California horses incur a bone fragility syndrome manifested by pathologic fractures. This study investigated gross, radiographic, and histologic features of the disorder as well as relationships with silicosis and levels of heavy metals and trace minerals through a postmortem study of 9 affected and 3 unaffected horses. Bones and soft tissues were evaluated grossly and histologically. Bones, lymph nodes, and lung tissue were evaluated radiographically. Tissues were evaluated for silicon levels, intracytoplasmic crystals, heavy metals, and trace minerals. All 9 affected horses had osteoporosis and clinical or subclinical pulmonary disease due to silicosis (8/9) or pneumoconiosis (1/9). All affected horses had radiographic findings consistent with osteopenia and histologic evidence of osteoporosis characterized by osteopenia, numerous resorption cavities, cement lines, and a mosaic lamellar pattern indicative of multiple remodeling events. Silicosis was characterized by widespread pulmonary granuloma formation with fibrosis; variable tracheobronchiolar and mediastinal granulomatous lymphadenitis; intracellular crystals within lung and lymph node macrophages; and pronounced lymph node fibrosis, focal necrosis, and dystrophic calcification. Crystals in lung (6/9) and lymph node (8/9) tissues were identified as cytotoxic silica dioxide polymorphs. Lung and liver tissue from affected horses had elevated levels of elemental silicon. Osteoporosis was highly correlated (r = 0.8, P < .01) with silicosis. No abnormalities in heavy metal or trace minerals were detected. This evaluation indicated that horses with bone fragility disorder have systemic osteoporosis associated with fibrosing pulmonary silicosis. The etiopathogenesis of the bone fragility syndrome is unknown; however, this study provides circumstantial evidence for a silicate associated osteoporosis.

Treatment with zoledronate appeared to be useful in improving clinical outcome and scintigraphic findings in horses with a BFD; however, future placebo-controlled studies are necessary to accurately determine efficacy and long-term safety.

Results suggested that horses with a BFS had osteopenia and compromised trabecular bone function, consistent with bone deformation and pathological fractures that develop in affected horses. Effects of this BFS may be systemic, and bones other than those that are clinically affected had changes in morphological and mechanical properties.

Scintigraphy remained the most informative diagnostic modality for BFS, providing insight into disease severity and distribution; however, physical examination and scapular ultrasonographic abnormalities were diagnostic in horses with moderate to severe disease. Proposed severity indices classified the spectrum of disease manifestations. Clearly defined criteria for interpretation of diagnostic tests aid in the detection of BFS. Severity indices may be useful for assessing disease progression and response to treatment.