Widespread exposure but relatively rare clinical disease makes Lyme disease diagnosis and treatment challengingMany horses in endemic areas test positive for Lyme antibodies without ever developing clinical illness. | Adobe stockLyme disease often leaves horse owners and veterinarians with more questions than answers. Caused by the bacterial spirochete Borrelia burgdorferi and transmitted by black-legged ticks (Ixodes spp), the infection is common in many regions of North America. Yet the jump from exposure to true clinical disease in horses remains inconsistent, controversial, and at times unclear.Priscila Serpa, DVM, MSc, DSc, Dipl. ACVP, assistant professor at Cornell Universitys College of Veterinary Medicine, in Ithaca, New York, says equine exposure to B. burgdorferi happens far more frequently than manifestation of illness. Figuring out whether a horse with subtle or nonspecific clinical signs is infected or merely exposed continues to be a core challenge.Clinical Signs of Lyme DiseaseWhen B. burgdorferi exposure does lead to clinical illness in horses, the disease most often presents as vague musculoskeletal, neurologic, or ophthalmic issues or behavioral changes, rather than a single defining sign.These manifestations differ from those seen in humans and dogs and frequently overlap with many other equine conditions. Horses rarely exhibit the acute fever and lethargy common in dogs, or the bullseye rash (erythema migrans) seen in humans, says Serpa. Chronic lameness and stiffness are far more prominent in horses than in dogs, where acute lameness and kidney disease are classic. Neurologic and ophthalmic signs are more notable in horses than in dogs. She adds that in humans, arthritis caused by Lyme disease typically involves a single joint, while in horses, its often a shifting polysynovitis affecting multiple joints.In its B. Burgdorferi Infection and Lyme Disease Guidelines the American Association of Equine Practitioners (AAEP) lists neurologic impairment (neuroborreliosis), ocular inflammation (uveitis), nodular skin lesions (cutaneous pseudolymphoma), and inflammation of a fluid-filled sac along the crest of the neck (nuchal bursitis) among the most documented clinical signs of Lyme disease.Neuroborreliosis involves nervous system inflammation and can produce a range of signs in horses such as the behavioral changes mentioned, head tilt, difficulty swallowing (dysphagia), cranial nerve deficits, and ataxia (incoordination). Uveitis leads to eye pain, tearing, light sensitivity, or more profound vision changes. Cutaneous pseudolymphoma refers to the benign lesions that can develop at or near tick bite sites. More recent research now supports a connection between B. burgdorferi and nuchal bursitis in some horses (more on this on page 25), an inflammatory condition of the structure cushioning the nuchal ligament.Black-legged ticks (Ixodes scapularis) are found across the eastern U.S., Midwest, and Canada. The greatest risk of being bitten exists in the spring, summer, and fall. | Adobe stockTiming adds another layer of complexity. After exposure to the agent, says Serpa, it can take three to six weeks for horses to develop antibodies when experimentally infected. The incubation timehow long it takes to develop symptomsis unpredictable, and infected horses might never show symptoms.Here are the signs Serpa says owners might notice:Musculoskeletal Reluctance to move and muscle wasting or twitching. The AAEP guidelines note that limited evidence exists to document stiffness, lameness, and malaise in horses as a result of B. burgdorferi infection, but these signs might be possible.Behavioral and neurologic lethargy, poor performance, irritability, difficulty swallowing, ataxia, and facial paresisOphthalmic squinting, tearing, cloudy eyes, redness of the scleraSystemic low-grade fever and weight lossThe AAEP guidelines note that while no Lyme vaccines are licensed for use in horses, reasonable evidence suggests vaccination with canine recombinant outer surface protein A (OspA) vaccine or the whole-cell vaccines are at least partially protective. Specific protocol and protective antibody levels have not been established.How Common Is Lyme Exposure?Researchers on serologic surveys in horses have found high exposure rates in endemic areas. For decades veterinarians have recognized that asymptomatic horses in certain areas of the U.S., such as the Northeast, Midwest, Mid-Atlantic, and Pacific Northwestanywhere Ixodes spp liveoften have positive antibody tests for B. burgdorferi, says Serpa. Some researchers believe many horses have been exposed to B. burgdorferi, even though clinical signs appear uncommon. This mirrors research in other species, where exposure appears widespread, while only a small subset of exposed individuals ever develop clinical disease.In a landmark comparative immunoassay study completed in Germany (Broeckl, 2024), researchers evaluated three serologic tests to detect B. burgdorferi-specific antibodies in clinically healthy vaccinated (with the canine vaccine off-label) and nonvaccinated horses. The researchers showed multiple assay platforms can detect exposure with varying accuracy and highlighted the complexity of interpreting serologic data.Interpreting Serologic TestingInterpreting serologic testing for Lyme disease remains challenging because, again, many horses in endemic regions have antibodies to B. burgdorferi without clinical illness, while true clinical disease affects only a small fraction.Serpa says veterinarians often use Lyme multiplex assays that identify antibodies to specific outer surface proteins (OspA, OspC, OspF), to help them estimate the timing of exposure and whether it might be ongoing. However, positive results, even with detailed profiling, still need clinical correlation. Serology alone does not confirm that clinical signs are caused by B. burgdorferi infection, she explains, noting a veterinarians clinical judgment and case context remain crucial.Nuchal Bursitis and Lyme InfectionIn a 2024 multicenter cohort study, scientists analyzed horses with nuchal bursitis. They found those with B. burgdorferi in the bursal fluid or tissue also had significantly higher blood serum antibodies against OspA than healthy control horses.Outer surface protein A antibodies are often associated with early infection or vaccination in other species but, in this context, where the horses had not been vaccinated, elevated serum OspA levels correlated with B. burgdorferi detection in fluid and tissue, suggesting the organism might play a larger role in some cases of equine nuchal bursitis than previously appreciated. Importantly, the researchers found similar histologic (visible under the microscope) inflammation in bursitis cases regardless of whether they detected B. burgdorferi, confirming bursitis pathology (disease or damage) alone isnt specific to Lyme infection.This work supports B. burgdorferis involvement in some cases of Lyme disease in horses and underscores the need for clinicians to include Lyme disease in differential diagnoses when evaluating poll-region swelling and neck pain.In a 2025 case report Serpa and colleagues describe a 1-year-old American Quarter Horse with acute hind-limb lameness, marked eosinophilic synovitis (joint inflammation with fluid containing an excess of eosinophils, a type of white blood cell), and positive serology for B. burgdorferi. Polymerase chain reaction testing (PCR, a method to detect bacterial DNA) of synovial fluid was negative. However, the horse improved with a treatment of Bute, minocycline, and oxytetracycline, and clinical signs recurred in a pattern consistent with ongoing disease. Based on the serology and clinical course, Serpa and her coauthors described this as a possible naturally occurring B. burgdorferi-associated synovitis, a condition previously unreported in horses. She says their report illustrates both the clinical variability and the diagnostic challenges of Lyme in horses. Positive serology and clinical response to treatment can be clues, even when direct molecular detectionas with the synovial fluid hereisnt available or conclusive.Indeed, clinical response to treatment can provide useful context, but it does not confirm disease cause on its own, particularly when medications used to treat Lyme also have anti-inflammatory effects. In other words, improvement of clinical signs alone should not drive antibiotic use.What This Means for Horse Owners and VeterinariansEquine researchers continue to advance their understanding of how B. burgdorferi infection manifests, how serology can be interpreted, and when Lyme might be playing a role in a horses health problems. However, no single definitive test or pathognomonic sign (specific to a certain disease) clinches the diagnosis without clinical signs of the disease.For people navigating these questions, several practical points emerge, says Serpa:Exposure is common, but disease is not. Many horses in endemic areas test positive for Lyme antibodies without ever developing clinical illness.A positive test alone does not justify a diagnosis or treatment. Veterinarians must interpret serologic testing alongside clinical signs, exposure risk, and overall case context.Lyme remains a differential diagnosis, not a default explanation. Documented associations exist between B. burgdorferi and clinical syndromes such as neuroborreliosis, uveitis, pseudolymphoma, and nuchal bursitis, but the conditions are not specific to B. burgdorferi infection.Clinical response matters. In the absence of definitive organism detection, clinical improvement with appropriate therapy after other causes are ruled out can be an indicator of successful management.The growing body of research offers perspective for veterinarians and horse owners but reinforces the importance of clinical judgment, comprehensive workups, and careful interpretation of serology when Lyme is in question.Take-Home Message Lyme disease exposure is common in horses living in endemic regions of North America, but true clinical illness remains relatively uncommon and difficult to confirm. Early recognition, thoughtful diagnostics, and realistic expectations remain essential for managing suspected Lyme disease in horses.This article is from the Spring 2026 issue ofThe Horse: Your Guide to Equine Health Care. We at The Horse work to provide you with the latest and most reliable news and information on equine health, care, management, and welfare through our magazine and TheHorse.com. Your subscription helps The Horse continue to offer this vital resource to horse owners of all breeds, disciplines, and experience levels. To access current issues included in your subscription, please sign in to theAppleorGoogleapps ORclick herefor the desktop version.