Oculus (pl. Oculi)
Horses have among the largest eyes of land mammals; axial globe length is about 4 cm front to back (roughly 39 to 42 mm in measured breeds). The eyes are positioned laterally on the skull, providing nearly 350° of monocular vision with roughly 65° of binocular overlap in front. The equine retina contains a horizontal visual streak (rather than a fovea), giving excellent horizontal panoramic vision. Horses have excellent night vision due to a large pupil and a reflective tapetum lucidum, but relatively poor color vision (primarily blue and green wavelengths: dichromatic). The third eyelid (nictitating membrane) provides additional protection and produces a significant portion of the tear film. The nasolacrimal duct drains tears from the medial canthus through the lacrimal bone to exit at the nostril floor. The large size and lateral prominence of the equine eye make it particularly vulnerable to trauma and infection.
Equine recurrent uveitis (ERU, “moon blindness”) is the leading cause of blindness. Acute signs include squinting, tearing, cloudiness, and a constricted pupil. Corneal ulcers are emergencies; certain bacterial (Pseudomonas) or fungal infections can perforate within 24–48 hours. Always perform a fluorescein stain before administering any corticosteroid eye medication. Subpalpebral lavage systems allow treatment of painful eyes without repeated eyelid handling. Appaloosas are at higher risk for ERU.
## Overview Equine recurrent uveitis (ERU), also called moon blindness, is an immune-mediated inflammatory disease of the uveal tract and the leading cause of blindness in horses. It is characterized
## Overview Corneal ulcers are defects in the corneal epithelium, often extending into the stroma. They are common in horses and can progress rapidly to deep infection, descemetocele, or perforation
The most common tumor of the equine eye and periocular tissues. UV radiation is the primary risk factor. Common sites are the third eyelid (nictitating membrane), the eyelid margin, and the limbus (co
## Overview Cataracts are opacities of the lens that interfere with light reaching the retina. In adult horses the majority are secondary to equine recurrent uveitis. Congenital and juvenile cataract
## Overview Blocked nasolacrimal duct, often accompanied by dacryocystitis (inflammation of the lacrimal sac), prevents tears from draining from the eye into the nose. The result is chronic epiphora
## Overview Conjunctivitis is inflammation of the conjunctiva, the mucous membrane lining the eyelids and covering the white of the eye. It produces redness, swelling, and discharge. Causes include i
## Overview Entropion is inward rolling of the eyelid margin so that hair contacts the cornea. It is most common in newborn foals due to dehydration, prematurity, or excess skin. Adult-onset entropio
## Overview Habronemiasis at the medial canthus produces a yellow to reddish-brown ulcerated granulomatous mass at the inner corner of the eye. It contains gritty sulfur granules, causes intense rubb
Any sign of eye pain (squinting, tearing, swelling, cloudiness, or a horse holding the eye shut) is an emergency; call the veterinarian immediately. Do not wait to see if it improves, and do not apply any eye medication (especially steroids) without veterinary guidance. Use UV-protective fly masks during turnout to reduce fly irritation, UV exposure (lowering SCC risk in light-colored eyes), and debris. Monitor horses with known uveitis daily for recurrence. Keep emergency medications on hand as directed by your vet.
## Overview Equine recurrent uveitis (ERU), also called moon blindness, is an immune-mediated inflammatory disease of the uveal tract and the leading cause of blindness in horses. It is characterized by repeated episodes of intraocular inflammation that progressively damage the eye. ## Causes Autoimmune response, often triggered by prior infection (Leptospira in some regions) or other ocular insults, in genetically susceptible horses. ## Diagnosis Clinical signs of active uveitis (squinting, tearing, cloudy eye, constricted pupil) recurring over time, plus secondary changes such as synechiae or cataract. ## Treatment Aggressive anti-inflammatory therapy during flares (topical and systemic). Long-term management may include cyclosporine implants or other immunomodulation. Enucleation for painful blind eyes. ## Recovery and Prognosis Guarded for vision. Many affected eyes eventually become blind despite treatment. Pain control is achievable. ## Prevention Prompt treatment of any uveitis episode. Leptospira control in endemic areas may reduce risk. Signs include: recurrent squinting, tearing, and light sensitivity; cloudy cornea; constricted pupil; progressive cataract or synechiae; eventual blindness
## Overview Corneal ulcers are defects in the corneal epithelium, often extending into the stroma. They are common in horses and can progress rapidly to deep infection, descemetocele, or perforation if not treated promptly. Trauma is the usual initiating event. ## Causes Trauma from foreign bodies, scratches, or entropion; secondary bacterial or fungal infection is frequent. ## Diagnosis Fluorescein staining confirms the ulcer. Culture and cytology guide antimicrobial choice in complicated cases. ## Treatment Topical antibiotics, antifungals when indicated, atropine for pain and synechia prevention, and systemic anti-inflammatories. Deep or infected ulcers may need surgical grafting. Frequent medication is essential. ## Recovery and Prognosis Uncomplicated superficial ulcers often heal in days. Deep or fungal ulcers carry a more guarded prognosis and may leave scarring. ## Prevention Fly masks, safe stabling, and prompt attention to any ocular trauma or squinting. Signs include: squinting; tearing; cloudiness of the cornea; light sensitivity; positive fluorescein stain
The most common tumor of the equine eye and periocular tissues. UV radiation is the primary risk factor. Common sites are the third eyelid (nictitating membrane), the eyelid margin, and the limbus (corneal-scleral junction). Horses with unpigmented eyelids, Appaloosas, Haflingers, and draft horses with pink skin are at highest risk. Early detection and treatment are critical; advanced cases may require enucleation (eye removal). Signs include: Pink to red raised mass on the third eyelid, eyelid margin, or corneal limbus; may be ulcerated and bleeding; chronic ocular discharge; progressive growth; may cause corneal irritation and secondary ulceration; advanced cases invade the orbit; metastasis to regional lymph nodes in 10–15% of cases
## Overview Cataracts are opacities of the lens that interfere with light reaching the retina. In adult horses the majority are secondary to equine recurrent uveitis. Congenital and juvenile cataracts also occur and may have a genetic component in some breeds, notably Standardbreds. Severity ranges from small incidental opacities to complete blindness. ## Causes Equine recurrent uveitis is the leading cause in adults. Congenital cataracts may be inherited or developmental. Trauma and aging contribute additional cases. ## Diagnosis Ophthalmic examination with pupil dilation reveals the lens opacity. Associated signs of uveitis (synechiae, aqueous flare) are commonly present. Electroretinography may be performed before surgery to confirm retinal function. ## Treatment Small, non-progressive cataracts may be monitored. Vision-impairing cataracts are treated by surgical lens removal (phacoemulsification). Control of underlying uveitis is essential before and after surgery. ## Recovery and Prognosis Many horses regain useful vision after uncomplicated cataract surgery. Pre-existing uveitis, retinal detachment, or glaucoma worsen the outlook. Some horses return to athletic use. ## Prevention Prompt treatment of uveitis reduces secondary cataract formation. Breeding decisions may eventually be informed by genetic research in predisposed breeds. Signs include: cloudiness in the eye; vision impairment or spooking; bumping into objects; signs of uveitis such as squinting or tearing
## Overview Blocked nasolacrimal duct, often accompanied by dacryocystitis (inflammation of the lacrimal sac), prevents tears from draining from the eye into the nose. The result is chronic epiphora (watering eye), secondary conjunctivitis, and sometimes facial skin irritation. Obstruction may be congenital (atresia) or acquired from debris, infection, trauma, dental disease, or sinus problems. ## Causes Mucoid plugs, dacryoliths, foreign material, strictures after trauma, tooth-root abscesses, and sinusitis are common acquired causes. Congenital atresia of the distal opening is seen in some foals. ## Diagnosis Failure of fluorescein dye placed in the eye to appear at the nasal punctum confirms obstruction. Flushing the duct, endoscopy, or contrast radiography (dacryocystorhinography) localizes the blockage. ## Treatment Most simple obstructions are relieved by careful flushing under sedation. Recurrent or permanent blockages may need stenting, surgical creation of a new drainage pathway (canaliculosinostomy), or treatment of underlying dental or sinus disease. Topical antibiotics address secondary infection. ## Recovery and Prognosis Prognosis is good when the duct can be reopened or bypassed. Chronic dacryocystitis may require repeated flushing or surgery. ## Prevention Prompt treatment of ocular or sinus infections and protection from facial trauma reduce acquired obstructions. Congenital cases cannot be prevented. Signs include: chronic tearing from one or both eyes; wet or stained hair below the eye; recurrent conjunctivitis; occasional swelling near the medial canthus
## Overview Conjunctivitis is inflammation of the conjunctiva, the mucous membrane lining the eyelids and covering the white of the eye. It produces redness, swelling, and discharge. Causes include infection, allergens, foreign material, and underlying uveitis or corneal disease. ## Causes Bacterial, viral, or fungal infection; environmental irritants; parasites; or secondary to uveitis or corneal ulceration. ## Diagnosis Ophthalmic examination including fluorescein staining to rule out corneal ulcer. Cytology or culture when infection is suspected. ## Treatment Depends on cause: topical antibiotics for bacterial infection, anti-inflammatories when appropriate, and removal of irritants. Underlying uveitis or ulcer must be treated specifically. ## Recovery and Prognosis Good for uncomplicated cases. Untreated or misdiagnosed conjunctivitis can progress to more serious ocular disease. ## Prevention Fly control, dust reduction, and prompt attention to any eye discharge. Signs include: redness of the conjunctiva; ocular discharge; squinting; swelling of the eyelids; rubbing the eye
## Overview Entropion is inward rolling of the eyelid margin so that hair contacts the cornea. It is most common in newborn foals due to dehydration, prematurity, or excess skin. Adult-onset entropion is usually secondary to ocular pain (spastic entropion). Corneal ulceration is a frequent complication. ## Causes Congenital or acquired (dehydration, pain-induced spasm). ## Diagnosis Visual inspection of the eyelid position and fluorescein staining to check for corneal damage. ## Treatment Temporary tacking sutures or lubrication in foals. Treat underlying pain in adults. Surgical correction for persistent cases. ## Recovery and Prognosis Excellent in foals once corrected. Adult cases resolve when the primary ocular pain is treated. ## Prevention Prompt treatment of ocular pain and monitoring of neonatal foals. Signs include: inward rolling of the eyelid; tearing; squinting; corneal ulceration from hair contact; most common in newborn foals
## Overview Habronemiasis at the medial canthus produces a yellow to reddish-brown ulcerated granulomatous mass at the inner corner of the eye. It contains gritty sulfur granules, causes intense rubbing, and can damage the nasolacrimal opening. It is seasonal and often recurs. ## Causes Flies depositing Habronema larvae at the moist medial canthus. ## Diagnosis Characteristic location and appearance. Biopsy if tumor is a concern. ## Treatment Systemic and topical ivermectin, anti-inflammatories, and surgical debulking when the mass is large. Protect the eye from self-trauma. ## Recovery and Prognosis Good, though annual recurrence is common. Nasolacrimal damage may persist. ## Prevention Fly masks and rigorous fly control. Signs include: ulcerated granulomatous mass at the medial canthus; sulfur granules; intense rubbing; possible nasolacrimal damage; seasonal recurrence
Equine recurrent uveitis (ERU, “moon blindness”) is the leading cause of blindness. Acute signs include squinting, tearing, cloudiness, and a constricted pupil. Corneal ulcers are emergencies; certain bacterial (Pseudomonas) or fungal infections can perforate within 24–48 hours. Always perform a fluorescein stain before administering any corticosteroid eye medication. Subpalpebral lavage systems allow treatment of painful eyes without repeated eyelid handling. Appaloosas are at higher risk for ERU.
Any sign of eye pain (squinting, tearing, swelling, cloudiness, or a horse holding the eye shut) is an emergency; call the veterinarian immediately. Do not wait to see if it improves, and do not apply any eye medication (especially steroids) without veterinary guidance. Use UV-protective fly masks during turnout to reduce fly irritation, UV exposure (lowering SCC risk in light-colored eyes), and debris. Monitor horses with known uveitis daily for recurrence. Keep emergency medications on hand as directed by your vet.
See the Eyes on our interactive 3D horse model. Open 3D Model