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Fungal Infection of the Eye in equines

Fungal Infection of the Eye

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Background

Fungal keratitis is a serious and painful corneal disease that is caused by fungal infection. Fungal keratitis in horses is a well-known and frustrating clinical problem and accounts for 31% of equine infectious keratitis. Under the best circumstances, the disease requires prolonged and expensive treatment; whereas under the worst circumstances, it can result in blindness or the need for enucleation.

Horses live in environments that harbor many fungal agents, and fungal organisms frequently reside in conjunctival flora. The most common fungal species isolated from the normal equine eye are Aspergillus, Penicillium, Alternaria, Cladosporium, and Fusarium. In addition, the equine globe is large, prominent, and easily injured. Corneal injuries in horses often are caused by plant material or soil, either of which could provide a fungal inoculum. Horses that have been treated with either topical antibiotics or corticosteroids are predisposed to fungal infection, because chronic topical antibiotic usage alters the microbial flora of the conjunctiva and corticosteroids suppress local immunity. Topical corticosteroid treatment preceded patient referral in 42% to 66% of fungal keratitis cases in the literature.

Clinical Presentation

The clinical appearance of fungal keratitis varies greatly depending on the duration and severity of infection. It may present as either an ulcerative or non-ulcerative corneal lesion. Ulcerative fungal keratitis may appear as a distinct lesion with a uniform white or yellow border and base; with a necrotic, dry, shaggy raised border; as a gray or white edematous necrotic central corneal lesion; or as multiple white or gray punctate lesions of the epithelium and superficial stroma. Occasionally fungal keratitis will appear as an abscess within the corneal stroma. These abscesses may be located very deep in the stroma and close to Descemet's membrane. It is possible for fungal organisms to penetrate Descemet's membrane and gain access to the anterior chamber. Deep corneal invasion of the fungi and concurrent bacterial infection can lead to corneal perforation and iris prolapse. Common clinical signs of fungal keratitis include ocular pain manifested by blepharospasm, epiphora and/or photophobia, fluorescein-positive corneal ulceration, corneal neovascularization, and uveitis manifested by miosis and aqueous flare.

Diagnosis

Diagnosis is based on the history and clinical signs, with confirmation from cytology and/or culture results. Corneal scrapings should be obtained for cytology and culture in all cases of suspected fungal keratitis. Samples can be collected with a sterile Kimura spatula or the sharp or dull side of a sterile No. 15 scalpel blade. It has been found that micro-brushes are also useful for collecting cytology samples. If a corneal stromal abscess is present, deep scraping is needed to obtain good samples. If the abscess is very deep, sample collection should be postponed until surgical treatment is possible and the sample should be obtained at that time. Both ulcerated and subepithelial lesions should be debrided to facilitate the collection of superficial stromal tissue for diagnosis and enhance drug penetration during treatment. Scraping samples should be placed on a glass slide and air-dried, then stained with Gram's, Wright's, or Giemsa stains. Culture specimens should be immediately inoculated onto Sabouraud's agar or transferred onto a sterile swab to be inoculated into a thioglycollate broth. The most common reason of failing to obtain a positive result is poor sample collection (a superficial scraping). Aspergillus and Fusarium are the most commonly isolated fungi.

Treatment

The three major goals for treating fungal keratitis are to iradicate the fungal infection, to prevent secondary bacterial infection, and to control ocular pain. Fungal infection, however, is very difficult to treat for several reasons. Few topical antifungal drugs have good corneal penetration, most are fungal static, many are irritating, and some are expensive. The inability to penetrate the cornea adequately and to kill fungi necessitates prolonged therapy and frequent (every one to two hours) application of antifungal drugs. Topical treatments generally require frequent application to maintain adequate concentrations at the ocular surface and within the cornea. Because of the difficulties encountered with frequent application of antifungal drugs and blepharospasm, placement of a lavage catheter is almost always necessary. A subpalpebral lavage catheter is usually preferred. The two most common topical antifungal agents are miconazole and natamycin, although silver sulfadiazine, iodine, amphotericin B, ketaconazole, and fluconazole have been tried. A mixture of DMSO and itraconazole has also been shown to be effective. Analgesic therapy includes cycloplegics and nonsteroidal anti-inflammatory drugs. Atropine 1% ophthalmic solution or ointment should be applied topically, as frequently as is necessary, to maintain pupillary dilation. It not only blocks painful ciliary spasm but also minimizes the development of synechiae. Ocular pain may also be controlled by the systemic administration of non-steroidal anti-inflammatory drugs, such as phenylbutazone (2.2 mg/kg every 12 to 24 hours) or flunixin meglumine (1.1mg/kg every 12 to 24 hours), for 5 to 7 days, thereafter gradually reducing the dose to 50% or less. Additional antibacterial therapy for individual cases should be guided by culture and sensitivity testing results. Because secondary bacterial invasion is likely, topical antibiotics should be included in the therapeutic regimen. Initial antibacterial therapy should be directed against both gram-positive and gram-negative organisms. Recommended antibiotics include gentamicin or triple antibiotic ointments or solution.

Medical treatment can be effective, provided that suitable drugs are administered appropriately. Medical therapy alone may be adequate but usually involves prolonged treatment of 4 to 8 weeks. Combinations of surgical and medical treatment usually reduce the duration of therapy, although surgical treatment can produce more scaring. Surgery is often chosen because of the shorter recovery time and potential better prognosis. Surgical treatment is also indicated if deep corneal ulceration (greater than 50% of corneal depth) or if a deep stromal abscess is present. Various recommended surgical procedures include keratectomy, conjunctival grafts, and penetrating keratoplasty. Any corneal tissue that is removed should be cultured and submitted for histopathologic evaluation. In some horses with severe endophthalmitis, chronic or severe pain, or blindness, enucleation should be the surgical option.

Many variables can affect the outcome of equine fungal keratitis. The pathogenicity of the fungal organism and previous treatment with topical steroids are two important factors. Accurate diagnosis and proper treatments (medical/surgical) are as important as well. Complications resulting from fungal keratitis include corneal scaring, pigmentation, corneal perforation and iris prolapse, uveitis, synechia formation, cataract formation, endophthalmitis, phthisis bulbi, and blindness. If proper diagnosis and treatment are applied early in the course, the visual outcome could be as high as 70% and overall ocular survival rate could be 80% of the cases. Early diagnosis and aggressive therapy can result in a successful result for equine fungal keratitis.

1 Ball MA: Equine Fungal Keratitis. Compend Contin Educ Pract Vet 22(2): 182-186, 2000.
2 Barton MH: Equine Keratomycosis. Compend Contin Educ Pract Vet 14(7): 936-944, 1992.

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