Ask the Infectious Disease Expert

What is the Latest on Lyme Disease in Horses, and What Information Do We Need to Communicate to Clients?

By Toby Pinn-Woodcock, DVM, DACVIM

Lyme disease in North America is caused by infection with the spirochete Borrelia burgdorferi sensu stricto and is transmitted to horses through the bite of infected Ixodes ticks. In horses, B. burgdorferi infection is associated with four proven syndromes: uveitis, neuroborreliosis, pseudolymphoma and nuchal bursitis. Although equine lameness is often attributed to Lyme disease, few confirmed cases of joint and tendon swelling due to B. burgdorferi exist. A recent study found no evidence of B. burgdorferi in over 100 equine limb joint specimens submitted to the Cornell Animal Health Diagnostic Center between 2007 and 2023.

CHALLENGES IN DIAGNOSING LYME DISEASE IN HORSES
Horse owners are often surprised to learn that the greatest hurdle veterinarians face when investigating Lyme disease in horses is linking clinical signs to B. burgdorferi infection. This difficulty arises for two reasons: First, some clinical signs in horses are non-specific, such as performance decline, neck pain or attitude changes, and are common in many other equine disease processes. Second, apparently healthy horses in Lyme-endemic areas frequently test positive for B. burgdorferi antibodies due to previous exposure, limiting the interpretation of serologic testing. While veterinarians may attempt to recover B. burgdorferi in samples like aqueous humor, cerebrospinal or joint fluid, the spirochete prefers to reside in the tissue, requiring surgical or post-mortem collection for polymerase chain reaction (PCR), in situ hybridization or immunohistochemistry.

As a result, veterinarians must make a diagnosis of exclusion, ruling out other diseases with similar clinical signs before presumptively diagnosing and treating horses for Lyme disease. Unwarranted antibiotic treatment for suspected Lyme disease can lead to other bacteria developing antimicrobial resistance.

NEW INSIGHTS ON NUCHAL BURSITIS AND LYME DISEASE
Recent research highlights B. burgdorferi’s role in equine nuchal bursitis (Pearson et al., 2024).1 Novel findings include elevated concentrations of outer surface protein A (OspA) antibodies in affected horses’ serum and frequent detection of B. burgdorferi in nuchal bursa fluid using PCR. While high OspA serum concentrations have been described in human Lyme arthritis cases, this is the first time such findings have been significantly associated with chronic Lyme disease in horses. Though other bacteria, such as the now-eradicated Brucella abortus, have been linked to equine nuchal bursitis, veterinarians are encouraged to consider B. burgdorferi infection in horses residing in Lyme-endemic regions.

Preventing B. burgdorferi infection in horses includes limiting access to wooded or brushy areas where ticks congregate, minimizing habitats for reservoir hosts on horse properties, applying insecticides labeled for tick control in horses, frequent grooming to remove ticks and off-label vaccination with canine Lyme vaccines. It’s important to set owner expectations in reference to the use of canine Lyme vaccines in horses, as there is limited research on their efficacy and equine antibody responses can wane after just a few months.

Research in the field of equine B. burgdorferi infection is ongoing and necessary to improve the veterinarian’s ability to diagnose, treat and prevent Lyme disease in horses.

1. Pearson EK, Guarino C, Cercone M, Divers T, Lambert J, García-López J, Johnson AL, Engiles JB, Marconi R, Smith J, Brown K, Pinn-Woodcock T. Association of Borrelia burgdorferi with nuchal bursitis and elevated outer surface protein A–specific serum antibodies in horses of the northeastern United States. J Am Vet Med Assoc. 2024; doi:10.2460/javma.24.05.0312.

For more information, visit https://www.merck-animal-health-usa.com/species/equine.

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