Presumptive Lyme disease-associated eosinophilic synovitis in a horse.
Authors: Serpa P B S, Bogers S, Byron C, Cardona G, Patton M
Journal: Journal of equine veterinary science
Summary
# Editorial Summary: Lyme Disease-Associated Joint Inflammation in a Young Horse A 1-year-old Quarter Horse presented with acute right hind lameness and synovial effusion of the tarsocrural joint, with synovial fluid analysis revealing marked eosinophilic inflammation—a notably unusual finding in equine synovitis. Serological testing confirmed both acute and chronic *Borrelia burgdorferi* (Lyme disease) infection, whilst synovial fluid PCR remained negative, suggesting a systemic immune response rather than direct joint infection. Initial treatment with phenylbutazone and oxytetracycline produced improvement, followed by 30 days of minocycline; however, recurrent bilateral tarsocrural effusion emerged eight months later, at which point synovial cytology showed macrophagic rather than eosinophilic inflammation, responding fully to a further 14-day minocycline course. This case represents the first documented report of eosinophilic synovitis associated with *B. burgdorferi* in horses, suggesting Lyme disease may trigger atypical inflammatory joint presentations beyond the classical septic or traumatic arthritides. Practitioners should consider Borrelia serology in cases of unexplained synovitis—particularly eosinophilic patterns—and recognise that extended antimicrobial courses may be warranted even when initial improvement occurs, as recrudescence can manifest months later with altered inflammatory profiles.
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Practical Takeaways
- •Consider Lyme disease (B. burgdorferi) in horses presenting with acute tarsocrural joint effusion and eosinophilic synovitis, particularly in endemic regions, even if PCR is negative.
- •Serology may be more sensitive than PCR for diagnosing B. burgdorferi-associated synovitis; combine diagnostic tests for better detection.
- •Extended antibiotic therapy (minocycline) may be necessary for Lyme disease-associated synovitis, and recurrence months later should prompt repeat serological testing and further treatment.
Key Findings
- •A 1-year-old Quarter horse presented with acute right hind lameness and synovial effusion of the tarsocrural joint with marked eosinophilic synovitis.
- •Serology confirmed evidence of acute and chronic Borrelia burgdorferi infection despite negative PCR of synovial fluid.
- •Initial treatment with phenylbutazone and oxytetracycline followed by 30 days of minocycline showed improvement, but recurrent bilateral tarsocrural effusion occurred 8 months later with macrophagic rather than eosinophilic synovitis.
- •This is the first reported case of presumptive Borrelia burgdorferi-associated eosinophilic synovitis in horses, responding to antibiotic therapy.