Equis ISSN 2398-2977

Keratitis: mycotic

Synonym(s): Keratomycosis, fungal keratitis

Contributor(s): Alison Clode, Paul Gerding, Graham Munroe, William C Rebhun

Introduction

  • Cause: saprophytic commensal fungal organisms in the external eye invade the cornea as opportunist pathogens following long-term topical antibiotic or corticosteroid therapy or corneal injury - immunosuppressive tear film deficiencies.
  • Incidence: should be suspected in any case of recurrent deep keratitis with/without surface defects   Keratitis: overview  .
  • Signs: ocular pain, photophobia, blepharospasm +/- hypophyon, increased lacrimation, characteristic ulcerated appearance and long-term resistance to treatment should arouse suspicions, miosis, flare, deep corneal neovascularization, conjunctival hyperemia.
  • Diagnosis: corneal scrapings   Cornea / conjunctiva: scraping  are essential for diagnosis by cytology, culture and histology.
  • Treatment: should include:
    All OfIntensive long-term topical antifungal, topical atropine   Atropine sulfate  and anti-ulcer medication.
    AndSystemic antifungals and NSAIDs   Therapeutics: anti-inflammatory drugs  .
    AndSurgical corneal debridement   Cornea: debridement  is essential to increase drug penetration.
  • Prognosis: guarded. If left untreated may   →   eventual loss of vision.

Pathogenesis

Etiology

  • The fungi involved are mainly saprophyte commensals of the external eye and the particular species involved reflects the horse's geographical environment   Eye: microbiology - overview  .
  • Those most commonly isolated include:
    • Aspergillusspp   Aspergillus spp  .
    • Penicilliumspp.
    • Fusariumspp.
    • Alternariaspp.
    • Cladosporiumspp.
  • Yeasts, eg:
    • Candidaspp.
    • Torulopsisspp (less commonly found).

Predisposing factors

General
  • Old age.
  • Corneal injury or disease.
  • Large surface area of prominent cornea.
  • Disruption of the balance of commensal micro-organisms in the external eye, usually by long-term therapy.
  • Increased exposure to environmental fungi.

Specific

  • Long-term application of antibiotics and, particularly, corticosteroids   Therapeutics: eye   to external eye   →   disruption of balance of micro-organisms. Corticosteroids decrease corneal surface defense mechanisms by inhibiting the cell-mediated immune response.
  • Direct innoculation of material, particularly vegetative, into cornea as a foreign body   Cornea: foreign body  .
  • Previous corneal disease, especially ulceration or abrasion   →   stromal exposure to mycotic pathogens.
  • Immunosuppressive tear film deficiencies.

Pathophysiology

  • Fungal organisms involved reflect those found in the immediate environment and are mainly commensals. These are opportunistic pathogens - require a predisposing corneal condition or injury, particularly if the corneal defence mechanisms (cell-mediated immunity) are decreased, eg previous topical corticosteroid use, to help them become established.
  • The fungal organisms are often capable of invading into the corneal tissue   →   protease release   →    tissue destruction   →   a deep keratitis or abscess sometimes without surface defects   Keratitis: overview  .
  • Fungal organisms have an affinity for Descemet's membrane.
  • More usually there is a keratitis with epithelial ulceration, stromal edema and deep corneal vascularization.
  • The corneal resistance is compromised and there is increased susceptibility to bacterial infection - mixed bacterial and fungal infections are common and serious   Keratitis: bacterial  .
  • Fungi may be able to suppress neovascularization   →    predisposes to abscess formation.

Timecourse

  • Clinical signs may be observed within several days.
  • Usually long-term corneal disease.

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Voelter-Ratson K, Pot S A, Florin M & Spiess B M (2013) Equine keratomycosis in Switzerland: A retrospective evaluation of 35 horses (January 2000-August 2011). Equine Vet J 45 (5), 608-612 PubMed.
  • Reed Z et al (2013) Equine keratomycosis in California from 1987 to 2010 (47 cases). Equine Vet J 45 (3), 361-366 PubMed.
  • Gilmour M A (2012) Subconjunctival voriconazole for the treatment of mycotic keratitis in a horse. Equine Vet Educ 24 (10), 489-492 VetMedResource. 
  • Malho P (2011) Equine fungal keratitis: a case report. UK Vet 16, 8-12 VetMedResource.
  • Utter M E, Wotman K L, Armour M & Bagel J (2010) Clinical findings and outcomes of ulcerative keratomycosis in 30 horses in the mid-Atlantic United States (2006-2007). Equine Vet Educ 22 (1), 31-39 VetMedResource.
  • Brooks D E (2009) Equine keratomycosis: An international problem. Equine Vet Educ 21 (5), 243-246 Wiley Online Library.
  • Clode A B, Davis J L, Salmon J H et al (2006) Evaluation of concentration of voriconazole in aqueous humor after topical and oral administration in horses. Am J Vet Res 67 (2), 296-301 PubMed.
  • Davis J L, Salmon J H & Papch M G (2005) Pharmacokinetics and tissue distribution of itraconazole after oral intravenous administration to horses. Am J Vet Res 66 (10), 1694-1701 PubMed.
  • Sanson J, Featherstone H & Barnett K C (2005) Keratomycosis in six horses in the United Kingdom. Vet Rec 156 (1), 13-17 PubMed.
  • Latimer F G, Colitz C M, Campbell N B & Papich M G (2001) Pharmacokinetics of fluconazole following intravenous and oral administration and body fluid concentrations of fluconazole following repeated oral dosing in horses. Am J Vet Res 62, 1694-1701 PubMed.
  • Hamor R E et al (1999) Equine infectious keratitis. Vet Clin North Am Equine Pract 15 (3), 623-646, ix PubMed.
  • Andrew S E, Brooks D E, Smith P J et al (1998) Equine ulcerative keratomycosis - visual outcome and ocular survival in 39 cases (1987-1996)Equine Vet J 30 (2), 109-116 PubMed.
  • Gaarder J E, Rebhun W C, Ball M A et al (1998) Clinical appearances, healing patterns, risk factors and outcomes of horses with fungal keratitis: 53 cases (1978-1996). JAVMA 213, 105-112 PubMed.

Other sources of information

  • Andrew S A & Willis A M (2005) Disease of the Cornea and Stroma. In: Equine Ophthalmology.Ed: Gilger B C. Elsevier Saunders. USA. pp 157-251.
  • Barnett K C, Crispin S M, Lavach J D & Matthews A G (2004) Equine Ophthalmology - An Atlas and Text. 2nd edn. W B Saunders.
  • Brooks D E (1999) Equine ophthalmology. In: Veterinary Ophthalmology. Ed: Gelatt K N. 3rd edn. Lippincott, USA. pp 1053-1116.


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