Lyme Disease in Horses: Symptoms, Treatment and Prevention

Lyme Disease in Horses: Symptoms, Treatment and Prevention

Lyme disease in horses is a mysterious and confusing equine disease, but is very treatable.

A blond girl grooms a bay horse.

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By Andrea Caudill

The signs can be vague and mysterious. A thumbnail-size lump on the body, general lethargy and sore joints could point to anything or nothing at all, which is exactly the problem posed by Lyme disease in horses, which can present as a number of rather generic symptoms. 

If you live in an area where ticks are common, Lyme disease is a possibility for humans and horses alike. While it is usually a treatable disease, it is one where an ounce of prevention is the best option

How Do Horses Get Lyme Disease?

Lyme disease gets its name from the town where it was first identified as a unique syndrome–Lyme, Connecticut–in 1975. 
The disease is caused by the spirochete bacterium Borrelia burgdorferi, and is transferred to horses through infected ticks. There are several types of ticks that can transfer the bacterium, but the most common one is known as a deer tick (Ixodes scapularis). 

To transfer the bacteria, the infected tick must bite the horse and, researchers believe, must stay attached to the horse for at least 24 hours to successfully transmit the bacteria to the horse. Horses do not transmit the disease to other horses. 

Because the bacteria must be transferred via tick, the disease is common in tick-hospitable environments, such as woodsy areas. 

Dr. SallyAnne DeNotta spent many years practicing in the Northeast United States where Lyme disease is well known, but now works as a clinical assistant professor of large-animal internal medicine at the University of Florida. 

 “The thing to know about Lyme disease in horses, and people and dogs too, is that many, many horses will get infected with borrelia but never develop clinical signs,” Dr. DeNotta says. “So you can be exposed, you can be infected, you can develop antibodies and even an immunity to borrelia, but never actually have any negative effects that could be attributed to Lyme disease.”

Research estimates about 30 percent of the tick populations in endemic regions are infected with the bacteria, the veterinarian says, but the impact of Lyme is much lower. 

“If it were a 100 percent relationship between infection with borrelia and Lyme disease, then most horses living in the Northwest would have Lyme disease,” Dr. DeNotta say. “But it’s not. It’s much, much lower. We don’t know why some horses get sick and some don’t, but we do know that many horses are exposed and infected with borrelia and never suffer any negative health consequences.”

It is very common for a horse to show evidence of antibody levels in their blood, but to be completely asymptomatic and healthy.

“That’s an important concept, because the way we test for this disease is actually to measure those antibody levels,” Dr. DeNotta says. “It can be very confusing to folks to see that the horse has antibodies, but the veterinarian may say, ‘We don’t think that’s a problem, we don’t think the horse has Lyme disease. Having antibodies in the blood just means the horse has been exposed to borrelia, which is very common in endemic regions. In order to truly diagnose Lyme disease, the horse needs to have compatible clinical signs, borrelia antibodies and have other common causes of illness ruled out.” 

How Does Lyme Disease Affect Horses?

Diagnosing Lyme disease is a difficult endeavor. 

 “The horses that do develop Lyme disease, the symptoms are vague and somewhat nonspecific, which makes this a difficult disease,” Dr. DeNotta says. “There’s a lot of confusion, there’s a lot of controversy.”

The signs of the disease are general and can be indicative of a number of other diseases.

Documented clinical signs include:

  • A swollen lump at the site of the tick bite
  • Swollen joints
  • Uveitis (eye inflammation)

Other undocumented but possible signs include:

  • Stiffness
  • Lameness
  • Lethargy
  • General changes in behavior


A rare but very serious form of Lyme is neuroborreliosis, which is when the bacteria invades the horse’s central nervous system, causing severe issues including

  • Neurologic signs
  • Fever
  • Muscle wasting
  • Skin sensitivity
  • Difficulty eating.

This form can be fatal.

How Lyme Disease is Identified in Horses

The identification and treatment of the most common form of Lyme disease starts with diagnostics.

“When we have a horse that has signs that we think could be compatible with Lyme disease, the next step is actually to rule out all the other causes that could be causing it,” Dr. DeNotta says. 

A veterinarian typically starts with a complete physical exam, blood work, lameness exam and diagnostic workup. 

“It’s what we call a diagnosis of exclusion,” Dr. DeNotta says. “The reason for that is most of the time, it’s something else (other than Lyme disease).”

The veterinarian first must rule out the most common reasons causing whatever abnormality the horse’s owner is reporting. 

An additional difficulty is that the bacteria doesn’t show in high concentrations in easily sampled fluids, such as blood, which would allow for a direct test. This is why the blood work is checked for, among other things, the antibody levels.

However, if all those reasons have not proven compatible, and the horse has antibody levels indicating a past or ongoing borrelia infection, the veterinarian at that point might look to Lyme disease and recommend treatment.

Can Lyme Disease Be Cured in Horses?

Fortunately, the most common, non-neurologic forms of Lyme disease are very treatable using antibiotics such as intravenous oxytetracycline or oral doxycycline.

Treatment typically lasts four to eight weeks. Note that a horse will likely still show antibody levels in its blood after treatment–they  indicate the body’s response to infection, not necessarily the immune system’s response to treatment. 

One treatment should typically be effective, and the horse would not be expected to relapse. While it could be possible for a horse to become symptomatic again in the long term, that is rare, the veterinarian says, and there would be no reason to expect the horse to have long-term effects.

The much more pressing dangers, Dr. DeNotta says, include two things. First, antibiotic resistance if the medications are overused, and second, if the horse is still showing problematic symptoms after being treated, it could be a warning that something else is amiss.

“Just to muddy the water a little more, tetracycline antibiotics are really good anti-inflammatories,” she says. 

This means that if there is an undetected lameness issue, the horse might appear to improve while on the medication, as its pain is relieved, and then gets worse when it is taken off the medication.

“One cycle (of antibiotic) should be plenty,” Dr. DeNotta says. “If the horse gets better on the antibiotic and then gets worse again when taken off, I start to wonder if that horse has pain that has been mitigated by the anti-inflammatory properties and not the bacteria infection.” 

Preventing Lyme Disease in Horses

There is no vaccine to prevent Lyme disease, so the best defense to protect against it is offense.

This means starting with the horse’s environment to make it as inhospitable to ticks as possible. This includes mowing pastures and removing brush, debris and other places ticks feel at home. 

“We call it ‘tick-scaping,’” Dr. DeNotta says. “If you have pastures and you live in an area where ticks are pandemic, then by removing the woodsy areas and low-lying brush, weeds and debris in your pastures, where ticks like to live and breed,  you can reduce the tick populations that your horse is exposed to.” 

Applying sprays to the horse that repel ticks, like DEET, picaridin or permethrin, is also a helpful deterrent, but the other biggest defense is grooming the horse. Because it takes 24 hours to transmit the bacteria, if you can remove a tick in less than that time, it will increase your horse’s chances of staying healthy. Find a time to groom the horse thoroughly every single day, running your hands over every part of him, including the tail, tailhead and ears. 

“You’ll feel a pretty big bump,” Dr. DeNotta says of how to locate the tick. “Grooming your horse every day, you can get pretty good at it.”

The good news for horse owners is that the most common form of Lyme disease is very treatable. While researchers are working to discover more about the disease, prevention can go a long way to protecting your horse. 

“You can see what a headache this disease is,” Dr. DeNotta says. “It’s a tough disease. It’s frustrating for owners, and veterinarians and researchers alike. We’re trying to figure it out, and there are lots of good people doing research and trying to figure it out. We know more than we used to, but there are a lot of questions still to be answered.”