Association of Borrelia burgdorferi with nuchal bursitis and elevated outer surface protein A-specific serum antibodies in horses of the northeastern United States.

Objective: Evaluate the incidence of Borrelia burgdorferi in cases of equine nuchal bursitis (NB) and investigate the relationship between elevated serum outer surface protein A (OspA) antibodies and the molecular identification of B burgdorferi in bursal tissue or synovial fluid. Additionally, describe clinical cases and compare the histologic changes in NB with and without detection of B burgdorferi.

Methods: This was a retrospective multicenter cohort study (2013 to 2022). Medical records from horses with a diagnosis of NB and B burgdorferi PCR testing on NB tissue or synovial fluid were reviewed. The study population included 11 horses with a postmortem diagnosis of NB, 19 horses from the northeastern US with an antemortem diagnosis of B burgdorferi PCR-positive NB, and 15 healthy controls without evidence of NB and unvaccinated for B burgdorferi. Where serum was available, Lyme multiplex assay results were compared with controls and ELISAs targeting individual B burgdorferi antigens were performed. Histologic findings in nuchal bursa tissue were compared between NB cases with and without B burgdorferi PCR detection.

Results: Serum OspA antibody values in B burgdorferi-positive NB cases (n = 13) were significantly elevated (P < .001) compared to controls (15), and OspA was the predominant antigen detected by ELISA (8). Histopathology did not vary between NB cases with (n = 9) and without (6) B burgdorferi PCR detection.

Conclusions: The presence of B burgdorferi in the nuchal bursa of horses is associated with increased serum OspA antibodies.

Clinical relevance: The role of B burgdorferi in equine NB may be underestimated, and targeted therapy requires investigation.