Pythium insidiosum keratitis - A review

Abstract

Pythium insidiosum is an oomycete and is also called "parafungus" as it closely mimics fungal keratitis. The last decade saw an unprecedented surge in Pythium keratitis cases, especially from Asia and India, probably due to growing research on the microorganism and improved diagnostic and treatment modalities. The clinical features such as subepithelial infiltrate, cotton wool-like fluffy stromal infiltrate, satellite lesions, corneal perforation, endoexudates, and anterior chamber hypopyon closely resemble fungus. The classical clinical features of Pythium that distinguish it from other microorganisms are reticular dots, tentacular projections, peripheral furrowing, and early limbal spread, which require a high index of clinical suspicion. Pythium also exhibits morphological and microbiological resemblance to fungus on routine smearing, revealing perpendicular or obtuse septate or aseptate branching hyphae. Culture on blood agar or any other nutritional agar is the gold standard for diagnosis. It grows as cream-colored white colonies with zoospores formation, further confirmed using the leaf incarnation method. Due to limited laboratory diagnostic modalities and delayed growth on culture, there was a recent shift toward various molecular diagnostic modalities such as polymerase chain reaction, confocal microscopy, ELISA, and immunodiffusion. As corneal scraping (10% KOH, Gram) reveals fungal hyphae, antifungals are started before the culture results are available. Recent in vitro molecular studies have suggested antibacterials as the first-line drugs in the form of 0.2% linezolid and 1% azithromycin. Early therapeutic keratoplasty is warranted in nonresolving cases. This review aims to describe the epidemiology, clinical features, laboratory and molecular diagnosis, and treatment of Pythium insidiosum keratitis.

Keywords: Keratitis; Pythium insidiosum; linezolid; parafungus; zoospore.

Figure 1

Image depicting the detailed mechanism of pathogenesis of Pythium insidiosum keratitis

Figure 2

(a) Slit-lamp image depicting the case of confirmed Pythium keratitis having anterior to mid stromal infiltrate with tentacular extensions. (b) Slit-lamp image depicting the image of the same case after resolution on medical treatment. (c) Slit-lamp image depicting the case of confirmed Pythium keratitis having anterior to mid stromal infiltrate having tentacular extensions extending till posterior stroma. (d) Slit-lamp image depicting worsening of infection as observed by increased density of endo-exudates with cotton wool-like fluffy infiltrates

Figure 3

(a) Shows a 10% KOH wet mount demonstrating the presence of long, sparsely septate hyaline hyphae. (b) Shows the gram stain image depicting the thick cell wall, a few septate, and ribbon-like folding patterns of fungal hyphae. (c) Shows a 5-day old culture of P. insidiosum at 37°C grown on 5% sheep blood agar. (d) Shows a confocal microscopy image depicting thin, hyperreflective, occasionally branching structures with varying angles

Figure 4

Depicts a proposed diagnostic algorithm for diagnosis and management of Pythium keratitis

Figure 5

(a) Slit-lamp image depicting graft reinfection with anterior chamber exudates. (b) Slit-lamp image depicting graft reinfection with host rim margin infiltrate. (c) Slit-lamp image depicting graft reinfection with full-thickness infiltrate of host and the donor cornea. (d) Slit-lamp image depicting graft reinfection with full-thickness infiltrate of host and the donor cornea