Which organism is associated with a ring infiltrate in keratitis?

• A. Acanthamoeba
• B. Pseudomonas aeruginosa
• C. Moraxella catarrhalis
• D. Staphylococcus aureus

Answer: (A)

The main concept here is that a ring-shaped corneal infiltrate is a classic clue for Acanthamoeba keratitis, an infection caused by a free-living amoeba that often affects contact lens wearers who have exposure to water. Acanthamoeba invades the cornea and nerves, triggering a peripheral inflammatory response that expands into a circular, ring-like infiltrate. This pattern helps distinguish it from bacterial keratitis, where you’d typically see more focal, suppurative infiltrates and rapidly progressive epithelial defects without this ring configuration. Clinically, Acanthamoeba keratitis often comes with severe pain out of proportion to the exam and can involve radial keratoneuritis (inflammation of corneal nerves). Diagnosis is confirmed with corneal scrapings using special stains or culture on non-nutrient agar with E. coli overlay, or through confocal microscopy. Treatment requires topical anti-amoebic agents (such as polyhexamethylene biguanide or chlorhexidine) and can be lengthy, sometimes needing adjunctive therapies or surgery.