questing throughout the warm seasons. Cattle begin to show signs of disease 4-6 weeks after infected ticks take their blood meal. Keep in mind that wildlife can serve as tick hosts and accelerate their spread in the absence of cattle. Ticks can remain infective on pasture for up to 2 years under favorable conditions so removing cattle from infested pastures for extended periods will not remove ticks from the area.
Regardless of life stage, whether larva, nymph or adult, the ALH tick acquires the Theileria orientalis blood parasite when it feeds on blood from an infected cow and the tick remains infected throughout its life stages. Fortunately, an infected adult tick cannot pass the Theileria organism to her eggs so newly hatched larvae can only acquire the parasite with their first meal. Therefore, only the nymphs and adult ticks can infect naïve cattle with Theileria orientalis and cause disease. Infected nymphs that go dormant during winter (known as “diapause”) in the grass and woods can transmit the disease to cattle the following spring. It is also possible for eggs, larvae and adults to survive mild winters and resume development when the weather warms. The Theileria orientalis organism is a protozoon that infects and destroys red blood cells, causing anemia in infected cattle. This disease, called “bovine theileriosis”, is often mistaken for anaplasmosis, another tick-borne disease caused by a blood bacterial parasite, Anaplasma marginale. Clinical signs seen in both Theileria-infected cattle and Anaplasma-infected cattle are due to severe anemia and include lethargy, anorexia, fever, exercise intolerance, difficulty breathing, foamy nasal discharge, an increased incidence of abortion, pale mucous membranes or jaundice, aggression, and death. Most theileriosis cases occur between April-June and September-November but can be seen year-round and in all ages of cattle. After initial infection, animals that survive become chronic carriers and can relapse during periods of stress. Anaplasmosis, on the other hand, usually occurs in the fall (September-November), only affects adult cattle, and cattle tend to show more aggressive behavior. There is no effective treatment for Theileriosis or vaccine to prevent infection. T. orientalis Ikeda is not a public health concern and contact with affected cattle does not pose a human health risk.
At the UKVDL, diagnostic testing for detection of T. orientalis in live animals is performed on whole, anticoagulated bovine blood (collected in purple top blood tubes) or from the spleen collected at necropsy. A duplex PCR, the “Anaplasma/Theileria Tick Panel PCR” ($68.50 + Accession fee) can detect both of the blood-borne organisms that cause anemia, Theileria orientalis and Anaplasma marginale. There are 11 different genotypes related to T. orientalis so a second test, the “Theileria Duplex Real-time PCR” ($40 + Accession) is necessary to confirm the Ikeda genotype.
Figure 3: Top: Asian longhorned ticks are light brown in color and are very small, often smaller than a sesame seed, as seen in the photo on the fingernail. Bottom left: The nymph and adult stages (a dime is in the background). Bottom right: The adult female is only about the size of a pea when it is full of blood.
Figure 3: Top: Asian longhorned ticks are light brown in color and are very small, often smaller than a sesame seed, as seen in the photo on the fingernail. Bottom left: The nymph and adult stages (a dime is in the background). Bottom right: The adult female is only about the size of a pea when it is full of blood.
Because ALHT can vector Theileria to cattle and can cause extreme blood loss in farm animals, careful monitoring of livestock and use of tick prevention methods is highly recommended. Routinely inspect livestock, pets, and humans for ticks. In cattle, check the head, neck, ears, flanks, armpit, groin, udder and under the tail (areas where the skin is thinner). Cattle that seem lethargic or unthrifty should be closely inspected for ticks. The most common areas on cattle that ticks will be found are around the tailhead, on the udder, inside the legs, on the brisket, in the ears (particularly near the insertion sites of ear tags) and occasionally on the face and neck. Tick identification is helpful for both disease diagnosis and premises assessment. Laboratory identification is the best way to confirm the identity of ALHT. The ticks are light brown and often smaller than a sesame seed. The adult female is about the size of a pea when full of blood (Figure 3). If concerned about the identification of the Asian longhorn tick, or if you find an unusual tick species on an animal, it can be submitted to the National Veterinary Services Laboratory, and they will appropriately identify the tick. Contact the UKVDL for further information.
Tick control should be approached from both the animal and the environmental perspectives. Integrated tick management includes a combination of tick surveillance, altering tick habitat, strategic application of insecticides to cattle, and careful management of cattle movement. Currently there are no known “acaricides” (tick pesticides) labeled for use against the ALHT. The use of pesticide impregnated ear tags, pour-ons, sprays, and back rubs labeled for control of the American dog tick and the Lonestar tick should also provide beneficial ALH tick control. Employing more than one control method for cattle (such as using ear tags and back rubbers) will yield better results. Control through treating cattle with acaricides alone is difficult due to the limited time ticks are attached to the host as ticks spend nearly 90% of their lifetime in the environment. The main environmental goals are to modify the habitat so ticks are unable to survive there, and hosts are not present in the tick-infested areas. Environmental control involves mowing pastures, especially overgrown grasses and weeds, and for extreme numbers of ticks, acaricides can be applied to vegetation. Apply acaricide using label instructions to sections of pasture with the highest number of ticks, such as woodland edges and grassy patches, during times when ticks are most actively seeking hosts. Although it varies by year, ALH ticks are generally active from March to November, with chemical applications on vegetation most successful in the spring. Perimeter fencing of a minimum of 20 feet from wooded areas will also help reduce cattle contact with ticks in the pasture. Perimeters can be treated with pyrethroid products (such as bifenthrin) though this should not be done to entire pastures. Bifenthrin 2E®, Paradigm VC®, (pyrethroids), and Sevin SL®, (carbaryl insecticide) are approved for pasture applications in certain states; check with your local county agent or regulatory official before using any pesticide.
Lastly, remember that when animals move, ticks move with them whether it is across state lines or across personal properties. When rebuilding or expanding herds, learn something about the source area and make sure to inspect and treat new purchases to remove ticks, quarantine them for observation and ask your veterinarian to conduct appropriate diagnostic tests before mixing the new cattle with the home herd. Virginia Cooperative Extension has produced a fact sheet entitled “Managing the Asian Longhorned Tick: Checklist for Best Management Practices for Cattle Producers” that covers animal inspection, chemical control, and herd management options. It may be downloaded at https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/ENTO/ento-382/ENTO-382.pdf
Figures 1 and 3: Accessed from https://www.aphis.usda.gov/livestock-poultry-disease/cattle/ticks/asian-longhorned/asian-longhorned-tick-what-you-need-know