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 39.1Â° C. A trial medical therapy with antibiotics and corticosteroids was unsuccessful in relieving
clinical signs and the ewe was euthanized.
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.Â 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.Â The mass was firm,
pale and fibrous.Â When cutting through the mass there was a gritty sensation and marked resistance.
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.Â Osteoblasts outline bone spicules, although a transition from collagenous fibrous matrix to bone matrix is
occasionally seen.Â In several places along bone margins osteoclasts are present, as well as adipose tissue.Â Within the
stroma streams of plump and slender spindle shaped fibroblast-like cells and abundant collagenous extracellular
matrix is seen.Â Dense collagen tissue is mixed with areas of myxoid matrix tissue.Â No mitotic figures were seen in
examined sections.Â Moderate numbers of scattered as well as smaller multifocal accumulations inflammatory cells
are present throughout the mass.Â Inflammatory cells consist of large numbers of plasma cells and smaller numbers of
lymphocytes and neutrophils, and occasional eosinophils.Â Multifocally there are dense accumulations of
inflammatory cells with a high proportion of neutrophils and these are sometimes centered on bone margins.Â Rarely
fusifom microorganisms are found (not present in all slides).Â 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.Â Extravasated erythrocytes are multifocally present.
1.Â Nasal cavity: Ossifying fibroma with plasmacytic and neutrophilic inflammation
2.Â Nasal cavity: Rhinitis, lymphoplasmacytic and neutrophilic, chronic, moderate, diffuse
Aerobic bacterial culture was performed and yielded no specific growth.
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.Â 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.Â 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.Â 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+ï¿½-ï¿½rez et al., who also described presence of inflammatory cells in the fibrous stoma in
part of the neoplasm.(6)
Nasal cavity: Ossifying fibroma.
Ossifying fibroma is an intraosseous lytic mass that destroys bone, and in early lesions is
often intramedullary and does not produce a mass effect.Â Although considered benign, they are expansile and
destroy adjacent normal bone.Â If this lesion was present in soft tissue it would be termed a fibroma with osseous
metaplasia.Â 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.Â 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.Â 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.Â 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.Â 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).
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2.Â Caswell JF, Williams KJ: Respiratory system.Â In: Jubb, Kennedy, and Palmer's Pathology of Domestic Animals ed.Â Maxie MG, 5th ed., pp.Â 533-534.Â Saunders Elsevier, Philadelphia, 2007
3.Â 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).Â Vet Pathol 48:706-712, 2011
4.Â Morse CC, Saik JE, Richardson DW, Fetter AW.Â Equine juvenile mandibular ossifying fibroma.Â Vet Pathol 24: 415-421, 1988
5.Â Nelson AM, Baker DC.Â Pedal osteosarcoma in a donkey.Â Vet Pathol.Â 1998 Sep;35(5):407-9.
6.Â P+ï¿½-ï¿½rez V, R+ï¿½Â°a P, Benavides J, Ferreras MC, Garc+ï¿½-ï¿½a Mar+ï¿½-ï¿½n JF: Osteoma in the skull of a sheep.Â J Comp Pathol 130: 319-322, 2004
7.Â Puff C, Ohnesorge B, Wagels R, Baumg+ï¿½-ï¿½rtner W: An unusual mucinous osteoma with features of an ossifying fibroma in the nasal cavity of a horse.Â J Comp Pathol 135: 52-55, 2006
8.Â Rogers AB, Gould DH: Ossifying fibroma in a sheep.Â Small Ruminant Res 28: 193-197, 1998
9.Â Rumbaugh GE, Pool RR, Wheat JD: Atypical osteoma of the nasal passage and paranasal sinus in a bull.Â Cornell Veterinarian 68: 544-554, 1978
10.Â Thompson KG, Pool, RR: Tumors of bones.Â In: Tumors in domestic animals, ed.Â Meuten DJ, 4th ed., pp 248-255.Â Iowa State Press, Ames, 2002
11.Â Thompson K.Â Bones and joints.Â In: Maxie MG, ed.Â Jubb, Kennedy and Palmers Pathology of Domestic Animals.Â 5th ed.Â Vol 1, New York, NY: Elsevier Saunders; 2007:111-2.