Anterior Uveitis, Cataracts, and Glaucoma
Anterior uveitis (inflammation of the iris and ciliary body) produces a variety of changes within the eye. These include decreased production of aqueous humor (decreased intraocular pressure), increased protein content of aqueous humor (flare), and accumulation of leukocytes (hypopyon) or erythrocytes (hyphema).
Constriction of the pupil is an important diagnostic sign, as is photophobia; the latter occurs because of painful inflammation of the ciliary muscle. Organophosphate toxicosis, which is also accompanied by miosis, is an important differential. Miosis has also been reported with urea poisoning (Schmidt 1973). Adhesions may form between the swollen iris and the lens or cornea. Retinal detachment and glaucoma are other potential sequelae.Causes of Anterior Uveitis
Septicemia
Hypopyon, deep neovascularization of the cornea, and other signs of anterior uveitis sometimes accompany neonatal septicemia. Common causative bacteria include enteric coliform bacteria and Mycoplasma spp. (Whitley and Albert 1984). The posterior segment of the eye or the brain may also be infected. Appropriate systemic antibiotic therapy is critical to survival of the animal. If the infectious agent is resistant to the antibiotic chosen, the corticosteroids used to treat the secondary anterior uveitis may exacerbate the primary infection.
Toxemia
Constriction of the pupil and anterior uveitis may be noted in goats with severe toxic mastitis or metritis.
Deep Keratitis
Severe keratitis resulting from infectious keratoconjunctivitis or exposure keratitis may extend to or cause reflex inflammation of deeper portions of the globe.
Retroviral Infections
A non-suppurative chorio-iridocyclitis, sometimes accompanied by granuloma formation, has been documented in the eyes of goats with chronic neurologic disease believed in retrospect to be caused by retroviral infection (Stavrou et al.
1969; Dahme et al. 1973). The prevalence of anterior uveitis in goats with caprine arthritis encephalitis or related virus infections (see Chapter 5) is unknown. Clinical blindness could be the result of a brain lesion rather than an eye lesion and few studies have reported histology of the eye. In one group of goats naturally infected with caprine arthritis encephalitis virus, no histologic lesions were found in optic nerves or eyes (Sundquist et al. 1981).Toxoplasmosis
At least in other species, toxoplasmosis can lead to iridocyclitis and necrotizing granulomas on the retina and ciliary body. In one study including sheep (but no goats), granulomatous ocular lesions were demonstrated in eyes from 12 of 18 sheep with experimental toxoplasmosis (Piper et al. 1970). The possible occurrence of ocular lesions in kids congenitally infected with toxoplasmosis needs to be investigated.
Trauma
Blunt trauma to the head may induce hyphema or an anterior uveitis. This might explain the occasional occurrence of fibrin tags attached to the iris of goats with central blindness because of polioencephalomalacia (Smith 1979). In general, it is wiser to administer thiamine whenever blindness is accompanied by any other neurologic signs, rather than to exclude from consideration a common disease based on the presence of an unusual sign.
Treatment of Anterior Uveitis
Even though the exact cause is rarely determined, in the absence of fluorescein uptake by the cornea, treatment of the eyeball showing such signs should include corticosteroids and atropine. The corticosteroids are generally given by bulbar subconjunctival injection with a tuberculin syringe and a 25-gauge needle, or by frequent application of topical medications. The atropine might be given by injection, but more commonly ointment or drops are instilled every few hours until the pupil dilates, then once or twice a day. Both painful spasm of the ciliary muscle and the danger of adhesion formation are decreased by using atropine.
The animal should be allowed access to a darkened stall until the ability to constrict the pupil is regained after discontinuation of therapy.Cataracts
Cataracts are opacities of the lens or its capsule. They can be documented by ultrasound (El-Tookhy and Tharwat 2013). Incipient cataracts are commonly present in apparently healthy adult goats (Broadwater et al. 2007). Uveal pigment may be deposited on the anterior lens capsule. Fluid accumulation within the lens or denaturation of lens protein may hinder light transmission. A cataract can occur as a sequela to any severe anterior uveitis if not vigorously treated. Referral to a specialist should precede any surgical attempt at cataract extraction, although enucleated goat eyes have been used as a training model for phacoemulsification (Dada and Sindhu 2000).
Glaucoma
The large ciliary cleft of the goat's iridocorneal angle (Kassab and El-Zoghby 2010) probably helps to explain why glaucoma is rare in this species.
Secondary glaucoma has been reported as occurring after severe anterior uveitis induced by M. agalactia (Moore and Whitley 1986). The intraocular pressure rises because the iridocorneal angle is closed by the inflammatory process. Both the anterior uveitis and the glaucoma may contribute to corneal edema, as does anterior lens luxation if it occurs. Buphthalmia (enlargement of the globe) may initially protect the retina. When intraocular pressure becomes high enough to damage the retina, permanent blindness occurs. Proper use of mydriatics and corticosteroids to prevent secondary glaucoma is paramount.
Tonometry
The only method available to most large animal practitioners for estimating intraocular pressure is digital tonometry. The index and middle fingers of one hand are used to slightly flatten the globe by applying pressure through the upper lid. The other hand simultaneously evaluates the second eye of the patient. At least until experience is gained, comparison with a normal goat is helpful.
The Schiotz tonometer, although previously available in many small animal practices, is not practical for use in goats. The goat must be in dorsal recumbency for the cornea to be accessible when the tonometer is held vertically. A topical anesthetic is required. Struggling or compression of the jugular veins might artificially elevate the intraocular pressure. By contrast, Tono-Pen® (applanation; Reichert Technologies, Depew, NY, USA) and iCare® TONOVET (rebound; Icare Finland, Vantaa, Finland) tonometers are easy to use on goats. There are a few reports of normal ranges for the goat using these devices, with means approximately 8-11mmHg (Broadwater et al. 2007; Ribeiro et al. 2010b). In contrast, in a study by Peche and Eule (2018) of 20 Toggenburg does with normal eyes, the TONOVET values were higher (median 23, range 9-37mmHg) than those obtained with the Tono-Pen (median 13, range 4-25 mmHg). Values from the patient could also be compared with intraocular pressure measurements from several normal goats of similar age.
Treatment
It is unlikely that glaucoma will be diagnosed in an affected goat until the cornea is becoming opaque and the globe is enlarging. In most instances, medical management is impractical. Prompt referral to an ophthalmic surgeon may be appropriate for certain valuable animals. Otherwise, the practitioner should plan to enucleate the glaucomatous eye when the goat appears to be in persistent pain or when the eyelids can no longer cover the enlarged globe well enough to prevent drying of the central cornea. An alternative to enucleation might be an injection of gentamicin plus corticosteroid into the vitreous to destroy the ciliary body.