LQCL2610 - CMLE - 68-Year-Old with Corneal Infection that Failed First-Line Therapy-LQCL2610

LQCL2610 - Educational Activity

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This LabQ Microbiology 2026 document reviews infectious keratitis with emphasis on diagnosing Acanthamoeba keratitis (AK) when standard therapy fails. A case is presented of a 68-year-old immunocompetent man who developed worsening unilateral eye pain after grass clippings hit his face. An initial dendritic corneal lesion led to empiric treatment for presumed herpes simplex virus (HSV) keratitis without microbiologic testing. Despite antivirals, antibiotics, and steroids, symptoms progressed over weeks, prompting expanded evaluation.<br /><br />The overview notes that most infectious keratitis is bacterial (eg, Staphylococcus aureus, coagulase-negative staphylococci, Pseudomonas aeruginosa), but viral (HSV/VZV), fungal (Candida; filamentous fungi), Microsporidium, and Acanthamoeba are important alternative causes. The American Academy of Ophthalmology recommends culture in severe, atypical, central/large, stromal, chronic, or treatment-unresponsive cases. Close coordination between ophthalmology and the microbiology lab is highlighted because ocular specimens are often scant and may be inoculated at the bedside.<br /><br />AK is rare (<2% of corneal infections) but Acanthamoeba are ubiquitous in soil, dust, and water. In high-income countries, the major risk factor is contact lens use, though traumatic inoculation can also occur. Pathogenesis involves trophozoite adherence and stromal invasion; organisms exist as trophozoites (irregular, granular cytoplasm) and highly resistant double-walled cysts.<br /><br />Definitive diagnosis relies on specialized methods: culture on non-nutrient agar overlaid with a lawn of gram-negative bacteria (eg, E. coli) incubated at 30–32°C and examined daily for trophozoite “tracks,” tissue microscopy (calcofluor white; histology with PAS/GMS), and/or PCR (often limited to reference labs). Specimens should be transported in saline at room temperature to avoid encystment.<br /><br />In the case, HSV/VZV PCR was negative; Acanthamoeba culture showed characteristic cysts and trophozoites, leading to anti-amoebic therapy (chlorhexidine plus PHMB) and adjunctive interventions. The key message is to specifically request AK testing when keratitis is atypical or unresponsive, because diagnosis and treatment differ substantially from more common etiologies.

Keywords

infectious keratitis

Acanthamoeba keratitis (AK)

treatment-refractory keratitis

corneal culture indications

contact lens risk factor

traumatic corneal inoculation

non-nutrient agar E. coli overlay culture

trophozoite and cyst morphology

calcofluor white staining

PCR diagnostics for Acanthamoeba