A 38-year-old woman from Bangalore with a history of type 2 diabetes mellitus (HbA1c 9.2%) presents with a 2-week history of progressive left eye pain, blurred vision, and a white corneal infiltrate that does not respond to topical antibiotics. On examination, visual acuity is 6/36, and slit-lamp biomicroscopy shows a 5 mm corneal ulcer with feathery, ill-defined borders, minimal inflammatory response, and a dense, dry appearance. KOH mount of the corneal scraping reveals septate hyphae. What is the most likely causative organism?

Explanation

## Fungal Keratitis: Clinical Diagnosis **Key Point:** The combination of a feathery, ill-defined corneal ulcer with minimal inflammatory response, septate hyphae on KOH mount, in a patient from a tropical region (Bangalore) with diabetes and failure to respond to topical antibiotics, is most consistent with **Fusarium species** keratitis — the most common cause of fungal keratitis in India and tropical/subtropical regions. ### Fungal Corneal Ulcer Classification | Organism | Morphology | Border | Inflammatory Response | KOH Mount | Risk Factors | |----------|-----------|--------|----------------------|-----------|---------------| | **Fusarium species** | Septate hyphae | Feathery, ill-defined | Minimal | Septate hyphae | Trauma (vegetative), tropical climate, diabetes, contact lens | | **Aspergillus fumigatus** | Septate hyphae | Feathery, ill-defined | Minimal | Septate hyphae | Trauma, immunosuppression, diabetes | | **Candida albicans** | Yeast + pseudohyphae | Raised, demarcated | Moderate | Budding yeast ± pseudohyphae | Immunocompromised, contact lens, temperate climate | | **Cryptococcus** | Yeast | Minimal infiltration | Minimal | Round yeast with halo | Immunocompromised, rare corneal involvement | **High-Yield:** Fusarium species (especially *F. solani* and *F. oxysporum*) are the **most common cause of fungal keratitis in India and other tropical/subtropical countries**, accounting for up to 40–70% of cases in studies from South India. Key associations include: - Trauma with vegetative/organic matter - Tropical and subtropical climates (high ambient fungal load) - Diabetic patients (impaired neutrophil function) - Prolonged topical antibiotic use (altered ocular flora) - Contact lens wear ### Distinguishing Fusarium from Other Fungi **Fusarium species characteristics:** - Septate hyphae (indistinguishable from Aspergillus on KOH alone; culture needed for definitive ID) - Feathery, ill-defined borders — classic appearance - Minimal inflammatory response despite large ulcer size - Dense, dry, "ground glass" appearance - Slow-to-moderate progression (days to weeks) - Poor response to topical antibiotics (key diagnostic clue) - Can cause satellite lesions and immune ring (Wessely ring) **Clinical Pearl:** Both Fusarium and Aspergillus produce septate hyphae on KOH mount and can appear morphologically similar. However, **Fusarium is the predominant filamentous fungal pathogen in tropical India** (Khurana; Garg et al., Indian J Ophthalmol), making it the most likely answer in this geographic and clinical context. Aspergillus is more common in temperate climates and post-surgical/immunosuppressed settings. ### Why Not the Other Options? - **Aspergillus fumigatus (C):** Also produces septate hyphae and feathery borders, but is less common than Fusarium in tropical India. More associated with post-surgical or severely immunosuppressed patients. Cannot be distinguished from Fusarium on KOH alone, but epidemiologically Fusarium predominates. - **Candida albicans (D):** Produces budding yeast and pseudohyphae (NOT purely septate hyphae). Presents with raised, demarcated borders and moderate inflammatory response. More common in temperate climates and immunocompromised hosts. - **Cryptococcus neoformans (B):** Primarily a systemic pathogen; corneal involvement is exceedingly rare. KOH shows round yeast with a thick capsular halo, not septate hyphae. ## Management Implications Fusarium keratitis requires: - **Topical natamycin 5%** — first-line agent; superior efficacy against Fusarium compared to amphotericin B - Topical voriconazole 1% — second-line or adjunct - Systemic voriconazole for deep/severe ulcers - Corneal scraping and culture on Sabouraud dextrose agar for species identification - Aggressive glycemic control (HbA1c 9.2% is suboptimal) - Avoid topical corticosteroids (worsen fungal disease) - Consider therapeutic keratoplasty if perforation risk **Mnemonic:** **FITS** — Fusarium In Tropical Settings (septate hyphae, feathery borders) [cite: Khurana Comprehensive Ophthalmology 7th Ed, Ch 3; Garg P et al. Indian J Ophthalmol 2012; Srinivasan M, Cornea 2004]