Acanthamoeba spp. is the Most Likely Pathogen
The correct answer is Acanthamoeba spp. This patient's presentation of severe keratitis following contact lens storage in tap water is pathognomonic for Acanthamoeba keratitis, which would be cultured from both the contaminated tap water and the patient's vitreous fluid if the infection has progressed to that severity.
Clinical Reasoning
Why Acanthamoeba is the Answer
Tap water exposure is the cardinal risk factor: The American Academy of Ophthalmology explicitly states that swimming with contact lenses has been associated with the development of Acanthamoeba keratitis, and patients should be instructed not to rinse contact lenses or lens cases with tap water 1.
Storage in tap water is a direct transmission route: Contact lens wearers should use only sterile products commercially prepared for contact lens care and never rinse lenses with tap water, bottled water, or homemade saline 1. The organism is ubiquitous in water sources and binds to contact lens materials 2, 3.
Severe keratitis presentation matches: Acanthamoeba keratitis characteristically presents with severe pain often disproportionate to clinical signs, with early superficial keratitis that progresses to central corneal epithelial loss and marked stromal opacification with vision loss 2. This matches the "severe keratitis" and "deterioration of vision" described in the question.
Why Other Options Are Incorrect
Cryptococcus neoformans: This fungal pathogen causes keratitis primarily in immunocompromised patients and is not associated with tap water exposure or contact lens wear. It would not be expected in tap water cultures.
Naegleria fowleri: This free-living amoeba causes primary amebic meningoencephalitis through nasal passage entry during swimming in warm freshwater, not keratitis. It does not cause ocular infections.
Strongyloides stercoralis: This is a parasitic nematode causing intestinal and disseminated disease in immunocompromised hosts, not ocular infections. It has no association with contact lenses or tap water exposure to the eye.
Toxoplasma gondii: While this can cause chorioretinitis, it is not associated with contact lens wear or tap water exposure, and would not be cultured from tap water.
Epidemiology and Risk Factors
Water contact is the primary modifiable risk factor: Recent epidemiological studies confirm the link between solution topping up and Acanthamoeba keratitis, and reinforce the importance of avoiding tap water either as part of care for the contact lens or storage case, handling lenses with wet hands, or showering while wearing lenses 3.
All contact lens types are at risk: Cases have been documented in disposable soft lens wearers, hybrid lens wearers, and rigid gas permeable lens wearers 2. Even cosmetic contact lens users who purchase lenses via the internet and use tap water irregularly are at risk 4.
Bilateral disease can occur: When both lenses are stored in the same contaminated case exposed to tap water, bilateral Acanthamoeba keratitis can develop 5.
Clinical Pitfalls to Avoid
Early misdiagnosis as HSV keratitis: The early superficial keratitis is often misdiagnosed as herpes simplex virus keratitis, delaying appropriate treatment 2. Maintain high clinical suspicion when water exposure history is present.
Manufacturer guidelines remain problematic: Despite decades of known risk, discrepancies between clinician recommendations and written instructions on solution packages persist, with some protocols still not explicitly warning against tap or distilled water use 6.
Prevention is superior to treatment: Despite significant advances in medical therapy over the past decade, prevention through patient education about proper lens care and absolute avoidance of water exposure remains the best approach 2, 3.