I. Eyes
As instinctive prey animals, horses are very dependent on vision, with a total visual field of almost 350 degrees. Ocular health is therefore critical to the well-being of horses, which have particularly large and prominent eyes, making them prone to injury. Whereas many ocular diseases have the potential to heal if detected early and treated adequately, they can just as easily progress to irreversible vision loss. This chapter summarizes both congenital and acquired inflammatory, degenerative, and neoplastic ocular diseases that affect the horse.
1. Congenital ocular diseases
Ocular anomalies usually affect multiple structures, reflecting the stepwise embryology and interdependence of the various parts of the developing eye.
Figure 14.1. Horse. Eye. Cornea Globosa. Horses with this alteration have a globular corneal contour with a protruding cornea and a deep anterior chamber. (Courtesy Comparative Ophthalmology Service, Michigan State University, East Lansing, MI.)
Cornea globosa can be seen in addition to multiple other anterior segment anomalies such as cataract, lens luxation, and iris hypoplasia as part of the multiple congenital ocular anomalies (MCOA) syndrome described in Silver-Colored Rocky Mountain horses, Kentucky Mountain saddle horses, Icelandic horses, Miniature horses, Morgan horses, and Shetland ponies.
Figure 14.2. Fetus. Eye. Severe Microphthalmia. No internal structures, such as lens and retina, are apparent in this malformed globe that consists solely of a scleral sheet and some pigmentary differentiation, typical of persistence of the primary optic vesicle. The adjacent plate of cartilage is likely derived from the third eyelid analog. (H&E)
Entropion and Ectropion. Inversion of the eyelid margin (entropion) may occur as a primary condition or may be secondary to enophthalmos associated with microphthalmos and prematurity or dysmaturity. It generally affects the lower eyelid, but in Miniature horses it may affect the upper eyelid only. Eversion of the eyelid margin (ectropion), rarely congenital, is more often secondary to scarring of the eyelids resulting from trauma or burns.
Aniridia and Iridal hypoplasia. Complete absence of the iris (aniridia) is a rare congenital anomaly that is usually bilateral. Underdevelopment and thinning of the iris (iridal hypoplasia) is, in contrast, relatively common in Appaloosas and eyes with heterochromia iridis. The thin iris stroma may balloon into the anterior chamber from the higher pressure in the posterior chamber.
Retinal dysplasia is an uncommon developmental lesion characterized histologically by retinal folds and rosettes. It may occur in association with other ocular anomalies in animals with anterior segment dysgenesis. Retinal detachments are generally secondary to posterior segment inflammation in equine recurrent uveitis (ERU), head trauma, perforating globe wounds, and tumors. Nonetheless, they may also be congenital, possibly associated with other ocular abnormalities.
2. Degenerative ocular diseases
Figure 14.3. Foal. Eye. Icterus. Neonatal Isoerythrolysis. The conjunctiva is one of the first tissues to change color as bilirubin levels rise in diseases that lead to jaundice.
Atrophia and phthisis bulbi. These terms refer to shrunken eyes at, respectively, the advanced and end stage of severe ophthalmitis, most commonly seen in association with corneal perforation. When ocular structures have still some recognizable orientation, the alteration is more properly termed atrophia bulbi. The phthisic eye is structurally more disorganized, with barely any recognizable content.
Figure 14.4. Horse. Eye. Phthisis Bulbi. Clinically, this condition is characterized by a shrunken globe within a normal sized orbit following severe trauma. The third eyelid is prominent. (Courtesy Comparative Ophthalmology Service, Michigan State University, East Lansing, MI.)
Figure 14.5. Horse. Eye. Atrophia Bulbi. This horse had previous trauma to this eye. Ophthalmologic exam revealed enophthalmos, corneal laceration near the medial canthus, and a shrunken globe. A stick was found lodged between the medial canthus and the globe.
Figure 14.6. Horse. Eye. Atrophia Bulbi (same as in Figure 14.5). On cut section, the globe is shrunken and distorted, but the internal structures are still partially recognizable. There is lens rupture and complete retinal detachment. Lenticular capsular and retinal fragments are surrounded by granulating fibrosis, hemorrhage, and inflammation. The limbus and sclera are focally perforated, with prolapse of the anterior uveal tract through the defect in the fibrous tunic.
Cataract. Lenticular opacity is a common sequela to uveitis, either as a result of uveal adhesions to the lens surface, altered aqueous flow with lenticular malnutrition, exposure to injurious inflammatory by-products, or stagnation of the aqueous humor in postinflammatory glaucoma. It is also a frequent congenital ocular defect in foals.
Figure 14.7. Foal. Eye. Cataract. The late immature cataract affecting more than 90% of the lens of both eyes of this animal and the absence of any other sign of ocular disease suggested a congenital inherited rather than an acquired condition. (Courtesy Comparative Ophthalmology Service, Michigan State University, East Lansing, MI.)
Prolapse or herniation of orbital fat. This is a rare condition that may be associated with weakened episcleral fascia or trauma. The resulting lobular subconjunctival masses can mimic neoplasms, from which they can be easily differentiated through cytologic or histologic evaluations.
Figure 14.8. Horse. Eye. Partial Prolapse of the Extraorbital Fat Pad. The prolapse involves the third eyelid and is complicated by secondary bacterial infection. (Courtesy Comparative Ophthalmology Service, Michigan State University, East Lansing, MI.)
Glaucoma. Once thought to be a very rare disease in horses, glaucoma has emerged as a relatively frequent, underdiagnosed condition in this species. Clinical claims that affected horses all have had previous bouts of uveitis are often difficult to substantiate by histologic assessment. Most horses with glaucoma have no histologic explanation for the development of this disease. Elevation of intraocular pressure in such cases may be due to obstruction of alternative uveal routes of aqueous outflow through the ciliary muscle interstitium into the suprachoroidal space and sclera.
Figure 14.9. Horse. Eye. Haab’s Striae. The numerous white linear streaks in the cornea of this horse with glaucoma represent breaks in the Descemet’s membrane. They are the result of corneal stretching due to enlargement of the glaucomatous globe (buphthalmos). Note also the concurrent presence of circumlimbal corneal neovascularization affecting approximately 50% of its circumference. (Courtesy Comparative Ophthalmology Service, Michigan State University, East Lansing, MI.)
Senile changes. Aged horses present a number of minor degenerative ocular alterations that, if extensive, may culminate in vision deficit.
Figure 14.10. Horse. Eye. Retina. Degeneration. Peripheral retinal degeneration is a finding related to old age characterized by neurosensory retinal degeneration with pigmentary changes. Additionally, the aged horse can have chorioretinal scars in the fundus, most commonly in the nontapetal region, which may represent the sequela of subclinical equine herpesvirus-1 infection. (H&E)
3. Inflammatory ocular diseases
Orbital cellulitis forms in response to infectious agents introduced into the orbit via a penetrating wound, a migrating foreign body, or an inflammatory focus in an adjacent location (e.g., sinusitis and tooth root abscess). Only rarely does uncontrolled endophthalmitis spread through the sclera into the orbit because the sclera is generally an effective barrier to leukocytes and infectious agents.
Figure 14.11. Horse. Eye. Severe Facial and Orbital Cellulitis. The severe unilateral swelling of the supraorbital fossa, exophthalmos, and hyphema in this horse were associated with an oral fistula. Similar lesions may also occur as the result of injuries of the orbital region on the racetrack, in trailers, or on the pasture, by rearing and hitting stall ceilings, or from kicks by other horses.
Figure 14.12. Horse. Face. Cutaneous Habronemiasis. The medial canthus is one of the preferred areas for aberrantly migrating larvae of Habronema muscae, H. majus, and Draschia megastoma to develop ulcerative lesions that expose granulation tissue containing eosinophilic granulomas. (Courtesy Dr. Rafael A. Fighera, Santa Maria, RS, Brazil.)
Corneal ulceration. Loss of corneal epithelium has a number of causes such as physical and chemical trauma and bacterial or fungal infection. Bacteria and fungi are generally secondary. Ulcers may heal, may persist as nonprogressive lesion, or may progress to involve more of the stroma, with the possibility of eventually leading to corneal perforation.
Figure 14.13. Horse. Eye. Cornea. Superficial Corneal Ulceration and Band Keratopathy. The nonadherent loose epithelium at the inferior ulcer margin is characteristic of a persistent or indolent ulcer. Additionally, there is also mild diffuse corneal edema and neovascularization. (Courtesy Comparative Ophthalmology Service, Michigan State University, East Lansing, MI.)
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