Lyme disease, caused by the organism Borellia burgdorferi is a potentially devastating disease in horses. It’s difficult to diagnose and even more difficult to treat. Symptoms overlap other conditions, which can lead to delay in treatment and decrease the odds of cure.
Horses are infected when bitten by a tick carrying the Lyme organism. The longer the tick remains attached, the higher the risk that the horse will be infected. The earliest symptoms include fever, lethargy, lymph-node swelling, possibly achiness. These are easily missed or written off as a ”virus.” Over the following weeks, symptoms will disappear, although they may briefly intensify to include vague neurological signs or heart rhythm abnormalities.
If not treated in these early phases, the infection will go dormant for a few months then reappear, often as shifting joint pains. In humans, headache and neurological symptoms are also common. It’s important to remember that once the disease has reached the arthritis stage, it’s more difficult to eradicate. Unexplained and often severe laminitis is another possible symptom. Symptoms may come and go, often in a spring and fall pattern.
To make matters worse, diagnostic tests aren’t perfect. Because most are based on antibodies, they only tell if the horse has been exposed, not if it’s currently infected. The IgG class of antibodies, tested by mixed ELISA and Western Blot (see chart) are slow to drop to 0 even if the horse was successfully treated. The CDC’s official recommendations regarding how to read a Western Blot differ from the opinions of many expert veterinarians. ELISA test results are often inconclusive. Titers over 1:110 but under 1:440 are read as ”equivocal.” Changing titers are most useful in determining if a horse has active disease and seeing a positive response to treatment.
For diagnosing a horse in the early stages (when horse is running a fever), the SNAP C6 and/or IgM ELISA are ideal. If negative at 2 weeks, wait a few more weeks and retest. Unexplained fevers should never be ignored, especially without coughing or nasal discharge. A horse showing clear signs of joint swelling can be PCR tested using the joint fluid.
Bottom Line. In our previous article on Lyme disease (September 2007), we surveyed veterinarians regarding the most common signs they saw for Lyme. Sensitivity to touch and shifting joint issues not related to level or type of work were at the top of the list, suggesting most horses aren’t diagnosed until their infections have become deep seated and chronic.
Researchers haven’t been able to reproduce Lyme symptoms with experimental infections, but they have successfully infected ponies. Intravenous tetracycline was far more effective in eliminating early infections than intramuscular Ceftiofur or oral doxycycline, but oral doxycycline continues to be the treatment of choice for price and convenience.
Interestingly, doxycycline has anti-inflammatory effects in joints. It’s possible the positive response that most horses show to doxycycline is more related to this than eliminating the organism. The duration of antibiotic treatment used in the study was 28 days, but these were ponies only recently infected with Borellia.
If the horse is showing joint symptoms, you may need longer than just 28 days of oral doxycycline to get it under control and you need to understand that symptoms may return. Lyme disease is treatable, but not always curable.
Article by Veterinary Editor Eleanor Kellon, VMD.