Lyme disease probably doesn't immediately spring to mind if your horse comes in from the pasture with a slight limp one day. But if the lameness can't be attributed to obvious injury and persists for more than a few days, and your horse acts grumpy or touchy when you groom him, you might want to ask your veterinarian about the tick-borne disease---especially if it's found in your region.
Lyme disease was first reported in 1975 near Lyme, Connecticut, and the causative organism---the spiral-shaped bacterium (spirochete) Borrelia burgdorferi---was found in deer ticks in 1982. In addition to the Northeast, the disease also occurs in the upper Midwest and on the West Coast.
Researchers estimate that only about 10 percent of horses exposed to B. burgdorferi develop illness, and when they do, the signs can be subtle and easily mistaken for other problems. But early treatment is critical. If the infection is not cleared within the first five months, it can become chronic and much more difficult to eradicate. What's more, long-lasting inflammation can lead to permanent changes in the body parts affected.
So even if you?ve been reading warnings about Lyme disease all your life, it's probably worth reviewing the topic. Then you'll be better able to spot it sooner should it appear on your farm.
HOW LYME DISEASE OCCURS
Lyme disease has not been studied as extensively in horses as in other animals, but B. burgdorferi causes similar illness in many mammals, including dogs, cats, cattle and people. ?Although there are some genetic variations of this bacteria, they cause much the same disease in whatever host they infect,? says Stephen Barthold, DVM, PhD, of the University of California?Davis.
When a tick carrying B. burgdorferi takes a meal from a host, the bacteria travel into the skin. The first response is usually inflammation---this is the characteristic red ?bull?s-eye? rash surrounding the tick bite. ?This is a localized infection, and the immune response puts an end to the spreading of spirochetes in the skin,? explains Barthold. ?Prior to or during the course of that rash, depending on the strain of the borrelia, the spirochetes get into the bloodstream and randomly spread to the joints, heart, brain or whatever.?
The bacteria may become established at different sites in the body, causing additional inflammatory reactions. ?This explains why one joint may be involved and not another,? says Barthold. And because B. burgdorferi can affect almost any tissue (see ?Lyme Disease on the Brain?? page 44) the signs of illness can vary from case to case, mimicking osteoarthritis and other inflammatory conditions that are far more common in horses. Signs of Lyme disease in horses include generalized stiffness, swollen joints, chronic weight loss, eye inflammation, mild fever, hepatitis, laminitis, muscle tenderness, head tilt, difficulty in swallowing, facial paralysis, sensitive skin (hyperesthesia), lethargy or hyper-reactivity.
DIAGNOSIS AND TREATMENT
Lyme disease is difficult to diagnose in horses. For one thing, the characteristic red bull?s-eye rash that marks the start of the disease is often hidden under hair, so the illness isn?t usually apparent until signs of spreading infection appear. As a result, your veterinarian will also focus on other factors, such as whether Lyme disease is endemic in your area, when making a diagnosis. He?ll also consider:
? Physical signs. Because B. burgdorferi can invade so many different areas of the body, the signs of Lyme disease can be frustratingly diverse. Nonetheless, says David Trachtenberg, DVM, of Ledgewood Equine Veterinary Clinic in Ontario, New York, ?The most common clinical sign is lameness. It's often an obscure lameness---the horse isn?t going right.?
Of course, even if other signs---such as hypersensitivity---are pointing toward Lyme disease, a veterinarian will run other tests to rule out other possible causes for the lameness or other signs of illness. ?Lameness examinations, radiography, nerve blocks, joint taps, etc., are usually done to try to pinpoint the problem,? says Sandra L. Bushmich, DVM, of the University of Connecticut. ?If they can't find anything else, and it's a highly endemic area for Lyme disease, they look for that.?
That said, Bushmich says, once you get used to seeing it, you know what to look for: ?A horse with Lyme disease usually has a shifting lameness---one leg today, and in a couple days he might be lame in another---and it's usually in the big joints.?
? Blood tests. An ELISA (enzyme-linked immunosorbent assay) can determine whether a horse has been exposed to B. burgdorferi by checking for antibodies in his blood; a Western blot analysis is also available that checks for antibodies to more specific proteins. However, although the presence of antibodies is an important clue, it is not by itself a cause for diagnosis: For reasons that are still not clear, the majority of horses exposed to B. burgdorferi do not develop Lyme disease.
?If your horse is exposed, he may become infected, but that doesn't mean he's going to get sick,? says Bushmich. In fact, in areas with high rates of Lyme disease, as many as 50 percent of horses may carry antibodies to the bacteria, but fewer than 10 percent of those show signs of illness.
?An animal could have lameness due to some other cause and still have a positive antibody test,? says Bushmich. ?This is important for people to understand. A lot of people see the positive test and want to call it Lyme disease.?
? Response to antibiotic treatment. In the end, the best way to be certain that a horse has Lyme disease is to see how well he responds to the standard therapy for the disease: tetracycline.
?A study in 2003, using an experimental model, showed that giving tetracycline IV for three weeks eradicated the disease in horses,? explains Trachtenberg. ?I typically do 21 days, but treatment [protocols] range from 21 to 30, and much longer for chronic cases.? Other drugs used include doxycycline and Naxcel, but, he adds, ?they seem to only suppress the infection and are not very good at eliminating it.?
Occasionally a horse may be diagnosed with Lyme disease, improve when given tetracycline, then become lame again after treatment is discontinued.
?There may be two reasons for this,? says Trachtenberg. ?First, it may not be Lyme disease. Tetracycline has been shown to have anti-inflammatory as well as antibiotic properties. You might see an improvement in soundness with tetracycline if it's just arthritis.
The second reason is that the horse may have chronic Lyme disease, which can be difficult to cure. If lameness returns after discontinuing tetracycline we need to reevaluate whether it is truly Lyme disease.?
Taken together, all of these risk factors add up, says Trachtenberg: ?Is there a history of exposure to ticks? Is there generalized stiffness, lameness, hyperesthesia? If you see these signs and the horse tests positive for Lyme disease, I usually treat that horse. If he responds to treatment, great. If he continues to get better after we discontinue treatment, then I feel fairly comfortable that it was indeed Lyme disease.?
Eliminating the risk of Lyme disease is difficult for horses who live in endemic areas. A vaccine against the illness has not been approved for use in horses, but in regions where it is common, some people choose to administer the canine version (see ?A Vaccine in the Works?? at right). This means that the best preventive strategy centers on limiting your horse's exposure to ticks. Here are some steps you ca n take:
? Examine your horse daily. Ticks typically crawl around on the body looking for a feeding site for hours or days before biting; simply brushing a horse each day and especially after riding in the woods may be enough to remove ticks that have not yet attached themselves to the skin.
Studies have shown that once a tick begins feeding, it does not transmit burgdorferi into the host?s skin for another 36 to 48 hours, so removing attached ticks daily will also go a long way toward preventing Lyme disease.
?You?ll find the ticks if you look under the throatlatch, around the ears, neck and belly---the undersides of the horse,? says Bushmich. ?They aren?t that hard to find if you are looking for them.? Keep in mind, though, that the larger brown ticks that are easier to spot are
? Apply appropriate repellents. Look for products that are labeled for use against ticks as well as flies and mosquitoes, and apply them as instructed. ?Preventing ticks from getting on the horse in the first place can help a lot,? says Bushmich. ?If you live in an area with a tick problem, apply insecticides on a horse's legs, head and neck, belly and tail area each time you turn him out.?
? Continue applying repellent late into the fall, even after fly populations dwindle. Ticks can bite at almost any time of year, but the adult deer ticks most likely to transmit B. burgdorferi to horses come out predominantly in the fall, and they continue to be active until temperatures drop below 35 degrees Fahrenheit.
? Keep pastures mowed. Ticks find hosts to feed on by climbing up onto tall grasses and brushy undergrowth, then catching a lift when large animals brush past them. If your pasture grass is high enough, the ticks are likely to get onto a horse's legs or face as he grazes. Keeping turnout areas mown to less than four or five inches will limit ticks? opportunities to get onto your horse.
? Reduce habitat for wild animals that host ticks. The white-footed mouse is an important reservoir for B. burgdorferi. Taking steps to limit mouse populations around your barn and pastures can help limit your horse's exposure to the bacteria.
Keep weedy areas mown down and remove woodpiles that give them cover. In the barn, keep grains and feeds stored in sealed, rodent-proof containers, and clean up any spills immediately. Also keep the ground clean under any bird feeders. Natural predators, such as blacksnakes, will help keep populations down.
Once B. burgdorferi get established in the local wildlife, the bacteria are there to stay, and surveys done in endemic areas have found the spirochetes in dozens of species, from small rodents, to foxes, opossums, raccoons and feral cats as well as horses. The bottom line, then, is that Lyme disease will always be a threat in regions where it has become entrenched. And until a more permanent solution, such as a vaccine, is approved, awareness of the disease and vigilance on your farm will be your best defenses against it.
A VACCINE IN THE WORKS?
Currently no equine vaccine against Lyme disease is available, but there is one for dogs.
The canine vaccine has an unusual mechanism for preventing infection, says Stephen Barthold, DVM, PhD, of the University of California?Davis. The vaccine stimulates the production of antibodies that travel from the host into the body of the tick when it starts sucking blood, attacking and disabling the bacteria before they enter the host?s body.
Cornell University researchers have investigated the use of a similar vaccine in horses, and the evidence suggests that it may work the same way. In the study, the researchers didn't see any rise in titers0 after ticks fed on vaccinated horses, so they speculate that the vaccine actually killed the bacteria before they left the tick. This indicates that it might be a very effective vaccine. ?I believe the existing [canine] vaccine would work, but it's a matter of licensing it for use in horses,? says Barthold.not the ones that carry Lyme disease. The deer ticks that carry the spirochete are much smaller, about the size of a pinhead. However, if you're in an area where the larger ticks are common, the deer ticks may well be present, too.
If you do find a tick feeding on your horse, use tweezers to grasp it gently at the head, as close to the skin as possible, then slowly and steadily pull it straight up and away from the skin. Avoid squeezing the body; this could force infected fluids from the tick?s body down into the horse's bloodstream. After the tick is removed, clean the bite area with a disinfectant.