Rabbits

Rabbits have big and prominent eyes, directed more laterally than are those of most mammals; this provides a visual field of almost 360°. In the retina rods predominate, which gives them a good nocturnal vision.

Rabbits do not have dorsal lacrimal punta. The third eyelid only covers 2/3 of the cornea. Behind it and separated from the deep part of the cartilage is the harderian gland, which has a small, white upper lobe and a large, pink lower lobe. The excretory ducts from both lobes converge into a single duct that opens on the inner surface of the third eyelid. The harderian gland is larger in males than in females and it is even larger during breeding seasons. The secretion is like milk. An important feature is the retrobulbar venous plexus, vital to note during enucleation.

The fundus of the rabbit is merangiotic in nature with a band of blood vessels and myelinated nerve fibers traversing the retina in a horizontal plane from the optic disc. The optic nerve is situated above the horizontal midline of the eye. Rabbits also have a depression or physiologic cup in their optic disc. Rabbits do not have a tapetum lucidum and the background pigmentation is either brown or pink, depending on the eye color. 40% of rabbits have atropinesterasas. After testing the Schirmer the values are 5,30±2,96 mm/min and intraocular pressure 15-23 mmHg.

Orbit Diseases

Exophthalmia could appear in a physiological way and because of being stressed, in males when they are in season or because of diseases that affect directly to the retrobulbar space (abscesos apicales de los molares) or when they compromise the venous return (masas en mediastino craneal como timoma o linfoma).

Aberrant Conjunctival Overgrowth

Circumferential conjunctival hyperplasia and placation, epicorneal conjunctival membranes and pseudopterygium are all terms used to describe this abnormality. It is a conjunctiva's fold which grows centripetally from the bulbar conjunctiva at the limbus to obscure the cornea. It remains attached at the limbus but the central fold of tissue is freely movable over the corneal surface. The vision is maintained until the central circular hole is occluded by the progression of the membrane.

Blepharitis, Conjunctivitis and Dacryocystitis

Rabbits are prone to inflammatory and infectious diseases of the conjunctiva and nasolacrimal drainage system. P. multocida are sometimes the causative agents, S. aureus, poxvirus, myxomatosis virus and allergies have also been implicated. In early stages, rabbits present epiphora and reddened eyelid margins. Later, white threads of mucus or pus appear at the medial canthus or in the ventral conjunctival. Nasolacrimal duct follows a tortuous route through the lacrimal and maxillary bones. Nasolacrimal duct is predisposed to the obstruction due to infectious diseases, dental disease or maxillary osseous change secondary to nutritional hyperparathyroidism.

Corneal Diseases

Rabbits are quite prone to corneal ulcers. Causes of ulcers are traumatisms, foreign bodies, systemic diseases (hypovitaminosis A) and dry eye disease. Rabbits can acquire cholesterol or other lipids in their corneas. These have a variety of appearances ranging from white perilimbal arcs to clouding of the ventral or medial cornea.

Uveitis, Glaucoma and Diseases of the Lens

Uveitis in rabbits can be shown as focal abscesses in the iris or as a diffuse inflammation of the eye, with hypopyon, corneal edema and severe episcleral infection. Glaucoma is hereditary in New Zealand white rabbits. It begins as a progressive buphthalmos, starting at age of 3-6 months. Spontaneous lens capsule rupture and resultant phacoclastic uveitis associated with Encephalitozoon cuniculi usually appear in young rabbits (younger than 2 years old) as a white mass originating at the lens capsule, with inflammation centered around the break in the capsule.

Ferrets

Ferret eye is similar to the canine eye, except that the pupil is horizontal slit. They are mostly monocular (270°) with very little binocular vision. The central visual pathways have been extensively studied because ferrets experience significant photoperiodism that influences their endocrine and reproductive systems.

The earliest signs of distemper, which is usually fatal in ferrets, are ocular: conjunctivitis, swollen and inflamed eyelids, and corneal ulcers or keratoconjunctivitis sicca. Corneal ulcers and cataracts are quite common. Cataracts may be hereditary, secondary to dietary factors (vitamin A deficiency), or idiopathic. Mature cataracts are commonly a result of cataract-induced uveitis, which causes miosis, mild corneal edema, squinting, glaucoma and anterior lens luxation.

Retinal degeneration is also commonly seen in ferrets and causes the same clinical and ophthalmoscopic signs as progressive retinal degeneration in dogs.

Guinea Pig and Chinchilla

Both of them, guinea pig and chinchilla, have a rudimentary nictitating membrane. The guinea pig has a paurangiotic retina, the tapetum is absent. The chinchilla has an anangiotic retina. The pupil is narrow and vertical, and it is very difficult to dilate.

They are precocial species, so they are born with open lids.

Blepharitis is shown and swelling, redness, flaking, crusting or alopecia of the lids. In guinea pigs vitamin C deficiency, resulting from either dietary deficiency or metabolic disease can cause the appearance of a flaky discharge on lids. Conjunctivitis can be caused by Pasteurella, Streptococcus or Chlamydia psittaci.

Guinea pigs are also subjected to the development of yellow subconjunctival fatty deposits, commonly called "pea eye". They also may develop white opaque growths in the anterior chamber (choristomas), sometimes associated with cataracts. This may be a form of ectopic mineralization, which is common in other tissues in guinea pigs.

Rats, Mice, Hamsters and Gerbils

Rats, mice, hamsters and gerbils have a cholangitic retina with 5-10 radiating retinal vessels that extend from the disk. The tapetum is absent.

Sialodacryoadenitis complex refers to a coronaviral infection of the many lacrimal glands that surround the globe of rodents. It usually affects young animals. Initial signs are blinking, red tears and exophthalmos or proptosis. When the infection is chronic results in keratoconjunctivitis sicca, corneal neovascularization and anterior synechiae. Its most severe form, it can result in retinal degeneration, retinal detachment, endophthalmitis or phthisis bulbi.

In older animals, red tears, conjunctivitis and blepharitis can be associated with excessively long lower incisors. Older rodents commonly develop calcification of the cornea too.

Transient cataracts are common in anesthetized rodents probably due to changes in the aqueous humor composition or temperature secondary to the eye's being open and nonblinking. Cataracts get resolved when the animal wakes up.

In young rodents, it is common for the hyaloid artery to persist up to the age of 3 months. This disease causes vitreal hemorrhages. Other congenital abnormalities includes optic nerve hypoplasia and myelinated retinal nerve fibers.

References

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3.  Harcourt-Brown F. Textbook of rabbit medicine. Elsevier Science Ltd. 2002.

4.  Kern TJ. Ocular disorders of rabbits, rodents and ferrets. In: Kirk RW (ed): Current veterinary therapy X: Small Animal Practice. WB Saunders Co., Philadelphia. 1989:681-686.

5.  Kirschner SE. Ophthalmologic diseases in small mammals. In: Hillyer EV, Quesenberry KE. Ferrets, rabbits and rodents, Clinical Medicine and surgery. WB Saunders Co., Philadelphia. 1997:344-354.

6.  Lawton MPC. Exotic species. In: Petersen S, Crispin S. BSAVA Manual of small animal ophthalmology. 2nd edition. 2002:285-294.

7.  Lewington JH. Diseases of the ferret ear, eyes and nose. In: Lewington JH. Ferret, husbandry, medicine and surgery. Butterworth Heinemann. 2000:177-198.

8.  Miller PE. Ferret ophthalmology. Sem Avian Exotic Pet Med 1997; 6:146-151.

9.  Williams DL. Laboratory animal ophthalmology. In: Gelatt KN. Veterinary ophthalmology. 4th Ed. Blackwell Publishing. Iowa. 2007:1336-1369.

10. Williams DL. Rabbits. In: Petersen S, Crispin S. BSAVA Manual of small animal ophthalmology. 2nd edition. 2002:276-284.