Lyme disease is an increasingly common diagnosis in horses, but what’s really known about this lurking illness?
If you find that you’ve been bitten by a tick, you likely know what to keep an eye out for in the days or weeks that follow: lethargy, fever, and the “bulls-eye” rash pattern that is characteristic of Lyme disease.
But if you find a tick on your horse, the indications of a brewing infection can be less obvious. For starters, you’re dealing with an animal that has fur and can’t tell you what hurts. And many times, you won’t find a tick at all, as they drop off after feeding on a horse’s blood. Beyond that, how Lyme disease presents itself in horses is extraordinarily variable, testing is unreliable, and veterinarians have different opinions about treatment.
“Overall, there’s little consensus on Lyme disease among equine veterinarians,” said Dr. Rachel Gardner, an equine veterinarian for B.W. Furlong & Associates in Oldwick, N.J. Dr. Gardner also received her board certification from the American College of Veterinary Internal Medicine. “There’s just very little concrete information understood about it.”
Although it’s most prevalent in the northeastern states, horses across the U.S. face exposure to the Borrelia burgdorferi organism. This organism – a bacterium – is what ticks can carry and transfer to horses through a bite. Bite rates are highest in the spring and fall, although climate change seems to play a role in the prevalence of Lyme due to altered hibernation cycles of ticks.
Regardless of when they’re bitten, horses can develop symptoms year-round. But the symptoms are anecdotal and not all are scientifically linked to Lyme.
“In horses, it’s been very difficult to prove what’s clinical disease and what’s not. People talk about shifting leg lameness and lethargy or joint swelling, and it’s very hard to prove that those things are actually due to Lyme,” Dr. Gardner explained.
Owners of horses with Lyme have reported a variety of symptoms that first tipped them off to an ailment, including sporadic and unexplained lameness, neck stiffness, sudden behavior change, generalized tenderness, and many others.
Eiren Crawford, a grand prix dressage rider in Upperco, Md., first noticed signs in a young horse when he was having difficulty moving at the canter.
“Things that were easy, I had to carry him around suddenly. We went through what had changed: switched to group turnout then to night turn out, so maybe he was tired? Maybe he got kicked? Or fell? Vet exam ended in treating the sacroiliac (joint) and we had brief improvement,” Eiren explained. “He then became worse again and next vet exam showed significant pain in back and one fore leg. We took a multitude of X-rays, took blood, did some treatment with minimal improvement.”
But according to Dr. Gardner, the only clinical syndromes that studies have linked to Lyme are neuroborreliosis (a neurologic manifestation of Lyme resulting in neurologic deficits like lack of coordination and balance), uveitis (a condition where the horse’s immune system attacks the tissues of the eye), and pseudolymphoma (a skin disorder caused by Lyme that just looks like a bump in the skin).
Diagnosing Lyme disease in horses often follows a process of elimination. A horse isn’t acting normally, and veterinarians must rule other possible ailments out before narrowing a possible diagnosis down to Lyme.
“Truly I think the hallmark of trying to establish infection is actually just to rule out everything else that can cause the horse’s symptoms,” said Dr. Gardner. “It becomes a diagnosis of exclusion. Because it’s so hard to prove, you have to prove that it’s not everything else until you’re kind of left with Lyme.”’
It’s important for horse owners to understand that exposure to the Lyme causing organism doesn’t necessarily mean the horse will contract the disease, Dr. Gardner explains.
“Infection is when a horse gets bitten by a tick and the organism is transmitted from the tick to the horse. If the horse’s immune system fights off that infection and therefore eliminates the Borrelia from the system, then we’d say that they had been infected but had not developed clinical disease. We would call that exposure.”
But when veterinarians suspect that the horse wasn’t able to ward of the organism and has full-blown Lyme disease, they’ll turn to a blood test – a multiplex. Unfortunately, more recent studies have found that even very high multiplex values aren’t necessarily linked to clinical Lyme disease in the horse, which is bad news for owners trying to gain a definitive diagnosis for their horse’s symptoms.
“The multiplex does seem to be a good indicator at least of exposure but it’s variable in terms of its usefulness in diagnosing Lyme disease,” Dr. Gardner described. “In one of the most recent studies of neuroborreliosis which is probably one of the more well-studied clinical syndromes, we found that there essentially no correlation between likelihood of infection and how high the multiplex values were – which is discouraging.”
When blood results showed the that Eiren’s young dressage horse tested positive for chronic Lyme disease, she began treating the horse immediately with strong antibiotics.
“The work in canter became more balanced again, and overall efforts under saddle were improved,” she explained. “The horse seems to be fine and progressing in the work, healthy and happy in life.”
Oxytetracycline, doxycycline, and minocycline are the most common antibiotics used to fight Lyme in horses. Treatment can last for a few weeks to months.
“In my opinion, right now the best treatment is minocycline,” she said. “It’s related to doxycycline but has better absorption and penetration into the tissues.”
Some veterinarians recommend supportive therapies for horses during treatment such as probiotics, acupuncture, or rest. While Dr. Gardner doesn’t discourage owners from pursuing these, she warns that none are scientifically proven to aid in recovery from the disease.
“There’s no supportive care that has been proven to help. If they have neurologic signs, anti-inflammatories are appropriate to decrease inflammation associated with infection and also the inflammation associated with hopefully killing off the infection.”
Dr. Gardner recalled when she was on a panel of authors for a consensus statement for the American College of Veterinary Internal Medicine last year.
“(The statement’s) purpose is to be a very comprehensive paper on something people don’t agree about, of for a bunch of vets to put their heads together on something complicated,” she said. “Last year’s was on Lyme, and what was amazing is that as we were writing it – there’s just no consensus. Everyone has a different idea, and almost nothing is proven.”