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INSIGHT INTO RABBIT EYE DISEASES

Author : ELISABETTA MANCINELLI

Categories : Vets

Date : August 12, 2013

ELISABETTA MANCINELLI discusses common conditions that present with this delicate and complex organ in the second of a two-part feature on the rabbit eye

IN part one of this article (VT43.30), an overview of how the rabbit eye is constructed and operates was given, followed by an explanation of examination techniques. In part two, common diseases attributed to the rabbit eye and treatment methods are discussed.

Conjunctivitis

Conjunctivitis is the inflammation of the outermost layer of the eye and inner surface of the eyelids. Redness, irritation, increased tear flow or ocular discharge are common clinical signs that might occur, with a variety of causes, such as bacterial (Pasteurella, Staphylococcus, Bordetellaand Treponema; that is, the bacterium responsible for rabbit syphilis) or viral (myxomatosis) infections.

Conjunctivitis may be a primary problem, but, in most cases, is secondary to other inciting causes such as irritation due, for example, to poor ventilation, a high ammonia level present in urine- soaked bedding, hay, dust or other irritant particles. Eyelid abnormalities, respiratory disease, traumatic injuries (especially fight wounds affecting the eye) and even more commonly, dental disease, could all predispose to secondary bacterial conjunctivitis (Van der Woerdt, 2012).

When elongated tooth roots – for example, maxillary incisors – obstruct the nasolacrimal canal, tears cannot be drained normally. They instead accumulate in the eye and start to overflow down the face. This condition, usually defined as epiphora, is also commonly associated with facial dermatitis – especially at the medial corner of the eye due to recurrent irritation caused by the

1 / 11 tears. Epiphora and conjunctivitis may or may not be associated.

Dacryocystitis

The term dacryocystitis is used when infection affects the nasolacrimal duct, causing a purulent, often malodorous, discharge within the duct itself. Sometimes, gentle pressure at the medial corner of the eye expresses purulent material into the conjunctiva. Secondary conjunctivitis and inflammation of the cornea may also be present in severe chronic cases. Dacryocystitis is commonly associated with dental disease and subsequent blockage of the nasolacrimal canal.

Treatment

Treatment always depends on the primary and underlying causes. In cases of conjunctivitis, a swab can be gently rolled on the affected conjunctiva to collect material to be submitted for cytology, bacterial and fungal culture to help in the identification of the infectious agents involved, and choosing the appropriate antibiotic to use. Topical eye ointments are frequently indicated, but any other underlying reason for inflammation or infection should be identified and corrected.

Improving the husbandry and providing good ventilation and a clean, dry bed are an important part in the prevention and treatment of a potentially serious condition. In cases of epiphora and dacryocystitis, a complete investigation should include skull x-rays to identify the presence of dental disease as a possible primary cause. Dilations or obstructions of the nasolacrimal canal can also be identified in x-ray studies after injection of an appropriate contrast substance directly into the nasolacrimal canal (dacryocystorhinography).

Flushing of the canal can be done to relieve partial or incomplete obstruction, but treatment of complete obstructions can be frustrating and elimination of the obstruction is rarely possible. Administration of local and systemic antibiotics is indicated where secondary infections exist, but underlying dental disease needs to be treated as well, for example by removing the incisors if maloccluded, or by burring the elongated molars with appropriate dental burrs. Husbandry and dietary changes may also be necessary. If epiphora is caused by dental disease alone, it is often difficult to resolve definitively as the underlying dental changes cannot be reversed (Van der Woerdt, 2012).

Cataracts

Despite being frequently associated with diabetes in other mammal species, cataracts are generally not an indication of this disease in pet rabbits. More commonly, this condition is present in cases of infection due to the parasite Encephalitozoon cuniculi (Williams, 2007).

A survey has shown 52 per cent of healthy UK rabbits are positive for E cuniculi, with antibodies

2 / 11 against this parasite found in their blood. These rabbits will not necessarily show clinical signs associated with the parasite infection, but a variable percentage of them will present symptoms associated with kidney and nervous system inflammation. If the infection occurs when the fetus is in utero, the eye is more commonly affected, with cataract and inflammation of the anterior portion of the eye later in life being one of the most common clinical signs (Keeble, 2006).

Pseudopterygium

Occasionally, progressive occlusion of the cornea with a conjunctiva-like membrane starting at the limbus is seen, known as pseudopterygium. In severe cases, only a small central portion of the cornea is left uncovered, limiting the visibility of an otherwise normal eye (Van der Woerdt, 2012). Medical and surgical interventions have often been unrewarding, but recurrence can be sometimes prevented up to one year.

Uveitis

Uveitis refers to a serious condition affecting the front part of the eye that could result from a severe corneal infection, a foreign body penetrating the eye, trauma or as a consequence of E cuniculi infection when the parasite causes rupture of the lens within the anterior chamber of the eye. Topical eye drops and systemic medical treatment might be of help in early or mild cases. In severe and specific cases, surgery can be performed to remove the affected lens. Enucleation may be required where recovery is unlikely.

Corneal problems

Corneal problems can arise in association with bacterial infections, trauma (as a result of fights with another rabbit or environmental factors such as hay fragments) or as a result of dental disease when this alters the protective tear film causing dacryocystitis, frequently associated with corneal damage and/or infections.

All these situations are usually extremely painful to the rabbit and often cause classic signs of eye pain, such as partial closure of the affected eye, avoidance of bright light and partial withdrawal of the globe deeper into the orbit. Good eye examination and simple tests – such as the instillation of fluorescein, which highlights areas of corneal damage – can be extremely helpful in identifying the exact problem.

Corneal ulcerations

Corneal ulcerations are common in rabbits, especially after a traumatic event. Many can be easily solved with topical antibiotic cover, pain relief and time to heal. In some cases, they can be a persistent and recurrent problem, so more specific or aggressive intervention may be required.

3 / 11 Samples can be collected from a diseased eye to be submitted for bacterial culture so appropriate topical and systemic antibiotics will be used.

Unfortunately, in some cases, healing can be difficult and treatment frustrating. Surgical techniques, such as debridement of areas of damaged corneal tissue, conjunctival grafts and temporary closure of the eyelids to prevent ongoing abrasion, can be useful in these refractory cases. Grid keratotomy or the placement of contact lenses has also been used to cure non-healing ulcers in rabbits (Andrew, 2002).

Exophthalmos

The term exophthalmos is used to indicate a condition in which the eye seems to be bulging out of the globe. These cases are rather complicated because the eye is usually just secondarily involved, but the primary cause can be located elsewhere – either behind the eye or in the thorax.

In the first instance, a retrobulbar abscess, tumour or blood clot takes up space within the orbit, pushing the eye out of its normal position. In the latter case, there is usually a mass – most likely to be a tumour – localised in the thorax that compresses the blood vessels around it, compromising the normal blood flow. The blood cannot flow freely back to the heart and pressure within the veins increases, leading to an accumulation of blood in the retro-orbital venous sinus, eventually resulting in exophthalmos.

Sometimes, exophthalmos is just a result of fear, with a subsequent natural increase in blood pressure. This may be the reason why many rabbits, presented to the veterinary surgeon for different reasons or even for a health check, may have protruding eyes with wide opened eyelids. In this case, usually both eyes are involved, the rabbit appears frightened and may also have a frozen look. This condition is not associated with any other abnormalities and tends to resolve within a few minutes.

When trying to reach a definitive and correct diagnosis, it is extremely important to distinguish between all these possibilities, considering that some conditions are absolutely benign, but others might carry a poorer prognosis. Thorax x-rays and ultrasound may be helpful in identifying the presence of a mass in the chest. Samples from any abnormal tissue found may be also collected under ultrasonographic guidance. An ultrasound of the eye can also be performed and be extremely useful to rule out the presence of a retrobulbar mass. More advanced techniques, such as MRI or CT, could also be used to help in the diagnosis, localise the problem and clarify its extensiveness.

Glaucoma

When the pressure inside the eye is higher than normal (a condition called glaucoma), a bulging appearance may also result. Glaucoma can occur as a result of disease damaging the normal

4 / 11 drainage within the eye, but some breeds (such as the New Zealand white) have a genetic predisposition, due to abnormal structure of drainage tissue present from birth (Williams, 2007). Measuring the eye pressure is a simple and non-invasive procedure that, within minutes, can allow a diagnosis of glaucoma. Glaucoma can be managed with specific ophthalmic solutions but, in many cases, the changes are advanced by the time they are noticeable and treatment acts only to slow progression, with blindness eventually resulting.

Sometimes, the rabbit appears normal when in a familiar environment but, if moved into an unfamiliar situation, its blindness may become clearly apparent. Blind rabbits benefit greatly from having a stable environment and companions may act as “guide bunnies”, assisting in finding food and navigating in a less familiar territory.

Conclusion

The rabbit eye can be affected by many different conditions, so it is important to perform a full diagnostic work-up to obtain a definitive diagnosis and provide more effective treatments for pet rabbits.

References

Keeble E J and Shaw D J (2006). Seroprevalence of antibodies to Encephalitozoon cuniculi in domestic rabbits in the United Kingdom, Vet Rec 158(16): 539-544. Van der Woerdt A (2012). Ophthalmologic diseases in small pet mammals. In Quesenberry K E and Carpenter J W (eds), Ferrets, Rabbits and Rodents Clinical Medicine and Surgery (3rd edn), Elsevier Saunders, St Louis: 523-531. Williams D L (2007). Rabbit and rodent ophthalmology, EJCAP 17(3): 242-252.

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Dacryocystitis can be associated with dental disease, blockage and nasolacrimal duct secondary infection.

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Dacryocystorhinography, following an appropriate contrast substance injection directly into the nasolacrimal canal, is useful to identify and localise dilation or obstruction.

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Cataract and uveitis can be a consequence of Encephalitozoon cuniculi infection.

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Progressive occlusion of the cornea with a conjunctiva-like membrane starting at the limbus (pseudopterygium).

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A retrobulbar abscess, neoplasia or haemorrhage can cause severe exopthalmos.

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Enucleation may be necessary in cases where the eye cannot be saved.

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