Tick season will be starting up again soon and with it the risk of Lyme Disease. The first report of horses carrying antibodies to the organism causing Lyme Disease, Borrelia burgdorferi, appeared in the scientific literature 35 years ago. Skepticism abounded regarding Lyme Disease (LD) actually existing in horses (antibodies only confirm exposure, not infection or disease) and continues to some extent today.
The disease is spread by the bite of infected Ixodes/hard-shelled ticks. In people, the first symptom is often a rash that expands around the bite site and resembles a bull’s eye target. This has not been observed in horses. Fever, fatigue and body aches may occur or there may be no other signs. Treatment at this stage can be curative.
Untreated infections can progress to more serious problems as the organism migrates to tissues other than the skin. It has an affinity for the joints and nervous system. It may also attack the eyes or heart.
LD is most frequently suspected in horses which show unexplained lameness/arthritis and stiffness that shifts in locations. This is the most common symptom of late LD in people. However, in horses these cases are often poorly documented as having been definitely caused by B. burgdorferi. A particular severe form of laminitis has also been suspected to be caused by LD and it may invade tendon sheaths.
There are more confirmed cases of LD involving the eyes (uveitis) and nervous system (neuroborreliosis) in horses. A skin condition, cutaneous pseudolymphoma, has been found in infected horses.
Diagnostic efforts usually involve blood tests for antibodies. Problems with this include cross-reaction with other infections in some assays, trouble differentiating between active infection versus past exposure, and false positives from vaccination. One of the best tests for sorting through these issues is the Lyme Multiplex https://www.vet.cornell.edu/animal-health-diagnostic-center/testing/protocols/lyme-multiplex-horses .
The organism is a bacterium of the spirochete type and treatment is with antibiotics. Unfortunately, by the time a horse is diagnosed they are in the advanced disseminated phase when actual cure is extremely difficult and even when remission of signs is achieved the horse may need to be treated again in the future.
Researchers have been unable to experimentally induce LD in horses using deliberate exposure to infected ticks. Although the infection was successful, no clinical signs developed. This makes it very difficult to study the effectiveness of various antibiotics. However, in one trial of experimental infection, in treatment started 3 months after the start of the infection only 4 weeks of intravenous tetracycline was successful in eliminating the organism. Half of the test ponies treated with intramuscular ceftiofur and 25% of those treated with oral doxycycline also cleared the infection.
Four weeks of intravenous treatment is both impractical and prohibitively expensive for most owners. Unless dealing with a serious neurological case, most practitioners rely on treatment with oral doxycycline or minocycline which is usually well tolerated. Some will start with a few days to a week of the intravenous tetracycline. Response is typically good although, as above, it may need to be repeated in the future.
Protective measures include keeping your horse’s environment closely mowed and free of brush. Use tick repellant chemicals especially when venturing into areas that may harbor ticks. Permethrin is best and also an excellent fly repellant. There are no equine LD vaccines. Veterinarians may use canine vaccines in horses but the protection has not been carefully evaluated. A typical course for best antibody levels is a 2 injection initial series with boosters at 6 month intervals. The Multiplex assay can be used to monitor vaccine titers.
Lyme is a complex disease with potentially serious consequences. Speak with your veterinarian about the best program for protection and monitoring in your area.
Eleanor Kellon, VMD