Pythium Insidiosum Keratitis: Histopathology and Rapid Novel Diagnostic Staining Technique

Purpose: To elucidate the histopathology of Pythium insidiosum keratitis and to describe a novel, simple, and rapid staining technique for identification of oomycete Pythium insidiosum and to differentiate it from fungi.

Methods: This is a laboratory investigation study of 38 nonconsecutive cases (37 ocular samples and 1 colonic biopsy); 14 microbiologically diagnosed as Pythium insidiosum keratitis and 24 as fungal keratitis. Review of clinical, demographic details, microbiological results, and identification of cases that necessitated evisceration was performed. Reevaluation of histopathology slides was done using stains such as hematoxylin-eosin, Gomori methenamine silver (GMS), periodic acid-Schiff (PAS), potassium iodide-sulfuric acid (IKI-H2SO4). Morphology, degree, and nature of inflammation and load, distribution, and staining results of Pythium insidiosum and its comparison with fungi were studied.

Results: Delay in zoospore formation, failure of growth, and delay in identification of Pythium were the main cause of evisceration. Corneal pythiosis showed epithelial ulceration, stromal destruction, and varying inflammation; load and distribution of Pythium were inversely proportional to inflammation. The filaments were commonly wide, with admixed narrower structures and uncommonly involved Descemet membrane. The oomycete was not discretely discerned with PAS stain and stained distinctly with GMS stain and IKI-H2SO4 stain (100% sensitive). In comparison, fungal organisms stained well with PAS and GMS stain, but not with IKI-H2SO4 stain (100% specific).

Conclusions: Pythium insidiosum keratitis is perhaps not more devastating than fungal keratitis but late diagnosis, misdiagnosis, and treatment as fungal infection are major heralds. Early diagnosis may markedly improve the patient outcome. IKI-H2SO4 is a cost-effective, simple, sensitive, and specific stain for the diagnosis of oomycete Pythium.