Amebic Keratitis

Amoebic Keratitis

Amoebic keratitis, also known as Acanthemoeba keratitis, is an inflammation of the cornea caused by the microorganism Acanthamoeba.  The cornea is the clear outer covering of the eye, and this keratitis may involve the superficial or deeper layers of the cornea.  The latter may cause scarring after it has healed, which may result in visual impairment.

Pathophysiology

Amoebic keratitis is caused by the organism Acanthamoeba castellani, which is an opportunistic protozoa usually found in soil and fresh water.  This kind of keratitis is most commonly found among people who wear contact lenses.  Risk of infection is greater for those people who wear lenses while swimming or who rinse their lenses using tap water.

This infection is commonly mistaken for other infections such as bacterial, herpetic, or fungal keratitis.  Amoebic keratitis may appear to have partially resolved with non-protozoan treatments, however, its clinical course may be on and off for several years before the correct diagnosis is made because of this microorganism’s tendency to encyst and becoe dormant. 

Signs and Symptoms

Patients affected with amoebic keratitis characteristically present with a red and painful eye, more commonly on one side.  The complaint of pain usually ranges from significant pain to mild irritation and a foreign body sensation.  Other symptoms can include light sensitivity and blurring of vision. 

Diagnosis

Scrapings from the cornea may be taken and cultured for Acanthamoeba, which can take 1-9 days or longer.  In most cases, it has a low diagnostic yield and cultures are false negative and only reveal the presence of bacteria.  This is because Acanthamoeba oftentimes coexist with bacteria upon which it feeds.  More often than not, this microorganism accompanies other diseases of the cornea, and may or not be active. 

Treatment

Treatment of amoebic keratitis tends to be difficult because of Acanthamoeba’s ability to encyst upon administration of topical medications.  Medications that are recommended are topical polyhexamethylene biguanide (PHMB), polymixin B, propamidine isethionate (Brolene), neomycin, chlorhexidine digluconate 0.02%, and clortrimazole 1%.  An eye patch may be worn by the patient as a protection from foreign bodies, other irritants, and bright light.

Prevention

It is imperative that patients who wear contact lenses use sterile lens cleaning and disinfecting solutions.  These have to be rinsed every night and air dried. Tap water is not recommended for cleaning.  Use of contact lenses should be limited, especially when the eyes become red and irritated. 

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