Signalment:  

Nineteen-year-old quarter horse mare (Equus ferus caballus).The horse presented to the teaching hospital for severe acute onset of right front limb lameness, from Middletown in Lake County, California. According to the owner, abnormal posturing began two weeks prior to presentation, and the horse was described as tucking its hind legs under itself. At that time, titers were negative for equine protozoal myeloencephalitis. Physical exam revealed toe dragging lameness of the right forelimb and asymmetric shoulder muscle atrophy, most pronounced on the right side. Radiographs of the proximal aspect of the right forelimb demonstrated a lytic bone lesion in the proximal scapula. In addition, multifocal heterogeneous mineral dense nodules were observed following the trachea from the thoracic inlet (cranial mediastinal lymph nodes), to the hilar region (tracheobronchial, caudal mediastinal lymph nodes). Ultrasound of the right scapula identified a severely irregular bone margin with mixed echogenicity, and soft tissue masses within the overlying skeletal muscle. A soft tissue sarcoma was suspected based on results from a fine needle aspirate of the masses.

Gross Description:  

Gross examination of the right scapula identified moderate hemorrhage and edema within the surrounding soft tissue as well as exuberant periosteal callus formation associated with a complete transverse, non-displaced fracture of the scapular neck. The skeletal muscle proximal to the fracture callus was diffusely atrophied, firm, pale, and intersected by bands of fibrosis. A second, more chronic complete fracture of the left supraglenoid tubercle was also identified. There were multifocal to coalescent regions of circum-ferential, porous cortical expansion of the ribs. Multiple, well-demarcated areas of bright red cortical discolouration were noted in the humeri. These lesions corresponded to regional osteolysis on clinical and post-mortem radiographs. The mineralized masses within the thorax were identified as enlarged, mineralized mediastinal and tracheobronchial lymph nodes

Histopathologic Description:

Bone: The slide contains a single section of demineralized bone that lacks cortico-medullary distinction and consists of variably oriented and shaped, anastomosing osteochondral trabeculae. The section is embedded within and partially encased by a regionally necrotic and inflamed fibrovascular stroma. Dense fibrous and mildly congested tissue interdigitates with trabeculae at one section margin (presumed periosteal surface). Eighty percent of the bony trabeculae are composed of immature (woven) bone, and some contain cartilage. Lamellar bone is only preserved within larger trabeculae. Sharp edges, numerous, coalescing resorption bays (Howship’s lacunae), and variation in trabecular shape and size is a result of resorption by large, hyperactive osteoclasts present in large numbers throughout the section. Osteoclasts contain up to 30 nuclei with often prominent eosinophilic nucleoli. Osteoclastic nuclei are occasionally arranged into circles resembling Langerhans giant cells.  Osteoclasts are particularly abundant around the necrotic focus engulfing small shards of necrotic bone (presumed region of previous trauma) and some are noted within the surrounding stroma not adhered to the bone. Hyperactive osteoblasts accompany the osteoclasts and line most of the bone surfaces. A mosaic pattern of cement lines indicate ongoing dysregulated bone remodeling characterized by exuberant, seemingly random osteolysis and compensating osteoblastic activity. Inter-trabecular spaces are largely devoid of hematopoietic elements and filled by edematous, congested and mildly inflamed fibrous connective tissue. Inflammation consists of plasma cells, lymphocytes, and macrophages. Macrophages occasionally contain granular brown to golden cytoplasmic pigment (hemosiderin). Thin interlacing trabeculae of woven bone extend outwards from the presumed periosteal surface into the surrounding dense fibrous tissue (periosteal new bone formation vs callus).

Morphologic Diagnosis:  

Bone, scapula: Severe, chronic, multifocal to coalescing osteolysis with atypical osteoclasts, aberrant bone remodeling, medullary fibrosis, and multifocal necrosis (consistent with silicate associated osteoporosis).

Lab Results:  

Biochemistry: (normal range)

Mild increase in creatinine kinase 348 IU/L (119-287)

Condition:  

Equine bone fragility syndrome

Contributor Comment:  

Histologic examination of the tracheobronchial lymph nodes confirmed the presence of numerous fibrosing and mineralizing granulomas that contained moderate amounts of birefringent crystalline material. This finding, in conjunction with the osteoporotic skeletal lesions, and the geographic origin of the horse (from a location known to contain toxic soil silicate dioxide (cristobalite)) are consistent with a diagnosis of silicate associated osteoporosis (SAO). Without additional clinical information and knowledge of the bone sample skeletal site, differential diagnoses for this histological slide should broadly include a metabolic bone disease. For example, hyperactive osteoclasts and poor construction of new bone may be seen with fibrous osteodystrophy as a result of Ca++/P imbalance, variable nutritional deficiencies, vitamin D deficiency, and endo-crinopathies.⁴

Pulmonary silicosis has been previously reported in both humans and horses, however, the associated osteoporotic syndrome appears unique to equids.¹ The underlying pathogenesis for this condition has yet to be elucidated. Affected horses present with vague acute or chronic signs of lameness, often accompanied by weight loss, and variably by clinically evident pulmonary disease. Horses may appear to have bowing of one or both scapulae and accentuated lordosis. Acute lameness may be observed secondary to pathologic fractures often with ineffective attempts at repair as observed in this case. If clinically silent, chronic fibrosing granulomas within the lung-draining lymph nodes and variably within the lungs are usually identified postmortem. Thoracic radiographs may detect an interstitial pattern of consolidated pulmonary nodules, but will often miss mineralized, lung-associated lymph nodes. Histologic examination of the granulomas reveals the typical pattern of central necrosis, marked fibrosis and mineralization surrounded by epithelioid macrophages and occasional giant cells. Small, angular, birefringent, intra- and extra-cellular crystals may be revealed under polarized light. The exorbitant reaction associated with such crystals is unique to the toxicity of cristobalite as compared to relatively innocuous accumulations associated with common anthracosilicotic nodules. Electron diffraction crystallography has been used to confirm the physical characteristics of crystals as cristobalite, (technique available in geology laboratories that analyze soil or stones).

Clinical and postmortem radiographs of the axilla and proximal appendicular skeleton are non-specific but may reveal poorly demarcated areas of osteoporosis and confirm bone deformity. Finding cervical facet joint osteoarthrosis is common in advanced cases of SAO. The radiographic appearance of bone lesions is variable but can be mistaken for neoplasia driven osteolysis as observed in this case. Currently, the most sensitivity pre-mortem diagnostic test for this condition is bone nuclear scintigraphy.⁴ The search for sensitive and specific clinical markers to detect early onset of the disease is ongoing.

Histologic examination of the affected bone reveals dysregulated resorption by morpho-logically atypical giant hyperactive osteoclasts.¹ Typical of SAO, the osteoclasts indiscriminately resorb pre-existing cortical and trabecular bone as well as newly formed woven bone. A mosaic pattern of cement lines is commonly observed in SAO bones; this microscopic feature is shared by human Paget disease of bone.⁶ Multifocal areas of bony lysis, aberrantly increased regions of bone remodeling, pathologic fractures and aberrant giant osteoclasts are other similarities observed between these two disorders.¹ However, polyostotic Paget disease is restricted to specific age groups (elderly humans) and exhibits cessation of osteoclast activity over time. In contrast, SAO is observed in horses of all ages and shows progression of disease with time.^1,5 In addition, the skeletal distribution of SAO does not correspond with that observed in Paget disease.¹ The lesions of SAO have also been compared to fibrous osteo-dystrophy, however, skull lesions are uncommon in horses with SAO and only a subset of affected horses exhibit elevations in parathyroid hormone.¹

Unusual features of this case include the effacement of hematopoietic tissue within the medullary cavity and the identification of gross and radiographic lesions with the humeri.

JPC Diagnosis:  

Bone: Abnormal bone remodeling with myelofibrosis, proliferation of numerous large atypical osteoclasts, and focally extensive necrosis, quarter horse, Equus ferus caballus.

Conference Comment:  

This challenging case confounded even the most senior and experienced conference participants. Due to the lack of a recognizable corticomedullary junction or physiologic tissue border combined with severe distortion of the tissue by the disease process, attendees were unable to determine the type of bone this tissue section represented. Participants were impressed by the extensive and aberrant bone remodeling and resorption by atypically large and randomly located osteoclasts with vacuolated foamy cytoplasm and up to 30 supernumerary nuclei. These nuclei are sometimes arranged in circles resembling Langerhans giant cells in this section of bone. The constellation of lesions in this case combined with the historical data raise the index of suspicion for equine bone fragility syndrome (BFS), also known as silicate associated osteoporosis (SAO), although, as mentioned by the contributor, other metabolic bone diseases should also be considered as differential diagnoses.^1,2 

The pathogenesis and cause of bone fragility syndrome in horses is unknown. The vast majority of affected horses present concurrently with pulmonary silicosis and there is likely a causal relationship between the two conditions. As mentioned by the contributor, pulmonary silicosis, defined as silicate pneumoconiosis with accompanying pulmonary fibrosis, occurs secondary to inhalation of cytotoxic silica dioxide (SiO2) crystal polymorphs, including quartz, cristobalite, and tridymite.^1,2,3 Pulmonary silicosis with concurrent bone fragility syndrome has been reported with increased frequency in horses from areas with high levels of soil cristobalite, such as Monterey, Napa, and Sonoma regions of California; however, affected horses have also been seen in Oregon, Texas, Virginia, Illinois, and Kentucky.^1,2,3 The cytotoxic crystals associated with pulmonary silicosis are found worldwide, perhaps suggesting that this disease may be more widespread.^1,2

Readers are encouraged to review 2015 Wednesday Slide Conference #3 Case 4 for an excellent review of a suspected case of pulmonary silicosis in a horse from Monterey, California. It is thought that the inhaled silicate stimulates the massive release of proinflammatory cytokines, IL-1, IL-6, and TNF-alpha, which stimulate inflammation and osteoclastogenesis via increased production of the RANKL and decrease in expression of the decoy receptor, osteo-protegerin.¹ Other proposed mechanisms of pathogenesis include hyperparathyroidism or an equine variant of Paget disease of bone in humans.^1,2

Horses with bone fragility usually present with chronic lameness and skeletal deformities such as lateral bowing of the scapulae, lateral bowing of the rib cage, lordosis, and decreased range of motion in the cervical vertebrae.^1,2 Bones of both the axial and proximal portions of the appendicular skeleton, such as the scapula, in this case, are typically affected. Radio-graphically, there is severe osteopenia with multiple bony lucencies and exostoses at the articular facets as well as thickening of the rib consistent with extensive bone remodeling.^1,2 As a result of the severe osteoporosis associated with this disease entity, most animals die from a catastrophic pathologic fracture.^1,2 Conference participants noted that the large focally extensive area of necrosis in this tissue section may be the site of a pathologic fracture of the scapula.

References:

1. Arens AM, Barr B, Puchalski SM, et al. Osteoporosis associated with pulmonary silicosis in an equine bone fragility syndrome. Vet Pathol. 2011; 48(3):593-615.
2. Arens AM, Puchalski SM, Whitcomb MB, et al. Comparison of the use of scapular ultrasonography, physical examination, and measurement of serum biomarkers of bone turnover versus scintigraphy for detection of bone fragility syndrome in horses. J Am Vet Med Assoc. 2013; 242(1):76-85. 
3. Berry CF, OBrien TR, Madigan J, Hager DA. Thoracic radiographic features of silicosis in 19 horses. J Vet Intern Med. 1991; 5:248-256
4. Carlson CS, Weisbrode SE. Bones, joints, tendons, and ligaments.  In:  McGavin MD, Zachary JF, eds. Pathologic Basis of Veterinary Disease. 5^th ed. St. Louis, MO, PA: Mosby Elsevier; 2012.
5. Symons JE, Entwistle RC, Arens AM, et al. Mechanical and morphological properties of trabecular bone samples obtained from third metacarpal bones of cadavers of horses with a bone fragility syndrome and horses unaffected by that syndrome. Aust Vet J. 2012; 73(11):1742-1751.
6. Seitz S, Priemel M, Zustin J, et al. Paget’s disease of bone: histologic analysis of 754 patients. J Bone Miner Res. 2009; 24:62-69.

Click the slide to view.

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