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).

Chlamydial conjunctivitis refers to an acute, chronic, or recurrent infection of the conjunctiva of a variety of animals with intracellular bacteria of the family Chlamydiaceae. Although often asymptomatic, infection may also lead to acute or chronic purulent inflammation of the conjunctiva with or without the presence of keratitis or other pathologies. Antimicrobial treatment typically involves administration of tetracycline-class antibiotics. Systemic administration is preferred because chlamydial conjunctivitis may arise as a result of systemic infection and/or involve shedding at other anatomic sites.

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.

Thelaziases are caused by spirurid nematodes (also known as eyeworms), which cause infestation of the orbital cavities and associated tissues of several animal species. These whitish nematodes are transmitted by nonbiting dipteran muscid flies or drosophila fruit flies (intermediate hosts) while feeding on the lacrimal secretions of infected animals. Adults of Thelazia spp localize under the eyelids and the nictitating membrane, as well as in nasolacrimal ducts, conjunctival sacs, and excretory ducts of lacrimal glands, depending on the species targeted. Thelaziases may be responsible for eye disease, with signs and symptoms of varying severity (eg, lacrimation, ocular discharge, epiphora, conjunctivitis, keratitis, and sometimes corneal opacity or ulcers). Levamisole and ivermectins (eg, doramectin) administered either subcutaneously or intramuscularly, are useful for the treatment of thelaziasis in cattle and horses. Some Thelazia spp may also infect humans.

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.