Avian and Exotic Animal Clinic, Faculty of Veterinary Medicine, University of Veterinary and Pharmaceutical Sciences Brno, Czech Republic
Ophthalmic and associated structure diseases are common problems in rabbits. In this presentation, the most commonly encountered ocular problems receive the most detailed attention and will be described thoroughly. Priority will be given to a thorough ophthalmic examination, normal physiology, disease recognition, diagnostic procedures and optimal medical and surgical treatment.
The orbits of the rabbit are situated in either side of the skull and their openings are directed at 85 degrees to the transverse plane of the head. The rabbit eye appears compressed in its anteroposterior dimension and possesses, in contrast to humans, an active retractor bulbi muscle and acino-tubular gland of the third eyelid (Harderian gland). The conjunctiva is divided into palpebral and bulbar conjunctiva and is relatively thin (10–40 µm). The cornea of an adult rabbit has a power of 40–43 dioptres. The rabbit lens accommodation is limited (0–1.5 dioptres). The retina is merangiotic. The external ophthalmic artery is the chief arterial supply to the orbital structures, including the bulbus. The venous sinus completely surrounds the muscle cone and covers the Harderian gland.
Rabbits are able to resist blinking for long intervals because they have a very stable tear film. Tears of a rabbit are a clear and slightly alkaline solution, with an average pH of 7.5 with electrolyte concentration similar to that of plasma. There is a single nasolacrimal punctum for each eye in the rabbit. Swabs taken from the conjunctival sac of healthy pet rabbits contain mostly DNase-negative Staphylococcus sp., Micrococcus sp., Bacillus sp., Stomatococcus sp., Neisseria sp., Pasteurella sp., Corynebacterium sp., Streptococccus sp. and Moraxella sp.
The ophthalmic examination should always follow history, physical and neurological examination. In rabbits it is often difficult to demonstrate reduced vision because individuals can compensate for severe visual disability. The general eye examination should start with gross observation of the position and symmetry of the eyes and adnexa. Aspects of neurological examination that relate to ocular diseases include assessment of vision, eye movement and cranial nerves. Reflexes which should be tested are menace response, visual placing reaction, palpebral and pupillary reflex.
The basic equipment requirements for examination consist of a room that can be darkened, a focal light source and some form of magnifying device. Additional equipment and various disposable items (direct and indirect ophthalmoscope, slit-lamp, tonometer, fluorescein, local anaesthetics, mydriatics etc.) are also needed.
Adequate manual restraint is essential during the ophthalmic examination. Sedatives should be avoided; however, if it is necessary, potential drug effects on eye structures should be taken into consideration. Corneoconjunctival cytology and culture are helpful in cases of conjunctivitis or dacryocystitis. The Schirmer tear test (2–11 mm/min) and phenol red tear test (15–27 mm/15 seconds) are applicable in rabbits. Normal intraocular pressure in rabbits is in the range of 15–23 mmHg. Examination of the bulbar surface of the nictitating membrane is performed after instillation of topical anaesthetic. The cornea, anterior chamber, iridocorneal angle and iris are examined by direct or indirect ophthalmoscopy. Mydriasis is achieved with the topical use of a 0.5–1% tropicamide solution. Although approximately 60% of rabbits have atropinesterase, the author has also had good experience with the use of atropine eye drops. Once the mydriatic agent has taken effect, the lens, vitreous and fundus are examined thoroughly. The fundus is evaluated with the use of an indirect ophthalmoscope and 30–40 D lenses, slit lamp or with oculoendoscopy.
Fluorescein staining of the cornea with the use of the full-strength 2% solution or direct touching of a fluorescein-impregnated strip to the cornea should be avoided because it can result in artificial staining of the cornea.
Electroretinography is receiving greater emphasis in experimental studies. Photography plays an important role in the documentation of ophthalmic lesions.
Corneal Erosion and Ulceration
Due to the relatively large and protruding globe, even minor problems associated with blinking or tear production or traumatic injury result quickly in corneal oedema and subsequent erosion or ulcer formation. Clinical signs could include blepharospasm, conjunctival hyperaemia, keratoconjunctivitis sicca or epiphora, and different stages of corneal opacity and corneal vascularisation. Fluorescein staining reveals diffuse uptake of the dye in the affected area. The presence of a small number of faint focal or hazy stained areas in rabbits is normal. Cytological examination and bacteriological culture will aid antibiotic selection. Superficial debridement is performed with cotton-tipped swabs. Treatment consists of administration of broad-spectrum antibiotics (gentamicin, fluoroquinolones, tobramycin, chlorampheniocol, sulphonamides), anti-inflammatory eye drops (indometacine, diclofenac) and agents supporting corneal healing (retinol). Acetylcysteine could also be used. In cases of non-healing ulcers autologous serum administration or surgery (keratectomy, corneal flaps) may be performed.
Epiphora is caused by an overproduction and/or inadequate removal of tears. In rabbits, it is frequently due to an inadequate removal of tears caused by blockage of the tear ducts by apical elongation of the incisors or periapical cheek teeth pathology. In cases of conjunctivitis and keratitis, epiphora is present due to overproduction of tears from eye irritation. Purulent epiphora is commonly associated with bacterial infection and keratoconjunctivitis. The skin in the area of the medial canthus may be wet and alopecic. Wet dermatitis could be the result of chronic skin irritation. In cases of nasolacrimal duct obstruction, milky discharge is commonly present. This secretion comes from the Harderian gland and physiologically contains lipids, which are macroscopically seen as a white opacity of tears.
Diagnosis is based on clinical examination, oral cavity examination, ocular examination or conjunctival swab examination. Imaging methods could reveal incisor apical elongation or periapical pathology. Dacryocystography with the use of 1 ml of iodine contrast medium is a useful procedure for the visualisation of the nasolacrimal duct. Epiphora may also be present in cases of myxomatosis, foreign body in the conjunctival sac or trichiasis.
If the epiphora is due to eye disease, then treatment of the disease will usually resolve the problem. If systemic infection is present, oral or parenteral antibiotic administration is recommended. For the treatment of pasteurellosis, trimethoprim-sulpha drugs (30 mg/kg orally/s.c. q12h for 2–3 weeks) are the first choice; for the treatment of treponematosis, penicillin G (60,000 U/kg i.m. q24h for 5 days) clears up the infection. Daily washing with anti-inflammatory solutions and clipping the hair in the affected skin area will minimise skin inflammation.
Epiphora caused by blockage of the tear ducts often responds to medical treatment with antibiotic eye drops, but many cases require flushing of the ducts under anaesthesia. If dental disease is a cause, proper treatment is necessary to solve the problem. An intravenous cannula, lacrimal cannula or metal irrigation cannula with a blunt end can be used for irrigation of the nasolacrimal duct, which should be performed under sedation or general anaesthesia. The syringe is filled with sterile saline and the cannula is inserted into the nasolacrimal duct or, where the diameter of the nasolacrimal duct is very small, close to the nasolacrimal punctum. Milky fluid or pus should come from the ipsilateral nostril if the procedure is successful. In some cases gentle digital pressure in the area of the lacrimal sac is necessary to force the fluid to go further along the duct to clear the blockage. If flushing is unsuccessful, administration of anti-inflammatory eye drops and/or antibiotics is recommended with a second attempt at flushing after 4–7 days of treatment. Flushed material can be used for cytological examination or culture. Depending on the severity of the disease, repeating this procedure is necessary at 3–10-day intervals. The author recommends applying eye protective gel containing retinol and anti-inflammatory drugs after each procedure to protect the superficial eye structures.
Documented cases of retrobulbar mass lesions include retrobulbar abscess associated with periodontal disease, malignant lymphoma and a coenurus cyst of Taenia serialis. Exophthalmos could also be associated with thymoma, hypertension, stress and Harderian gland neoplasia. To determine the extent and nature of retrobulbar mass or exophthalmos, ophthalmoscopy, radiography, computed tomography, ocular ultrasonography (10–20 MHz probe) and lesion biopsy for cytological and histopathological examination are necessary.
A cataract is any opacity within a lens which could vary in size with variable vision impairment (incipient, immature, mature or hypermature). Aetiology could be congenital, post-inflammatory, parasitic (Encephalitozoon cuniculi), metabolic or idiopathic.
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