By Kim Campbell Thornton • Boehringer Ingelheim The longhorned tick (Haemaphysalis longicornis) is a medically important invasive tick species that now calls the United States home, joining the plethora of other domestic disease-causing ticks that affect companion animals, livestock, and humans. Native to Southeast Asia, the longhorned tick was first identified in the U.S. on sheep in 2017 in Hunterdon County, New Jersey. However, specimens were found upon subsequent review of archived tick collections as early as 2010 suggesting this tick was introduced into the U. S. long before being recognized in 2017. Every year this tick is found in more counties and states, with 259 counties across 21 states now reporting confirmed populations. Longhorned ticks have a unique reproductive capability and a broad host range, making them a risk to animals and humans and difficult to control in the environment. Dr. Kathryn Reif is a parasitology expert with a focus on ticks and tick-borne pathogens of public health, veterinary concern, and agricultural importance. She explains why this emerging tick species is of concern, presents potential health risks to animals and humans, and the challenges in managing it. This interview has been edited for length and clarity. Q: How significant from a veterinary perspective was the discovery of presence of the longhorned tick in the United States? A: It’s very significant that this tick now has established populations in the U. S. for both livestock and companion animals. This tick species doesn’t discriminate and will happily bite a cow, dog, deer, or many other potential hosts. They can cause massive infestations on hosts resulting in anemia, and in cases of severe infestations, exsanguination of the host. There have been several reports of healthy cattle exsanguinated from overwhelming longhorned tick infestations. These ticks can pose a similar risk for our companion animals. Because this tick feeds on a variety of wildlife species and domestic animal species, our ability to contain its spread or eradicate it from the U. S. is going to be very challenging, if not impossible. Q: How is this tick different from other established ticks in the U. S.? A: This tick is unique in its ability for parthenogenic reproduction: Females do not require a male to reproduce. In fact, no male longhorned tick has been found in the U.S. If a single female is introduced into a new area and finds a host to feed on, she then has the potential to lay anywhere from 2,000 to 4,000 eggs and establish a population in the new area. Because the females essentially ‘clone’ themselves, the geographic expansion of unique lineages (three lineages recognized in the U. S.) can be traced. One case study documented a dog with longhorned ticks of the Westchester County, NY lineage in New Hampshire. That dog had visited Westchester County, New York and accidently carried longhorned tick larvae back home to New Hampshire. Q: What potential health risks does the longhorned tick pose to pets and livestock? A: Upon identification in the U. S., there was significant concern that the longhorned tick could potentially be another vector for some of our endemic tick-borne pathogens like Borrelia burgdorferi (agent of Lyme disease) and Anaplasma phagocytophilum (agent of anaplasmosis). The Centers for Disease Control have conducted several studies to evaluate its competence to transmit Lyme disease and anaplasmosis agents, and they found that it was not a capable vector for those endemic pathogens. So, it’s good news that longhorned ticks won’t likely contribute to the transmission of some of our most common zoonotic tick-borne pathogens in the U. S. However, the longhorned tick was experimentally demonstrated to acquire and transmit the agent of several tick-borne viruses including Powassan virus, Heartland virus, and Bourbon virus which raises concern that we may start to see more cases of these rare tick-borne pathogens in areas where the longhorned tick is endemic. Notably, for cattle producers, this tick is a vector of Theileria orientalis, including the Ikeda strain, the agent of bovine theileriosis, a disease that can produce severe anemia in cattle that clinically resembles bovine anaplasmosis. As the longhorned tick was being recognized in New Jersey, the Theileria orientalis Ikeda strain pathogen was being identified from cattle in Virginia, again suggesting that this tick and its associated pathogens had been introduced into the U. S. several years before either the tick or pathogen were recognized. A study by the United States Department of Agriculture confirmed that the longhorned ticks found in the U. S. could acquire and transmit the Theileria orientalis pathogen found in U. S. cattle. Q: Does the tick also pose a health risk to humans? A: While longhorned ticks can and do occasionally bite humans, they appear to prefer cattle, deer and dogs over humans. They are less likely to transmit tick-borne pathogens to people compared to other U. S. tick species like the blacklegged tick (vector of Lyme disease agent) and the lone star tick (vector of ehrlichiosis agents). However, based on experimental data, this tick has the potential to expand transmission of rare tick-borne viruses, so we’ll need to keep an eye on this. While veterinarians often educate pet owners on ways to reduce the risk of tick exposure and tick bites for their patients, it is also important to take steps to reduce your own risk (avoiding areas of high risk, reducing areas in your surroundings that serve as ideal habitat for ticks and their hosts, performing regular tick checks, etc.). Q: How long does it take for disease transmission to occur after a bite? A: For the pathogens associated with the longhorned tick, there have not been any transmission-timing studies looking at the minimum amount of time post infestation required for pathogen transmission. But in general, tick-borne viruses can be transmitted within a matter of hours, maybe even as quickly as a matter of minutes. For tick-borne bacterial pathogens, transmission typically occurs within a day or two, although it may occur more quickly for some bacterial pathogens. For protozoal pathogens like Theileria orientalis for cattle, transmission tends to take a few days because protozoal pathogens often have to progress through several life stages in the tick before they can be transmitted to the host. To understand the minimum transmission time for Theileria orientalis, specific transmission timing studies still need to be performed. Q: Does tick species influence treatment or control? A: Some ticks species are considered more challenging to treat or control compared to others. In the U.S., the lone star tick (Amblyomma americanum) is considered one of the more challenging tick species to treat and control. As new ticks are discovered in the U. S., like the longhorned tick, there may be concern as to whether our existing tick control products will work for these new ticks as well. Based on available data, currently available tick-control products should be able to protect pets from the longhorned tick. That being said, when a new tick does emerge, it is reassuring when manufacturers of tick-control products take the extra step to evaluate the efficacy of their product against the new tick species. Recently, the effectiveness of afoxolaner has been examined against US isolates of the Longhorned tick. Based on the results of these studies, the label indications for NexGard® (afoxolaner) and NexGard Plus (afoxolaner, moxidectin, and pyrantel chewable tablets) have been updated to include this relatively new threat to dogs. As a general practice, it’s always a good idea for veterinarians and pet owners, especially in areas where novel tick species are emerging, to consider the tick species included on product labels. Q: What are some of the notable advances in treatment or control of ticks in recent years? A: We have many excellent tick control products available in the U. S. to protect our pets, and these products offer the best protection for our pets against ticks. The isoxazoline class of ectoparasiticide drugs, often referred to as ‘orals’ or ‘systemic’ products are included in the newest tick control products. The isoxazoline class of drugs provides excellent ‘full body’ protection to treated pets, since they are distributed systemically in the body of the dog and ticks will become affected and ultimately die upon feeding on a treated dog’s blood. For pet owners that prefer topical products, there are several effective topical tick control products as well. Topical tick control products work on the outside of the pet as acaracides and contact dependent irritants for ticks. In the U. S., the two tick species that I always look for on product labels are Ixodes scapularis and Amblyomma americanum, the most infamous and the most challenging tick species to kill in the U. S., respectively. In addition to tick control products, there is a lot of activity and interest in vaccines developed for tick-borne pathogens and ticks themselves. Presently, there are several vaccines on the market to protect dogs from Lyme disease and we’re about to see some of these vaccines roll out for humans as well. Outside of Borrelia burgdorferi, multiple research groups are evaluating vaccines for other tick-borne pathogens such as Anaplasma phagocytophilum and Ehrlichia species. There are even some groups working on anti-tick vaccines to kill the tick before it’s able to transmit pathogens to the host. Right now, outside of the commercially available Lyme vaccines, these other experimental vaccines are still being refined and evaluated, but hopefully some may be ready for wider clinical testing in the near future. Q: What are the greatest challenges in diagnosing tick-borne infections/diseases in pets? A: Diagnosing tick-borne pathogen infections and disease in pets can be tricky. For the most common tick-borne pathogens we typically rely on either detection of a specific antibody response to the pathogen, the genetic material of the pathogen, and/or a specific clinical presentation. It’s important to understand how to interpret the results of these different assays and their limitations. Many of our diagnostic assays for tick-borne pathogens rely on seroconversion, which means the pet had a tick on it long enough to transmit the pathogen and cause a response in the body, but a seropositive test result doesn’t mean that pet is actively infected or that they will develop clinical disease. Animals can remain seropositive for an extended period of time from many months to potentially many years. Not only does this complicate decisions on treatment, but it can interfere with accurately identifying re-exposure or re-infection of that pet from subsequent tick bites. This is important because even if that pet didn’t develop clinical disease from the first exposure, it doesn’t mean they can’t develop clinical disease if they are exposed again. Although we have some great patient-side diagnostic options for the most common tick-borne pathogens, we don’t have them for all important tick-borne pathogens (e.g. spotted fever rickettsial pathogens, Cytauxzoon felis, relapsing fever borreliae, tick-borne viruses, etc.). Molecular-based tests that screen for DNA or other genetic material of a pathogen can be helpful to confirm active infection; however, these tests typically need to be sent out to diagnostic laboratories to perform. Tick-borne pathogens that remain in the blood tend to be easier to diagnosis compared to pathogens that escape into tissues or other places in the body that are more difficult to collect samples from. Q: How can veterinarians stay informed about longhorned ticks and tick-borne diseases in general? A: Great resources for keeping up to date on the longhorned tick and tick-borne diseases in general include, the Companion Animal Parasite Council, Centers for Disease Control and Prevention, Animal and Plant Health Inspection Service (APHIS), local universities, local public health agencies, and national or regional continuing education conferences. It’s important to remember that ticks are on the move and as they expand their range, they carry their pathogens with them. For veterinarians practicing in or adjacent to areas with longhorned ticks, this may be a tick species you will start to see more. Gathering a good history of where a patient has been or will be traveling can also inform the risk of encountering longhorned ticks (or other less common ticks for your area) and potentially bringing them or their associated pathogens back home. Most of the eastern U. S. and many areas in the Pacific coastal states have suitable habitats for longhorned tick, so expect this tick to continue to expand into new areas. Another way to stay aware of what ticks are in your area is to maintain a tick jar and reach out to folks at these resource centers to help identify which ticks are biting your patients. Again, remember that many tick species, including the longhorned tick, are expanding into new areas, and just because you didn’t see a tick species in your area in the past, even a few years ago, doesn’t mean it’s not there now or cannot show up at any time. Finally, pets and pet owners share many of the same risk factors for encountering ticks and potentially being infected with a tick-borne pathogen, so pets can be important sentinels for pet owner risk. IMPORTANT SAFETY INFORMATION: NexGard® (afoxolaner) and NexGard® PLUS (afoxolaner, moxidectin, and pyrantel chewable tablets) are for use in dogs only. Use with caution in dogs with a history of seizures or neurologic disorders. The most frequently reported adverse reactions include vomiting, diarrhea, itching, lethargy, and lack of appetite. The safe use of NexGard and NexGard PLUS has not been evaluated in pregnant, breeding, or lactating dogs. For more information, please see the NexGard and/or NexGard PLUS full prescribing information included on pages ( 30 & 31 ). NexGard® is a registered trademark of Boehringer Ingelheim Animal Health France, used under license. ©2024 Boehringer Ingelheim Animal Health USA Inc., Duluth, GA. All rights reserved. US-PET-1062-2024
