Multifocal discrete osteolysis in a horse with silicate associated osteoporosis

Silicate associated osteoporosis (SAO) was diagnosed post mortem in an adult horse with the shortest documented exposure to cytotoxic silicates of 2 years. The horse was evaluated for a 6-months history of progressive back tenderness and acute onset of lameness. The horse had a marked (4/5) [American Association of Equine Practitioners scale] left forelimb lameness, moderate (2/5) hindlimb ataxia and weakness, and cervical pain upon palpation. Physical examination did not reveal clinical skeletal deformities or respiratory compromise. Radiographs revealed widespread, discrete, sharply delineated, osteolytic lesions in the skull, vertebral column, ribs, scapulae and middle phalanx (P2) of the left forelimb and a diffuse bronchointerstitial lung pattern. The presumptive clinical diagnosis was widespread, metastatic osteolytic neoplasia. Due to the poor quality of life and grave prognosis, the horse was humanely euthanised. Post mortem examination revealed pulmonary silicosis in the lungs and hilar lymph nodes and osteolytic lesions with numerous, large osteoclasts and disorganised bone remodeling both consistent with SAO. SAO should be included as a differential diagnosis for horses with widespread, multifocal, discrete osteolysis and history of exposure to endemic regions with possible cytotoxic silicate inhalation. Exposure time of 2 years is potentially sufficient to develop SAO.