Acanthamoeba keratitis — DTM&H MCQ

Correct answer: E — Corneal scraping for microscopy, culture and specialist testing

The correct answer is E, corneal scraping for microscopy, culture and specialist testing. The history of contact lens wear with water exposure (swimming while wearing lenses), disproportionate pain, photophobia and a ring-shaped corneal infiltrate is classic for Acanthamoeba keratitis, and UK guidance mandates urgent same-day ophthalmology referral for corneal sampling to allow microscopy (looking for double-walled cysts), culture on non-nutrient agar with E. coli overlay, and confocal microscopy or PCR where available. Confirming the diagnosis at the cornea itself is essential because Acanthamoeba is a sight-threatening free-living amoeba acquired from contaminated water rather than a systemic parasitic infection, so systemic stool, blood or urine tests have no diagnostic role. Prompt corneal sampling before starting empirical anti-amoebic biguanide therapy maximises culture yield and avoids the visual loss associated with delayed diagnosis. Why the other options are wrong: D. Stool ova and parasite examination: this detects intestinal parasites (e.g. helminths, Giardia) and has no bearing on a corneal infection; Acanthamoeba is not shed in stool in a way that informs keratitis diagnosis. B. Blood film for trypomastigotes: this screens for trypanosomiasis (African or Chagas disease), which causes systemic illness or chagoma, not a localised ring corneal infiltrate in a contact lens wearer. A. Urine microscopy for schistosome eggs: relevant to Schistosoma haematobium urinary tract disease, not to a water-associated corneal infection; it does not sample the infected tissue. C. Serum dengue IgM: dengue causes a febrile systemic illness with rash and thrombocytopenia, not a focal ring corneal infiltrate, and serology here is clinically irrelevant to the eye findings described. Key point: a ring corneal infiltrate with pain out of proportion to signs in a contact lens wearer exposed to water demands urgent corneal scraping for Acanthamoeba, not systemic parasitology or serology.

Reference: College of Optometrists, Clinical Management Guidelines: Microbial keratitis (Acanthamoeba sp.), https://www.college-optometrists.org/clinical-guidance/clinical-management-guidelines/microbialkeratitis_acanthamoebasp