The cornea, or transparent front part of the eye, is a thin dense fiber-like structure characterized by 4 distinctive layers in rabbits:

• A keratinized layer of epithelium (30 to 40 mm);

• The stroma, formed by parallel bundles of collagen;

• The Descemet’s membrane (7 to 8 mm);

• A single layer of endothelium, which is rich in Na+-ATPase pumps.

The cornea covers up to 30% of the rabbit eye. Due to its large size, the cornea is prone to trauma or other damages, including drying-out. If the epithelial layer of the cornea is scratched or wounded, it may become locally opaque and neo-vascularization has been observed.

Damage and ulceration of the cornea are painful, as the surface of the cornea is well innervated. As a result, a rabbit will rub the affected eye, which can lead to the development of an ulcer, and keep the eye closed. It is often accompanied by dacryocystitis. Contraction of the pupil, conjunctival hyperemia (presence of excess blood), and epiphora (increased production of tears) can furthermore be the result of pain caused by the ulcer. The rabbit is depressed and can stop eating or drinking.

Ivy

Two year old French lop rabbit suffering from entropion (red arrow) and eye lashes falling on the cornea, accompanied by secondary dacryocystitis (green arrow). This condition can cause damage to the cornea and can be corrected surgically.

Causes

For an extended list of causes for keratitis, corneal damage and ulceration, see: Corneal ulcers in rabbits.

Diagnosis

An opacity and/or inflammation on the surface of the eye can be indicative of a superficial corneal abrasion. The use of a fluorescent dye (fluorescein) helps determine the extent of the damage, and its depth. It can be accompanied by a secondary bacterial infection (abscess, uveitis).

If an infection is present, it is necessary to take a sample for a bacterial culture and antibiotic sensitivity test, before the application of the dye.

Akira Yamanouchi

Rabbit eye with a corneal ulcer as seen with normal light

Akira Yamanouchi

Same eye, seen with black light (Wood's light), after application of the fluorescein dye

The size of a corneal ulcer varies and necrotic tissue may be present. It may accompanied by a temporary constriction of the pupil (miosis), or inflammation of the uvea (uveitis). In rare cases the ulcer becomes indolent, without healing.

The presence of corneal damage may be accompanied by an overflow of tears (epiphora), involuntary closing of the eyelids (blepharospasm), accumulation of blood (conjunctival hyperemia) or secondary abscessation.

MediRabbit

Grijsje, a female rabbit with a scar, after she suffered damage to the cornea and edema at young age.

Underlying disorders or diseases should not be ruled out. Indeed, corneal ulceration can be the result from exophthalmia (protruding eyeball), leading to the impossibility to blink, the presence of a retrobulbar abscess, neoplasia, cellulitis, or tooth root related problems, like the presence of an abscess or abnormal elongation of the tooth root in the direction of the eye socket. Abnormal growth of the eyelashes (e.g. entropion, distichiasis) is a further cause for corneal abrasion and ulceration.

Treatment

The treatment depends on the type of ulceration (abrasion, ulcer or descemetocele) as well as the cause, if it is superficial or deep, and its extent.

The treatment of superficial abrasion and ulcers includes the application of a topical antibiotic solution 4 to 6 times a day. Their effect may last a few minutes only in rabbits. Topical atropine has good healing properties and needs to be given twice a day only. The treatment should be accompanied by the administration of analgesics. Healing is usually observed within 3 to 5 days.

In the case of a corneal ulcer or descemetocele, the eye must be protected. The treatment should be aggressive, with frequent application of topical antibiotics (e.g. ciprofloxacin 3%, ofloxacin 0,3%, norfloxacin 0,3% are antibiotics of choice) and the use of pain relief medication (e.g. meloxicam).

Amy Carpenter

Accidental peroxide in the eye of a cottontail. The eye has a deep corneal ulcer up to the Descemet’s membrane or descemetocele. If there is rupture of this structure, the fluid contained in the eye will leak out and the eye will collapse. If rupture, the iris may stick or protrude out of the hole. Although healing is possible, the damage is irreversible and the eye usually becomes blind

Amy Carpenter

Detailed view

Persistant non-healing corneal ulcers are characterized by the accumulation of dead cells at the edge of the ulcer, which will prevent healing. In this case, the area must be debrided, so that cells from the healthy corneal surface can start migrate towards the ulcer and fill the gap. A local or full anesthesia is necesary before corneal debridement can be started with e.g., a dry cotton-tipped applicator.

Grid keratotomy, superficial keratotomy or the placement of contact-lenses have also been used to cure non-healing ulcers in rabbits.

When the abrasion or the ulcer is related to underlying anatomical or pathological causes (e.g., abscess, dacryocystitis, blepharo- or keratoconjunctivitis), these must be treated or corrected too, either medically or surgically (e.g. entropion, distiachiasis).

Acknowledgement

Thanks are due to Ivy (Serbia), to Amy Carpenter (USA) and Akira Yamanouchi (Veterinary Exotic Information Network, http://vein.ne.jp/, Japan), for the permission to use their pictures. Thanks also to Grijsje.

Further information

Andrew SE. Corneal diseases of rabbits. Vet Clin North Am Exot Anim Pract. 2002; 5:341-56. Review.

Flecknell P., editor Gloucester, BSAVA Manual of Rabbit Medicine and Surgery, UK: British Small Animal Veterinary Association2000.

Hillyer E.V. and Quesenberry K.E., Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery, New York: WB Saunders Co. 1997.

Fox JG, Shalev M, Beaucage CM, Smith M. Congenital entropion in a litter of rabbits. Lab Anim Sci. 1979; 29:509-11.

Kern T.J., Ocular disorders of rabbits, rodents and ferrets. In: Kirk R.W., Bonagura J.D., eds. Current veterinary therapy X. Philadelphia, WB Saunders, 1989.

Manning P.J., Ringler D.H., Newcomer C.E. The Biology of the Laboratory Rabbit, New York: Academic Press1994.

Okerman, L: Diseases of Domestic Rabbits, 2nd Edition, Blackwell Scientific Publications, London, 1996.

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