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Acanthamoeba Keratitis Alert
The recent increase of Acanthamoeba keratitis cases has received timely and comprehensive attention from the Centers for Disease Control and Prevention (CDC) and the American Optometric Association (AOA). For that reason, the American Academy of Optometry's Section on Cornea and Contact Lenses and Communications Committee believe that the following background information may be helpful to you in the event you see a patient with this condition.
The CDC issued a report on May 26, 2007, citing results from their initial analysis of a case control study using case data from the first 46 completed patient interviews of the 138 confirmed cases and control data from the 2006 Fusarium keratitis outbreak investigation. Of the 46 cases, 85 percent wore soft contact lenses, 7 percent wore rigid lenses, 9 percent reported no contact lens use, 38 percent reported swimming while wearing contact lenses, and 83 percent reported showering while wearing contact lenses during the month before symptom onset.
As Acanthamoeba keratitis symptoms can mimic other keratopathies, we’d like to review some clinical questions regarding risk, diagnosis, and management of this relatively rare infection.
What causes Acanthamoeba keratitis?
Acanthamoeba keratitis (AK) is caused by a free-living parasite commonly found in water, soil, air, ventilation systems and sewage systems. The organism feeds on bacteria and human tissue.
Acanthamoeba have two stages of life cycle: cyst, a sessile form, which is more resistant to treatment, and trophozoite, a free living, motile form. It can remain in cyst form for long periods of time and then assume the trophozoite form when exposed to a food source or other ideal environment like a favorable pH and reduced cell crowding.
Dentritiform lesion can present early in Acanthamoeba keratitis.
What is the incidence of Acanthamoeba keratitis?
Incidence of AK is generally rare, particularly in the United States. Since Acanthamoeba keratitis is not officially reported or required by any government or other national agency, the true prevalence is undetermined. However, U.S. studies have shown incidence of 1.65 to 2.01 per million in contact lens wearers and only 0.58 to 0.71 in the general population. Only 208 cases were reported between 1973 and 1988, 85 percent of which were among contact lens wearers. CDC has received reports of 138 cases of confirmed AK since January,2005.
What are the distinguishing clinical characteristics of AK?
The clinical presentation of Acanthamoeba keratitis varies greatly, which may lead to misdiagnosis and delayed treatment. Patients with AK may complain of unilateral foreign body sensation, photophobia, decreased visual acuity, tearing, and pain or redness of the eye. Infection can be unilateral or bilateral. Pain out of proportion to clinical findings is a classic feature of Acanthamoeba keratitis. However, early in the disease a lack of pain does not preclude the diagnosis. A high percentage of AK occurs in contact lens wearers. Because of similarities to the clinical manifestations of viral, fungal, or bacterial corneal infection, patients may be misdiagnosed and treated with improper antimicrobial or corticosteroid therapy. Such therapy may initially alleviate symptoms and further obscure the clinical picture and diagnosis.
Ring infiltrate may be a late onset sign of Acanthamoeba keratitis
How do we make a definitive diagnosis?
The first step in diagnosing Acanthamoeba keratitis is to have a high degree of suspicion, especially in a contact lens wearer with a recent diagnosis of another form of keratitis, such as HSV, who is not responding to therapy. Corneal scrapings can be stained with the following: Giemsa, periodic acid-Schiff, hematoxylin and eosin, Wright’s or calcofluor white which may expedite identification of the organism. Best culture results are obtained on non-nutrient agar with an E. coli overlay. Impression cytology with bright field microscopy has been successful in identifying early infection. Once deep involvement occurs, a biopsy may be required. However, a negative culture does not necessarily rule out Acanthamoeba infection. Consider confocal microscopy and polymerase chain reaction assays to help make the diagnosis.
Slit lamp features of Acanthamoeba keratitis, Courtesy of Northwest Eye Surgeons
How do I treat AK?
Early diagnosis is key to effectively treat AK. Treatment can be difficult due to the resilient nature of the cyst form. Current treatment regimens usually include a topical cationic antiseptic agent such as polyhexamethylene biguanide (0.02%) or chlorhexidine (0.02%) with or without a diamidine such as propamidine (0.1%) or hexamidine (0.1%). Therapy duration can be protracted, often requiring extended treatment from 6-12 months. Pain control can be helped by topical cyclopegic agents and oral analgesics; however, treatment with corticosteroids to control inflammation is controversial. Penetrating keratoplasty may be required to help restore visual acuity and corneal integrity.
How should we educate our patients to minimize the risk of AK?
As we know, contact lenses are medical devices and should be cared for as such. Unless wearing daily disposable contact lenses, they should be removed daily, digitally cleaned in the palm of the hand, and stored in a prescribed overnight disinfection solution. Washed hands are a prerequisite to handling the contact lenses and this needs to be reinforced. Avoid additional contact with tap or well water by drying hands after they are washed. The disinfection solution should be replaced daily and the storage container regularly cleaned and air dried. Lens case should be replaced every 3 months. Patients should remove their contact lenses before swimming or using a hot tub. Instruct patients to discontinue contact lens wear and consult their eye practitioner if they experience emergent symptoms of ocular pain, sudden blurred vision, redness, increased tearing, and photophobia.
When compared to the Fusarium keratitis controls in the CDC study, the only brand of contact lens solution that revealed a significant association was AMO Complete MoisturePlus. A new control database is currently being established for the AK investigation to assess more specific risk factors. Until then, a cautious approach to risk factors has been established by the CDC based on the behaviors and contact lens usage of the 46 completed case patients.
Clinicians should report cases of AK to state and local health departments or directly to CDC at 770-488-7775.
For further information, you can access the CDC and AOA websites at
http://www.cdc.gov/ncidod/dpd/parasites/acanthamoeba/hcp_images_keratitis.htm
and
http://www.aoa.org/.
American Academy of Optometry Section On Cornea & Contact Lenses and Communications Committee
