Why Acanthamoeba diagnosis is often challenging
Pay attention to epidemiology, risk factors, signs/symptoms, tests to minimize corneal infections
Many cases of Acanthamoeba keratitis go undiagnosed, and consequences can be devastating, according to Jeremy Keenan, MD, MPH.
“It’s an uncommon cause of cornea infections, but it’s a really bad one,” said Dr. Keenan, associate professor of ophthalmology, University of California, San Francisco. “If you were to pick, you would not want this one.”
Ever since an outbreak was traced to a solution for contact lens care (AMO Complete Moisture Plus) in 2004, reports of Acanthamoeba have infection remained higher than before that time, Dr. Keenan said.
“This could be because people are more aware of it,” he said. The availability of the confocal microscope has made it easier to identify the organism.
In Western countries, about 85% of cases occur in contact lens wearers. In the United States, the incidence of infection is about 0.15 per million in non-contact lens wearers and 1 per million in contact lens wearers.
By contrast, in the United Kingdom, the incidence is about 1 per million in non-contact lens wearers and 20 per million in contact lens wearers. The organism lives in drinking water, and one explanation for the higher incidence in the United Kingdom could be the use of water storage tanks above houses, Dr. Keenan said.
The incidence of infection increases in the summer months, most likely because water sports such as swimming and boating are more common then, he added.
Poor contact lens hygiene, such as washing lenses in tap water or showering while wearing contact lenses, can increase the risk.
Researchers have identified 25 species based on morphology, with A. castellanii and A. polyphaga the most common found in keratitis. The organism takes 2 forms. Under harsh conditions it forms a cyst about 10-30 microns in size. As a cyst, it does not require food, and becomes resistant to extremes in temperature, pH balance, dessication and chemicals and is difficult to kill.
In the presence of food it becomes a trophozoite, which is more susceptible to treatment. As a trophozoite it is 20-50 microns in size and motile. It replicates by binary fission. It feeds on algae, bacteria, other protozoans and possibly keratocytes in the cornea.