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

Overview

• This study examined the cranial nuchal bursa and ligament in healthy horses to detect subtle abnormalities and explored whether Lyme disease bacteria (Borrelia burgdorferi) are involved.
• Researchers found mild to moderate inflammation and other tissue changes in horses without symptoms, indicating subclinical (hidden) changes that may have clinical relevance.

Study Purpose and Background

• The cranial nuchal bursa and ligament are anatomical structures in the horse’s neck that can be prone to inflammation (bursitis).
• Clinically apparent cranial nuchal bursitis causes neck pain and dysfunction, but subclinical (no outward signs) inflammation has not been well characterized.
• The study also aimed to evaluate the role of Borrelia burgdorferi, the bacteria responsible for Lyme disease, since it is known to cause joint and tissue inflammation in horses in endemic regions.

Study Design and Methods

• Fifteen horses of various breeds and ages (4 to 29 years old), all clinically normal without any history of neck bursitis, were euthanized for reasons unrelated to this study.
• Tissue samples were collected postmortem from the cranial nuchal bursa, synovial fluid within the bursa, and nuchal ligament.
• Samples underwent detailed gross (visible) and microscopic histopathologic examination, focusing on inflammation, edema, fibrosis, hemorrhage, and mineralization, each scored on a scale from 0 (normal) to 3 (severe).
• Blood sera were tested with a Lyme multiplex assay to detect antibodies against Borrelia burgdorferi, indicating possible exposure or infection.
• Polymerase chain reaction (PCR) testing for Borrelia burgdorferi DNA was performed on bursa, ligament, and synovial fluid samples to detect active infection.
• 16S rRNA gene sequencing, a broad bacterial detection method, was conducted on three samples to further explore microbial presence.

Key Findings

• Gross abnormalities were observed in 4 out of 15 horses, including mineralization (calcium deposits), thickening of the bursa, excess synovial fluid (effusion), amorphous material deposits, and adhesions.
• Histopathology revealed inflammation in 5 horses, often along with other abnormalities such as edema (fluid accumulation), tissue necrosis (death), fibrosis (scar tissue), and mineralization.
• Overall histopathology severity scores ranged from mild (1) to moderate (8), indicating subclinical tissue changes.
• Five horses had positive antibody titers for Lyme disease, suggesting past or present exposure to Borrelia burgdorferi.
• However, PCR testing for Borrelia burgdorferi DNA in bursa, ligament, and synovial fluid was negative in all horses, implying no active infection in these tissues.
• No correlation was found between histopathology severity and either the horse’s age or Lyme disease antibody status.

Interpretation and Implications

• Healthy, clinically normal horses can harbor mild to moderate tissue changes in the cranial nuchal bursa and ligament indicative of subclinical inflammation.
• These findings suggest that some degree of inflammation or structural alteration may be common and undetected in horses without symptoms.
• The absence of Borrelia burgdorferi DNA in affected tissues despite positive antibody tests indicates that while exposure to Lyme disease bacteria occurs, it might not directly cause these subclinical changes.
• The study highlights the need for further research to determine what predisposes horses to these subclinical lesions and what potential clinical impacts they might have over time.
• Understanding these subclinical changes could improve the diagnosis and treatment of neck pain and inflammation in horses before overt clinical signs develop.