Ophthalmology has become an important medical specialty for managing the health of exotic and wild animals. Because of the current explosion in popularity of the exotic pet market, improved medical attention to endangered species in zoos and the public demand for novel exotic pets, awareness of pertinent clinical signs, diagnostic methodology and treatment of ocular diseases involving exotic and wild animals became a true a necessity for veterinary ophthalmologists.
Practicing Ophthalmology in Wild and Exotic Animals
It is true that the eyes of birds and reptiles present some important anatomic and physiologic differences from those of domestic pets. Nevertheless, even considering these 2 classes (birds and reptiles), eyes of all exotic or wild animals still show far more similarities than differences. Basically, the eye of all vertebrates follows the same basic structure consisting of 3 concentric layers and the lens. These tissue layers, or tunics, are: 1) the outermost fibrous tunic, divided into the cornea and sclera (with or without cartilaginous/bony tissue); 2) the middle tunic, called the uveal tract, a richly vascular layer with or without a tapetum lucidum, pecten (birds), conus papillaris (reptiles) or falciform process (fish); and 3) the innermost, or neural, tunic, composed of the retina and optic nerve.
It is not an exaggeration to state that ophthalmic diseases in wild and exotic pets are very similar to those observed in domestic species. The main advices for practicing exotic or wild animal ophthalmology are: 1) Knowledge of specific anatomy, behavior, nutritional habits and common health problems of the various classes and orders of exotic and wild animals; 2) Knowledge of restraint methods commonly used with various species; 3) Awareness that limited medical literature is available on specific exotic animal ophthalmic disease compared to the amount of literature available for domestic animals. Information regarding normal physiologic parameters for each species is also scarce.
The Ophthalmic Examination
Assessing vision in exotic species can be difficult, because reliance on menace responses and avoidance behavior are often meaningless in some of the lower vertebrates and wild mammals. However, careful observation of the animal's behavior, tests using a cotton ball (visual tracking), optokinetic devices and maze tests are still very useful in some species.
Particularities About Instruments, Clinical Tests and Materials for Exotic and Wild Animal Ophthalmology
In snakes and some geckos, the presence of the spectacle renders Schirmer Tear Tests, as well as tonometric evaluation, impossible. Some exotic animals, mainly small mammals and passerines, have very small palpebral apertures and conjunctival sacs. For pupil dilation a mixture of equal parts 1% atropine and 10% phenylephrine should be used in exotic or wild mammals with darkly-stained iris tissues, because the melanin granules can potentially bind to tropicamide and inhibit its action. Since the ciliary muscles of birds and reptiles are composed of striated muscle rather than smooth muscle fibers, conventional mydriatic agents (parasympatholytics), such as tropicamide and atropine, or even sympathomimetic drugs, are ineffective in producing pupillary dilation. Mydriasis (lasting 2-4 hours) may be achieved with inhalant or injectable anesthesia or topical administration of curariform drugs.
Selected Ophthalmic Disorders of Exotic Animals
Conjunctivitis and Dacryocystitis in Rabbits
Conjunctivitis in lagomorphs is commonly secondary to infections of the lacrimal drainage apparatus (dacryocystitis). The lacrimal duct in rabbits is narrow; with suppurative inflammation it becomes blocked, resulting in epiphora. Brown tear staining, hair loss and periocular dermatitis are common signs of conjunctivitis in lagomorphs. With dacryocystitis, mucopurulent material may be seen emerging from the lower lacrimal punctum or may be expressible by digital pressure over the canthal region. The treatment involves flushing the lacrimal ducts with a small (24-gauge) lacrimal cannula, which is inserted in the nasolacrimal punctum (lower only in lagomorphs), and prescribing topical antibiotics and steroids (the latter if the cornea is not ulcerated). In some cases, the origin of conjunctivitis or just chronic conjunctival discharge may, in fact, be due to periodontitis involving tooth roots anatomically positioned close to the lacrimal duct. Radiographs can help determine the source of the problem. Dacryocystorhinography, using 0.2 ml of non-ionic contrast medium, is also useful in diagnosing diseases of the lacrimal apparatus. In some cases of severe tooth root disease, protrusion of the globe (exophthalmos) is visible. The condition is treated by extraction of the teeth with affected roots. Careful systemic treatment should be implemented.
The opacification of the lens, lens capsules or both (cataracts) is a common eye disease in most exotic and wild animals. The general etiology of cataracts is basically the same as for domestic animals. Trauma is always a common cause of cataracts regardless of the species. Heritability and genetics are common causes of cataracts in exotic animals and in inbred wild animals. Inherited cataracts occur in canaries, rodents, guinea pigs, ferrets, guanacos, vicunas and presumably other species. Diabetes mellitus also is a common cause of metabolic cataract in several exotic species. There are, however, some unique causes for lens opacification in some species. For instance, reversible cataracts develop in some rodents during heavy sedation or anesthesia, and the same phenomenon can occur in fish subsequent to osmotic changes. Cataracts in tortoises have been associated with freezing episodes. Some parasites, such as Diplostomum spathaceum in freshwater fish, cause cataracts by direct larval invasion of the lens. Senile cataracts have been reported in various species, including Amazon parrots and macaws. In the absence of uveitis, cataract removal may be very effective (using phacoemulsification) in larger species. The lens of birds and reptiles are substantially softer than in other species, which can make cataract surgery relatively easier in these species. There is, however, considerable variation in the reported success rate when removing cataracts in exotic species.
Stromal Keratitis (Stromal Abscess) in Lagomorphs and Rodents
Stromal keratitis or abscesses are quite common in lagomorphs and rodents, chinchillas in particular. In these lesions epithelial migration proceeds over a traumatic corneal injury, which then seals infectious agents (bacterial, fungal or both) in the underlying stroma. Patients presenting with this condition usually have conjunctivitis and a thick mucopurulent discharge. Often, a white plaque can be observed on the patient's cornea, corresponding to purulent stromal infiltrate, which is sometimes surrounded by an intense stromal keratitis and vascularization. The presence of corneal blood vessels may partly obscure the abscess. Uveitis may be present as well. Treatment includes intensive topical and occasional systemic antibiotics (and if indicated, antifungals), iridocycloplegics, and topical and systemic anti-inflammatory drugs. In some cases surgical drainage of the abscess is necessary. Surgery also provides material for culture and cytology to obtain an etiological diagnosis. These stromal abscesses are often septic, but on occasion, they are sterile. In most septic cases Staphylococcus spp. or Pasteurella spp. are isolated from the infection site.
Periorbital Swelling or Abscesses in Birds and Reptiles
Some psittacines can voluntarily inflate the infraorbital sinus as a display of aggression in order to look bigger; this is a normal process, though. The infraorbital sinus opens dorsally into the nasal cavity and is a common site of bacterial infections in birds. Consequently, infectious material contained in the sinus cannot drain easily to the nasal cavity and often results in severe periorbital swelling. These infections can be primary (sinusitis) or an extension of ocular or respiratory disease. The main causes for periorbital swelling are bacterial infections, fungal infections and vitamin A deficiency. Neoplasia is far less common than infection. Cultures and cytologic analysis, using a needle to aspirate material from the sinus, are often necessary to establish the etiology. When aspirating the sinus, the needle should be inserted below the zygomatic arch. Drainage, systemic medication, nebulization to treat the underlying respiratory disease, sinus flushes and surgical debridement are often necessary components of the treatment. Topical antibiotics should be ideally selected based on culture and sensitivity testing. When hypovitaminosis A is involved in the process, the bird often presents with keratoconjunctivitis sicca (KCS, or "dry eye"). If vitamin A deficiency is suspected, vitamin A should be administered and the necessary dietary changes implemented. Keratoconjunctivitis sicca should be treated with topical 0.2% cyclosporine in conjunction with vitamin A supplementation. Caseous orbital and periorbital abscesses are commonly encountered in some groups of birds, such as the chopi blackbird (Gnorimopsar chopi). Reptiles also develop a similar clinical condition, often related to respiratory disease. Periocular inflammatory disease in reptiles may also be associated with septicemia. Periorbital caseous abscesses are quite common in lizards (especially chameleons), and treatment consists of surgical debridement and systemic antibiotic therapy based on culture and sensitivity results.