[Pythium keratitis: a case report].

A 31-year-old female presented with a 3-week history of right eye redness, ocular pain, photophobia and decreased vision after tap water exposure. Slit-lamp examination revealed corneal lesions with pseudopodia and hill-shaped hypopyon. In vivo confocal microscopy (IVCM) detected filament-like structures in the corneal stroma. The patient was initially suspected of having fungal keratitis, but the condition deteriorated despite standardized antifungal treatment. Repeated corneal scraping culture and targeted DNA sequencing confirmed the infection of Pythium insidiosum. After switching to linezolid-based antibacterial therapy, corneal perforation still occurred, and total keratoplasty with a scleral ring was performed. The infection relapsed at 2 weeks postoperatively and was finally controlled by multiple intracameral and intrastromal injections of linezolid. A secondary penetrating keratoplasty was subsequently performed. No recurrence was observed during the 11-month follow-up; the corneal graft remained transparent, and the best corrected visual acuity improved to 0.6. Pythium species are oomycetes instead of true fungi, which are intrinsically resistant to conventional antifungal agents but susceptible to specific antibacterial drugs such as linezolid. For clinically suspected Pythium keratitis unresponsive to antifungal treatment, early targeted DNA sequencing is essential for rapid pathogen identification. Timely administration of sensitive antibacterial agents and individualized surgical intervention are critical to control infection and optimize visual prognosis.