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 these horses have an infection that will cause obvious signs of Lyme disease is unknown. However, skepticism over whether or not Lyme disease exists in horses is long gone. It's real.
The progression of Lyme disease 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
• 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.
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.