Hidden in the Bones: Widespread Osteolysis Reveals a Rare Manifestation of Equine Sarcoidosis

Widespread Osteolysis as an Uncommon Manifestation of Equine Sarcoidosis

Devon DiBello, Nicolette Cassel, Viktoria Granacka, Laurie Beard, Stephanie Cruz Rosado, Kristen Hill-Thimmesch, Haileigh Avella

Background

Equine sarcoidosis is an uncommon multisystem granulomatous disease in horses that most commonly affects the skin but may involve multiple organs. Musculoskeletal involvement has been rarely reported, and prior studies have not clearly correlated imaging findings with histopathology. This report describes a case of a horse with progressive equine sarcoidosis in which widespread osteolytic bone lesions were detected on imaging and later confirmed histologically as granulomatous inflammation, suggesting that osseous involvement may be an underrecognized manifestation of the disease.

Methods

This imaging diagnosis report describes a single case involving a 15-year-old Quarter Horse mare presented for chronic left forelimb lameness and diffuse dermatitis. Initial evaluation included physical examination, blood work, and radiography of the distal limbs. Additional diagnostics included serum protein electrophoresis, thoracic and abdominal ultrasound, skin biopsy, follow-up appendicular radiographs, post-mortem computed tomography (CT) of selected limbs, necropsy, and histopathologic examination of multiple tissues and bone samples to correlate imaging findings with pathology.

Results

Initial radiographs revealed bilateral navicular degeneration and multiple discrete osteolytic lesions within the phalanges. Subsequent radiographs identified extensive polyostotic lytic lesions affecting multiple long bones, some associated with cortical thinning, expansion, and mild periosteal reaction. Post-mortem CT confirmed widespread osteolysis involving cortical, corticomedullary, and medullary regions of numerous bones. Histopathology demonstrated granulomatous inflammation composed of histiocytes, occasional multinucleated giant cells, and fibrous stroma replacing normal bone at sites corresponding to the radiographic and CT lesions. Necropsy also revealed widespread granulomatous disease affecting the skin, mesentery, peritoneum, skeletal muscle, colon, nervous system, and lymphatic tissues.

Limitations

This report describes a single case, limiting the ability to determine the prevalence or clinical significance of osseous involvement in equine sarcoidosis. Additionally, complete sampling of all skeletal lesions was not feasible due to the size of the horse, meaning the full extent of bone involvement may have been underestimated.

Conclusions

The case demonstrates that widespread osteolysis can occur in equine sarcoidosis and may correlate directly with granulomatous inflammation in bone. These findings suggest that skeletal lesions may represent an underrecognized manifestation of the disease. Recognizing this imaging pattern could aid diagnosis and improve understanding of disease progression, particularly in horses with systemic granulomatous disease. Further investigation is needed to determine the prevalence, clinical impact, and fracture risk associated with osseous involvement in equine sarcoidosis.

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