4-year- old female Leister cross, Ovis aries, ovineThe ewe was presented to the clinic with severe dyspnea and weight loss. Respiratory signs of variable degree had been present during the last month, no fever was recorded. At clinical examination there was marked inspiratory and expiratory dyspnea with respiratory noise from the upper airways. The sheep was depressed and the temperature 39.1° C. A trial medical therapy with antibiotics and corticosteroids was unsuccessful in relieving clinical signs and the ewe was euthanized.

Gross Description:  

A space occupying mass was found in the caudal part of the nasal cavity, filling a large proportion of both the left and right side of the caudal nasal cavity. It markedly compressed the ethmoid bone and middle nasal concha on the left side, and was attached to the caudal region of the ethmoid bone. The mass was firm, pale and fibrous. When cutting through the mass there was a gritty sensation and marked resistance.

Histopathologic Description:

Nasal mucosa: In the lamina propria is a large expansile, partially encapsulated and demarcated mass composed of numerous spicules of woven bone separated by an abundant fibrous stroma. Osteoblasts outline bone spicules, although a transition from collagenous fibrous matrix to bone matrix is occasionally seen. In several places along bone margins osteoclasts are present, as well as adipose tissue. Within the stroma streams of plump and slender spindle shaped fibroblast-like cells and abundant collagenous extracellular matrix is seen. Dense collagen tissue is mixed with areas of myxoid matrix tissue. No mitotic figures were seen in examined sections. Moderate numbers of scattered as well as smaller multifocal accumulations inflammatory cells are present throughout the mass. Inflammatory cells consist of large numbers of plasma cells and smaller numbers of lymphocytes and neutrophils, and occasional eosinophils. Multifocally there are dense accumulations of inflammatory cells with a high proportion of neutrophils and these are sometimes centered on bone margins. Rarely fusifom microorganisms are found (not present in all slides). In the respiratory mucosa there is also subepithelial and periglandular diffuse moderate to marked infiltration of plasma cells and lymphocytes with smaller numbers of neutrophils. Extravasated erythrocytes are multifocally present.

Morphologic Diagnosis:  

1. Nasal cavity: Ossifying fibroma with plasmacytic and neutrophilic inflammation
2. Nasal cavity: Rhinitis, lymphoplasmacytic and neutrophilic, chronic, moderate, diffuse

Lab Results:  

Aerobic bacterial culture was performed and yielded no specific growth.


Ossifying fibroma

Contributor Comment:  

Benign lesions arising from membranous bone include ossifying fibroma, osteoma and fibrous dysplasia.(10) The morphologic features of the present case are most consistent with an ossifying fibroma. Ossifying fibromas show an intermediate morphologic architecture between osseous metaplasia of fibrous connective tissue, with predominate fibrovascular stroma separating poorly differentiated bone (as seen in fibrous dysplasia), and dense accumulations of well differentiated cancellous or compact bone with delicate intervening fibrovascular tissue (as seen in osteoma).(10) However, osteomas may show features of both ossifying fibromas and fibrous dysplasia, and it has been suggested that osteomas could represent the end stage lesion of other processes.(10) Ossifying fibromas are rare in all species.(10) In sheep, ossifying fibroma has previously been reported in the mandible of a young adult.(8) Also involving the mandible, ossifying fibroma has been described in several cases of young horses.(4) Furthermore, osteoma of the nasal cavity and frontal bone has been described in a sheep, and recently myxomatous fibroma with presence of bone spicules rimmed by osteoblasts and fewer osteoclasts was described in bighorn sheep.(3,6) In addition, mucinous osteoma with features of ossifying fibroma has been reported in the nasal cavity of a horse.(7) In the present case, there was a marked inflammatory reaction within the mass and in the respiratory mucosa. The neoplasia may be related to chronic inflammation, although it is also possible that inflammation could arise secondary to disturbed airway function due to the presence of neoplasia. Chronic rhinitis is known to induce proliferative lesions including polypoid thickening of nasal mucosa, which may cause obstruction of the nasal passages.(2) Metaplastic ossification of connective tissue components incepted in inflamed nasal mucosa was discussed in the pathogenesis in an atypical osteoma in a bull.(9) Chronic rhinitis was also found in the ovine case of skull osteoma described by P+�-�rez et al., who also described presence of inflammatory cells in the fibrous stoma in part of the neoplasm.(6)

JPC Diagnosis:  

Nasal cavity: Ossifying fibroma.

Conference Comment:  

Ossifying fibroma is an intraosseous lytic mass that destroys bone, and in early lesions is often intramedullary and does not produce a mass effect. Although considered benign, they are expansile and destroy adjacent normal bone. If this lesion was present in soft tissue it would be termed a fibroma with osseous metaplasia. This neoplasm differs from osteoma or exostoses, which arise from the periosteum and are proliferations of bone rather than fibrous tissue(1,11).

Fibrous dysplasia looks and behaves in a very similar manner to ossifying fibroma, is also more common in young animals, and is an important differential diagnosis for this case. However, fibrous dysplasia does not have spicules and trabeculae of woven to lamellar bone lined by osteoblasts, which is a defining feature of ossifying fibroma, as in this case. Another distinguishing feature is that fibrous dysplasia typically has lamellar bony trabeculae concentrated centrally in the mass, with predominantly woven bone at the periphery(1,5,11).

Fibrous osteodystrophy (FOD) was also discussed as a possibility, but in this case there is lamellar bone overlying woven bone in the spicules and trabeculae, and lamellar bone is usually not seen in FOD. Developing cortical bone occurs with woven bone laid down first which becomes compacted with overlying lamellar bone, and the lamellar bone present in this case likely represent preexisting bone. FOD also presents as a more fibroblast-rich lesion with less collagenous matrix as compared to ossifying fibroma, and is bilateral(1,11).

Because of the areas of inflammation, osteoclasis, and fibrosis, conference participants discussed chronic osteomyelitis as a possibility in this case; however, in osteomyelitis there is a concentric orientation of the inflammation to the fibrous connective tissue or the reactive bone, and in this case there is relatively little inflammation compared to the amount of fibrous connective tissue(1).


1. Carlson CS, Wesibrode, SE. Bones, joints, tendons, and ligaments. In: McGavin MD, Zachary JF, eds. Pathologic Basis of Veterinary Disease. 5th ed. St. Louis, MO:Mosby; 2011:951-2, 956-8.

2. Caswell JF, Williams KJ: Respiratory system. In: Jubb, Kennedy, and Palmer's Pathology of Domestic Animals ed. Maxie MG, 5th ed., pp. 533-534. Saunders Elsevier, Philadelphia, 2007

3. Fox KA, Wootton SK, Quackenbush SL, Wolfe LL, LeVan IK, Miller MW, Spraker TR: Paranasal sinus masses of Rocky mountain bighorn sheep (Ovis canadensis canadensis). Vet Pathol 48:706-712, 2011

4. Morse CC, Saik JE, Richardson DW, Fetter AW. Equine juvenile mandibular ossifying fibroma. Vet Pathol 24: 415-421, 1988

5. Nelson AM, Baker DC. Pedal osteosarcoma in a donkey. Vet Pathol. 1998 Sep;35(5):407-9.

6. P+�-�rez V, R+�°a P, Benavides J, Ferreras MC, Garc+�-�a Mar+�-�n JF: Osteoma in the skull of a sheep. J Comp Pathol 130: 319-322, 2004

7. Puff C, Ohnesorge B, Wagels R, Baumg+�-�rtner W: An unusual mucinous osteoma with features of an ossifying fibroma in the nasal cavity of a horse. J Comp Pathol 135: 52-55, 2006

8. Rogers AB, Gould DH: Ossifying fibroma in a sheep. Small Ruminant Res 28: 193-197, 1998

9. Rumbaugh GE, Pool RR, Wheat JD: Atypical osteoma of the nasal passage and paranasal sinus in a bull. Cornell Veterinarian 68: 544-554, 1978

10. Thompson KG, Pool, RR: Tumors of bones. In: Tumors in domestic animals, ed. Meuten DJ, 4th ed., pp 248-255. Iowa State Press, Ames, 2002

11. Thompson K. Bones and joints. In: Maxie MG, ed. Jubb, Kennedy and Palmers Pathology of Domestic Animals. 5th ed. Vol 1, New York, NY: Elsevier Saunders; 2007:111-2.

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