Postmortem characterization of cranial nuchal bursa and ligament in healthy horses reveals subclinical gross and histopathologic abnormalities.

Abstract: To investigate the presence of subclinical cranial nuchal bursitis and characterize its histopathologic features and association with Borrelia burgdorferi. Unassigned: This was a prospective descriptive cadaver study on a convenience population of horses in a B burgdorferi-endemic region (15 horses: 5 geldings and 10 mares of various breeds; 4 to 29 years old). Horses without history or clinical signs of cranial nuchal bursitis underwent euthanasia and tissue donation. Cranial nuchal bursa, synovial fluid, and nuchal ligament were collected postmortem. The bursa and ligament were evaluated via histopathology, grading inflammation, edema, fibrosis, hemorrhage, and mineralization (each scored 0 = normal to 3 = severe). Lyme multiplex assay on serum and B burgdorferi polymerase chain reaction on bursa were performed. 16S rRNA gene sequencing was performed on 3 samples. Unassigned: Gross abnormalities were found in 4 horses (mineralization, thickened bursa, synovial effusion, deposits of amorphous material, and adhesions). On histopathology, inflammation was detected in 5 horses, nearly all with 1 or more additional abnormalities (edema, necrosis, fibrosis, and mineralization). The overall histopathology score ranged from 1 to 8. Five horses had positive Lyme multiplex assay antibody titers. Nuchal bursa, ligament, and synovial fluids were negative on B burgdorferi polymerase chain reaction. There was no association between histopathology score and age or Lyme multiplex titers. Unassigned: Our study identified mild to moderate cranial nuchal bursa and ligament histopathologic changes suggestive of subclinical inflammation. Unassigned: The detection of subclinical gross and histopathologic changes in clinically normal horses warrants further investigation of the predisposing causes and clinical implications.