The current knowledge of Lyme's disease leaves many unanswered questions. In the northeastern United States approximately 75% of horses are serologically positive to Borrelia burgdorferi. The vast majority of these horses will not show any clinical signs. A positive test means the horse has been exposed and may be harboring the organism but it cannot determine if the clinical signs that are present are Lyme's disease.
There have been several studies that have tried to correlate a positive test with clinical signs. One study done in Germany was unable to show a correlation between horses with uveitis and a seropositive test. Two subsequent studies examined the correlation between lameness and a seropositive test. One of these studies found horses (n=22) with clinical signs were more likely to be positive than horses (n=21) without signs. The other study found no correlation.
There are only a few reported cases of confirmed Lyme's disease in the horse. However many believe the disease is present in a subset of horses. The most common clinical signs thought to be associated with Lyme's disease are low-grade fever, stiffness and lameness in more than one limb, muscle tenderness and wasting, hyperesthesia, swollen joints and lethargy.
Neuroborelliosis has been described in four cases. These horses presented with ataxia, lumbar muscle wasting, lymphohistiocytic meningitis, radiculoneuritis, occasional fasciculations and neck stiffness. All horses had a Borrelia burgdorferi antibody titer in the CSF. There is one reported case of panuveitis in a horse in which the organism was found in the chamber fluid.
In experimental infection in ponies, there was evidence on necropsy of lymphocytic inflammation in the skin, fascia, muscle, peripheral nerves, and blood vessels. The organism could be isolated from these tissues. The synovial membrane was most commonly affected. However no clinical signs were present except for transient reaction at tick feeding sites.
What does a positive test mean? It means, the horse has been infected with the organism and developed an antibody response. It is unclear once a horse has been infected if the organism is cleared or if the horse harbors the organism without having clinical signs. The multiplex test provided by the Animal Diagnostic Lab at Cornell reports antibodies to three Borrelia antigens. OspA is expressed by the organism in the tick gut but is not expressed once the tick begins feeding. Recently it has been shown that infected and unvaccinated horses may have antibodies to this antigen. Researchers are unclear what significance this antigen has. Antibodies to OspC are present starting at 2 weeks post infection and may be present for at least 5 months post infection. OspF are first seen 6-8 weeks after infection and may persist indefinitely. The prolonged persistence of antibodies to OspF may be due to re-exposure.
Treatment consists of intravenous Oxytetracycline. The duration of treatment is not clear. Experimentally infected ponies required 28 days of intravenous oxytetracycline to clear the organism. Due to its calcium chelating and potential nephrotoxicity one should use precaution before choosing a prolonged course of intravenous oxytetracycline. Doxycycline has poor bioavailability in the horse and is unlikely to be effective by itself.
There is not an approved Lyme's disease vaccine for the horse. When an experimental recombinant OspA antigen vaccine was used in 4 ponies, all ponies had demonstrable anti-OspA antibodies at 3 weeks after vaccination. Following experimental challenge, the Borrelia organism could not be isolated from skin cultures. Whereas, the organism was found in 3 of 4 control ponies. This is the only published experiment examining vaccination in the horse. Results from this study show that vaccination with recombinant OspA protected ponies against infection after experimental challenge with B. burgdorferi-infected ticks. The canine vaccines, which are on the market, contain OspA antigen but are not the same formulation as used in the experimental model. Efficacy of these vaccines is unknown at this time.
The take home message is that a horse should not be treated on the sole basis of a positive test. The incidence of horses with clinical signs compatible with Lyme's disease is much smaller than the percent of horses that test positive. Intravenous oxytetracycline is the drug of choice. Duration of treatment is unknown but it appears that at least a 7-day course followed by doxycline should be used in suspected clinical cases. Kidney function should be monitored before, during and after treatment with intravenous oxytetracycline. It should not be used in dehydrated horses and should be diluted in saline and given slowly. The canine vaccine appears to be safe but the efficacy for preventing Lyme's disease in the horse is unknown.
