Microsporidia and Acanthamoeba: the role of emerging corneal pathogens

Abstract

Parasitic organisms are increasingly recognized as human corneal pathogens. A notable increase in both well-defined Acanthamoeba keratitis and a more dramatic increase in reported cases of Microsporidia keratitis have suggested significant outbreaks of parasitic keratitis around the world. Historical and contemporary baselines as well as a familiar associated clinical presentation reinforce the significant outbreak of Acanthamoeba keratitis in the United States. The remarkable rise in cases of Microsporidia keratitis, however, lacks these established baselines and, further, describes a disease that is inconsistent with previous definitions of disease. While a well-defined, abrupt increase strongly suggests temporally related risk factors, most likely environmental, involved in the Acanthamoeba outbreak, the rise in Microsporidia keratitis suggests that increased awareness and improved diagnostic acumen are a significant factor in case ascertainment. Regardless, recent evidence indicates that both parasitic diseases are likely underreported in various forms of infectious keratitis, which may have unrecognized but significant implications in the pathogenesis of both primary protozoal and polymicrobial keratitis. Further understanding of the incidence and interaction of these organisms with current therapeutic regimens and more commonly recognized pathogens should significantly improve diagnosis and alter clinical outcomes.

Figure 1

(a) A slit lamp photo of superficial microsporidial keratitis in an AIDS patient. (b) Photo of a histopathological specimen from a therapeutic penetrating keratoplasty performed for Acanthamoeba keratitis showing both full and empty cysts (arrows) in the posterior third of the stroma. (c) Slit lamp photo of epithelial Acanthamoeba keratitis with radial keratoneuritis. (d) Confocal microscopy of the same patient (c) demonstrating round double-walled cysts with a bright center. (e) A case of chronic stromal keratitis associated with Acanthamoeba. (f) Diff-Quick smear of a corneal scraping of the same patient (e) with a single characteristic double-walled cyst centrally with multiple pores.