Oculus (pl. Oculi)
Horses possess the largest eyes of any land mammal (approximately 65 mm in diameter, similar in size to a human eye but with a much larger globe). The eyes are positioned laterally on the skull, providing nearly 350° of monocular vision with approximately 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; they see primarily in blue and green wavelengths (dichromatic). The third eyelid (nictitating membrane/membrana nictitans) provides additional protection and produces approximately 30% of the tear film. The nasolacrimal duct drains tears from the medial canthus through the lacrimal bone to exit at the nostril floor.
The equine eye is particularly vulnerable to trauma and infection due to its large size and lateral prominence. Equine recurrent uveitis is the leading cause of blindness. Any sign of eye pain (squinting, tearing, swelling) is an emergency; corneal ulcers can perforate within 24–48 hours with certain bacterial (Pseudomonas) or fungal infections. Always perform a fluorescein stain before administering any corticosteroid eye medication. The subpalpebral lavage system allows treatment of painful eyes without repeated handling of the eyelids.
The #1 cause of equine blindness worldwide, affecting 2–25% of horses depending on the region. An immune-mediated inflammatory disease of the uveal tract (iris, ciliary body, choroid) with recurring e
Loss of corneal epithelium and potentially deeper layers (stroma, Descemet's membrane). Causes include trauma (most common), foreign bodies, entropion, and secondary infection. Simple superficial ulce
The most common tumor of the equine eye and periocular tissues. UV radiation is the primary risk factor. Most common on the third eyelid (nictitating membrane), followed by the eyelid margin and limbu
Opacity of the lens. May be congenital (present at birth, some inherited in certain breeds), developmental, traumatic, or secondary to ERU (most common acquired cause; inflammatory debris and synechia
Obstruction of the nasolacrimal duct that drains tears from the eye to the nostril. Causes include congenital atresia, foreign body, inflammation, dental disease (upper tooth root abscess extending in
Inflammation of the conjunctiva (the mucous membrane lining the eyelids and covering the sclera). Common causes include allergies, dust/irritants, flies, bacterial or viral infection, foreign bodies,
Inward rolling of the eyelid margin causing the eyelashes and hair to contact the corneal surface. Most common in neonatal foals (due to dehydration, prematurity, or redundant periocular tissue). Usua
Granulomatous lesion at the inner corner of the eye caused by aberrant Habronema or Draschia larvae deposited by flies attracted to the moist conjunctival secretions. Intensely pruritic and disfigurin
EYE EMERGENCIES; TIME IS CRITICAL: Any sign of eye pain (squinting, tearing, swelling, cloudiness) should be treated as an emergency. Do NOT wait to see if it gets better; corneal ulcers can perforate within hours. Call your vet immediately. SIGNS OF EYE PAIN: Squinting (blepharospasm), excessive tearing, swollen eyelids, cloudiness or blue haze, constricted pupil (compared to the other eye), avoiding light, rubbing the eye. FLY MASKS: Use UV-protective fly masks during turnout; they reduce fly irritation, UV exposure (reducing SCC risk), and protect from debris. Ensure proper fit (no rubbing on eyes). Remove and clean daily. UV PROTECTION: Horses with unpigmented eyelids and light-colored eyes are at higher risk for SCC. Use fly masks with UV protection rated >70%. Provide shade in pastures. ERU MONITORING: If your horse has been diagnosed with uveitis, monitor both eyes daily for signs of recurrence. Early treatment of flare-ups minimizes cumulative damage. Keep emergency eye medications on hand as directed by your vet. DO NOT: Apply any eye medication (especially steroids) without veterinary guidance. Never use expired eye medications. Never assume 'it's just allergies'; get any painful eye examined.
The #1 cause of equine blindness worldwide, affecting 2–25% of horses depending on the region. An immune-mediated inflammatory disease of the uveal tract (iris, ciliary body, choroid) with recurring episodes of painful inflammation. Associated with Leptospira spp. infection, Onchocerca cervicalis microfilariae, and autoimmune mechanisms. Appaloosas are 8x more likely to develop ERU and tend to have more severe, bilateral disease. Three forms: classic (recurrent episodes), insidious (chronic low-grade; common in Appaloosas and draft breeds), and posterior (primarily choroidal/retinal). Signs include: Acute episodes: blepharospasm (squinting), excessive tearing (epiphora), photophobia, miosis (constricted pupil), aqueous flare (cloudy anterior chamber), conjunctival injection (redness), corneal edema (blue haze); Chronic changes: posterior synechiae (iris adhesions to lens), cataracts, vitreal degeneration, retinal detachment, phthisis bulbi (shrunken globe); eventual blindness in 50%+ of affected eyes
Loss of corneal epithelium and potentially deeper layers (stroma, Descemet's membrane). Causes include trauma (most common), foreign bodies, entropion, and secondary infection. Simple superficial ulcers heal in 5–7 days. Complicated ulcers (infected, deep, melting) are emergencies. Pseudomonas and fungal (Aspergillus, Fusarium) infections can cause rapid corneal melting (keratomalacia) and perforation within hours to days. Signs include: Blepharospasm; epiphora (tearing); photophobia; corneal cloudiness; visible corneal defect (may need fluorescein stain); yellow/green discharge if infected; 'melting' appearance with gelatinous corneal surface (EMERGENCY); hypopyon (pus in anterior chamber); corneal vascularization in chronic cases; descemetocele (deep ulcer with only Descemet's membrane remaining; impending perforation)
The most common tumor of the equine eye and periocular tissues. UV radiation is the primary risk factor. Most common on the third eyelid (nictitating membrane), followed by the eyelid margin and 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
Opacity of the lens. May be congenital (present at birth, some inherited in certain breeds), developmental, traumatic, or secondary to ERU (most common acquired cause; inflammatory debris and synechiae damage the lens). Nuclear sclerosis (normal age-related lens hardening) should not be confused with cataracts. Significant cataracts affecting vision can be treated surgically (phacoemulsification) with good outcomes in selected cases. Signs include: White, blue, or gray opacity visible in the pupil; may be partial (focal) or complete; reduced vision or blindness depending on extent; horse may be more spooky or reluctant in unfamiliar environments; may bump into objects; secondary ERU changes often present if uveitis-related; congenital cataracts may be noted in foals
Obstruction of the nasolacrimal duct that drains tears from the eye to the nostril. Causes include congenital atresia, foreign body, inflammation, dental disease (upper tooth root abscess extending into the duct), or trauma. Leads to chronic epiphora and predisposes to conjunctivitis. Signs include: Chronic tearing (epiphora) with tear staining on the face; mucopurulent discharge from the medial canthus; recurrent conjunctivitis; may see swelling below the medial canthus; discharge may be expressed from the nasal punctum; flushing the duct from the nasal end is both diagnostic and therapeutic
Inflammation of the conjunctiva (the mucous membrane lining the eyelids and covering the sclera). Common causes include allergies, dust/irritants, flies, bacterial or viral infection, foreign bodies, and poor ventilation in stabling. Usually secondary to an underlying cause. Must rule out corneal ulceration in any red, painful eye. Signs include: Red, swollen conjunctiva; serous (clear) to mucopurulent (yellow-green) discharge; mild blepharospasm; chemosis (conjunctival edema); may be unilateral or bilateral; allergic conjunctivitis tends to be bilateral with serous discharge; infectious tends to be unilateral with purulent discharge
Inward rolling of the eyelid margin causing the eyelashes and hair to contact the corneal surface. Most common in neonatal foals (due to dehydration, prematurity, or redundant periocular tissue). Usually affects the lower eyelid. Causes corneal irritation and ulceration if not corrected. Neonatal entropion often resolves with temporary eyelid eversion (skin staples or mattress sutures). Signs include: Visible inward rolling of the eyelid; epiphora (tearing); blepharospasm (squinting); corneal ulceration from hair contact; mucopurulent discharge; photophobia; most common in newborn foals within the first few days of life; adult-onset suggests spastic entropion from ocular pain (rule out underlying cause)
Granulomatous lesion at the inner corner of the eye caused by aberrant Habronema or Draschia larvae deposited by flies attracted to the moist conjunctival secretions. Intensely pruritic and disfiguring. May damage the nasolacrimal punctum and duct. Seasonal (summer). Part of the broader habronemiasis complex. Signs include: Yellow to reddish-brown, ulcerated granulomatous mass at the medial canthus; contains gritty calcified material ('sulfur granules'); intense rubbing and self-trauma to the eye area; may damage the nasolacrimal duct opening; conjunctival inflammation; recurs annually in summer; may require surgical debulking and ivermectin (systemic and topical)
The equine eye is particularly vulnerable to trauma and infection due to its large size and lateral prominence. Equine recurrent uveitis is the leading cause of blindness. Any sign of eye pain (squinting, tearing, swelling) is an emergency; corneal ulcers can perforate within 24–48 hours with certain bacterial (Pseudomonas) or fungal infections. Always perform a fluorescein stain before administering any corticosteroid eye medication. The subpalpebral lavage system allows treatment of painful eyes without repeated handling of the eyelids.
EYE EMERGENCIES; TIME IS CRITICAL: Any sign of eye pain (squinting, tearing, swelling, cloudiness) should be treated as an emergency. Do NOT wait to see if it gets better; corneal ulcers can perforate within hours. Call your vet immediately. SIGNS OF EYE PAIN: Squinting (blepharospasm), excessive tearing, swollen eyelids, cloudiness or blue haze, constricted pupil (compared to the other eye), avoiding light, rubbing the eye. FLY MASKS: Use UV-protective fly masks during turnout; they reduce fly irritation, UV exposure (reducing SCC risk), and protect from debris. Ensure proper fit (no rubbing on eyes). Remove and clean daily. UV PROTECTION: Horses with unpigmented eyelids and light-colored eyes are at higher risk for SCC. Use fly masks with UV protection rated >70%. Provide shade in pastures. ERU MONITORING: If your horse has been diagnosed with uveitis, monitor both eyes daily for signs of recurrence. Early treatment of flare-ups minimizes cumulative damage. Keep emergency eye medications on hand as directed by your vet. DO NOT: Apply any eye medication (especially steroids) without veterinary guidance. Never use expired eye medications. Never assume 'it's just allergies'; get any painful eye examined.
See the Eyes on our interactive 3D horse model. Open 3D Model