Bit & Bridle

Equine Keratoconjunctivitis (EK)

Other Names

  • Eosinophilic Keratoconjunctivitis (EK)
  • Equine eosinophilic keratitis
  • Proliferative keratoconjunctivitis

What is Equine Keratoconjunctivitis?

Equine keratoconjunctivitis refers to inflammatory disease affecting both the cornea (keratitis) and the conjunctiva (conjunctivitis) of the horse’s eye. The most clinically significant and distinctive form in horses is Eosinophilic Keratoconjunctivitis (EK) — an immune-mediated inflammatory condition characterised by the infiltration of eosinophils (a type of white blood cell associated with allergic and parasitic immune responses) into the cornea and conjunctival tissues.

Eosinophilic keratoconjunctivitis was first comprehensively described in horses in the 1990s and has since been recognised as a distinct clinical entity. It can cause significant corneal pathology, including vascularisation, plaque formation, and ulceration, and can threaten vision if not managed appropriately. It is distinct from simple bacterial or viral conjunctivitis and requires a different treatment approach.

The condition can affect one or both eyes and can be acute or chronic in presentation. Unlike routine conjunctivitis, it is not primarily caused by bacterial infection, though secondary infection can occur. Equine eye conditions are discussed broadly in our equine eye anatomy guide.

Causes

The precise underlying cause of equine eosinophilic keratoconjunctivitis is not fully established. Several theories have been proposed:

  • Parasitic hypersensitivity: The most widely accepted hypothesis. Onchocerca cervicalis microfilariae — the larval stage of a filarial worm transmitted by Culicoides midges — are found in the conjunctival tissue of affected horses. An immune hypersensitivity reaction to these microfilariae, or to their Wolbachia endosymbionts, is thought to trigger eosinophilic inflammation in susceptible individuals.
  • Allergen exposure: Seasonal variation in some cases, and the eosinophilic nature of the inflammation, suggest that environmental allergens (pollen, dust, insect proteins) may contribute in a manner analogous to atopic disease
  • Equine Herpesvirus type 2 (EHV-2): Has been detected in lesions of some affected horses, suggesting it may act as a trigger or cofactor, though its causal role is debated
  • Immune dysregulation: An aberrant local immune response in the conjunctival and corneal tissue may underlie susceptibility regardless of specific triggers

Symptoms and Signs

Owners and handlers should be alert to the following signs:

  • Ocular discharge: Often mucoid or mucopurulent — a sticky, cloudy discharge from one or both eyes
  • Epiphora: Excessive watering or tearing of the eye
  • Blepharospasm: Squinting or partial closure of the eyelid due to ocular discomfort
  • Photophobia: Sensitivity to light — the horse may seek shade or turn the affected eye away from light
  • Conjunctival redness and swelling: Hyperaemia and chemosis of the conjunctiva
  • Corneal changes: In established cases, white or pale plaque-like lesions on the corneal surface, superficial corneal vascularisation (new blood vessel growth into the clear cornea), and corneal oedema (cloudiness). These are the hallmarks of active eosinophilic keratitis.
  • Corneal ulceration: May develop in active lesions
  • Eosinophilic plaques: Chalky white, irregular raised deposits on the corneal surface are characteristic of this condition

Diagnosis

Diagnosis should be made by a veterinary surgeon, ideally in consultation with or by referral to an equine ophthalmologist:

  • Ophthalmic examination: Thorough slit-lamp biomicroscopy and fluorescein staining (to identify corneal ulceration). The characteristic appearance of corneal plaques with eosinophilic infiltrate is often strongly suggestive of EK.
  • Conjunctival cytology: A scraping or swab from the conjunctival surface examined under the microscope confirming eosinophilic infiltration is diagnostic. This is the most definitive diagnostic test and can be performed in the standing horse.
  • Conjunctival or corneal biopsy: Histopathological examination confirming eosinophilic infiltrate and any associated granulomatous reaction
  • Microfilarial testing: Skin biopsies or conjunctival samples may be examined for Onchocerca microfilariae
  • Bacterial culture and sensitivity: Where secondary bacterial infection is suspected

Treatment

Treatment of eosinophilic keratoconjunctivitis combines antiparasitic therapy (where microfilarial involvement is suspected) with anti-inflammatory treatment:

  • Topical corticosteroids: Ophthalmic prednisolone or dexamethasone drops or ointment reduce the eosinophilic inflammatory response and corneal plaque formation. Must be used cautiously and only under veterinary supervision — contraindicated in the presence of corneal ulceration.
  • Topical ciclosporin (cyclosporine): An immune-modulating agent that reduces eosinophilic inflammation without the corneal-thinning risk of steroids. Particularly useful for long-term maintenance therapy. Available as ophthalmic drops or ointment.
  • Ivermectin: Antiparasitic treatment targeting Onchocerca microfilariae. Administered systemically. Care is required — killing large numbers of microfilariae can cause a marked local inflammatory reaction (similar to the Mazzotti reaction described in other species) that temporarily worsens ocular signs before improving them.
  • Topical antibiotics: Where secondary bacterial infection is present
  • Moxidectin: An alternative antiparasitic agent to ivermectin, also effective against Onchocerca microfilariae
  • Surgical debridement: In some cases of extensive corneal plaque formation, superficial keratectomy (surgical removal of corneal plaques) may be required to improve comfort and clarity
  • Fly and midge control: Reducing Culicoides exposure through fly masks, insect repellents, and stabling at peak midge activity times (dusk and dawn) may help reduce reinfection and re-exposure

Prevention

  • Routine equine anthelmintic programmes that include ivermectin or moxidectin help control Onchocerca microfilarial loads — though no programme completely eliminates the risk of EK in susceptible horses
  • Use of well-fitting fly masks to protect the eyes from insect contact during high-risk periods
  • Reducing Culicoides midge exposure through stable management (fine-mesh screens, stabling at dusk and dawn, insect repellents)
  • Early treatment of any ocular signs to prevent progression to severe corneal pathology

Prognosis

With prompt and appropriate treatment, the prognosis for maintaining vision and controlling the condition is generally good. However, EK is frequently a recurring condition — many horses require ongoing maintenance treatment with ciclosporin or periodic courses of topical anti-inflammatories to prevent relapse. Horses that develop severe corneal scarring from delayed or inadequate treatment may have permanent visual impairment in the affected eye. Regular ophthalmological monitoring is recommended in horses with a history of EK.

When to Call the Vet

  • Any horse with a painful or weeping eye — prompt veterinary assessment is important to identify whether corneal ulceration is present, as this changes treatment significantly
  • Visible white or pale deposits on the cornea (the front surface of the eye)
  • Recurring eye problems despite treatment — persistent or relapsing ocular disease in a horse warrants referral to an equine ophthalmologist
  • Signs of severe photophobia (refusing to open the eye in daylight) or marked swelling around the eye

Frequently Asked Questions

Is equine keratoconjunctivitis contagious?

Eosinophilic keratoconjunctivitis is not a contagious condition in the conventional sense — it is immune-mediated and parasitically associated rather than directly infectious between horses. However, if Onchocerca cervicalis is involved, all horses in an area share exposure to the same Culicoides midge vector. Good anthelmintic protocols across the herd make sense where the condition is prevalent.

Can I use human eye drops on my horse?

No — you should not apply any eye medication to your horse without veterinary advice. Some human preparations are not suitable for equine use, and incorrect treatment (such as applying corticosteroids to an eye with an undiagnosed corneal ulcer) can dramatically worsen the situation, potentially resulting in corneal rupture. Always seek veterinary assessment of any equine eye problem before commencing treatment.

How is eosinophilic keratoconjunctivitis different from recurrent uveitis (moon blindness)?

Equine Recurrent Uveitis (ERU) is an inflammatory condition affecting the inner structures of the eye (the uveal tract — iris, ciliary body, and choroid), whereas eosinophilic keratoconjunctivitis primarily involves the surface structures (cornea and conjunctiva). Both cause recurring ocular pain and discharge and can ultimately lead to vision loss, but the treatment approaches differ significantly. Accurate diagnosis by a veterinarian — ideally an ophthalmologist — is essential to distinguish them.

How often does equine keratoconjunctivitis recur?

Recurrence is common. Many horses with EK have repeated episodes, particularly during warm seasons when midge activity is highest. Proactive management — fly mask use, anthelmintic protocols, and ready access to topical ciclosporin on veterinary prescription — allows most horses to be managed successfully despite their tendency to relapse. Long-term monitoring by an equine vet is advisable. See BEVA ophthalmology resources for further guidance.