Features of Acanthamoeba
Acanthamoeba is a free-living protozoan that is ubiquitous in nature worldwide1, 2. Although many individuals are exposed to Acanthamoeba, only a few people become infected. Acanthamoeba infections occur in both healthy and immunosuppressed people with the potential to infect the eyes, skin, and CNS. Acanthamoeba keratitis, or infection of the cornea with Acanthamoeba, can lead to visual impairment or blindness if not treated promptly3.
Infections most frequently occur in healthy contact wearers with approximately 85% of Acanthamoeba keratitis cases involving contact lens wearers3, 4. The incidence is estimated to be 1-2 individuals per million, but this may underestimate the true number of cases5. Individuals are at greater risk of Acanthamoeba keratitis with improper care of contact lenses, coming into contact with contaminated water, and minor damage or trauma to the cornea2, 6.
Acanthamoeba exists in two forms: cyst and trophozoite7. The cysts measure 10-25 micrometers in diameter and contain two cell walls (wrinkled, fibrous outer wall and inner wall). This dormant form has only one nucleus with a large karyosome. Trophozoites are pleomorphic in shape and are the infectious form of Acanthamoeba. They measure 15-45 micrometers and have a large nucleus with a centrally located karyosome but no peripheral chromatin. Although trophozoites are the infective form, both trophozoites and cysts can gain entry into human hosts. The incubation period varies based on the initial inoculum but is hypothesized to be from several days to several weeks.
When invading the cornea, Acanthamoeba binds to mannose glycoproteins on the corneal epithelium8, 9,10. Secretion of cytolytic proteins and proteases then allow entry into the cornea11. Infections vary greatly in presentation from: foreign body sensation, photophobia, decreased vision, tearing, pain, and ocular erythema2. Although many of these findings are found in other bacterial, fungal, and viral infections, pain out of proportion to the clinical findings is key to the diagnosis1. Testing for Acanthamoeba with confocal microscopy, targeted PCR amplification, and cultures may assist with the diagnosis2.
The next day, the result from real time polymerase chain reaction (qPCR) testing demonstrated Acanthamoeba the 18S ribosomal RNA gene at approximately 11-fold higher levels than the 1,000 copy number standard (Figure 1). Cultures also verified the pathogen three days later. The patient's follow-up appointments demonstrated no signs or symptoms of continued infection and further cultures remained negative for Acanthamoeba.
Answers: 1. A, 2. C, 3.B, 4.C, 5.D
Contributed by Waseem Anani, MD; Charleen Chu, MD, PhD; Tim Oury, MD, PhD