What Causes Lyme Disease in Horses

brown horse standing in high grass in sunset light

If you’ve ever gone hiking in the northeastern United States, you’ve probably had someone remind you, “Check yourself for ticks!”

After all, ticks can carry a myriad of diseases, including the infamous Lyme disease.

Just like humans, horses are susceptible to tick bites and, therefore, to Lyme disease. While this isn’t the most common equine disease, it can have profound effects that range from stiff joints to underperformance and beyond.

In this short guide, we’ll cover the characteristics of equine Lyme disease, treatment strategies, and the best way to protect your horse and yourself from Lyme disease.

Lyme Disease 101

Lyme disease is a bacterial infection carried by ticks. In the U.S., it’s most frequently caused by the spirochete Borrelia burgdorferi, although ticks in Europe and Asia carry other similar bacteria that can result in Lyme-like infections1.

Ticks throughout the midwestern and northeastern United States carry B. burgdorferi. Lyme disease is the most common vector-borne disease (a disease in which another animal serves as a vector for transmission to humans).

Unfortunately, this nasty bacteria can be transmitted to humans, horses, cats, dogs, and other mammals.

  • Hard-bodied ticks—including deer ticks and Western black-legged ticks—are carriers for B. burgdorferi. 30-50% of adult ticks carry the bacteria, with a slightly lower incidence amongst nymphs. However, it seems that nymphs are most likely to transmit the disease.
  • Ticks feed on blood. They can thrive in residential areas, fields, yards, and woodlands. When people, cats, dogs, and horses move through tick-infested regions, there’s a high risk of tick bites.
  • Not all ticks carry Lyme disease. Even if a tick is infected, it won’t necessarily transmit the bacteria to its animal host.
  • The risk of infection with Lyme disease is highest in spring throughout summer, all before nymphs fully mature.
  • The longer a tick feeds, the higher the chance that it will transmit bacteria to its host. If ticks are immediately detected and removed, this can lower the likelihood of contracting Lyme disease.

Unfortunately, the incidence of Lyme disease is only spreading. The number of cases per year has doubled since 1991, and there are now an estimated 20,000-30,000 human cases per year.2

No one collects nationwide data on Lyme disease in horses, but veterinarians find that it occurs in the same places as human Lyme disease.3 

However, Lyme disease can have a range of symptoms, making it potentially difficult to detect and diagnose both horses and humans. Next, we’ll go over the potential symptoms.

Lyme Disease Symptoms in Humans

Lyme disease was first discovered in Old Lyme, Connecticut, in 1975 when 51 residents of Old Lyme and the surrounding area had a unique form of arthritis. Eventually, scientists traced the common cause back to B. burgdorferi.

Now, we know that Lyme disease can present a wide range of symptoms, including:

  • Fever
  • Fatigue
  • Aching muscles
  • Headache
  • Joint discomfort
  • Swollen lymph nodes

Many of these symptoms are associated with illnesses like the flu, and people can easily assume they have a different disease if they haven’t found a tick on their flesh.

However, ticks naturally detach after feeding. Most people diagnosed with Lyme disease discover they have the illness without ever seeing signs of a tick bite. 

If you find a tick on your skin, you can place it in a bag and have it tested for Lyme disease. Unfortunately, it may be long gone before symptoms arise. To that end, one tell-tale sign of a tick bite on humans is a large, red bullseye-shaped rash. This can appear anywhere from three days to one month after a tick bite. 

If you have this symptom, see a doctor immediately.

Lyme Disease Symptoms in Horses

Unfortunately, your equine friend can’t tell you if they’ve had a bug bite—nor if they’re experiencing headaches or joint discomfort. Likewise, their coats can hide any side of redness, bites, and irritation.

Therefore, signs of Lyme disease may initially present as other illnesses. In which case, potential symptoms of Lyme disease in horses include:

  • Swollen joints – Swelling, arthritis, and lameness occur in horses for various reasons, despite their average age. If there are no Lyme disease symptoms, your veterinarian would likely rule out other possible causes before testing for Lyme disease.
  • Fatigue and mood changes – Likewise, low energy levels and moodiness could be associated with primary infection or even an emotional issue. However, fatigue can also be a sign of Lyme disease.
  • Eye issues – Horses can develop equine recurrent uveitis (ERU)—aka “moon blindness”—after exposure to some bacteria. Some studies suggest that Lyme disease can create similar issues in the visual field.

Besides the above issues, be on the lookout for symptoms like the following:4

  • Sensitivity to touch
  • Glazed eyes
  • Excessive sweating
  • Stiff tail
  • Difficulty swallowing
  • Confusion (i.e., wandering)
  • Fever
  • Weight loss
  • Tilted head

If you think your horse may have Lyme disease, it’s time to consult your veterinarian.

Screening Horses for Lyme Disease

Horse Stud and her beautiful foal on a field

It can be difficult to diagnose Lyme disease since symptoms often occur after the immune system has mounted an initial response to the invading bacteria. Rather than testing for B. burgdorferi bacteria, Lyme screening tests an antibody response to some of the proteins that make up this bacteria.

Therefore, the main diagnostic criteria for Lyme disease in horses include:5

  • Exposure to an area with B. burgdorferi infected ticks
  • Displaying signs of a tick bite
  • Showing symptoms associated with Lyme disease
  • Ruling out issues with training, behavior, or orthopedic disease
  • Testing for a high level of B. burgdorferi antibodies

It can be challenging to interpret Lyme disease test results since different factors can impact the presence of antibodies. These include:

  • The horse’s vaccination history – Some studies suggest that vaccinated horses can present false positives to tests for the bacterium’s outer surface protein A.
  • The recency of tick bite – While recently infected horses may exhibit outer surface protein C antibodies, these diminish over time.
  • Duration of infection – Only horses with infections lasting three months or longer will display outer surface protein F antibodies.

However, a qualified veterinarian should be able to interpret the results and get a sense of how long your horse has been infected with Lyme disease (if at all).

Lyme Disease Treatment

Because Lyme disease is a bacterial infection, it is usually treated with antibiotics. Consult with your veterinarian to develop a treatment plan for your horse.

Common choices include:

  • Oral antibiotics – A 30-day course of doxycycline or minocycline can help kill any lingering bacteria.
  • Intravenous antibiotics – Some veterinarians recommended a more aggressive course of tetracycline or oxytetracycline delivered via IV.

Your vet may also recommend anti-inflammatories to help with any lingering discomfort.

Preventing Lyme Disease

While dogs can receive a Lyme vaccine, there is no vaccine for people or horses. Therefore, the best line of defense against Lyme disease is visual infection.

Getting a single tick bite does not guarantee Lyme disease, especially since tics need to feed for over 24 hours to transmit the bacteria.

In which case, try out the following strategies to prevent and remove ticks:3

  • Use fly and tick repellants around the perimeter of your property. Depending on your preferences, choose a commercial insecticide or a natural alternative. 
  • Make your horse’s turnout space less hospitable to ticks by regularly mowing the grass and trimming overhanging branches.
  • Inspect your horse daily for signs of tick bites. It’s essential to be vigilant in the spring and summer months when the risk is highest.
  • Remove and bag any ticks. These can always be tested if your horse does develop signs of Lyme disease.

Supporting Health in Horses

Cute little girl with her horse on a lovely meadow lit by warm evening light

If your horse exhibits signs of joint stiffness or swelling, Lyme disease