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Acanthamoeba Keratitis in 2007: Stay Informed but Calm

BY EDWARD S. BENNETT, OD, MSED

Contact lens wear has often been associated with the development of Acanthamoeba keratitis over the last 30 to 35 years, with surgical intervention and vision loss not uncommon. However, the overall overall risk of Acanthamoeba keratitis has been extremely low, estimated at one to two cases per million contact lens wearers.

Recently the Centers for Disease Control and Prevention (CDC) reported that the incidence of Acanthamoeba keratitis has increased in recent years. Through numerous sources, it has obtained reports of a total of 138 patients with onset of symptoms and positive Acanthamoeba cultures from 35 states and Puerto Rico between January 1, 2005 and May 24, 2007. These findings supplement previous reports of 40 cases reported in the Chicago metropolitan area from June 1, 2003 to November 30, 2005 and 19 cases reported from Wills Eye Hospital from January 2004 to August 2005.

Risk Factors

The No. 1 predisposing factor is contact lens wear, which has been reported in 85 percent to 91 percent of patients exhibiting this condition. Absence of or infrequent disinfection has been implicated. In addition, washing storage cases with tap water and not allowing cases to air dry after rinsing have been associated with Acanthamoeba keratitis. Likewise, numerous recent reports have associated some isolated cases of Acanthamoeba keratitis with overnight orthokeratology.

However, much of the recent concern pertains to one common denominator: water. It's evident that Acanthamoeba is inherent in water systems around the world. Having worked in David Seal's laboratory in Scotland, his extensive research stemmed from a relatively high incidence of Acanthamoeba keratitis in the United Kingdom, with tap water contamination often implicated.

The recent outbreak in the Chicago area reported by Joslin and colleagues provided the hypothesis that recent changes by the U.S. Environmental Protection Agency to reduce the allowable amount of carcinogenic by-products in the water supply (in this case, in Lake Michigan) may have increased the risk of Acanthamoeba proliferation in the water supply and, ultimately, the risk of Acanthamoeba keratitis. Whether that has been a factor in the increased number of cases managed at Wills Eye Hospital hasn't been determined; however, exposure to well water (42 percent of cases) and swimming without lens removal (28 percent) were reported. The recent CDC report also found that 38 percent of contact lens wearers with Acanthamoeba keratitis had been swimming during lens wear.

Impact of Lens Care Systems

Silicone hydrogel lenses have minimized hypoxia as a factor in compromised corneal health, but haven't necessarily reduced the risk of contact lens-related corneal infections, including Acanthamoeba keratitis. The effectiveness, or relative lack thereof, of care systems does appear to have a significant impact. A May 26th report by the CDC stated that of 46 culture-confirmed patients who were interviewed (of the 138 patients about whom they had obtained reports), 42 were contact lens wearers. Of this 42, 39 were soft lens wearers with 21 of these individuals (58 percent) reporting using Complete MoisturePlus Multi-Purpose Solution (Advanced Medical Optics) within one month of their diagnosis of Acanthamoeba keratitis. As the CDC determined that the risk of developing Acanthamoeba keratitis was seven times greater for those consumers who used Complete MoisturePlus as compared to those patients who used another solution, AMO has issued a voluntary recall of this solution.

This is consistent with reports that Complete MoisturePlus hasn't been as effective as other multipurpose solutions against Acanthamoeba. ReNu care systems (Bausch & Lomb) and Opti-Free Express (Alcon) have demonstrated greater effectiveness against both cysts and trophozoites.

It's also important that Shoff and colleagues have found that certain environmental strains — notably isolates of Acanthamoeba from a South Florida beach — were more resistant to disinfection from several popular multi-purpose solutions.

Reducing Patient Risk

Numerous sources, including the American Optometric Association's (AOA) Contact Lens and Cornea Section and the CDC, have useful guidelines for minimizing the risk of Acanthamoeba keratitis. A rub and rinse step after lens removal has demonstrated much effectiveness in reducing microbial bioburden on lens surfaces. Emphasize the appropriate use of the recommended care regimens: don't re-use or top off solutions, avoid tap water and practice good hygiene.

Regular case care and replacement is especially important. For example, see B&L's Wear & Care Healthy Contact Lens Care Program (www.bausch.com).

Educate patients about environmental factors such as avoiding contact lens wear during swimming (or wearing airtight goggles), showering, while in a hot tub or during exposure to well water. Table 1 provides an overview of important guidelines to minimize the risk of Acanthamoeba keratitis.

Summary

The recent reports of Acanthamoeba keratitis outbreaks have prompted the CDC, in conjunction with the AOA and the American Academy of Ophthalmology, to ask eyecare practitioners to join in a comprehensive study on the incidence of Acanthamoeba keratitis and its possible relationship to contact lenses. You can report cases at www.aoa.org/acanthamoeba.xml.

Nevertheless, remember that the overall incidence of Acanthamoeba keratitis is quite low. There has been no mandate from the Food and Drug Administration or the International Standards Organization for contact lens disinfection systems to show efficacy against Acanthamoeba. Acanthamoeba do not readily attach to contact lenses, especially GP and non-ionic soft lenses and, according to Fleiszig, the combined efforts of the corneal epithelium, the tear fluid and the basal lamina result in an effective barrier against infection. The impact of changes in water purification/disinfection and its ultimate impact on this condition should become evident in the future.

But the selection of an appropriate care system, supplemented by comprehensive patient education on lens care and compliance, should minimize the risk of this rare condition and maintain the accurate perception that contact lenses are, in fact, a safe and valuable modality for correcting refractive error. CLS

For references, please visit www.clspectrum.com/references.asp and click on document #140.

Dr. Bennett is an associate professor of optometry at the University of Missouri-St. Louis and is executive director of the GP Lens Institute.