ISSN 2398-2977      

Keratitis: mycotic

pequis

Synonym(s): Keratomycosis, fungal keratitis


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

This article is available in full to registered subscribers

Sign up now to obtain ten tokens to view any ten Vetlexicon articles, images, sounds or videos, or Login

Treatment

This article is available in full to registered subscribers

Sign up now to obtain ten tokens to view any ten Vetlexicon articles, images, sounds or videos, or Login

Prevention

This article is available in full to registered subscribers

Sign up now to obtain ten tokens to view any ten Vetlexicon articles, images, sounds or videos, or Login

Outcomes

This article is available in full to registered subscribers

Sign up now to obtain ten tokens to view any ten Vetlexicon articles, images, sounds or videos, or Login

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.

Can’t find what you’re looking for?

We have an ever growing content library on Vetlexicon so if you ever find we haven't covered something that you need please fill in the form below and let us know!

 
 
 
 

To show you are not a Bot please can you enter the number showing adjacent to this field

 Security code