Fungal Keratitis

Risk Factors & Stratification

Corneal trauma involving plant or vegetable matter is the classic risk factor for fungal keratitis. A branch, a piece of straw, or garden debris can introduce filamentous fungi such as Fusarium and Aspergillus directly into the stroma, which is why the mechanism is overrepresented in agricultural work and warm climates. Any corneal foreign body or abrasion history involving organic plant material should raise suspicion for a fungal cause, even when the eye initially looks like a routine abrasion. Contact lens wear (see Contact Lens-Related Corneal Disease) and chronic topical steroid use round out the major risk factors; together with agricultural or outdoor occupational exposure, they define the population where fungal keratitis needs to stay on the differential.

The risk factor itself also predicts the likely organism. Yeast forms such as Candida typically cause keratitis in eyes with pre-existing ocular surface disease, chronic corticosteroid use, or systemic immunosuppression. Filamentous fungi (Fusarium, Aspergillus), by contrast, track with trauma in an otherwise healthy cornea, the classic vegetative-injury presentation above. The distinction matters clinically: it sets the pretest suspicion for a given patient and, as covered below, determines the first-line antifungal.

Clinical Features

Fungal keratitis often signals itself on exam before any culture result comes back. The corneal ulcer classically has feathery, irregular margins, produced by hyphae invading adjacent healthy stroma beyond the primary infiltrate. That pattern is distinct from the sharper-edged infiltrates typical of bacterial keratitis. Satellite lesions, small secondary infiltrates near but separate from the main lesion, arise from that same outward hyphal spread and are an equally useful clue.

Diagnosis

Whenever fungal keratitis is suspected clinically, on the basis of feathery margins, satellite lesions, or a history of vegetative trauma, corneal scraping should be cultured on Sabouraud dextrose agar, a medium selectively optimized for fungal growth, alongside standard blood and chocolate agar. In vivo confocal microscopy, discussed at greater length on the Acanthamoeba keratitis page, can also help in select fungal cases: it images hyphal elements at the bedside directly, without waiting on culture.

Management

Natamycin 5% solution is first-line for keratitis caused by filamentous fungi such as Fusarium. It has strong activity against filamentous organisms and penetrates the corneal epithelium well. Amphotericin B 0.15% is reserved for yeast (Candida) keratitis rather than used first-line for filamentous disease. Voriconazole is another option, including for cases refractory to first-line therapy.

Fungal keratitis generally needs a longer treatment course than bacterial keratitis, and carries a worse prognosis overall. The antifungal response is slower, and the presentation is often delayed and less acute than bacterial disease, so more damage tends to accumulate before treatment starts and takes effect.

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