Lyme Disease in Horses

Prevention is the best cure when it comes to Lyme disease in horses.

Talk about opportunistic: The tiny tick is highly adaptable and focused on survival.

Although often wrongly identified as insects, ticks are actually eight-legged parasitic arachnids, making them relatives of the spider.

Unfortunately, ticks can transmit a variety of harmful pathogens, including Borrelia burgdorferi, the infective organism that causes Lyme disease in humans, dogs, and horses.

The disease takes its name from Lyme, Ct., where it was first identified in the U.S. in the mid-1970s.

Lyme disease is transmitted by Ixodes scapularis (the eastern black-legged tick), found in the north and eastern part of the U.S., and Ixodes pacificus (the western black-legged tick), found in the Pacific Coast states of the western U.S. Both are also referred to as “deer ticks,” as deer are common hosts.

Ticks have four life stages: egg, larvae, nymph, and adult. From larvae stage onward, blood meals are needed to survive.

It is through feeding that ticks become infected with disease-causing pathogens. For example, the black-legged tick can become a carrier of B. burgdorferi after feeding on a white-footed mouse, which is known to carry the bacterium. If that tick then feeds on a horse, human or dog, it can transmit B. burgdorferi.

Shaped like tiny corkscrews known as spirochetes, this bacterium can colonize in the bloodstream, brain, joints, organs, and central nervous system.

At Cornell University College of Veterinary Medicine in Ithaca, N.Y., Thomas J. Divers, DVM, has devoted years of research to studying bacterial tick-borne infections in horses.

Divers notes that the rise in tick numbers and their expanding geographic range has no doubt increased the exposure of humans, horses and dogs to B. burgdorferi.

Clinical Signs of Lyme Disease in Horses

Lyme disease is the most common of several tick-borne diseases affecting horses. Diagnosing Lyme, however, is not straightforward, and can in fact be extremely challenging.

In humans, Lyme disease can affect the entire body with an array of troubling symptoms, often causing misdiagnosis. A similar problem is found in the equine world, as the clinical signs of equine Lyme disease can mimic other diseases.

Another difficulty with diagnosing Lyme in horses is that the clinical signs aren’t consistent. These signs may include:

◆ Nodular skin disease at the tick bite site

◆ Neurologic disease affecting the spinal cord, brainstem or brain

◆ Uveitis (inflammation within the eye)

◆ Overall tenderness/sensitivity

◆ Vague lameness

◆ Neck and back stiffness with pain

◆ Muscle atrophy

◆ Behavioral changes

◆ Weight loss

◆ Ataxia (lack of voluntary coordination)

Adding to the challenge of diagnosis is that clinical signs may not appear for weeks—if not much later—and the horse may only exhibit random signs.

Diagnosing Lyme

In areas of the U.S. where B. burgdorferi exposure is common, 20 to 50 percent of adult horses have likely been exposed.

“The infection rate in horses is high, but clinical disease following infection appears to be very low in horses,” says Divers.

Exposure does not prove Lyme infection, so if your horse is exhibiting clinical signs related to the disease, your veterinarian should conduct a thorough exam to rule out any other causes.

If your veterinarian suspects Lyme, blood is drawn for serologic testing to determine if the horse’s immune system has produced antibodies to B. burgdorferi.

Having a high antibody suggests the horse has been exposed to the bacteria, but doesn’t necessarily confirm the horse has Lyme disease, since numerous equine diseases share similar clinical signs.

“It’s important to remember that most horses exposed to the bacteria and having a positive antibody test show no signs of Lyme disease,” says Divers. “If the antibody remains high without a drop in titer after several months, this suggests chronic infection, but most horses with this laboratory finding appear completely normal.

“I still believe clinical Lyme disease is much less common in horses than humans,” he adds. “There might be a slight increase in confirmed equine Lyme cases, but some of this is likely due to increased diagnostics, rather than a true increase in the disease.”

Treatment

Veterinarians admit that there is no “ideal” treatment protocol for equine Lyme disease.

Drug protocol and the duration of treatment should be determined by the attending veterinarian based on where the infection is in the horse’s body.

Divers emphasizes that it isn’t appropriate to use antibiotics to treat a horse that is antibody positive but doesn’t have a confirmed Lyme diagnosis, as this can contribute to widespread antimicrobial resistance.

Drugs in the tetracycline family, including doxycycline and minocycline, have been used to treat the disease. For a horse that truly has Lyme disease, a course of antibiotics may result in improvement of clinical signs within a few days. However, completely eliminating the B. burgdorferi organisms likely requires long-term treatment of weeks to months.

Divers generally recommends rechecking the antibody levels after six weeks. If no decline in titer is visible, the veterinarian should reassess the treatment protocol.

“Recovery from neurologic disease and uveitis is difficult,” cautions Divers, adding that prognosis is guarded in horses with such clinical signs from Lyme disease.

Protecting Horses from Exposure

No vaccines for Lyme disease are currently labeled for use in horses, although a vaccine is available for dogs. Some horse owners in areas where Lyme disease is prevalent have tried using these canine vaccines, but veterinarians may be hesitant to recommend such off-label use.

The best way to protect horses from possible infection is by limiting their exposure to ticks, which typically occurs when horses are turned out in or ridden through wooded areas.

When not on a host, ticks prefer shaded, sheltered areas with vegetation. If it’s not possible to keep horses out of wooded areas completely, maintain a 10-foot mowed boundary strip between wooded areas and open pasture. Keep that strip mowed very short, since ticks usually avoid open, sunny areas with little cover.

You should also protect your horse by:

◆ Keeping pastures well mowed; don’t let them get overgrown

◆ Removing weeds/leaf litter around barns, fence lines and under trees

◆ Practicing rodent control in and around barn

◆ Applying repellent products specifically labeled for use against ticks and, ideally, approved for use on horses

◆ Checking horses thoroughly after any exposure to areas that may harbor ticks

Unless you see a tick moving on a light-colored horse, you aren’t usually aware of their presence without careful observation. Check closely in thin-skinned areas of the horse’s body, including the chest, neck, flank, under the mane and tail, inside ears, and under the belly.

It’s often said that a tick must be attached for 24 hours before the Lyme disease organism can be transmitted, but this is not black and white. The best protection plan to is keep ticks off horses as much as possible.

Even if they don’t transmit disease, the ticks can cause inflammation, swelling and itching at the bite site.

After removing a tick, clean the site thoroughly with an antiseptic made for equine use, then wash your hands thoroughly with soap and water.

Removing a Tick Ignore colloquial advice about “smothering” attached ticks with petroleum jelly or burning them with a match. Such tactics can backfire and make the tick release more saliva, increasing the odds of disease transmission. If you find a tick on your horse, dog, or yourself, use a pair of fine-tipped tweezers to remove it. Hold the tweezers as close to the skin as possible and grasp the tick by its mouthparts. Pull back with steady, even pressure. Don’t twist or squeeze the tick’s body, as this can release infectious organisms. After removal, drown the tick in a dish of soapy water or flush it. Or, to confirm the species, save the dead tick and use a free online identification option, such as offered by the Tick Research Lab of Pennsylvania. (ticklab.org/tick-identification; ticklab.org/identify-my-tick)