Equine Lyme disease is caused by infection of a horse with the spirochete bacteria Borrelia burgdorferi.  B. burgdorferi is spread to horses by ticks that are infected with the bacteria, transmitting the bacteria through their saliva when they bite a horse.  The most common tick to carry Lyme is Ixodes scapularis (also called the black-legged tick and commonly known as the deer tick).  As the range and population of the deer tick has been expanding in Manitoba, the prevalence of Equine Lyme Disease has also been increasing.

Equine Lyme Disease a somewhat controversial and frustrating disease.  Within medicine as a whole, there is relatively little known about the disease and how to effectively prevent and treat it.  There is a good understanding of how many horses are exposed to the bacteria, but it is unclear how many will actually develop Lyme Disease.

Horses with Lyme disease can exhibit many potential symptoms, such as:

–          Chronic weight loss

–          Sporadic lameness

–          Shifting leg lameness

–          Low grade fever

–          Muscle tenderness

–          Poor performance

–          Swollen joints and arthritis

–          Skin hypersensitivity

–          Neurologic signs such as depression, head tilt, difficult eating/swallowing

The most common signs seen are lameness, stiffness, and chronic weight loss.

There are two common tests to consider when testing a horse for Lyme Disease.  The first is called a SNAP 4DX test.  This test involves drawing a blood sample from the horse and the test can be run on the farm or in a veterinary clinic.  This is a convenient, inexpensive test that is a good choice for screening horses for Lyme disease if they are not exhibiting symptoms.  However, within Manitoba, it seems that the SNAP 4DX test can have false negative results.  Furthermore, this test only gives a result of positive or negative, and does not provide a quantitative result that can be monitored over time.

If a horse is suspected of having Lyme disease, the best test to perform is a Lyme Disease Multiplex Test, which is offered by Cornell University.  This test involves drawing a blood sample and sending it to Cornell University for testing.  This test is much better at eliminating false positive and false negative test results.  In addition, unlike the SNAP 4DX test, the Multiplex test can differentiate positive results to be from vaccination, early infection, and chronic infection.  This test also gives quantitative antibody results, that can be monitored through repeated tests to gauge response to treatment.  The Lyme Multiplex Test is considered the current gold standard test.

Treatment of Lyme disease requires treatment with an antibiotic to kill the Lyme bacteria.  The most common medications used to treat Lyme are oxytetracycline and doxycycline.  The current recommended protocol is to treat a horse with a 7-day course of intravenous oxytetracycline, and then follow up with a 3-6 week course of oral doxycycline.  Preferably, an additional Lyme Multiplex Test will be performed about 3 months after treatment to evaluate for a decrease in the level of antibody present, signifying successful treatment.

B. burgdorferi tends to be a difficult bacteria to rid the body of.  For this reason, treatment failure is relatively common and it seems to be more difficult to treat Lyme the longer the disease goes on.  One important aspect of the disease is that removal of the bacteria will not necessarily resolve the damage that has already occurred within the body.  Therefore, if the bacteria is successfully cleared from the body, it is possible for the horse to have some symptoms remain after successful treatment. Other adjunctive therapies may also be used to control symptoms, such as non-steroidal anti-inflammatory drugs (ex. phenylbutazone) and muscle relaxants (methocarbamol).

Currently, there is no vaccine for Lyme disease licensed for use in horses.  However, there is research that shows that vaccination with a vaccine licensed for dogs will result in a high level of antibody.  There is also research that shows that a high level of antibody in horses will prevent transfer of B. burgdorferi from the tick.  Therefore, many veterinarians who practice in Lyme endemic areas have been administering canine Lyme vaccines to horses.  Anecdotally, these practitioners have found adverse reactions to the vaccine to be uncommon.  They have also found that the vaccination seemed to be protective.  In Manitoba, the prevalence of Lyme disease is high enough that vaccination of horses with canine Lyme vaccine is recommended in high risk areas.  However, it must be noted that this is off-label use of this vaccine. The current protocol is to give an initial vaccine, administer a booster vaccine in 3-4 weeks, and then booster the vaccine annually.

Equine Lyme disease continues to be a frustrating disease to deal with, which is increasing in prevalence from year to year.  Veterinary medicine has a lot more to learn about Equine Lyme disease.  Thankfully, research into Equine Lyme disease is becoming more common and the future hopefully will hold more information to effectively prevent, diagnose, and treat this disease.

Written by Dr. Mackenzie Marks
Learn more about Dr. Mackenzie Marks here.