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Ocular diseases of nontraditional and exotic pets (Proceedings)

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Aug 01, 2010

When presented with exotic species with ocular disease it is important to remember the concept of "the same and different". Eye are eyes regardless of the species and many of the conditions we are presented with in pocket pets and other exotic species are in many ways the same as those we have come to recognize in the more usual domestic species and can be diagnosed and treated empirically in the same way.

However differences in anatomy, husbandry and nutritional requirements and susceptibility to unusual pathogens in the more exotic pet species do result in ocular diseases unique to exotics species which need more general awareness and knowledge of exotic or laboratory animal medicine than may be taught in conventional veterinary curricula.

The general concepts of a thorough and logical ophthalmic examination apply as much in exotics as domestic species although particular aspects of the anatomy or biology may affect how the examination is conducted. Many non-domestic and exotic species are likely to require an understanding of restrains and sedation techniques in order to effectively conduct the ophthalmic examination. Use of methods of examination from a distance and maximizing the use of photography can be very helpful in evaluation of ocular lesions.

Amphibians

Larval stages have eyes like teleost fish. Eyelids generally poorly developed in adult stages. Strong retractor bulbi muscles involved in eye protection and swallowing. Third eyelid may limit examination of the globe. Globe has cartilage present. Considerable variation in the appearance of the iris. Avascular retina with membrane vasculosa retinae (vitreal)

Ocular trauma is common (corneal ulcers). Cautious use of antibiotics due to skin absorption. Often difficult to medicate due to aquatic component of lifestyle. Panophthalmitis is seen bilaterally associated with systemic bacterial disease and stress 9often in overcrowded and recently shipped groups of animals).

Lipid keratopathy occurs commonly in captive species. White deposits in the corneal tissue with progression of keratitis with vascularization and pigmentation. This is diet related (often fed insects with high fat content. Reversal of the condition is not often effective and systemic involvement may be fatal.

Occasionally encounter cataracts and glaucoma of unknown etiology. Enucleation is possible for glaucoma.

Reptiles

Large lower eyelid which may have a clear window in some lizard species. Very strong lid closure in crocodilians and chelonians (also head retraction in shell) makes examination of the eye very difficult. This is compounded in species with a third eyelid which may obscure a view of the globe. Examination under anesthesia may allow for lid opening. Mydriasis requires non-depolarizing neuromuscular blocking drugs.

Lids in snakes and many gecko fused to form spectacle (which is a vascular structure) – this affects type of ocular diseases encountered (few obvious corneal ulcers, common to get infectious disease of the conjunctival space and problems associated with ecdysis). Lack of nasolacrimal duct in chelonians. Globe has sclera cartilage and in some species sclera ossicles. Considerable interspecies difference in the iris appearance and function. Variation in lens accommodative mechanisms. Fundus is avascular but lizards have a conus papillaris.

Ocular malformations are quite common – particularly in captive bred collections of animals. Causes are usually unknown.

Eyelid diseases may include viral, bacterial and fungal infections. These may reach epidemic proportions in both wild and captive animals. Predisposing cause are the stresses of shipping and poor environmental hygiene. Not infrequently bacterial and fungal ocular lesions may reflect other more insidious systemic infections. Surgical debridement and use of both systemic and topical anti-biotic and antifungal therapy is required.

Spectacle disorders are common in geckos and snakes. The spectacle is normally shed at each ecdysis. Spectacles may become indented and irregular in shape but usually self correct at the next shed. Retention of the spectacle during shedding can become a problem if the layers of spectacle accumulated. There is a predisposition in very dry environments and where ectoparasites are present. Increasing the humidity just before shedding and treatment of ectoparasites often is effective in resolving the problem. The retained spectacle should be removed manually with great care to avoid corneal exposure.

Infections in the space between the spectacle and cornea and conjunctiva are very common in geckos and snakes. These are often associated with systemic septicemia, necrotic stomatitis but may occasionally arise as congenital conditions associated with abnormal conjunctival sac drainage via the lacrimal duct to the mouth. Common bacterial isolates are typical of reptile infectious diseases (usually Gram –ve species). The significance of protozoa found in the exudates is unknown. Drainage and flushing of the exudates via a window in the spectacle may allow topical treatment – however recurrence is likely. In resolved cases scarring of the spectacle and cornea may result. Systemic antibacterial and antiprotozoal therapy is usually required.

Conjunctivitis is common is reptiles and diagnosis of the etiology is often difficult. Hypovitaminosis A may be associated with some of these cases. Many will however respond to empirical antibiotic, corticosteroid and NSAIDs therapy. Swelling of the eyelids and conjunctiva is commonly seen in chelonians due to hypovitaminosis A which reflects enlargement of the lacrimal glands due to squamous metaplasia. Systemic lesions of the viscera can be fatal if untreated. Correction of the diet to include vitamin A is usually curative.

Species without spectacles are commonly affected with traumatic corneal abrasions and ulcers.

Treatment is the same as in domestic mammals. Lipid keratopathy is seen commonly in older animals and does not warrant treatment.

Cataracts are common in reptiles – diet, environmental factors and age are the most likely causes. Uveitis occurs associated with systemic infectious disease. Glaucoma is a rare finding in reptiles.