A masquerader? Paecilomyces must be distinguished from Penicillium in fungal keratitis: a report of two contrasting cases

Abstract

We describe the clinical outcomes of two contrasting cases of fungal keratitis due to Paecilomyces spp. The first case involving a 58-year-old woman was complicated by an initial laboratory misidentification as Penicillium and consequently a delay in treatment with an optimised antifungal regimen. The patient had a protracted clinical course that required a total of four penetrating keratoplasties. However, an accurate diagnosis was promptly made in the second case, a 46-year-old woman, which resulted in a satisfactory outcome after penetrating keratoplasty. Our principal aim was to highlight a diagnostic challenge relating to the accurate microbial identification of Paecilomyces spp. This can be difficult given its morphological similarity to Pencillium, and confusion over the two genera has resulted in misdiagnoses reported previously. Our report aims to raise awareness of this potential laboratory misidentification, which can affect clinical decision-making in guiding antimicrobial therapy.

Figure 1

Histopathology of the host cornea in case 1. (A) Low-power view showing acute ulcerative keratitis with perforation. Arrowheads mark perforation site, while arrows indicate a mixture of hypopyon and iris tissue prolapsing through the perforation, with a portion (perhaps artefactually) extending onto the corneal surface (H&E stain, original magnification, ×20). (B) High-magnification view of the area outlined by box in A, demonstrating stromal necrosis/hypopyon at perforation site, with abundant neutrophils (H&E stain, original magnification, ×400). (C) Grocott-Gomori methenamine silver stain showing septated filamentous fungi (arrows; original magnification, ×400).

Figure 2

Slit-lamp photograph of the left eye of case 1 showing a new inferior infiltrate at the deep corneal stroma and graft-host interface 4 weeks after the first corneal transplantation.

Figure 3

Histopathology of the excised failed graft from case 1. (A) H&E stain demonstrating acute inflammatory cells at the edge of the corneal button. The epithelium is eroded. (The absence of Descemet's membrane at this edge of the button was consistent with artefact, original magnification, ×100). (B) Grocott-Gomori methenamine silver stain showing septated filamentous fungi (arrows), similar in morphology to those seen in the original corneal button (original magnification, ×400).

Figure 4

Histology of excised corneal button from case 2, H&E (A) and Grocott-Gomori methenamine silver (B–D) stains. (A) Low-power view showing acute and chronic ulcerative keratitis, with re-epithelialised portion centrally exhibiting compensatory epithelial thickening. (Only a portion of Descemet's membrane is visible, possibly due to artefactual turning of the tissue, original magnification, ×40.) (B) GMS stain under higher magnification showing filamentous fungi extending to left edge of corneal button and invading deep stroma and Descemet's membrane (d) (original magnification, ×100). (C) Higher magnification of posterior cornea demonstrating filamentous fungi invading Descemet's membrane (d), with several fungi spilling into anterior chamber. Endothelial cell loss is also apparent (original magnification, ×400). (D) Higher magnification showing fungi with septations (arrowhead; original magnification, ×600).

Figure 5

Slit-lamp photograph of the right eye in case 2 with a history of Paecilomyces spp keratitis after receiving a therapeutic corneal transplantation. The corneal graft has been clear of keratitis for over 12 months.

Figure 6

Micrographs of Penicillium and Paecilomyces from unrelated isolates. Penicillium spp (A) has broad and then blunted phialids (brackets) with generally circular conidia (arrows). This differs slightly from the tapered phialids in Paecilomyces spp (B).