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Equine guideLyme disease in horses
How horses get Lyme disease, the signs equine vets look for, why diagnosis is so tricky, and how treatment and tick control work.
Quick answer
Horses can get Lyme disease from infected blacklegged ticks, and exposure is common in areas where Lyme is established. Many infected horses show no signs. When illness occurs, vet sources describe joint swelling, eye inflammation (uveitis), skin nodules at bite sites and, rarely, nervous system disease. Diagnosis is hard, so work closely with your equine vet.
- AAEP: Borrelia burgdorferi infection is common in horses living in Lyme-endemic areas, and the range of exposure appears to be growing. Confirming that a horse’s illness is actually caused by Lyme remains a diagnostic challenge.
- Equine Disease Communication Center (EDCC): A positive blood test shows exposure, current or past, and does not confirm Lyme disease in a horse without signs. Treatment is not recommended for horses without signs.
- Merck Veterinary Manual: Most seropositive horses show no clinical signs.
How horses get Lyme disease
Like dogs and people, horses are infected through the bite of an infected blacklegged tick: the deer tick (Ixodes scapularis) in the Northeast and Midwest, or the western blacklegged tick on the Pacific coast. Wild animals, especially rodents, keep the bacteria circulating. The Merck Veterinary Manual notes that farm animals, horses included, are not reservoirs of the bacteria.
Signs of Lyme disease in horses
Signs in horses are often vague and can look like many other problems. Vet sources describe these better-documented signs:
| Form | What may be seen |
|---|---|
| Joint inflammation | Swollen joints (synovitis) |
| Eye inflammation (uveitis) | Painful, cloudy or watery eye, squinting |
| Skin (pseudolymphoma) | Nodules or masses where a tick bit |
| Neuroborreliosis (rare, serious) | Fever, muscle wasting, trouble swallowing or eating, skin sensitivity, wobbliness, facial weakness, behavior changes |
Owners and vets also report things like stiffness, shifting lameness, low-grade fever, weight loss, poor performance and behavior changes. But the EDCC cautions that these general signs are more often caused by other diseases, and Merck notes lameness has not been conclusively linked to Lyme. That’s why a positive test alone shouldn’t end the search for a cause.
Call your veterinarian promptly if your horse has an eye that looks painful or cloudy, trouble swallowing, weakness, wobbliness or sudden behavior changes. These signs need urgent attention whatever the cause.
Why diagnosis is difficult
- Exposure is common. In endemic areas many healthy horses test positive, so a positive result may have nothing to do with current illness.
- Signs overlap with many other equine problems.
- Direct tests are limited. The bacteria live mainly in tissues rather than blood, and Merck notes that testing spinal fluid by PCR is not very sensitive.
- Horses are often diagnosed long after infection, which Merck says makes treatment harder.
Cornell University’s diagnostic lab offers an equine Lyme multiplex test. It measures antibodies to three proteins: OspA (a vaccination marker), OspC (early infection, from about 3 weeks) and OspF (longer-term infection, from about 5 to 8 weeks). Your vet may also examine samples from affected tissue, such as a skin nodule, to support the diagnosis.
Treatment
Treatment is for horses with signs, not horses that only test positive. Vet sources list these options, chosen by your veterinarian:
- Tetracycline-class antibiotics, such as oxytetracycline (given IV), or doxycycline or minocycline (given by mouth). Merck describes a typical course of about four weeks.
- For nervous system disease, Merck notes injectable penicillin or cefotaxime is preferred.
We don’t list doses. Recovery varies, and some horses respond better than others. Cornell’s lab says vets may compare antibody levels before and after treatment; a drop of at least 50% suggests treatment worked, while flat or rising levels can point to treatment failure or reinfection.
Is there a Lyme vaccine for horses?
According to Cornell, there is no Lyme vaccine approved for horses. Some owners use canine Lyme vaccines off-label, but how well they work in horses has not been studied. Talk this through with your equine vet.
Prevention
The EDCC calls controlling tick exposure the key prevention measure. Practical steps to discuss with your vet:
- Check horses for ticks during grooming, especially after time in wooded or brushy pastures.
- Remove ticks promptly by grasping close to the skin and pulling straight out without twisting.
- Keep pasture edges mowed and create a barrier between grazing areas and brush or tall grass.
- Ask your vet about tick control products labeled for horses.
Ticks on horses and around barns can bite people too. See which ticks carry Lyme, what to do after a tick bite and Lyme prevention for people.
Frequently asked questions
Can horses get Lyme disease?
Yes. Horses can be infected through the bite of an infected blacklegged tick. Exposure is common in endemic areas, but many infected horses never show signs.
What are the signs of Lyme disease in horses?
Better-documented signs include swollen joints, eye inflammation (uveitis), skin nodules at tick bite sites and, rarely, nervous system disease. Vague signs like stiffness or lameness are more often caused by other problems.
How is Lyme disease in horses diagnosed?
Vets combine signs, tick exposure, blood tests and ruling out other causes. A positive blood test shows exposure, not necessarily disease. Cornell’s multiplex test can help separate early infection, longer-term infection and vaccination.
How is Lyme disease treated in horses?
Horses with signs are treated with antibiotics chosen by the vet, often from the tetracycline class for about four weeks. Horses that only test positive are generally not treated.
Is there a Lyme vaccine for horses?
Cornell says no Lyme vaccine is approved for horses. Dog vaccines are sometimes used off-label, but their effectiveness in horses has not been studied.