Corneal ulcers, also known as ulcerative keratitis, represent one of the most common and serious ocular conditions in horses. [1][2] This condition involves the erosion of the corneal epithelium, the outermost layer of the cornea. [1]

The severity of corneal ulcers in horses can vary widely, ranging from superficial cuts or abrasions to full thickness corneal punctures (perforations) accompanied by prolapse (dislocation) of the iris. [2]

Since the cornea contains a large number of nerves, ulceration to this structure is very painful for the horse. [3] Additional signs of keratitis include redness, excessive tearing, eye discharge, and a cloudy appearance to the eye. [3]

If not treated promptly and appropriately, a corneal ulcer can lead to bacterial or fungal infection and potential vision loss. [2] Other complications include globe rupture and phthisis bulbi (end-stage eye). [2]

Immediate veterinary intervention is critical to preserve vision. With proper treatment, most horses can recover fully, maintaining both their health and quality of life.

Corneal Ulcer in Horses

Horse eyes are prominent, making them prone to injury. This is especially true of the cornea, which is the outermost layer of the eye.

A corneal ulcer or keratitis occurs when there is a loss of the corneal epithelium, the outermost layer of the eye, accompanied by inflammation of the cornea. In horses, this is often due to trauma, foreign bodies, or infections. [1][2]

This injury irritates or damages the epithelium, leaving the underlying structures in the cornea unprotected. [2] This condition is highly painful and can lead to a deeper infection. [2]

Equine Eye Anatomy

To better understand the different types of corneal ulcers, it’s helpful to familiarize yourself with equine eye anatomy.

The cornea is the transparent front part of the horse’s eye that covers the iris, pupil, and anterior chamber. It is a clear dome on the front surface of the eye, measuring 0.8 to 1.0 mm (0.03 to 0.04 in) thick.

The cornea protects the front of the eye, allows light in, and helps focus light on the retina at the back of the eye. [3][4] It is made of three primary layers: [3][5]

  • Epithelium: The epithelium is the outermost layer which repels tears and keeps organisms from entering the deeper layers of the cornea.
  • Stroma Layer: The stroma layer makes up approximately 90% of the cornea’s thickness, lies under the epithelium and is mainly composed of water.
  • Deep Endothelium: The deep endothelium lies beneath the stroma layer. Descemet’s membrane is the basement membrane of the deep endothelium.

In addition to the cornea, it’s important to understand the significance of several other parts of the eye anatomy in horses:

horse-eye-anatomyIllustration:
  • Iris: The iris is the circular, colored area in the center of the eye. It controls the amount of light entering the eye by making the pupil larger or smaller. [4]
  • Pupil: The pupil is the black area in the middle of the iris, and is a small hole in the eyeball that allows light in. In horses, the pupil is shaped like a horizontal oval. [4]
  • Lens: The lens sits behind the iris and changes its shape to focus light onto the retina. [4]
  • Retina: The retina contains photoreceptors, which are cells that sense light. Each photoreceptor is attached to a nerve fiber. When light hits the retina, the photoreceptors are stimulated, which creates electrical impulses in the nerve fibers. [4]
  • Optic Nerve: All of the nerve fibers are bundled together to form the optic nerve, which carries electric impulses from the retina to the brain, where images are translated from light signals into the sense of vision. [4]

Types of Corneal Ulcers

There are three types of corneal ulcers in horses: simple, recurrent and complex.

Simple Corneal Ulcers

Simple corneal ulcers are superficial, involving the outer layer of the cornea and up to one-third of the stroma layer.

This type of ulcer may not be noticeable to the naked eye, but does appear under fluorescein stain during ophthalmic evaluation. [3]

With simple ulcers, deep infection is not present, but the eye could still be contaminated with bacteria or fungi. [3] Simple ulcers usually heal quickly without complication. [7]

Recurrent or Refractory Corneal Ulcers

Recurring ulcers, also known as indolent ulcers, persist because the corneal epithelium is not able to adhere to the underlying stroma, preventing it from healing properly. [3]

Recurrent ulcers are visible under fluorescein stain. They are distinct from simple ulcers as staining is also seen beneath the periphery of the corneal epithelium, indicating detachment of the cornea from the stroma. [3]

Complex Ulcers

This category of corneal ulcers involves deep ulcerations that reach the stromal layers of the cornea. In these cases, bacterial or fungal infections are typically well-established, leading to moderate to severe swelling of the cornea. [3]

Complex ulcers are known for their prolonged healing times. [7]

Causes

Corneal ulcers in horses typically begin with a micro-puncture injury to the eye, often caused by small, sharp pieces of plant material like hay. Horses that are active or kept in environments where branches, dust, or sand are prevalent are particularly at risk for such injuries. [3]

When the corneal epithelium is punctured, bacteria or fungal organisms can become implanted in the tear film (fluid layer on the surface of the eye), complicating the healing process. [3]

The corneal epithelium and tear film normally serve as strong barriers against pathogens. However, because the corneal tissue is quite soft, it is vulnerable to physical injuries, making the entire eye susceptible to infections. [5]

While nonpathogenic bacteria are typically present in the eye and pose no threat, pathogenic bacteria such as Staphylococcus spp., Listeria spp., and Streptococcus spp. can proliferate when the cornea is injured, leading to infectious keratitis. This type of infection is particularly common in young horses, who may have less developed immune defenses.[5]

Fungal pathogens like Aspergillus spp. and Fusarium spp. are also common culprits in corneal ulcers, thriving in the compromised corneal tissue and contributing further to the severity of the ulceration. [6]

Symptoms

Symptoms of corneal ulcers vary depending on the severity of the ocular damage. The most common signs in horses include: [3][8]

  • Painful eye
  • Squinting
  • Bulging of the eye
  • Excessive blinking
  • Excessive tearing
  • Eye discharge
  • Sensitivity to light
  • Constricted pupils
  • Cloudy eye
  • Redness or swelling of the eye
  • Bluish tint to the cornea

All corneal ulcerations are accompanied by signs of anterior uveitis, which is inflammation of the inner eye. [3]

As the infection progresses, microorganisms can invade the stromal layers, giving the eye a melted or gelatinous appearance. [3]

Significant destruction and perforation of the cornea is likely unless the disease process is stopped through aggressive medical therapy or surgical intervention. [3][5]

Without prompt and aggressive medical treatment or surgical intervention, there is a high risk of substantial corneal destruction and potential perforation. [3][5]

In some cases, this can lead to the formation of a descemetocele — a crater-like defect in the cornea that absorbs fluorescein dye around its edges while remaining clear at the center. The presence of a descemetocele is a critical indicator that the integrity of the globe (eyeball) is severely compromised, often necessitating surgical repair to prevent rupture. [2]

Sudden changes in vision are considered medical emergencies. Promptly seeking veterinary care is critical for preserving a horse’s vision and preventing further complications.

Diagnosis

Diagnosing corneal ulcers involves a complete ophthalmic examination performed by a veterinarian. This is usually performed under IV sedation and a nerve block, ideally in a darkened room. [3] Referral to a specialist may be necessary in some cases.

In addition to physically examining the eye for defects, a Schirmer tear test may be performed to detect keratoconjunctivitis sicca, chronic inflammation of the cornea due to inadequate tear film. [3]

Fluorescein stain is used to detect the exact location of the corneal ulcer. [3] Corneal cytology, which involves examining a smear from a swab of the eye under a microscope, along with bacterial and fungal cultures, is typically conducted on all chronic and complex corneal ulcers. [3]

These diagnostic tests help identifying the specific infections present, allowing veterinarians to target treatment effectively to the underlying pathogens.

Prompt diagnosis of corneal ulcers is essential to start effective treatment quickly and minimize tissue loss. [3]

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Treatment

Treatment of corneal ulcers in horses aims to achieve several goals, including: [5]

  • Address any underlying conditions that may contribute to the ulcer
  • Treat existing infections and prevent new ones
  • Slow the breakdown of corneal collagen
  • Address any secondary uveitis
  • Provide structural support to the weakened cornea, if needed
  • Provide pain relief to the affected horse

Medical therapy for corneal ulcers is usually aggressive in order to reduce the chance of infection and decrease inflammation in the eye. [5] Treatment is often required for several weeks, with the type of medication and frequency of dosing varying depending on the individual case. In more severe cases, surgery may be recommended. [1]

During treatment, horses also need physical protection of the affected eye with a fly mask or cup mask. [3]

Treating Simple Ulcers

For simple ulcers, the goal of treatment is to reduce contamination and prevent bacterial colonization in the stroma. Typical treatment includes using topical antibiotic therapy until a fluorescein stain shows the corneal ulcer has resolved. [3]

Treating Recurrent Ulcers

For recurrent ulcers, surgical correction of the eyelid and/or conjunctival (third eyelid) anomalies is often recommended. The cornea may also need dead or damaged tissue removed with a dry cotton swab prior to the procedure. [3]

Topical antibiotics may be applied based on cytology and culture results. [3]

Treating Complex Ulcers

Complex ulcers may require other surgical interventions such as corneal grafts to heal completely. [3] Corneal grafts can involve placing a thin flap of conjunctiva (mucous membrane around the eye), corneal tissue, or equine amnion (placental tissue) over the ulcer. [3]

Medical Management

Treating equine eye conditions is challenging, as most horses are not tolerant of topical administration of eye medications. There are multiple options available to support treatment of corneal ulcers in horses, and your veterinarian may recommend a combination of approaches depending on your horse’s individual case.

Medication Administration

Though treating the cornea seems straightforward, it is often difficult due to the extreme sensitivity of the area, and the presence of tissue barriers which can block drug penetration.

Medication is most often delivered to the horse’s eye by the following methods, each with its own advantages and disadvantages: [5]

  • Topical ocular administration
  • Ocular injection or implant
  • Systemic administration

Topically applied medications may have limited penetration into the cornea and deeper structures of the eye. They may also have rapid dilution and wash out of the eye due to tear production. [5]

Topical medications generally have a short contact time with the treatment area, so they must be delivered frequently to be effective. The horse owner is often responsible for administering these medications, and failure to do so can lead to treatment failure. [5]

Subpalpebral Lavage System

A Subpalpebral Lavage System (SPL) is an ophthalmic catheter that delivers topical liquid medication to the surface of the equine cornea.

A SPL system consists of a silicone tube that passes through the upper or lower eyelid and is secured to the horse’s face. The tubing is woven through the mane and draped along the neck, ending in a closed port. Medication is then injected into the port, putting some distance between the caretaker and the horse’s eye. [9]

SPLs are often used with horses that are difficult to treat or those that need frequent treatment. They allow for repeated long-term therapy and are generally easier and safer than trying to apply eye medications without it. [2][3]

An SPL is critical for treating serious equine eye conditions, including deep ulcers that are infected or melting. It can also be used for severe uveitis. [9]

SPLs can be used either in the veterinary hospital setting or at home. They can be left in place for as long as 10-12 weeks, if necessary. [9]

Most horses tolerate SPLs well. The most commonly reported complications includes leaks or breaks in the tubing, which can be repaired or replaced. Using a fly mask or hood can help prevent damage to SPL tubing. [9]

Controlling Inflammation

Anterior uveitis, which is inflammation of the uvea, occurs alongside all corneal ulcers. Treatment with topical atropine sulfate and systemic NSAIDs is often needed to control inflammation.

Oral or intravenous NSAID therapy with medications such as phenylbutazone (“bute”) or Banamine® (flunixin meglumine) can help with pain. [3]

Other medications that can be used to reduce inflammation and prevent stromal melting include acetylcysteine and sodium ethylenediamine tetra-acetic acid (EDTA). Additional treatments may be prescribed by a veterinarian based on the specific needs of the horse. [2]

Consult with your veterinarian to determine the best treatment protocol and before administering pain medications to a horse being treated for corneal ulcers. Some research indicated that corticosteroids and topical NSAIDs should be avoided because they may delay epithelial healing. [3]

Clearing Infections

To fully resolve corneal ulcers, any bacterial and fungal growth must be effectively contained and treated. Based on culture results, broad-spectrum topical antibiotics are typically prescribed. [2]

For bacterial infections, the following topical antibiotics are often used: [2]

  • Chloramphenicol
  • Gramicidin-neomycin-polymyxin B
  • Gentamicin
  • Ciprofloxacin
  • Tobramycin

Anti-fungal therapy may also be needed if fungal infection is present. This type of therapy should also be considered if the ulcer is unresponsive to antibiotic therapy. [3]

Common topical anti-fungal medications used in horses include: [3]

  • 5% natamycin
  • 1% miconazole
  • 1% itraconazole
  • 30% dimethyl sulfoxide
  • 1% voriconazole

Anterior uveitis may initially worsen after beginning anti-fungal therapy. These medications should be used 4-6 times daily for a minimum of 2-4 weeks to eliminate fungal infection. For deep infections, they may be needed for 6-8 weeks. [3]

Surgical Treatments

Depending on the type and severity of the corneal ulcer, surgical treatments may be necessary alongside medication.

Specifically, if a corneal ulcer is deeper than one-third of the corneal stroma’s thickness or does not respond to medical therapy, surgical intervention is typically required to preserve the eye. [3]

Conjunctival Grafts

Conjunctival grafting is a common surgical procedure for treating corneal ulcers and is typically performed at a referral center. This surgery involves suturing healthy donor tissue over the damaged area of the cornea to promote healing and provide structural support. [3]

Cornea Transplant (Keratoplasty)

Corneal transplantation, also known as keratoplasty, may be needed for deep or perforating corneal ulcers, complex, and/or deep stromal abscesses. This procedure can help preserve the eye and remove diseased tissue. It has been used successfully with many horses. [3]

Enucleation

If surgical intervention is needed for the corneal ulcer but financial or other reasons prevent referral to a specialist, enucleation (removal) of the diseased eye may be recommended. This procedure can be done under general anesthesia or standing sedation. [3]

Most horses can adapt well to having only one eye and enucleation is a way to stop ongoing pain caused by severe corneal ulcers.

Adjunct and Emerging Therapies

Given the high incidence of eye injuries in horses, there is significant interest in developing new medical technologies to simplify management for both horses and caretakers. These therapies might complement existing medications or serve as alternatives, depending on your horse’s individual needs.

Autologous Serum

An emerging treatment for corneal ulcers is administering the horse’s own blood serum, known as autologous serum, to the eye. Studies show that this procedure promotes corneal healing. [8]

Autologous serum may also prevent or inhibit stromal melting. [2][7]

Corneal Collagen Cross-Linking

Corneal collagen cross-linking (CXL) is a treatment that involves applying vitamin B2 eye drops to the cornea and then exposing it to ultraviolet light. This process creates “crosslinks” in the corneal stroma, stabilizing the cornea and preventing further melting. [1]

CXL can be an alternative to antibiotics, which helps avoid the risk of developing antibiotic resistance. [1]

However, CXL carries its own risks, including potential direct damage to corneal cells and intra-ocular structures due to UV radiation, as well as possible harm to surrounding eye cells. [1] To mitigate these risks, vitamin B2 drops are used in CXL protocols to limit the transmission of UV light to deeper eye structures. [1]

Corneal Debridement

Recurrent corneal ulcers in horses can be treated with corneal debridement, a procedure where the cornea is superficially cleaned of dead and damaged tissue using dry, sterile cotton swabs. This is done while the horse is sedated, and both frontal nerve blocks and topical anesthesia are administered for pain management. [3]

After debridement, a keratotomy (incision in the cornea) is performed to expose healthy corneal stroma and promote healing. However, deep or infected ulcers should not be treated in this manner. [3]

For further support of the corneal repair, biologic “bandages” made from equine amnion or porcine small intestinal submucosa can be applied to the debrided areas. These bandages provide additional structural support to the healing cornea. [3]

Prognosis

Corneal ulcers are common in horses, but fortunately, most are superficial. These uncomplicated ulcers typically heal within 3-7 days, though treatment may continue for 1-2 weeks. [8]

However, more severe corneal ulcers can require weeks or even months to heal completely. Challenges such as antibiotic resistance and difficulties in treating the eye often contribute to slower healing processes. [3][10]

Repeated fluorescein staining during recovery can help monitor healing. [2]

For management, simple and recurrent ulcers should be reevaluated 5-7 days after treatment begins, while complex ulcers need a follow-up within 24-48 hours. Additionally, any worsening of symptoms should prompt immediate reevaluation of all types of ulcers. [3]

Prevention

While corneal ulcers are common in horses, owners can take proactive steps to minimize the risk of these injuries. Ensuring that stalls and paddocks are free of hooks or sharp points is essential. Maintaining fences in good repair and removing low-hanging tree branches or debris in pastures can also help prevent scratches or abrasions to the eyes. [8]

Using fly masks is another effective measure, as they protect the horse’s eyes from insects, dirt, and seed heads. Additionally, regular checks of the horse’s eyes are crucial for early detection of any potential issues, allowing for prompt treatment and minimizing complications. [8]

Summary

Corneal ulcers are among the most frequent eye injuries in horses, potentially leading to infection, severe pain, and potential vision loss if not treated promptly and appropriately.

  • Any sudden changes in vision are a medical emergency. Prompt veterinary evaluation provides the best chance of preserving vision.
  • Suspected corneal ulcers require a thorough eye exam, including cytology, bacterial and fungal culture, and sensitivity testing.
  • The eye should be continuously monitored during treatment to ensure proper healing.
  • Corneal ulcer treatment must be aggressive, with surgery considered if the ulcer fails to heal, worsens, is at high risk for perforation, or has already perforated.
  • If medical or surgical treatments fail, or if surgery isn’t possible, enucleation is strongly recommended to stop infection and alleviate pain.

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References

  1. Hellander-Edman, A. et al. Corneal cross-linking in 9 horses with ulcerative keratitis. BMC Vet Res. 2013.View Summary
  2. Brooks, D.E. Equine Ophthalmology. AAEP. 2002.
  3. Williams, L.B. and Pinard, C.L. Corneal Ulcers in Horses. Vetlearn.com. 2013.
  4. Gelatt, K.N. Eye Structure and Function in Horses. Merck Veterinary Manual. 2013.
  5. Brooks, D.E. and Plummer, C.E. Diseases of the Equine Cornea. Equine Ophthalmology. 2022.
  6. Conrado, F.O. et al. What is your diagnosis? Scraping from a corneal ulcer in a horse. Vet Clin. Path. 2022.View Summary
  7. Peyrecave-Capo, X. et al. Equine Umbilical Cord Serum Composition and Its Healing Effects in Equine Corneal Ulceration. Front Vet Sci. 2022.View Summary
  8. Wickens, C. Corneal Ulcers in Horses: The Importance of Keeping an Eye on Your Horse’s Eyes. IFAS Extension, University of Florida. 2023.
  9. Dwyer, A.E. How to Insert and Manage a Subpalpebral Lavage System. Proceedings of the American Association of Equine Practitioners. 2013.
  10. Prucha, V.J.S. et al. Equine non-healing corneal ulcers: a retrospective evaluation of 57 cases (2001–2017). Tierarztl Prax Ausg G Grosstiere Nutztiere. 2020.