Lyme Time

When I was recently in Massachusetts walking in the woods with my friend and her dog, a constant refrain I heard was, “Stay on the path! Do not venture into the grass or the woods.” The reason: ticks. Perhaps you’ve been similarly advised to be particularly observant in looking for ticks after riding your horse through woods or fields. This is good advice for those of you living in areas where Lyme disease is present. Lyme disease is caused by infection with the bacteria species Borrelia burgdorferi. The disease is passed to horses, people, and dogs through insect carriers, namely ticks. 

The particular culprits are the Ixodes ticks (black-legged and deer ticks), which flourish in the Northeast, mid-Atlantic, upper Midwest, and in pockets of the Northwest. The disease is named after Lyme, Conn., where it was first identified in 1975. 

Climate change is expanding not only the range of the Ixodes ticks but also their numbers as they thrive due to extended warm seasons in many areas; ticks favor temperatures above 40 degrees Fahrenheit. More cases of exposure to Lyme disease are documented each year, and almost every state is now seeing cases in people and dogs, meaning that horses, too, can become infected.

How is it Contracted?
Currently there is only a vaccine for dogs, so protection for your horse is based on knowing how the disease is contracted and how to avoid tick bites. Not all horses bitten by an infective tick actually develop the disease, but they do develop antibodies, which are identified through testing.

The areas of the country where Lyme ticks are widespread are also home to white-footed mice and gray squirrels, birds, and large wild mammals (like deer) that serve as a reservoir for Borrelia bacteria. When Ixodes ticks obtain a blood meal from an infected mammal, they can then spread Borrelia bacteria to the next animal they bite. 

One bit of good of news is that transfer of bacteria from a tick bite to a horse generally requires about 24 hours of tick attachment. Finding and removing ticks immediately improves the chances that your horse won’t develop an infection. If a tick is present for longer than 24 hours, Borrelia spreads through the horse’s connective tissue and into the blood to cause systemic infection.

Blood tests are able to detect antibodies that indicate exposure, but such tests are not reliable until three to four weeks following a tick bite. Exposure does not always mean a horse develops active infection. The experts recommend testing only horses with clinical signs that point to Lyme disease.

Treatment
Treatments for horses come from multiple sources: human treatment guidelines, testing the B. burgdorferi bacteria in the lab to see which antibiotics kill it or limit growth, as well as anecdotal reports. In general, tetracycline and derivatives, as well as penicillin and cephalosporins, are used to combat Lyme disease infection. 

There haven’t been enough clinical trials to evaluate which drugs best treat the myriad of clinical signs associated with Lyme disease. While some drugs may work well in humans, oral medications work very differently in humans and horses, so using human treatments is not reliable for horses. 

Length of treatment time is also not well-defined. Current recommendations suggest basing treatment duration primarily on the horse’s clinical response, and to a lesser extent on decline in blood antibody levels. Horses may test positive for Borrelia antibodies for months, and even years, despite aggressive and prolonged antibiotic treatment and recovery.

The longer a horse has an active infection before being treated, the less successful the treatment will be. There’s also a poor prognosis for horses that have developed neuroborreliosis. Lyme-induced uveitis also has a poor prognosis for vision restoration. 

It’s likely that Lyme disease is overdiagnosed in endemic areas. Exposure to the bacteria does not guarantee that a horse will develop clinical disease. For horses that test positive to Borrelia antibodies, it’s important to exclude other potential causes before starting antimicrobial therapy.

Prevention
Tick control is essential to prevention. Check your horse often and remove ticks as soon as possible. Environmental control is helpful through landscaping practices. Ticks prefer woodland habitats and areas abundant with tall grass and piles of leaves, especially at the boundaries of these areas. 

Fewer ticks are found in areas that are clean, dry, sunlit, regularly disturbed, and well-maintained. When possible, keep horses away from wooded areas and transition zones into wooded areas. Mow pastures and clear away leaves and debris. Exclude deer as much as possible from areas with horses. 

However, ticks can still survive in stalls and pastures even during cold temperatures, although they don’t do well with freeze-thaw cycles. Inspect your horse daily year-round if you live in an area endemic with Lyme disease. 

Meticulously check the belly, groin, under the tail and mane, beneath the chin, within the armpits, and the lower legs. While ticks can attach anywhere, they are often found in softer areas with finer hair. 

If you find a tick on your horse, remove it in its entirety, taking care not to leave any mouthparts embedded in your horse’s skin. Wear gloves and grasp the tick’s mouthparts with tweezers as close to the skin as possible. Apply gentle upward traction without twisting. Once removed, you can destroy the tick by any number of methods: a) with flame; b) immersion in a jar of rubbing alcohol; or c) flushing it down the toilet. 

Application of insecticides, like permethrin or cypermethrin wipe-on or spray products, shampoos or powders to your horse’s hair coat are helpful, although there is no guarantee that insecticides prevent ticks from attaching, biting, and transmitting disease. Topical insecticide treatments are effective for only four to eight hours and need frequent reapplication. 

Off-label use of canine Lyme disease vaccine has been attempted in horses, but protective antibody levels drop significantly within four months.

The Bottom Line
For areas of the country where Lyme disease is prevalent, you can still enjoy riding your horses out on wooded trails and through the fields. Just be aware that you’ll need to spend extra time going over your horse carefully to locate and remove any ticks that may have shared the ride with your horse or attached while your horse was out in pasture. Prompt removal within hours is key to curtailing risk of infection with Lyme disease. 

NANCY S. LOVING, DVM, is a performance horse veterinarian based in Boulder, Colo., and is the author of All Horse Systems Go.

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This article originally appeared in the July 2019 issue of Horse Illustrated magazine. Click here to subscribe!