Ticks can infect horses with an array of diseases including equine piroplasmosis, ehrlichiosis, and Lyme disease.
In the first article of this series, we learned that mosquitoes have been touted the “deadliest” animal on earth because they can transmit a plethora of life-threatening infectious diseases to both humans and animals. Because ticks spread what might be the widest variety of disease-causing agents to animals, including bacteria, parasites, and viruses, it is therefore reasonable to crown ticks “First Deadly Runners-Up.”
“There is an array of infections that North American horses can acquire from ticks, including equine piroplasmosis, ehrlichiosis, and Lyme disease, to name a few,” says Robert Mealey, DVM, PhD, Dipl. ACVIM, associate professor of immunology and infectious diseases at Washington State University’s College of Veterinary Medicine. “These diseases can be challenging to diagnose, are potentially life-threatening to the infected horse as well as horses residing nearby, and in some cases are able to infect humans.”
Avoidance is one of the most important ways to minimize your horse’s chances of acquiring a tick-borne disease, but would you recognize a tick if you saw one? In this article three veterinarians will describe ticks–where they live and how they are detrimental to your horse–and review ways to protect your horse (and you) from being their next meal ticket.
How Ticks Spread Disease
Ticks, which are actually mites, are large, external blood-sucking parasites of terrestrial vertebrates. Two main tick families reside in North America: hard ticks and soft ticks. “Hard ticks are literally ‘hard’ because they have a shield on their back called a scutum that soft ticks do not,” explains Larisa Vredevoe, PhD, professor at California Polytechnic State University’s Department of Biological Sciences, in San Luis Obispo.
Tick species differ in terms of their geographic distribution, their life cycles, the hosts on which they feed, and the disease agents they might transmit. In this article we will focus primarily on hard ticks because they parasitize horses more commonly than soft ticks.
Hard ticks climb grass stems and perch on leaf edges to find a host–a process called questing. “When the tick senses a suitable host, by heat, movement, or carbon dioxide produced by the host, they extend their front legs and climb aboard as the host brushes past them,” says Vredevoe.
“All hard ticks have similar life cycles in that the engorged female lays thousands of eggs after feeding on a host,” she explains. “Tiny larvae, which have six legs, emerge from the eggs and subsequently attach to a host to feed. After feeding, larvae molt into eight-legged nymphs, feed again, and then develop into eight-legged adults. Hard ticks can either spend their entire lives on a single vertebrate host or rely on up to two or three vertebrate hosts to complete their life cycle.” The larvae, nymphs, and ticks all require blood meals (whereas mosquitoes ony feed as adults).
She notes that a tick’s mouthparts contain a hypostome it can insert into a host’s skin. The hypostome has backward-pointing projections that prevent it from being removed easily from the host while it’s feeding. Hard ticks also release compounds in their saliva that, for example, help “glue” the hypostome in place and prevent the blood from clotting while feeding.
Let’s use equine piroplasmosis (EP) as an example to explain how ticks spread disease. “EP is caused by two distinct protozoal parasites: Babesia caballi and Theileria equi,” Mealey says. “Horses and other equids are the only reservoir for T. equi because infected ticks do not transmit T. equi to their offspring. This means an uninfected tick must feed on a horse that is infected with T. equi, acquire the parasite, then feed on a second (uninfected) horse to transmit the infection. In the case of B. caballi, however, ticks can serve as the reservoir.”
This is because ticks can pass B. caballi to their offspring transovarially, which means larvae that emerge from the ticks’ eggs are already infected and able to transmit the disease agent immediately. Female ticks can lay thousands to tens of thousands of eggs at once, offering abundant transmission opportunities. Keep in mind that even uninfected larvae and nymphs can become infected (not just adults) while feeding on infected horses. Those larvae and nymphs (and later adults) remain infected for life (and pass infection on to subsequent generations in the case of B. caballi), potentially spreading disease agents to future hosts.
With Lyme disease, ticks spread the disease agent Borrelia burgdorferi. Whereas in equine ehrlichiosis cases, deer ticks transmit the bacterium Ehrlichia equi.
Because different microorganisms cause the various tick-borne diseases, there is no classic sign of disease, notes Mealey. Instead, signs of infection vary depending on the exact pathogen (infection-causing organism) the mite transmits to the horse.
Signs of EP are often few, but they can include fever, anemia, icterus (jaundice), and anorexia. Lyme can cause an infected horse to develop shifting leg lameness, myalgia (muscle pain), dermal hypersensitivity, behavior changes, weight loss, uveitis, and neurologic signs. Clinical signs of ehrlichiosis include depression, anorexia, weakness, limb edema, fever early in the disease, and immunosuppression.
Thwarting the Tick
“One major problem with the tick-borne diseases, particularly EP, is that many horses do not show any signs of infection and are, therefore, ‘inapparent carriers,’ ” says Mealey. “This means that ticks can spread disease surreptitiously, potentially resulting in an increase of sporadic cases or even epidemics.”
Although veterinarians can treat some infections, such as Lyme disease, successfully, treatment is not usually an option for other tick-borne diseases such as EP. Because EP is considered a foreign animal disease in the United States, any infected horse must be managed under state quarantine, says Angela Pelzel-McCluskey, DVM, a Western region epidemiologist with the USDA-Animal and Plant Health Inspection Service (APHIS). “Currently, the long-term options for management of EP-infected horses include humane euthanasia, export from the country, life-time quarantine on the owner’s premises, or long-term quarantine with enrollment in the EP treatment research program administered by ARS (Agricultural Research Service) and APHIS-VS (Veterinary Services),” she says.
Similar to arboviruses (mosquito-borne diseases), the main ways to prevent tick-borne diseases are to avoid the vectors and/or to vaccinate against them.
Avoiding tick exposure completely is likely impossible. But if you reside in a region where certain ticks are known to exist, then taking a few simple measures can markedly limit your horses’ chances of acquiring an infection.
“Be familiar with the types of ticks found in your geographical area, what they look like, and where on the horse they are typically found,” recommends Pelzel-McCluskey. “To do this, locate an equine extension specialist in your area and contact them for more information. Examine your horse daily at the times of year that ticks are active in your area, use appropriate acaricides (pesticides that kill arachnids such as ticks and mites) on your horse that are EPA-registered as effective against ticks, and follow the label directions for those products.”
To conduct an effective tick hunt, try following these steps:
- Start at your horse’s head, and look and feel in the nose and false nostril;
- Move to the forelock, ears, and mane;
- Examine under the jaw, and feel for any irregularities in the skin;
- Move to the left forelimb, examining the “armpit” down to the fetlock and coronary band, then back to the belly/udder/scrotum;
- Examine the tail, tailhead, anus, and perineum;
- Examine and feel the left fetlock and coronary band on the left hind limb;
- Move to the right side and complete the exam in reverse.
If you see or feel a tick, do not crush, pinch, or puncture it. Wearing gloves to avoid becoming the tick’s next host, use tweezers to gently remove the entire tick. Do not twist or jerk the tick because the mouthparts could break off and remain lodged in the skin. If this happens, remove the mouthparts with the tweezers. Do not use any tick removal technique that could further harm the horse. This includes using a hot match or petroleum jelly; these approaches can stimulate the tick to release additional saliva, increasing infection risk.
As if finding a tick on your horse and having to remove it isn’t bad enough, some ticks can transmit more than one infection simultaneously. It is also theoretically possible for horses to be co-infected with more than one tick-borne disease (e.g., EP and Lyme).
As discussed by veterinary Lyme disease researchers during the 2011 American Association of Equine Practitioners’ (AAEP) Annual Convention, veterinarians have used the canine Lyme disease vaccine in horses, but it is not licensed for this purpose. Researchers have not yet conducted safety studies and do not currently know how frequently practitioners would need to administer the vaccine to at-risk horses. Additionally, the AAEP does not currently include the Lyme disease vaccine in their vaccination recommendations.
Because EP is a reportable and regulated disease in the United States, veterinarians can only perform treatment under strict ARS-APHIS-VS supervision after enrolling in an official treatment program. Veterinarians have reported very promising results treating EP-infected horses with the antiprotozoal drug imidocarb dipropionate.
Mealey notes that although imidocarb dipropionate appears to be effective in clearing infection in most of the U.S. horses treated so far, issues with toxicity and protozoan resistance do exist. In addition to ongoing work focused on improving treatment options, Mealey, along with USDA-ARS-ADRU (Animal Disease Research Unit) colleagues, are looking at ways to create an effective EP vaccine.
“An effective vaccine for EP would help protect horses against infection transmitted from inapparent carriers that already reside in the U.S.,” he relays. “It could also protect horses that travel to endemic countries for competitions, etc., ensuring that they would not have regulatory problems when returning to the U.S. Some of the major challenges include our incomplete understanding of how the immune system ‘handles’ T. equi and B. caballi during either the early or late stages of infection.”
He says researchers must also be certain the vaccine doesn’t interfere with current EP tests, to ensure vaccinated horses are not mistaken for being EP-positive.
Although ticks might not annoy us and our horses as much as mosquitoes do or induce quite the same level of angst as snakes or spiders (which will be discussed in Part 3 of this series), they should not be taken lightly. As Mealey attests, tick can spread many important diseases to and among horses, and signs of infection vary depending on the exact pathogen transmitted.