Pythium keratitis: experience from Thailand

Pythium insidiosum keratitis is a rare but highly aggressive corneal infection that closely mimics fungal keratitis yet responds poorly to conventional antifungal therapy. Although previously under-recognised, it is increasingly reported in tropical and subtropical regions, particularly Southeast Asia, and is associated with high rates of therapeutic keratoplasty and globe loss. This review summarises current evidence on the epidemiology, risk factors, pathogenesis, clinical presentation, diagnostic strategies and management of Pythium keratitis, with particular emphasis on data from endemic regions. The disease typically affects otherwise healthy individuals with antecedent ocular trauma, agricultural exposure or contact with contaminated water. Clinically, it is characterised by rapid progression, tentacular or reticular stromal infiltrates, early deep stromal involvement, and frequent limbal extension, often mimicking filamentous fungal keratitis. Definitive diagnosis requires a combination of direct microscopy with specialised staining, culture with zoospore induction, histopathology and molecular techniques, particularly PCR assays targeting the internal transcribed spacer region. The absence of ergosterol in the organism's cell membrane renders antifungal agents largely ineffective. Emerging evidence supports targeted antibacterial therapy, especially linezolid- and azithromycin-based regimens, which have been associated with improved outcomes and reduced need for surgical intervention. Early recognition, organism-specific diagnosis and prompt initiation of appropriate therapy are essential to improve visual and anatomical outcomes in affected patients.