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Ferrets: ophthalmology

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Abstract

The anatomy of the ferret eye is largely similar to that of other carnivores. However, the size of the ferret eye is proportionally and absolutely small, having an axial length of about 7mm. The conjunctiva lines the posterior face of the eyelids, the entire semilunar fold of the conjunctiva (third eyelid) and the exposed sclera before terminating at the corneal limbus (corneoscleral junction). Ferrets have a well developed third eyelid covered by tightly adherent conjuctiva on its bulbar and palpebral surfaces and it is usually either non-pigmented or pigmented at the margin. The third eyelid is reinforced by a T-shaped piece of cartilage that has a gland at its base that is responsible for secreting part of the aqueous layer of the tear film. There is no deep gland of the third eyelid. This chapter discusses Anatomy; Restraint and ophthalmic examination; and Ophthalmic diseases.

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Figures

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29.1 Photomicrograph of a 5 μm thick histological paraffin-embedded section of a ferret’s cornea. Note the presence of an epithelium (Ep), a stroma (St), a posterior limiting membrane (also called Descemet’s membrane, Dmt) and an endothelium (arrowed). The ferret’s corneal epithelium is about 4–5 cells thick. Gomori’s Trichromic stains collagen tissue blue; thus it is possible to see how rich in collagen the corneal stroma is. (Bar size: 350 μm; stain: Gomori’s Trichromic; original magnification: ×400).
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29.2 Detail of a typical small ferret eye with its brown iris. Detail of the ferret’s pupil, clearly demonstrating that it is ovoid in shape and horizontally oriented.
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29.3 Adult male ferretskull and bony orbit elements. Right rostrodorsolateral view of the skull without the jaw. Close-up of the right rostrodorsolateral view of the orbit (rectangular area of (a)). Caudodorsolateral view of the orbit. CPF = caudal palatine foramen; EF = ethmoidal foramen; FoLS = fossa of the lacrimal sac; FPr = frontal process of the zygomatic bone; IoF = infraorbital foramen; LP = lacrimal process; MF = maxillary foramen; OC = optic canal; OF = orbitorotundum foramen; SF = sphenopalatine foramen; ZPr = zygomatic process of the frontal bone. Note the exposure of the apical end of the palatine root (asterisk) of the superior first (and unique) molar tooth through the pterygopalatine surface of the maxillary bone, on the floor of the pterygopalatine fossa. (Courtesy of Marcello Machado.)
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29.4 Protocol for ophthalmic examination of ferrets. (Modified from Montiani-Ferreira, 2001.)
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29.5 Demonstration of selected ophthalmic procedures. Use of a commercial Schirmer tear test strip. Use of Tono-Pen XL (Mentor, Santa Barbara, California).
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29.7 Large chronic non-healing corneal ulceration of the right eye of a ferret. Note the corneal oedema and the presence of non-adherent epithelium surrounding the main central defect. Fluorescein dye clearly under-ran the surrounding loose epithelium in this case. (Courtesy of Giuseppe Visigalli.)
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29.8 Cataracts in ferrets. Mature cataracts. Hypermature cataracts and lens-induced uveitis. Note the small size of the lens due to reabsorption of the lens cortex. The lens cortical material is reabsorbed faster than the nucleus. Note the presence of a blood clot and pigment in the anterior surface of the lens. Lens-induced uveitis typically affects ferrets with mature or hypermature (‘resorptive’) cataracts. (a, courtesy of Paolo Selleri; b, courtesy of Yasutsugu Miwa.)
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29.9 Chronic glaucoma has led to a massive buphthalmia and forward displacement of the globe of the right eye. Due to the magnitude of the globe enlargement, it was believed to be possibly secondary to an intraocular neoplastic disease. Further diagnostic tests were not possible at the time because the owners declined the recommendation of enucleation and/or histopathological analysis. Thus, the actual cause of this massive globe enlargement was unknown. The case was lost to follow-up care. (Courtesy of Angela Duke.)
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29.11 Exophthalmos in the left eye of a ferret due to an orbital tumour. Note the soft tissue proliferation near the medial canthus region (arrowed). The neoplastic tissue was diagnosed as adenocarcinoma originating from paranasal sinus epithelium. (Courtesy of Yasutsugu Miwa.)

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