Nowadays birds are considered an important part of veterinary ophthalmology practice and the knowledge of the differences between bird's and mammal's eyes is necessary to make a good approach.

Birds possess great visual acuity. Relatively to their body the globe is very large, asymmetrical, has 10-18 ossicles and may have one of three shapes: flat (most common), globose or tubular. Posterior segment is relatively larger than the anterior segment. The orbit is large, incomplete (open) and can be evaluated radiographically. Eyes are separated by a thin bony septum and are anatomically related to the sinuses and rhinarium. Therefore, infection may lead to or be indicative of infection in any of those segments. Birds have 3 eyelids: upper, lower and the nictitans membrane. The lower eyelid is more movable than the upper one. Nictitans membrane is well developed, almost transparent, blinks from nasal to temporal direction and has voluntary movement. Lacrimal gland is located inferotemporally to the globe. The main source of tears comes from harderian gland, located adjacent to the base of the nictitans membrane, but doesn't belong to it. Meibomian glands are absent. Cornea has the same layers as mammals. Iris contains a myoepithelium, striated sphincter, dilator and smooth muscles. Pupil is circular and influenced by retinal stimulation and/or voluntary control. A soft lens allows rapid accommodation. Vitreous body is large and transparent. Retina is thick and has no direct blood vessels, as so the pecten (a pigmented vascular structure protruded into the vitreous) is responsible for retinal nutrition. Retina contains many cone cells allowing a notorious colour vision; its fovea is well developed, permitting a good visual acuity. Eye movements are limited because extraocular muscles are poorly developed; on the other hand, it is compensated by greater head mobility and their ability to move both eyes independently.

Ophthalmic Examination

Clinical examination of pet bird's eyes utilizes the same instrumentation as in mammals, albeit with modification dictated by the eye size and physiological characteristics of the iris. Complete ophthalmic examination includes anamnesis, function and morphologic examination. Several tests can be performed, such as: palpebral and pupillary reflex, Schirmer tear test, Fluorescein and Rose Bengal staining tests, tonometry, direct and indirect ophthalmoscopy, ultrasonography, electroretinography, cytology and cultures.

The eyes must be clear, round, centrally placed, and moist and shine, presenting no epiphora or ocular discharge. An abnormal tear film may indicate xerophthalmia associated with hypovitaminosis A. There should be no evidence of periocular or periorbital swellings. Medial swelling may be associated with a primary sinusitis; swellings above or below the eye are usually associated with hypovitaminosis A or lacrimal glandular epithelial tissues metaplasia. Every swelling should be further investigated submitting swabs, needle biopsies or flushings for cytology, bacteriology or histopathology tests. Routine ophthalmoscopy should disclose abnormalities of cornea or lens; however, assessment of fundus lesions requires more experience. The lower lid should be pulled away from the eye and examined for foreign bodies or inflammation of the palpebral conjunctiva; cornea must be examined for evidences of scratch or ulcers. Fluorescein staining test is used to confirm the presence of ulcers, and is mandatory prior to the application of any topical agent to the eye. The anterior chamber should be examined for hyphaema, iris prolapse and anterior synechia. The lens can be examined for evidence of cataracts, and the retina must be examined for evidence of detachment.

Orbital Diseases

Orbital diseases usually induce exophthalmos and are frequently related to trauma, inflammation of the Harderian gland, infection of paranasal sinuses or neoplasias. Infraorbital sinusitis, generally associated with respiratory diseases, is frequently diagnosed in psittacines, resulting in swelling of the medial and ventromedial areas of the globe. Exenteration of the orbit is recommended when neoplasia is confirmed (Bayon et al, 2007).

Conjunctivitis

Conjunctivitis is the commonest ocular disease in captive birds, nonraptors species and is very uncommon in raptors, unless when trauma is associated. There are numerous causative agents of avian conjunctivitis, many of which are associated with systemic diseases. A serous or seropurulent discharge is frequently present. Different from mammals, a mucoid component is not present. The conjunctiva becomes chemotic, erythremic and with its chronicity, discoloration of periocular skin may occur. Bacteria and viruses are the most commonly infectious isolated agents. Bacterial conjunctivitis may be an extension of the upper respiratory system infection or secondary to an opportunistic infection of the normal conjunctival flora. Conjunctivitis has been associated with Chlamydophila psittaci, Streptococcus spp, E. coli, Pseudomonas aeruginosa, Bordetella avium, Clostridium botulinum and Erysipelothrix rhusiopathiae.

Avian poxvirus is responsible for most cases of viral conjunctivitis in birds; clinical signs appear approximately 10 to 14 days after infection.

Hypovitaminosis A may predispose to diseases of the mucous membranes, like keratitis or conjunctivitis. Conjunctival hyperkeratosis results in eyelid edema. Treatment with supplementation of vitamin A is effective.

Keratitis

Poxvirus is responsible for the majority of viral infections in birds' eyes and results in proliferative multifocal keratoconjunctivitis. Some sequelae of the resolution of this disease are lid deformity, corneal vascularization, symblepharon, cataracts, phthisis and others. Corneal degeneration can be observed in Amazon parrots following poxvirus infection. Diagnosis can be confirmed histologically. The treatment is directed toward prevention of secondary bacterial infection.

Uveitis

The main causes of uveitis in birds are trauma, infections, immune-mediated inflammation and neoplasia. Treatment should include the elimination of the cause, control of inflammation and preservation of the pupil. Topical antibiotics and topical and systemic antiinflammatories are usually prescribed.

Cataracts

Cataracts have been frequently diagnosed in domestic, captive and wild birds. Heritable cataracts were reported in small kindred Yorkshire and Norwich canaries. Senile cataracts were identified in Amazon parrots and macaws. Cataracts secondary to nutritional deficiencies, trauma and retinal degeneration can be either observed in birds. Lens opacities may be capsular, cortical and/or nuclear. Phacoemulsification has been performed in birds, like as in mammals but depends on the eye size. In birds with very small eyes, complete blindness is not a reason for euthanasia since the interior of the cage is not modified at all.

Glaucoma

Glaucoma can be secondary to uveitis. Buphthalmia is not severe, due to the inflexibility of sclera ossicles. Topical treatment is unproven for birds. Enucleation of the eye may be performed in some cases.

Retinal Diseases

Retinal diseases include congenital anomalies, degeneration, inflammation and detachment. Observed signs are depigmentation, hyperpigmentation and loss of the choroidal vascular patterns. Intraretinal haemorrhages or detachment can be observed after ocular trauma.

Neoplasia

Neoplasia involving the avian eye and adnexa seems relatively uncommon. Eyelid and conjunctival neoplasia has been rarely reported.

Trauma

Ocular trauma is frequently observed in raptors and may be very complex, involving periorbital tissue, the anterior and posterior ocular segments. Ophthalmic signs includes: periocular bruising, subconjunctival hemorrhage, conjunctivitis, hyphaema, anterior uveitis, and posterior segment abnormalities. Fundus examination is relatively easy in raptors because of their eyes size. High prevalence of retinal injury, intravitreal hemorrhage and perforation of the posterior segment has been found. When only anterior segment abnormalities are present, prognosis for vision is sometimes good. When the posterior segment is involved, prognosis is usually guarded or poor.

References

1.  Abrams GA, Paul-Murphy J, Murphy CJ. Conjunctivitis in birds In: The Veterinary Clinics of North America--Exotic Animal Practice p. 287-309, 2002.

2.  Bayon A, Almela RM, Talavera J. Avian Ophthalmology. European Journal Companion Animal Practice vol. 17, Issue 3 December, p. 2007.

3.  Holmberg B. Ophthalmology of exotic pets In: Slatter's Fundamentals of Veterinary Ophthalmology p. 427-441, 2008.

4.  Kern TJ. Exotic animal ophthalmology In: Veterinary Ophthalmology, p. 1370-1405, 2007.

5.  Lawton, M. The physical examination In: Avian Medicine p. 26-42, 2003

6.  Macwhirter P. Basic anatomy, physiology and nutrition In: Avian Medicine p. 31, 2003.

7.  Ofri R. Retina In: Slatter's Fundamentals of Veterinary Ophthalmology p. 285-317, 2008.

8.  Redig TP, Ackermann J. Raptors In: Avian Medicine p. 180-214, 2003