NAVLE Special Senses

Equine Corneal Ulcer Study Guide

📅 March 28, 2026 ⏱ 25 min read

Corneal ulcers (ulcerative keratitis) represent one of the most common and potentially vision-threatening ophthalmic emergencies in equine practice.

Overview and Clinical Importance

Corneal ulcers (ulcerative keratitis) represent one of the most common and potentially vision-threatening ophthalmic emergencies in equine practice. Horses are uniquely predisposed to corneal disease due to their large, laterally positioned eyes with prominent corneas, inquisitive nature, and frequent exposure to environmental pathogens and trauma. The equine cornea is approximately 0.8-1.0 mm thick and consists of three primary layers: epithelium, stroma (comprising 90% of thickness), and endothelium with Descemet's membrane.

Unlike other domestic species, horses have a higher incidence of fungal keratitis (keratomycosis), particularly in warm, humid climates such as the southeastern United States. The avascular nature of the healthy cornea limits immune response and drug delivery, making aggressive treatment essential. Untreated or improperly managed corneal ulcers can progress to keratomalacia (melting), descemetocele formation, perforation, or endophthalmitis within 24-48 hours.

Classification	Characteristics	Clinical Features
Simple/Superficial	Epithelial defect only; no stromal involvement; no infection	Heals in 3-7 days; minimal corneal edema; no cellular infiltrate
Complicated/Infected	Stromal involvement; bacterial or fungal infection present	White/yellow stromal infiltrate; purulent discharge; severe uveitis; hypopyon possible
Melting (Keratomalacia)	Enzymatic stromal degradation by bacterial/fungal/inflammatory proteases	Gelatinous gray/yellow appearance; rapid progression; EMERGENCY
Descemetocele	Complete stromal loss; only Descemet's membrane remains	Central clearing on fluorescein (Descemet's doesn't stain); bulging; SURGICAL EMERGENCY
Indolent/Non-healing	Superficial ulcer with redundant non-adherent epithelial edges (SCCED)	Fails to heal beyond 7 days; requires debridement; may be EHV-2 associated
Stromal Abscess	Infection sealed within stroma beneath re-epithelialized surface	Yellow/white deep opacity; NO fluorescein uptake; severe uveitis

Etiology and Pathophysiology

Common Causes

Trauma: Direct corneal injury from hay, bedding, foreign bodies (thorns, plant material), stall fixtures, or rubbing. This is the most common initiating cause.

Bacterial Infection: Secondary colonization of traumatic ulcers. Common pathogens include Pseudomonas aeruginosa, Streptococcus equi subspecies zooepidemicus, Staphylococcus spp., and gram-negative enterobacteriaceae.

Fungal Infection (Keratomycosis): Aspergillus spp. (33-73% of cases) and Fusarium spp. (6-45% of cases) are predominant. Candida spp. and other yeasts are less common (4-10%). Fungi are normal conjunctival flora in horses and opportunistically invade damaged cornea.

Eyelid Abnormalities: Entropion, ectropion, distichiasis, ectopic cilia, or eyelid tumors causing chronic irritation.

Neurologic Deficits: Cranial nerve VII (facial) paralysis causing inadequate blink reflex and corneal exposure.

Viral Infection: Equine herpesvirus-2 (EHV-2) can cause chronic, recurrent superficial ulceration.

High-YieldOn the NAVLE, remember that geographic location matters for fungal keratitis. Southeastern and Mid-Atlantic states have significantly higher prevalence of keratomycosis compared to western states. Always consider fungal infection in horses from humid climates with complicated ulcers.

Classification of Corneal Ulcers

Test	Indication and Interpretation
Corneal Cytology	Gram stain and Diff-Quik to identify bacteria (cocci vs. rods, gram-positive vs. gram-negative) or fungal hyphae. Guides initial empirical therapy.
Bacterial Culture	Aerobic culture and sensitivity for all complicated ulcers. Collect BEFORE applying topical anesthetics or fluorescein (contain preservatives).
Fungal Culture	Sabouraud dextrose agar for suspected keratomycosis. Results may take 1-2 weeks; don't delay empirical antifungal treatment.
Schirmer Tear Test	Rarely abnormal in horses (KCS uncommon), but evaluate if chronic/recurrent. Normal equine STT is greater than 15 mm/min.
Rose Bengal Stain	Detects devitalized epithelial cells and tear film deficiencies. Useful for early fungal detection and dendritic (herpes) ulcers.
Tonometry	Measures IOP. Expect DECREASED pressure (hypotony) with uveitis. CONTRAINDICATED if deep ulcer or descemetocele (risk of rupture).
Seidel Test	Concentrated fluorescein applied to suspected perforation. Positive: aqueous humor dilutes dye, causing streaming green fluorescence indicating full-thickness defect.

Clinical Signs and Presentation

Primary Clinical Signs

Blepharospasm: Involuntary squinting or closure of the eyelids due to pain. Often the first sign noticed by owners.

Epiphora: Excessive tearing and ocular discharge (serous to mucopurulent depending on infection).

Photophobia: Light sensitivity causing avoidance of bright environments.

Corneal Edema: Blue-gray haziness surrounding the ulcer due to stromal water accumulation.

Miosis: Constricted pupil due to reflex anterior uveitis (axon reflex uveitis from trigeminal nerve stimulation).

Conjunctival/Episcleral Hyperemia: Reddening of conjunctival and scleral vessels indicating inflammation.

Signs of Complicated Ulcers

White/yellow stromal infiltrate indicating infection
Hypopyon (white blood cell accumulation in anterior chamber)
Aqueous flare (protein in anterior chamber)
Corneal neovascularization (blood vessel ingrowth from limbus)
Gelatinous, gray appearance with melting ulcers
Fungal plaques: tan/brown proliferative surface lesions

NAVLE TipThe classic triad for equine corneal ulcer presentation is: blepharospasm + epiphora + photophobia. If a NAVLE question describes a horse holding its eye closed with excessive tearing and avoiding light, think corneal ulcer first and confirm with fluorescein staining.

Drug Class	Examples	Coverage and Notes
Triple Antibiotic	Neomycin/Polymyxin B/Bacitracin (NPB)	First-line for simple ulcers; broad-spectrum; Q6-8h
Fluoroquinolones	Ofloxacin, Moxifloxacin, Ciprofloxacin	Excellent Pseudomonas coverage; good stromal penetration; Q1-4h for complicated
Aminoglycosides	Tobramycin, Gentamicin (fortified 1.5%)	Excellent gram-negative including Pseudomonas; fortified preparations for severe infections
Cephalosporins	Cefazolin (5% compounded)	Good gram-positive including Streptococcus; often alternated with aminoglycoside

Diagnostic Approach

Ophthalmic Examination

A complete ophthalmic examination should be performed systematically. Horses with painful eyes often require sedation (detomidine or xylazine) and regional nerve blocks to allow adequate examination. The auriculopalpebral nerve block (motor block of orbicularis oculi) prevents forceful blinking, and the supraorbital nerve block (sensory block) reduces pain sensation in the upper eyelid.

Fluorescein Staining

Fluorescein dye is hydrophilic and adheres to the exposed hydrophilic corneal stroma when the lipophilic epithelium is disrupted. Best visualized with cobalt blue light or in a darkened environment.

Positive stain uptake: Indicates epithelial loss (corneal ulcer present)
Negative stain uptake: Intact epithelium OR stromal abscess (epithelium has re-healed over infection)
Descemetocele pattern: Ring of fluorescein uptake around central clear area (Descemet's membrane doesn't retain dye)

Additional Diagnostic Tests

Drug	Mechanism	Clinical Notes
Natamycin 5%	Polyene; binds ergosterol	Only FDA-approved ophthalmic antifungal; good Fusarium coverage; poor stromal penetration
Miconazole 1%	Imidazole; inhibits ergosterol synthesis	Broad-spectrum; effective for Aspergillus, Fusarium, Candida; good tolerance
Voriconazole 1%	Triazole; inhibits 14-alpha demethylase	BEST in vitro activity; excellent penetration; drug of choice for deep/stromal infections
Silver Sulfadiazine 1%	Disrupts cell membrane; releases silver ions	Combined antibacterial and antifungal; often added empirically in endemic areas

Medical Treatment

Treatment Goals

Control infection (antibacterial and/or antifungal therapy)
Manage pain and secondary uveitis (cycloplegics, NSAIDs)
Prevent enzymatic stromal degradation (anticollagenase therapy)
Promote epithelial healing and minimize scarring

Antimicrobial Therapy

Antibacterial Agents

Antifungal Agents

High-YieldVoriconazole has demonstrated the BEST in vitro susceptibility against equine corneal isolates of both Aspergillus and Fusarium. It should be considered first-line for keratomycosis, especially deep stromal infections or stromal abscesses. Can be administered topically (1%), orally, or via intrastromal injection (0.5 mL of 5% solution).

Anticollagenase Therapy

CRITICAL for melting ulcers. Collagenases and matrix metalloproteinases (MMPs) are released by bacteria (especially Pseudomonas), fungi, and degranulating neutrophils. Ulcerated corneas contain at least 3 times normal MMP concentrations in the tear film.

Pain Management and Uveitis Control

Atropine 1% (Topical Cycloplegic): Paralyzes ciliary muscle, dilates pupil, relieves painful ciliary spasm. Administer to effect (until mydriasis achieved), typically Q12-24h. Monitor for decreased GI motility with prolonged use.

Systemic NSAIDs: Flunixin meglumine (1.1 mg/kg IV/PO) or phenylbutazone (2.2-4.4 mg/kg PO) for systemic anti-inflammatory effect and analgesia.

CONTRAINDICATED: Topical corticosteroids and topical NSAIDs are ABSOLUTELY CONTRAINDICATED in corneal ulcers. They delay epithelial healing, potentiate infection, and promote stromal melting.

Subpalpebral Lavage (SPL) System

Intensive treatment of complicated ulcers (Q1-4h) requires a subpalpebral lavage system for practical medication delivery. This catheter system allows topical medications to be administered remotely without touching the painful eye or face.

Placement: Through upper or lower eyelid into conjunctival fornix under sedation and local anesthesia
Tubing: Woven through mane to injection port at base of neck
Duration: Can remain in place for weeks to months
Complications: Iatrogenic ulceration from footplate, cellulitis, tube breakage (13% overall complication rate)
Lower eyelid preferred: Studies show lower complication rates compared to upper eyelid placement

Agent	Mechanism	MMP Activity
Autologous Serum	Contains alpha-2 macroglobulin and growth factors	Better activity vs MMP-2; promotes epithelial healing
EDTA 0.05-0.2%	Calcium chelator; inhibits calcium-dependent MMPs	Better activity vs MMP-9
N-Acetylcysteine 10%	Reduces disulfide bonds; mucolytic; antioxidant	Better activity vs MMP-9
Doxycycline	Direct MMP inhibition (independent of antimicrobial effect)	Better activity vs MMP-2; 10 mg/kg PO BID

Surgical Treatment

Indications for Surgery

Stromal loss greater than 50% of corneal thickness
Descemetocele formation
Corneal perforation with iris prolapse
Progressive melting despite aggressive medical therapy
Deep stromal abscess unresponsive to medical treatment
Non-healing indolent ulcers requiring debridement

Surgical Options

High-YieldThe equine cornea has tremendous regenerative capacity. Even after large perforations, if treated surgically with corneal transplantation and aggressive medical therapy, 65% of horses with iris prolapse retain vision. The key is early referral to a veterinary ophthalmologist when medical therapy is failing or surgical criteria are met.

Procedure	Indication	Key Points
Grid Keratotomy	Indolent/non-healing superficial ulcers	Creates grooves for epithelial adhesion; performed with 25G needle under sedation
Diamond Burr Debridement	Indolent ulcers; removes non-adherent epithelium	Creates smooth basement membrane surface for re-epithelialization
Conjunctival Pedicle Graft	Deep ulcers; descemetoceles; perforations	Provides vascular supply, structural support, and anticollagenase factors; results in corneal scarring
Amniotic Membrane Transplant	Melting ulcers; promotes healing with minimal scarring	Antifibrotic, anti-angiogenic, antiprotease properties; better cosmetic outcome than conjunctival graft
Penetrating Keratoplasty	Full-thickness defects; iris prolapse; deep abscesses	Full-thickness corneal transplant with frozen donor tissue; 65% visual outcome in iris prolapse cases
Lamellar Keratoplasty	Deep stromal abscesses; keratectomy defects	Partial-thickness corneal transplant; lower rejection risk than penetrating keratoplasty

Prognosis and Complications

Ulcer Type	Healing Time	Visual Prognosis
Simple superficial ulcer	3-7 days	Excellent; minimal to no scarring
Complicated bacterial ulcer	2-6 weeks	Good with aggressive treatment; scarring depends on depth
Fungal keratitis	3-12 weeks (median 40 days)	Guarded; 25% blindness rate; 33% require surgery
Stromal abscess	Weeks to months	Guarded; often requires surgical intervention
Perforated ulcer/iris prolapse	Variable; requires surgery	Guarded to fair; 65% visual with surgery; 10% enucleation

Prevention

Environmental modification: Remove sharp edges, hooks, and protruding objects from stalls and pastures
Fly masks: Protect eyes from insects, dust, UV light, and minor trauma
Regular monitoring: Daily observation of eyes for early signs of injury or disease
Prompt veterinary attention: Any eye abnormality should be evaluated within 24 hours
Avoid pre-treating: Never apply old eye medications without veterinary guidance (steroids can be catastrophic)

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