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Basics ⬇

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BASICS

Definition!!navigator!!

Keratomycosis may present clinically in the horse as superficial keratitis, ulcerative keratitis (to varying degrees and appearances), or stromal abscess. Ulcerative keratitis refers to a disruption of the corneal epithelium with varying amounts of stromal loss, which may have concurrent bacterial and/or fungal infection. Ulcers infected with fungi range from minor corneal epithelial abrasions/erosions, to superficial plaques, to deep and severe interstitial keratitis.

Pathophysiology!!navigator!!

  • Fungi are normal inhabitants of the equine conjunctival microflora, but can become opportunistically pathogenic following corneal injury. Fungal organisms are ubiquitous in the equine environment, although regional geographic differences exist to account for variation in the presence of particular fungal species to specific regions. Exposure to vegetative material (hay, grasses, shavings, straw) and dust in the horse environment may influence exposure to fungi
  • The pathogenesis of ulcerative fungal keratitis commonly begins with corneal trauma, resulting in an epithelial defect and stromal invasion by the commensal fungal organism or seeding of fungi from a foreign body of organic origin. Tear film instability also predisposes to fungal keratitis or is induced by the fungi prior to fungal attachment and invasion. Stromal destruction results from the release of proteases and other enzymes from the fungi, leukocytes, and keratocytes. Fungi appear to have an affinity for Descemet's membrane, with hyphae frequently found deep in the equine cornea. Deeper corneal invasion by the fungi can lead to sterile or infectious endophthalmitis

Systems Affected!!navigator!!

Ophthalmic

Genetics!!navigator!!

N/A

Incidence/Prevalence!!navigator!!

Keratomycosis is more common and more aggressive in warm climates.

Signalment!!navigator!!

All ages and breeds of horses may be affected.

Signs!!navigator!!

  • Clinical signs associated with ulcerative keratomycosis include blepharospasm, epiphora, photophobia, corneal opacity (edema, infiltrate, fibrosis, vascularization), and signs of anterior uveitis (miosis, aqueous flare)
  • Slight downward deviation of the upper eyelashes may be a subtle sign of ocular pain
  • The cornea can be dry in appearance, or display cellular invasion with varying amounts of vascularization or cellular infiltrate

Causes!!navigator!!

Septate filamentous fungi associated with ulcerative keratomycosis include several species common to the equine eye (Fusarium, Aspergillus, Penicillium spp.). Yeasts (Candida spp.) may also contribute to keratomycosis.

Risk Factors!!navigator!!

  • Horses may be more susceptible to fungal invasion and infection of the cornea owing to the large surface area and prominence of the equine eye and some weakness in the corneal immune system
  • Topical antibiotic and/or corticosteroid therapy of a noninfected corneal ulcer may predispose to fungal colony invasion and colonization

Diagnosis ⬆ ⬇

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DIAGNOSIS

Differential Diagnosis!!navigator!!

Ocular pain may also be found with bacterial corneal ulcers, uveitis, conjunctivitis, blepharitis, and dacryocystitis.

CBC/Biochemistry/Urinalysis!!navigator!!

N/A

Other Laboratory Tests!!navigator!!

N/A

Imaging!!navigator!!

N/A

Other Diagnostic Procedures!!navigator!!

The diagnosis of keratomycosis is based on finding fungal hyphae, mold, or yeast on at least 1 of the following: (1) cytologic examination of a corneal scraping, (2) culture of the corneal lesion, (3) PCR of corneal cytologic specimens, or (4) surgical histopathologic examination of a keratectomy specimen.

Pathologic Findings!!navigator!!

  • Fungi show marked affinity for the deep corneal stroma and Descemet's membrane
  • Hyphae are often found with neutrophils in the stroma and are rarely found free in the anterior chamber

Treatment ⬆ ⬇

Outline

TREATMENT

Activity!!navigator!!

  • Horses with keratomycosis and secondary uveitis should be stall-rested until the condition is healed
  • Intraocular hemorrhage and increased severity of uveitis are sequelae to overexertion

Diet!!navigator!!

Diet should be consistent with the activity and training level of the horse.

Client Education!!navigator!!

  • Ulcerative keratomycosis is a serious sight- and globe-threatening disease in the horse
  • Long duration of antifungal drug exposure (minimum of 4 weeks, often longer) is required for complete fungal destruction and resolution of the clinical signs

Surgical Considerations!!navigator!!

  • Combined medical and surgical therapy is indicated if ulcers are deep, if they are not responding to medical treatment, or if they worsen despite medical treatment
  • Surgeries for keratomycosis include conjunctival grafts and full- and split-thickness penetrating keratoplasty. While surgical treatment may leave the horse with a larger scar, conjunctival grafts usually prevent corneal rupture and allow for physical support, a regional blood supply, and a supply of endogenous antiproteases to the ulcer site

Medications ⬆ ⬇

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MEDICATIONS

Drug(s) of Choice!!navigator!!

  • Treatment must be directed against the fungi, as well as against the corneal and intraocular inflammatory responses that occur following fungal replication and hyphal death
  • Miconazole (1%) has been used successfully and frequently as a topical antifungal agent. The IV form is preferred, but human vaginal products may be used
  • Natamycin 3.33–5% is the only FDA-approved topical antifungal agent. It is effective against many ocular fungal pathogens
  • Voriconazole 1% is an effective broad-spectrum antifungal agent and has reasonably good corneal penetration
  • Amphotericin B (1.5 mg/mL) may be administered topically
  • Silver sulfadiazine (dermatologic preparation) is a topical antimicrobial agent with both antifungal and antibacterial activity that is believed to be fungicidal
  • Dilute (1:50) povidone–iodine is effective topically against some fungal isolates. It can be irritating if used too frequently
  • Itraconazole and fluconazole are used successfully topically for keratomycosis in horses in some cases
  • Topically administered antifungal therapy for equine keratomycosis is administered 4–6 times per day
  • Corneal ulcerations result in massive increases in tear film protease activity. Topical serum and topical EDTA are critical to speed healing. They should be administered as many times a day as possible. EDTA is believed to have a synergistic effect with antifungal agents when used together for keratomycosis
  • Iridocyclitis is present any time a horse has a corneal ulcer, and can escalate in intensity following hyphal death after antifungal therapy is initiated. Flunixin meglumine (1 mg/kg BID IV, IM, PO) is the most frequently used NSAID in horses for systemic treatment of iridocyclitis. It may also reduce the speed of corneal vascularization
  • 1% atropine sulfate, a parasympatholytic agent, is used in all cases for its mydriatic and cycloplegic effects to dilate the pupil and diminish ciliary body muscle spasms associated with the axon reflex uveitis that occurs with corneal ulceration in the horse. It may be administered every 4–6 h until the pupil is dilated, and then the frequency of administration reduced

Contraindications!!navigator!!

N/A

Precautions!!navigator!!

N/A

Possible Interactions!!navigator!!

N/A

Alternative Drugs!!navigator!!

N/A

Follow-up ⬆ ⬇

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FOLLOW-UP

Patient Monitoring!!navigator!!

  • The eye should be protected from self-trauma with hard- or soft-cup hoods
  • Patients should be monitored for colic and persistent signs of eye pain

Prevention/Avoidance!!navigator!!

N/A

Possible Complications!!navigator!!

  • Persistent pain, uveitis, endophthalmitis, globe rupture, iris prolapse, and blindness are complications
  • Topically administered atropine must be used cautiously in horses as it may alter gastrointestinal motility

Expected Course and Prognosis!!navigator!!

  • Vision following keratomycosis in horses may be retained in as few as 50% of eyes if treatment is not aggressive
  • Aggressive medical and surgical therapy for ulcerative keratomycosis in horses should, however, result in a positive visual outcome and ocular survival in >90% of eyes. Despite this success, therapy is prolonged and scarring of the cornea may be prominent
  • Enucleation may be necessary in horses that become blind and continue to experience ocular pain

Miscellaneous ⬆ ⬇

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MISCELLANEOUS

Associated Conditions!!navigator!!

Severe uveitis.

Age-Related Factors!!navigator!!

N/A

Zoonotic Potential!!navigator!!

N/A

Pregnancy/Fertility/Breeding!!navigator!!

N/A

Synonyms!!navigator!!

N/A

Abbreviations!!navigator!!

  • FDA = US Food and Drug Administration
  • NSAID = nonsteroidal anti-inflammatory drug
  • PCR = polymerase chain reaction

Suggested Reading ⬆ ⬇

Brooks DE. Ophthalmology for the Equine Practitioner, 2e. Jackson, WY: Teton NewMedia, 2008.

Brooks DE, Matthews AG. Equine ophthalmology. In: Gelatt KN, ed. Veterinary Ophthalmology, 4e. Ames, IA: Blackwell, 2007:1165–1274.

Gilger BC, ed. Equine Ophthalmology, 3e. Ames, IA: Wiley Blackwell, 2017.

Author(s) ⬆

Author: Caryn E. Plummer

Consulting Editor: Caryn E. Plummer