Equine Lyme disease is a growing concern for horses in North America, especially during the summer months when tick bites are more common.
Lyme disease is primarily caused by infection with Borrelia burgdorferi, a bacteria that is carried by ixodid, or hard-bodied, ticks.  You may recognize this tick by its more popular name, the black-legged or deer tick (Ixodes scapularis).
While tick prevention is common practice for many of us that own dogs and/or outdoor cats, it has not historically been a concern for horses. However, this is changing as tick populations continue to expand.
Lyme disease often targets the muscles and can lead to lameness in horses.  However, when a horse presents as lame, Lyme disease is not usually the first thought, making for a sometimes tricky diagnosis.
Horses affected by Lyme disease can also experience chronic weight loss, fever, muscle tenderness, poor performance, joint issues and behavioural changes such as depression.
Not all tick bites will lead to infection with B. burgdorferi. but in the cases that do, treatment may require a course of antibiotics over several months. Treatment can be expensive which is why prevention is so important.
This article will discuss the growing tick populations in North America, diagnosis and symptoms of Lyme disease in horses, treatment as well as how to prevent this condition in your horse.
Lyme Disease and Ticks in North America
The black-legged tick is found throughout the southeastern and southcentral regions of Canada. It can be found in the provinces of Manitoba, Ontario, Quebec, New Brunswick and Nova Scotia, as well as along the West Coast in British Columbia. 
Human cases of Lyme disease in Canada increased by more than 18-fold from 2009 to 2019, reflecting the increasing range of the black-legged tick.
In the United States, the black-legged tick is found across central and eastern states with a huge range covering all the way from Texas to Maine. 
A northward spread of the black-legged tick to northern states as well as southern and central Canada has been documented and climate change has been thought to play a role in this spread. 
Tick Life Cycle
The black-legged tick follows a three-host life cycle that can take between 2-3 years to complete. Below is the typical life cycle for a black-legged tick.
At each stage, any animal or human can be the host and the tick can attach and feed on multiple different hosts or individuals. 
- Adult females lay eggs, usually in the fall, after feeding on a host. The eggs hatch and spend the winter in the larval stage.
- In the spring, the larvae attach to a small rodent, such as a squirrel.
- After spending spring and summer feeding on the first host, larvae fall off the first host and molt into nymphs in the fall. They then spend the winter in the nymph form.
- The following spring, the nymphs attach to the second host, often a small mammal or rodent.
- The nymphs then fall off the second host in the fall and spend the winter in the adult form.
- The next spring, adults often seek a larger host, like a horse, cow or human. The adults then feed and mate on the third host, completing the life cycle.
The tick can pick up the bacteria, B. burgdorferi, from any of the hosts it feeds on. It can then transmit that bacteria when it attaches to a new host.
It will often take approximately 12-24 hours for the tick to transfer the bacteria after it has bitten the host. The key to reducing the chance of infection with Lyme disease is to remove the tick quickly.
Equine Lyme Disease Prevalence
Although not commonly reported in horses, several studies have investigated the prevalence of B. burgdorferi infection in clinically healthy horses. Researchers have measured the following rates of infection: 
- 14.8% of horses in the Pacific Northwest of the USA in 2008
- 45% of horses in Northeastern USA in 2000
- 33% of horses in Virginia, USA in 2016
These horses were all presumed healthy and not displaying clinical symptoms of Lyme disease. However, these reports suggest that exposure to B. burgdorferi is perhaps higher than expected and on the rise.
A recent study investigated the prevalence of B. burgdorferi infection in clinically healthy horses in Ontario, Canada, focusing on areas known to be inhabited by black-legged ticks. 
The authors reported that of the 551 healthy horses, 91 (17%) tested positive for B. burgdorferi infection. Horse owners also filled out a questionnaire describing their environment and some of their equine management practices.
The odds of a horse testing positive were increased when oak trees were present in the pasture and decreased when the horses were regularly checked for ticks by their owners.
This highlights the importance of environmental and management practices for the prevention of Lyme disease, which will be discussed later in this article.
Diagnosis & Testing
Diagnosis of past or present infection with B. burgdorferi can be done with a simple blood test looking at serum antibodies. 
However, this test only informs you if the horse has been exposed to B. burgdorferi at any point in its life and is not correlated with symptoms of Lyme disease.
It takes at least 3 weeks after a tick bite for antibodies to be detected in the blood. That is why some clinics recommend testing up to 8 weeks after tick exposure and every year in areas where ticks are frequently seen. 
A positive result doesn’t predict the onset of symptoms of Lyme disease. A horse may have been exposed to B. burgdorferi many years ago and still have a positive test result.
Because of this, the actual diagnosis of Lyme disease is quite tricky and other diseases need to be ruled out first.
Symptoms of Lyme Disease in Horses
Many symptoms of Lyme disease are similar to symptoms of other diseases. Usually, a combination of symptoms is present when a clinical diagnosis is made.
The most common signs of Lyme disease reported in horses include:
- Muscle tenderness
- Joint discomfort/swelling
- Lameness (sporadic or shifting leg)
- Weight loss and low appetite
- Reluctance to move/work
- Low grade fever
- Behavioural changes or irritable mood
- Generalized Sensitivity
- Uveitis (moon blindness)
If your horse displays several of these signs and has had a likely exposure to ticks carrying B. burgdorferi, your veterinarian may conduct tests to rule out other diseases and to confirm Lyme disease. 
Clinical symptoms of Lyme Disease and other diseases to rule out are outlined below.
Neuroborreliosis: Occurs when B. burgdorferi infects the central nervous system. Can present as:
- Behavioural changes
- Hyperesthesia: an increase in the sensitivity of the senses
- Ataxia: a lack of muscle control or coordination
- Trouble swallowing and respiratory distress
- Muscle atrophy and neck stiffness
These symptoms are similar to Equine Protozoal Myeloencephalitis and Viral Encephalitis which should be ruled out to confirm Lyme disease.
Uveitis: Presents with inflammation and/or discharge from the eyes. These symptoms are similar to Leptospirosis and Cutaneous lymphoma.
Cutaneous pseudolymphoma: Described as small masses on the skin where the tick attached to the horse.
If the horse is presenting with one of these symptoms and is positive for B. burgdorferi exposure, other tests can be done to confirm active infection.
If the horse displays signs of neuroborreliosis, a sample of cerebrospinal fluid can be taken and analyzed.  Often in horses with Lyme disease, the cerebrospinal fluid comes back with high counts of white blood cells, neutrophils and lymphocytes. 
Similar tests can be performed in ocular fluids and biopsies can be taken if cutaneous pseudolymphoma is present. 
Lyme Disease Treatment
The most common drugs used to treat equine Lyme disease are tetracycline antimicrobials, such as doxycycline, oxytetracycline or minocycline. 
These drugs also have anti-inflammatory properties which may help alleviate other symptoms. Other antibiotics that are effective against B. burgdorferi include penicillin and cephalosporins. 
In general, the earlier you start treatment after confirmation of Lyme disease, the better. Cases that are caught and treated early tend to have a better prognosis than chronic cases or cases with neuroborreliosis in which the prognosis is generally poor. 
Treatment of equine Lyme disease is often difficult, especially if the infection is chronic. Overall, the treatment protocols are poorly defined because the disease is rare and there is a lack of clinical data in horses. 
Treatment should be continued until the clinical signs resolve. Re-testing antibody levels may provide some valuable therapeutic information, but should not be used alone as an indicator of resolution of the disease. Horses may have a positive test years after resolution of clinical symptoms. 
Unfortunately, even after symptoms resolve a horse can contract Lyme disease again if bitten by another B. burgdorferi-carrying tick. Contracting Lyme disease once does not convey immunity. 
Prevention of Lyme Disease
Those of us who are dogs owners will be quite familiar with tick prevention practices. Although there are limited equine products available to prevent tick bites, similar products do exist and canine preventatives can be used off-label as well.
As ticks often inhabit tall and/or dense grassy and wooded areas, there are a number of landscaping techniques you can use to reduce the chance of ticks inhabiting the land where your horses frequent, including:
- Removing leaf litter
- Clearing tall grass/brush
- Discouraging animals like raccoons and deer from the property as they may be carrying ticks 
Chemical Prevention and Dewormers
Chemical wipe-on, pour-on and spray-on products, such as Co-Ral, Deep Woods Off, and Frontline spray can all be applied to the horse’s body and are effective for several hours. 
Alternatively, oral dewormers such as ivermectin or moxidectin can be used to kill the tick once it has bitten the horse.  The benefit of oral dewormers is that they are effective for much longer periods of time, usually 4 weeks.
A tick must be attached to the horse for 12-24 hours before transmitting B. burgdorferi bacteria that can cause Lyme disease. So the best way to prevent this disease is to carefully monitor and remove any ticks on your horse as soon as possible.
There are several canine Lyme vaccines that have been tested off-label in horses, some of which were found to be at least partially protective in the horse. However, specific vaccine protocols for horses have not been established.
One study investigated three different canine vaccines in horses. The first trial tested each of the three vaccines in unvaccinated horses, giving three doses on days 0, 25 and 108.
This vaccine regimen resulted in low levels of antibodies in the horses that lasted for less than 16 weeks.  This suggests that vaccination would need to be done regularly to maintain protective status for the horse.
The second trial used only one type of vaccine and compared a 2mL dose to a 1mL dose given on days 0 and 24. On this regimen, horses that received 2mL had higher levels of antibodies. In both cases, levels of antibodies declined less than 16 weeks after vaccination.
The third trial used the same population of horses as the second trial and looked at a different vaccine. In this trial, half of the horses were vaccinated one year after prior vaccination and the other half of horses were not re-vaccinated.
A second vaccine administered one year later induced a short-lived immune response, resulting in increased antibodies. The authors concluded that while the canine vaccines do produce antibodies in horses, they are very short-lived.
As such, they recommend vaccinating horses approximately 4 weeks before ticks are abundant in the area each year and possibly multiple times per year. More studies are needed to evaluate this approach.
If you suspect your horse may have lyme disease, contact your veterinarian for a diagnosis and treatment plan.
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- CDC Ticks. Centers for Disease Control and Prevention. 2021.
- Neely, M. et al. Seroprevalence and evaluation of risk factors associated with seropositivity for Borrelia burgdorferi in Ontario horses. Equine Vet J. 2021.
- Divers, T.J. and DeNotta, S. AAEP Infectious Disease Guidelines: Borrelia burgdorferi infection and Lyme disease. American Association of Equine Practitioners. 2020.
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- Guarino, C., et al. Vaccination of horses with Lyme vaccines for dogs induces short-lasting antibody responses. Vaccine. 2017.