Lyme disease in horses is tricky. It’s notoriously difficult to diagnose, and it can pose a serious threat to a horse’s health and well-being. The causal bacterium, Borrelia burgdorferi, which is transmitted by deer ticks, travels throughout the bloodstream and may cause localized inflammatory reactions in various tissues around the body— including muscles, joints, organs, even the brain—which means that the specific signs of illness can vary, too.
Clinical signs of Lyme disease typically develop several months after infection during the chronic phase of the disease. One frequent sign of Lyme disease in horses is a vague lameness that shifts from limb to limb. An affected horse may also have general stiffness, fever, lethargy or weight loss. He may become sensitive and jumpy when touched or just be grumpy and perform poorly. Some horses may develop uveitis0, have behavioral changes or begin headshaking.
The problem is that these signs mimic more common conditions, such as arthritis, as well as other viral or neurological diseases. Blood tests can determine whether a horse has been infected with B. burgdorferi, but these must be interpreted carefully. It is estimated that as many as 45 to 50 percent of horses in areas where Lyme disease is endemic may be exposed to the bacteria, but fewer than 10 percent ever show signs of illness. In other words, a positive result on a simple blood test for antibodies does not necessarily mean that the horse has Lyme disease—something else could still be causing his clinical signs.
A more specific test now available, the Lyme Multiplex Assay from Cornell University, quantifies antibodies against three different B. burg– dorferi proteins: OspA antibodies are higher among horses vaccinated with a canine formula; OspC antibody levels peak early after a natural infection and begin to decline at seven to 11 weeks post-infection; OspF antibody levels rise after five to eight weeks and remain high in the face of infection—this is considered a sign of long-term chronic infection.
These tests, too, must be interpreted carefully—even with positive results for chronic infection, a horse’s current illness or lameness could be due to something else. To diagnose Lyme disease, a veterinarian will rule out other potential causes and consider various factors, including the horse’s history and risk of exposure and his response to treatment.
In other words, veterinarians may partly “diagnose by treatment.” Lyme disease responds to antibiotics, often doxycycline although your veterinarian may choose other drugs. If Lyme is suspected, a veterinarian may prescribe antibiotics. Then, if the lameness or other signs go away—and the horse remains normal with reduced antibody counts after the drug treatment ends—a diagnosis of Lyme will be presumed.
Although severe cases of Lyme disease can be debilitating, most horses who receive prompt treatment during the early phase of the infection recover fully. If the disease becomes chronic, however, outcomes are more variable, depending on the body tissues affected and how much damage was done before treatment began.
Currently, there is no vaccine against Lyme disease approved for use in horses. However, in regions where the disease is common, some owners opt to administer the canine vaccine to their horses. Talk to your veterinarian about the risks and benefits if you want to consider this option.
Trials at Cornell University showed that vaccination with a canine product can stimulate the production of antibodies in horses, but responses varied in individual horses, ranging from weak to high, and the effects were short-lived. The typical schedule for vaccinating horses includes initial boosters at three weeks and then again at three months after the initial injection. OspA anti-bodies will begin to drop in less than six months, so biannual boosters are needed to achieve year-round protection in Lyme-endemic areas. No safety studies have ever been conducted for this vaccine in horses, but no adverse effects have been reported.
Apart from the canine vaccine, the best defense against Lyme disease is to limit your horse’s exposure to ticks:
• Remove ticks from your horse daily. As you groom your horse each day, check all the “secluded” places where ticks like to hide, such as the ears, under the jaw, under the mane and inside the legs. Keep a pair of tweezers on hand while you groom. If you find a tick that has latched on, you’ll want to remove it carefully: Grasp it gently at the head, as close as possible to the horse’s skin, and lift it straight up. Do not squeeze the body of the tick—this may push disease-causing agents down into the horse’s bloodstream.
Removing ticks daily, even after they have bitten, will help reduce your horse’s risk of Lyme disease. Studies have shown that it takes 16 to 24 hours for a feeding tick to begin transmitting B. burgdorferi to its host.
• Apply repellents wisely. Look for products that are labeled for use against ticks as well as flies and other pests. As you apply them, be careful to protect parts of the horse’s body where ticks are likely to climb on—the legs, head and neck, belly and tail.
Remember that ticks can continue biting well after flies and mosquitoes have started to die off in the fall months. Continue applying repellents to your horse until temperatures drop below freezing.
• Protect your dog with repellents, too. Your dog can be vaccinated against Lyme disease, which is a wise precaution in endemic areas. But it’s a good idea to apply tick repel- lents as well: Dogs are inclined to roam in weedy areas and roll in leaf litter where they can pick up ticks, which they could then carry back to your house and barn.
Cats can also be infected with Lyme disease, although naturally occurring cases are rare. If you want to apply tick repellents to your barn cats, choose products made for felines. Some products for dogs are toxic to cats.
• Keep pastures mowed. Ticks crawl up onto tall grasses and shrubs and wait to climb onto any large animal that brushes by. Keeping pastures and other turnouts mowed to five inches or less will help prevent ticks from getting on your horse.
• Control the host species. The white-footed mouse is an important reservoir for B. burgdorferi. Take steps to keep rodent populations around your farm under control. Eliminate old brush piles and mow down weedy areas that give them cover. Keep grains and other feeds in sealed containers, and clean up spills promptly. Barn cats and natural predators such as black snakes and barn owls can help keep down rodent populations.
Keeping deer away from your farm might not be possible, but you can take steps to avoid attracting them in large numbers. For example, if you have fruit trees, keep fallen fruit cleaned up, and choose deer-resistant plants for landscaping. Deer repellents—products that include ammonia compounds as well as natural substances such as blood meal —may be helpful, but they need to be reapplied regularly.
This article was originally published in EQUUS 486, March 2018