Pasture Perils


You know tick bites can make horses sick—but did you know there’s more than one kind of tick-borne equine disease? Two veterinarians explain what they are, the damage they do, and the remedies that work against them.

Reprinted from the June 2015 issue of USDF Connection

By D.J. Carey Lyons

Ticks are opportunists. They lurk on leaves, twigs, grass, weeds, and brush piles, ready to hop onto any passing horse for a free meal. And the tick bite that siphons out a few drops of the horse’s blood can also inject a disease-causing organism into his circulatory system.

Three tick-borne illnesses are known to infect horses: Lyme disease, equine anaplasmosis (previously named Ehrlichia equi), and equine piroplasmosis. Fortunately, “Piroplasmosis is not found in this country—which is good, because it’s a very difficult disease to deal with,” says Janice Kritchevsky, VMD, MS, professor of large-animal internal medicine at Purdue University College of Veterinary Medicine in West Lafayette, IN. So only Lyme disease and anaplasmosis are likely to affect horses here.

The carriers for both ailments are what are generally called “deer ticks” or “black-legged ticks.” But the infecting agents are two different bacteria: Borrelia burgdorferi for Lyme; Anaplasma phagocytophilum for anaplasmosis. And the differences go on from there.

According to Kritchevsky, “In fewer than fifty percent of the tick-borne disease instances we treat does an owner say, ‘Oh, yeah, I saw a tick on the horse.’ By the time symptoms develop, typically the tick is long gone. But if a horse is out on pasture where there are ticks—which is pretty much anywhere in the country—tick-borne disease is a possibility.”

So, with tick season under way, let’s take a detailed look at these two likely “pasture perils.”

Lyme Disease: Better Known, but . . .

Perhaps because it also affects humans, Lyme disease is better known than anaplasmosis. However, “better known” doesn’t mean “well understood.” Kritchevsky calls Lyme disease “one of the big question marks in equine medicine. We don’t tend to see horses with what we might call ‘acute’ Lyme disease, where they’ve just been infected and they develop fever.” Instead, she says, victims are more typically identified after “seeming ‘off’ in some way for a while.”

She adds: “If you survey horses in, say, the Midwest and Northeast, the vast majority will test positive for some level of Lyme antibodies. But many of those same horses won’t have had even a day when they looked sick.” And the puzzlement just goes on from there.

FIRST LINE FOR LYME: The antibiotic doxycycline is the treatment of choice for Lyme disease PHOTO CREDIT: DUSTYPERIN.COM

“We don’t know why some horses show the signs” of Lyme—including low-grade fever, stiffness, lameness, muscle soreness, skin sensitivity, weight loss, an overall dull attitude, and others—“and some don’t,” Kritchevsky says. “If horses show high antibody levels, we treat them and they seem to get better. But there are also horses diagnosed with Lyme disease because no one can figure out what’s wrong with them” until a blood test shows antibodies. “And the completely ‘normal’ horse in the next stall over” from that positive horse “might also show antibodies if tested.

“If a horse has had a recent high infection, or shows high antibody levels and clinical signs consistent with Lyme disease,” Kritchevsky continues, “we’ll treat the animal for Lyme”—usually with oral doxycycline, the first-choice antibiotic for the disease. An initial course of the drug typically lasts four to six weeks; then the animal’s titers are rechecked. If needed, treatment continues until the titers decrease. With medication, “many horses improve,” some fairly promptly. “But some don’t; they either backslide or don’t respond the way they should.”

“Horses don’t make very good experimental subjects,” Dr. Kritchevsky concludes. “They’re big, and they’re hard to make get sick. So a lot of questions about Lyme haven’t been answered yet—for example, questions relating to chronic poor performance or long-term illness. I hope that before too long we’ll know a lot more than we do now.” Still, veterinarians do know enough to counsel owners that “if you treat Lyme quickly, your horse will tend to be OK. It’s when you let the condition go that you risk longer-term problems.”

Anaplasmosis: More Obvious—and More Definitive

Every year, Complete Equine Health Service, Coatesville, PA, sees about a dozen cases of anaplasmosis—which looks quite different from Lyme disease, according to Annika Frederickson, DVM, one of the practice’s three veterinarians. “With Lyme, the signs are subtle; it kind of sneaks up on you. But equine anaplasmosis happens rather acutely: One day the horse is normal; the next day he’s depressed, lethargic, maybe off his feed, and with a pretty significant fever. In some cases there’s also an incidence of swollen limbs.”

Timing of symptoms is another tipoff. “Anaplasmosis tends to show up in the summer or early fall. If a horse has a fever then, ticks and a fairly acute type of tick-borne disease are the first thing I think of, rather than a more typical respiratory disease or some other cause,” Frederickson says.

Kritchevsky agrees that anaplasmosis appears “at a very odd time of year: August to December. It’s very seasonal, even though obviously the ticks are around at other times. Another unusual thing: The younger a horse is, the less likely it is to show symptoms. With most diseases, younger animals are more susceptible.” And an affected horse’s blood sample typically reveals yet more oddities: “a low platelet count” (which can cause spontaneous bleeding and tiny hemorrhages on the gums), “anemia, and a low white-cell count. There’s not much else that will make all a horse’s different cell lines low.”

However, veterinarians experienced in dealing with anaplasmosis don’t always test for it before initiating treatment with oxytetracycline, the intravenous form of tetracycline— which, according to Frederickson, tends to produce “a fairly immediate response. By the following day, the horse may be observably better.” Although her practice doesn’t confirm every case through blood tests, “we do test some. But usually by the time we get the results, it’s after we’ve treated the horse.”

According to Kritchevsky, “You need ‘an index of suspicion’” that anaplasmosis might be the problem “because it can look like a lot of other things. Some signs—reluctance to move, for example—can appear with both Lyme and anaplasmosis. But I think a veterinarian would be able to tell pretty quickly that ‘This horse doesn’t want to move because of joint problems or muscle problems’”—indicating Lyme—“but ‘that horse doesn’t want to move because he has high fever and is really depressed’”—suggesting anaplasmosis. “Looking at all the different body systems, you’d notice differences.” She calls anaplasmosis “very gratifying to treat: When you first see the horse, you can tell he feels horrible. But once he’s on the antibiotic, by the next day he looks a hundred percent better. If you’re on the right track, you know right away.”

For the length of the treatment, Frederickson strongly advises keeping the horse under veterinary supervision. Although her practice will, if needed, arrange to administer the antibiotic at an owner’s barn, “a vet has to come out there to give it. And the horse is getting a very large volume of IV fluid—45 cc’s a day for a typical large warmblood— which needs to be given very slowly. So we don’t advise clients doing it themselves.” She adds, “Early in the treatment we usually also give Banamine to help lower the fever, enabling the horse to feel better more quickly and regain his appetite. A horse that is eating hay again, or is able to go out and eat grass, is a happier horse that can heal faster.”

As for length of treatment, “We usually do at least three days of IV administration, but we really prefer to do a full five days—seven, in some cases, if the client is willing. However, we also consider practicality. We have a variety of clients; some can’t repeatedly have us come out to give oxytetracycline but would be happy to keep their horse on antibiotics a little bit longer. We’ll put that horse on oral doxycycline,” which is less expensive and easier to administer, though some horses may not absorb it as easily as the IV form. “Likewise, if a horse doesn’t bounce back quite as quickly as we’d expect—maybe the fever takes two or three days to go away—we’ll keep the animal on the oral drug for a while following the IV course. Or the client may be the one who feels a little better ‘covering the bases’ by continuing antibiotics a little longer. This is a problem I feel good dealing with, because I can help. Once we have the appropriate treatment started, I see the horse getting better every day. That makes me very happy, and I think it makes clients happy.”

Frederickson adds: “Anaplasmosis is a self-limiting disease, meaning that the horse will eventually get better on his own. I’ve certainly had people spend a few days sitting on what turns out to be anaplasmosis before calling me in, thinking it’s ‘just a fever.’ In my experience, the delay doesn’t make a difference in the outcome; once defeated, this disease goes away. Whereas Lyme—if you’re not on top of it promptly—seems to have much stronger tenacity and more aftereffects, with greater risk of becoming a chronic issue.”

Reducing Risk

The horses most exposed to tick-borne diseases, says Kritchevsky, are “the broodmares and ‘pasture pets’ that are on turnout most of the time—and maybe aren’t groomed and watched as closely as horses in their prime working years. Certainly in a bad tick year, it’s a good idea to check over even those horses regularly for ticks.”

Tick-borne diseases “all have an incubation period,” Frederickson says. “If you can remove the tick within twenty- four hours, the horse probably won’t come down with disease. But even with good scrutiny on a daily basis, ticks are easy to miss. Prevention is hard. Insecticidal sprays can help; so can mowing fields closely. But some things are just beyond our control because our horses live outside—and they should live outside.”

That being so, “I think a lot of problem prevention comes down to being very astute: recognizing symptoms when you see them and reacting promptly. And many owners are really on top of things. They know when a horse isn’t quite right, even when what they’re describing over the phone sounds pretty much within the ‘normal’ range to me.”

Frederickson’s recommendations: “Learn first-aid techniques, and get to know your horse’s ‘normal.’ Learn how to take his temperature, pulse, and respiration; take and record his readings several times over a couple of weeks when he’s in good health. Know how his mucous membranes normally look, too. All that gives you a baseline for comparison—and useful data for when calling your veterinarian.”

Piroplasmosis: An International-Level Problem

In equine piroplasmosis, the third major tick-transmitted equine disease, the infective agent is a protozoal organism: either Babesia caballi or Theileria equi (formerly called Babesia equi). A tick can pick it up by biting an already infected horse, then pass it on to other horses it bites. However, this disease, unlike Lyme or anaplasmosis, can also spread through any action resulting in the transfer of infected blood: for example, a blood transfusion, reuse of an inadequately cleaned syringe or needle, or even an intrauterine infection from mare to foal. The fatality rate is high. And any horse that survives remains a carrier of the organism.

Although piroplasmosis is quite common in parts of Europe, and in some tropical and subtropical regions (including parts of the Western Hemisphere), Purdue University’s Dr. Janice Kritchevsky says, “I’ve never seen a case of it because it really is not found in this country.” She’s grateful for that fact because “there isn’t any drug that is completely effective against it.”

For that reason, the US Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) focuses on preventing piroplasmosis from getting a foothold here. Horses being imported from other countries face rigorous quarantine regulations, and a number of states deny entry to horses without papers showing negative test results. Any positive test for piroplasmosis on a horse in the US must be reported to the USDA; then the horse must either be enrolled in a USDA-recognized study of the disease, be exported out of the US, be placed in permanent quarantine, or be euthanized.

But horse sports are international, and some piroplasmosis survivors from other countries are successfully competing internationally at top levels. So the USDA has devised—and revised—safeguards to reduce infection risk from visiting competitors. For example, in the 1984 Los Angeles and 1996 Atlanta Olympics, piroplasmosis-positive horses were allowed to compete in dressage and jumping but not in eventing (the crosscountry phase was considered to present too great a risk for tick-borne disease transfer). Positive horses were also stabled well apart from all other horses.

Before the 2010 Alltech FEI World Equestrian Games in Lexington, the host-venue Kentucky Horse Park’s management focused on eradicating all environmental elements within the park where ticks could lurk. Piroplasmosis-positive horses were permitted to compete in all sports but were required to go directly from their pre-WEG quarantine to the Games, were restricted to facilities for them alone when not competing, and were shipped home right after competing. The precautions worked, enabling the WEG and its participants to finish with a clean bill of horse health.

D.J. Carey Lyons is a freelance writer based in West Chester, PA. Her website is

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