Eye Diseases and Disorders

The initial examination of the eye should assess symmetry, conformation, and gross lesions; the eye should be viewed from 2–3 ft (~1 m) away, in good light, and with minimal restraint of the head. The anterior ocular segment and pupillary light reflexes are examined in detail with a strong light and under magnification in a darkened room. Baseline tests like the Schirmer tear test, fluorescein staining, and tonometry (intraocular pressure measurement) may be followed by ancillary tests such as taking corneal and conjunctival cytology and cultures, everting the eyelids for examination, and flushing the nasolacrimal system to evaluate the external parts of the eye, including the anterior segment. Diseases of the vitreous and ocular fundus are evaluated by direct and indirect ophthalmoscopy (usually performed after inducing mydriasis) and vision testing (menace reflex, obstacle course, dazzle reflex, etc).

Equine recurrent uveitis (ERU) is an important ophthalmic condition, with a reported prevalence of 2%–25% worldwide. The classic form of ERU is characterized by episodes of active intraocular inflammation followed by variable quiescent periods. However, some horses experience insidious ERU, in which subclinical ocular inflammation persists without obvious signs of discomfort. With chronicity, the inflammatory bouts cause secondary ocular changes such as cataracts, lens luxation, glaucoma, phthisis bulbi, and retinal degeneration. As a result, ERU is the most common cause of blindness in horses in the world.

Thelazia californiensis is found in dogs, cats, and deer in western USA; T callipaeda is found in dogs, cats, foxes, wolves, martens, and rabbits in Europe and Asia. T callipaeda appears to be spreading in some regions of Europe. Both species are zoonotic. The worms are whitish, 7–19 mm long, and move in a rapid serpentine motion across the eye. Up to 100 eyeworms may be seen in the conjunctival sac, tear ducts, and on the conjunctiva under the nictitating membrane and eyelids. Filth flies (Musca spp, Fannia spp) serve as intermediate hosts and deposit infective larvae on the eye while feeding on ocular secretions. Zoophilic fruit flies have emerged as potential intermediate hosts of T callipaeda. These include Phortica variegata in Europe and Amiota spp in Asia.

Infectious keratoconjunctivitis of cattle, sheep, and goats is a common ocular condition characterized by blepharospasm, conjunctivitis, lacrimation, and varying degrees of corneal opacity and ulceration. In severe cases, ocular rupture leading to blindness can result. Affected animals can present at any age, but young stock are most commonly affected. The disease can occur at any time of year, but outbreaks are most often associated with warmer times of year, when risk factor exposure is greatest.

The different tissues of the eye and associated structures can be the site of primary or metastatic neoplasms. Ophthalmic neoplasms vary in histologic type, frequency, and importance in different species and are a significant group of diseases in veterinary ophthalmology.