Presumptive Lyme disease-associated eosinophilic synovitis in a horse

A 1-year-old American Quarter horse was presented with acute onset of right hind lameness. On physical examination, there was synovial effusion of the right tarsocrural joint. Synovial fluid cytology revealed a marked eosinophilic synovitis. Serology indicated evidence of acute and chronic infection with Borrelia burgdorferi, although PCR of the synovial fluid was negative. The filly was treated with phenylbutazone and oxytetracycline, and repeated synovial cytology indicated improvement. The filly was discharged with a prescription of minocycline for 30 days. Despite initial improvement, recurrent lameness with bilateral tarsocrural effusion without radiographic abnormalities occurred 8 months later. Repeated synovial cytology showed macrophagic synovitis without an eosinophilic component. The filly was discharged with instructions to complete a 14-day course of minocycline, resulting in complete recovery. Based on the serology results and response to therapy, this report describes a possible naturally occurring eosinophilic synovitis with likely involvement of B. burgdorferi, a condition previously unreported in horses.