MOHRSBy: Susan Keil, DVM, MS, DACVO
Pet rabbits may experience a variety of ophthalmic diseases. These ophthalmic conditions are often different than eye diseases of the dog, cat, or other pet rodents. The more common etiologies (causes) of eye problems in the rabbit include nutritional deficiencies, infections, environmental and management problems, genetic influences, and congenital malformations.
The eyeball (globe) is housed within the orbit. The orbit is made of numerous bones and soft tissue structures (fat, glands, muscle). The orbit typically does a superb job of protecting the eye. The extraocular muscles on the surface of the eye are responsible for eye movement. The upper, lower, and third (nictitans) eyelids protect the eyeball, particularly the cornea. The conjunctiva is a vascular, protective tissue that lines the eyelids and the sclera.
The sclera is the fibrous, white outer layer of the globe. The sclera gives the globe shape and has the very important job of protecting the intraocular contents. Compared to some species, such as man, the rabbit sclera is not very visible. The cornea is the clear continuation of the fibrous sclera. The cornea is clear so light can enter the eye and a visual image can be formed.
The rabbit also has the following basic mammalian eyeball structures: iris, pupil, lens, ciliary body, vitreous, choroid, and retina. Albino rabbits lack pigment in their iris and choroids. This gives the albino rabbit a normal, reddish reflection from within the eye. The eyelids open in the rabbit about 10 days after birth.
Anatomical ophthalmic features unique to the rabbit (i.e. normal variation) include the following:
1. Four orbital glands within the orbit. These are called the lacrimal, gland of the nictitating membrane (third eyelid), Harderian gland, and intraorbital gland. Dogs and cats only have the first two. These glands contribute to the tear film
2. A single, large, inferior nasolacrimal punctum opening. This punctum is the opening to a tortuous nasolacrimal (NL) duct of variable diameter. The NL duct drains tears from the eye into the nose/mouth.
3. A visible dorsal (superior) extraocular (rectus) muscle through the dorsal (top) conjunctiva. This muscle is responsible for rotating the eye upward. This muscle is not visible in the human, dog, cat.
4. Retinal blood vessels which are present only medially and laterally to the optic nerve head (disc). This would be a species variation from the dog and cat that your veterinarian would take note of on examination. This is called a merangiotic retina. Dogs and cats have blood vessels throughout the retina. This is called a holoangiotic retina.
5. The optic disc is known for having abundant myelination (myelin is analogous to insulation around an electrical wire). Another species variation of the rabbit is the location of the optic disc. It is superior to the visual. To visualize the optic disc, your veterinarian must look up, rather than straight back into the eye with instruments.
6. An extensive orbital venous plexus. This means behind the eye there is a mass of veins; important to note when contemplating removing the eye (enucleation).
Examination of the eye by your veterinarian may include a Schirmer tear test (checking tear production quantity), a fluorescein stain test (checks for corneal ulcers and NL duct patency), and tonometry (measurement of eye pressure). Topical anesthesia and dilating agents can be used to facilitate the examination. Vision assessment in rabbits can be difficult. They are often scared and reluctant to move in the examination room.
Important ocular disorders of the rabbit include epiphora, conjunctivitis, blepharitis, uveitis, and glaucoma.
Epipora is either caused by excessive tearing or inadequate tear drainage. The clinical signs include a milky, watery discharge and a crusting of the facial hair near the medial corner (canthus) of the eye. Excessive lacrimation results from irritation to the eye. A variety of external or intraocular conditions may be responsible. Acquired nasolacrimal (NL) duct obstruction maybe associated with chronic rhinitis (nasal infection) or dacryocystitis (inflammation/infection of orbital glands). The most common infectious agents are bacterial. Dental disease could also be the culprit.
Your veterinarian may recommend a skull radiograph to evaluate the dental arcade, nasal cavity, and sinuses. Sometimes a NL duct obstruction may need to be cannulated with suture or a catheter to open the duct. This should be done with caution, however, as the NL duct is very tortuous. Bacterial rhinitis and dacryocystitis may resolve with systemic antibiotic therapy (enrofloxicin, gentamicin, or injectible penicillin).
According to the literatature, conjunctivitis is commonly caused by Pasteurella multocida and Staphylococcus sp. Clinically, however, other common bacterial organisms responsible for disease included Pseudomonas and Streptococcus sp. In addition, these bacterial organisms may also be responsible for a variety of other diseases, such as orbital cellulites, uveitis, pneumonia, otitis media and interna (middle and inner ear disease), pyometra (uterine infection), subcutaneous abscesses, and genital infections. This organism may be present an otherwise normal conjunctival surface or contaminate the ocular surface by extension from the nasal cavity through the NL duct.
Therapies include topical chloramphenicol, enrofloxicin, or gentamicin ophthalmic solution/ointment four times daily combined with systemic broad-spectrum antibiotics. If the conjunctivitis is associated with systemic Pasteurellosis, the prognosis for a cure is poor. However, management of Pasteurellosis can be very good with appropriate medical therapy. Some rabbits appear to be chronic carriers of this organism.
Other causes of conjunctivitis include: unsanitary husbandry conditions, Chlamydia, and Staphylococcus and Pseudomonas bacteria.
Blepharitis can be caused by an infection with the spirochete Treponema cuniculi, the causative agent for rabbit syphilis. Clinical signs most commonly observed include lesions on the vulva and prepuce. Other structures which may develop lesions include the lips, nares, anus, and eyelids. This disease is transmitted to young rabbits by an infected mother (dam).
Diagnosis is confirmed by identification of spirochetes and examination of the skin scrapings. The infection may be successfully treated with three injections of benzathine/procaine penicillin G given at 7-day intervals. Eradication of treponematosis may require treatment of even asymptomatic animals.
Blepharoconjunctivitis (inflammation of the eyelid and conjunctiva) can be caused by self-trauma, environmental problems (filthy and/or dusty bedding), eyelid malformations (extra hairs and/or abnormal lid carriage), rabbit pox virus, or rarely myxomatosis virus. Obtaining a definitive diagnosis is necessary to determine the effective therapy. Additional diagnostics may be recommended by your veterinarian. Eyelid malformations may require surgery.
Uveitis is the inflammation of the uveal tract. The uveal tract is the middle, vascular layer of the eye. The iris, ciliary body, and choroids make up the uveal tract. Uveitis can be caused by corneal ulcer, ocular trauma, systemic infection (especially pasteruellosis and staphylococcosis), and spontaneous lens rupture (known as phacoclastic uveitis). Encephalitozoon cuniculi organisms are often associated with and believed to be the cause of phacoclastic uveitis. The pathophysiology of this latter condition is unclear.
Therapy for rabbit uveitis is similar to other species. Treatment of the underlying disease, if possible, is the first step. Treatment with topical anti-inflammatories (steroids and/or non-steroidals) is also required. Depending on the etiology, topical and/or systemic antibiotics may or may not be indicated.
Glaucoma is a disease where an excessive intraocular pressure (IOP) causes damage to the retina. Blindness results when the IOP is too high for too long. Normal eye pressure is 15-25 mmHg. Glaucoma values can be interpreted as 26-95 mmHg. In New Zealand White (NZW) rabbits, the condition is inherited as an autosomal recessive trait. The onset of disease in the NZW is often between 3-6 months of age.
Clinical signs include buphthalmos (enlargement of the globe), generalized corneal edema (cornea appears blue), and blindness. Treatment of glaucoma is often frustrating. There are numerous topical medications (carbonic anhydrase inhibitors, prostaglandin antagonists, beta-blockers, parasympathomimetics) that can be utilized alone or in combination. Glaucoma is a disease that will require chronic medications and frequent rechecks. The target IOP for a visual eye with glaucoma is below 20-25 mmHg. If the IOP starts to rise, then additional medications will be recommended. The goal is to keep a visual eye visual.
Sometimes surgery may be recommended for either the visual (diode laser cycloablation, cyclocryoablation) or blind eye (enucleation, evisceration). Another option for a blind glaucoma eye is an injection of gentamicin antibiotic into the eye. This is called a chemical ablation. A chemical ablation “kills” the eye’s ability to produce fluid. Without fluid production, the IOP drops permanently. The goal with a blind glaucoma eye is to keep the eye comfortable. Pressures below 35-40 mmHg are considered comfortable.
Keratitis (inflammation of the cornea) may or may not be associated with ulcers. Corneal ulcers may be superficial or deep, chronic or acute, infected or non-infected. They may be associated with trauma or eyelid/lash abnormalities. Identifying the underling cause is important to the success of therapy. Therapy may include topical antibiotics and cycloplegics (atropine) to dilate the pupil. The frequency of these medications depends on the severity and duration of the ulcer. Surgery may be required if an eyelid/lash problem is identified.
Causes of non-ulcerative keratitis include trauma, eyelid abnormalities, dietary and inherited ocular lipidosis, and an inherited corneal dystrophy in the Dutch-Belted rabbit). Therapy may include topical steroids and/or non-steroidals, cycloplegics, and possibly antibiotics (solution or ointment). The prognosis for ulcerative and non-ulcerative keratitis depends on the etiology, extent of disease, and age of the rabbit.
Primary cataracts in the rabbit are uncommon. By contrast, primary (genetic) cataracts in the dog are quite common. Secondary cataracts (cataracts caused by something else) most commonly occur from chronic uveitis. Diabetic rabbits may develop cataracts similar to dogs. Cataract surgery is possible.
The best approach when it comes to your rabbit’s ocular health: use your common sense. If you notice epiphora, pain, squinting, increased redness, a bluish cast to the cornea, a generalized unusual appearance to the globe/orbit, and/or vision loss, call your veterinarian within 24-48 hours to discuss the problem. Your veterinarian will probably recommend an examination to evaluate the problem. The eye is particularly difficult to assess over the phone. It is always best to catch a problem when it is still a minor one!