Lyme Disease In Horses

As many as 75% of horses living in areas of the country where Lyme organism infection of ticks is high will test positive for antibodies to the organism. How many of them have an infection that will cause obvious signs is unknown. However, skepticism over whether or not Lyme exists in horses is long gone. It’s real.

The progression of Lyme symptoms has been best studied in people. The first symptom is a characteristic bull’s eye rash around the bite site, anywhere from three to 30 days after the tick is removed.

At this time, there is also fever, fatigue, chills, headache, muscle and joint pains, enlarged lymph nodes, making it a flu-like syndrome. Over the following days to weeks, the symptoms may intensify and expand to include neurological signs and heart-rhythm disturbances.

If untreated, the symptoms typically die down on their own, only to reappear several months later with shifting joint swelling and pain, possibly neurological symptoms of shooting pains, headache, and ”brain fog.”


According to our experience and the information we gathered from practicing veterinarians (see page 13), the real-life symptoms associated with Lyme infection in horses include:

• Fever (probably early infections)

• Ill-defined, shifting lameness not explained by injury or level of work

• Poor performance

• Personality changes

• Laminitis

• Anterior uveitis (ERU/moonblindness-like eye changes).

Varying degrees of insulin resistance can be found in laminitic Lyme horses. This isn’t surprising, since infections are known to induce insulin resistance in other species. When the horse is already predisposed to being insulin resistant, the insulin resistance is difficult to control by diet alone.

Diagnosing Lyme

Early Lyme symptoms, such as fever and irritability, are nonspecific and easy to miss or dismiss as a virus, work-related arthritis or various causes of muscle pain. Lyme is also likely to be put lowest on the list of possibilities if a horse is showing neurological signs, if it is considered at all.

To make matters worse, early Lyme symptoms may appear before the antibody tests are even a low positive. This makes it difficult to confirm the disease at the time when it ideally should be treated. The horse may also have antibodies from a prior exposure complicating interpretation of tests.

Horses at pasture are the least likely to be diagnosed at an early stage, while horses in active use will probably have their symptoms attributed to one or more other problems.

Fever can be an important clue, since horses with things like osteoarthritis or EPSM aren’t going to be running a fever. Persistent and/or recurrent fevers coupled with stiff or painful muscles and joints will raise the index of suspicion for Lyme before blood work changes.

The two most widely available diagnostic tests are ELISA tests for antibodies and Western Blot. Most ELISA tests use cultured organisms as their antigen source. The horse’s blood is mixed with the antigen bound to an enzyme. If there are antibodies that bind to the test, a color change occurs. Problems with this test include the inability to diagnose early infections and false positives from cross-reactions. All equivocal or positive reactions on this type of ELISA need to be confirmed by Western Blot.

Western Blot is a technique that binds antibody to antigen then uses electrophoresis to separate out the antigen-antibody complexes into specific bands, each corresponding to an antibody to a specific component of the organism. A minimum of three bands is required to call a sample positive. The more bands there are, the more strongly positive the result and likely the more chronic the infection is, since the Lyme organism is known to change its outer coat multiple times during the course of an infection.

A newcomer to ELISA testing is directed to a Lyme antigen called C6. This test was first available for human testing seven years ago. It uses a synthetic antigen that is derived from an outer surface protein called C6.

Researchers found that the C6 antigen is present in all Lyme organisms from around the world and doesn’t change when organism goes through the various mutations inside the body. The C6 antigen can also distinguish between vaccinated and actively infected animals.

Data from confirmed human Lyme infections suggests it may become positive earlier in the course of an infection that other antibody tests. There are virtually no false positives with the C6 ELISA (no cross-reactions), 37% false negatives in early disease but 0 false negatives in later stages.

This test is currently being offered by Idexx Laboratories as either an on-the-farm quick Lyme test (the SNAP 3Dx or SNAP 4 Dx — developed for dogs originally but work, for equine samples as well), or as an in laboratory C6 ELISA, which also gives a titer. The in-laboratory test is useful since it allows your veterinarian to follow the titers to see if an infection is active (rising titer) and/or if the horse is responding to treatment (dropping titer).

The IgenX Laboratory in California offers testing for Lyme such as:

• Lyme Dot-Blot Assay (LDA) ??? Uses antibodies against the organism to check for antigens in the urine. False positives possible because of cross-reactions with other organisms.

• Reverse Western Blot ??? Tests for antigen as above, but separates out the complexes into specific bands for confirmation that it is truly Lyme.

• Multiplex PCR ??? Very sensitive test that can detect either whole organisms or pieces of them by detecting bits of DNA. This test can be run on tissues, blood, urine, cerebrospinal fluid or joint fluid.


Lyme disease in horses is usually treated by intravenous tetracycline or oral doxycycline.

In a Cornell University study published in 2005 (Veterinary Microbiology), ponies were experimentally infected with Lyme organisms by infected adult ticks. Twelve weeks later four ponies were assigned to each of three treatment groups, tetracycline, doxycycline or Ceftiofur for 28 days. A fourth group was left untreated. ELISA antibody titers dropped in all the treatment groups, but began to rise again after three months in three of the four doxycycline-treated ponies and two of four Ceftiofur treated. The ponies were necropsied five months after treatment.

The untreated ponies and those that showed a rise in titer after treatment were confirmed to still be infected while there was no evidence of infection in th e tetracycline-treated ponies or ponies receiving treatment with another antibiotic whose titers did not rise again.

This study makes intravenous tetracycline the ”gold standard” treatment for Lyme disease, but tetracycline is extremely damaging to tissues if it gets outside the vein. The treatment must be administered by a veterinarian. Oral doxycycline is much easier to give, but in that study a 28-day treatment was not sufficient to kill the organism in 75% of the ponies. It’s currently unknown whether longer treatment time with doxycycline might more effective.

The ponies’ treatment was delayed until three months after the initial infection, so they were beyond the acute stage, but what about horses that might not be diagnosed for much longer’ In the human disease, the longer treatment is delayed, the worse the prognosis becomes for cure.

There is also a subset of patients that develop chronic arthritic or neurological problems that persist even with intensive and prolonged antibiotic therapy, including intravenously, and when the infection seems to have been cleared.

There is strong evidence to suggest that in some of these cases auotoimmune disease has developed. The Lyme antibodies are cross-reacting to the horse’s own tissue. In other instances, it may well be that the infection has not been completely treated.

Bottom Line

Lyme infection in horses is real. When untreated, the organisms can be found in a wide variety of body tissues for at least nine months. Muscles and joints are most often infected.

If the index of suspicion for Lyme is high, because you are in an endemic area and the horse’s symptoms are suggestive (e.g. unexplained muscle and/or joint pain, with fever), treatment may be advisable even in the absence of a positive test if other causes have been ruled out.

Whatever treatment course you and your vet decide upon, recheck the titer three months after treatment has stopped. A rising titer at this time likely means the horse still has an active infection.

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