Decoding the Rare Diagnostic Enigma of Pythium Insidiosum Keratitis – A Narrative Review

Abstract

Pythium insidiosum, an oomycete, is frequently referred to as a “parafungus” due to its striking similarity to fungal keratitis. Over the past 10 years, there has been a significant rise in Pythium insidiosum keratitis (PIK) cases, notably in Asia and India. This trend is potentially attributable to heightened research interest in this microorganism and advancements in diagnostic and therapeutic techniques. Clinically, its features, such as subepithelial infiltration, fluffy stromal infiltrates resembling cotton wool, satellite lesions, corneal perforation, endoexudates, and anterior chamber hypopyon, mirror those of fungal infections. However, hallmark clinical features like reticular dots, tentacular extensions, peripheral indentation, and prompt limbal progression distinguish Pythium from other pathogens and necessitate vigilant clinical scrutiny. From a morphological and microbiological perspective, Pythium closely mirrors fungi when subjected to routine smear tests, often revealing perpendicular or obtuse septate or non-septate branching hyphae. Culturing on nutritional media like blood agar, chocolate agar, and so on remains the diagnostic gold standard, with the organism presenting as cream-white colonies that form zoospores. This can be further verified through the leaf incarnation technique. Culture and polymerase chain reaction (PCR) are the gold standard diagnostic modalities for PIK. Historically, due to constraints in diagnostic techniques and the slow growth of cultures, there has been a trend toward employing advanced molecular diagnostic tools, including PCR, confocal microscopy, enzyme-linked immunosorbent assay, and immunodiffusion. When corneal scraping tests (10% KOH, Gram stain) depict fungal-like hyphae, antifungal treatments are often initiated even before culture results are ready. However, recent molecular research recommends the use of antibacterials, specifically 0.2% linezolid and 1% azithromycin, as the primary therapeutic agents. In cases that do not respond to treatment, early therapeutic keratoplasty is advised. This narrative review aims to provide a comprehensive overview of the epidemiology, clinical manifestations, laboratory and molecular diagnostic procedures, and therapeutic approaches for PIK.