Merck Manual

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Amebic Keratitis (Eye Infection)


Richard D. Pearson

, MD, University of Virginia School of Medicine

Last full review/revision Oct 2019| Content last modified Oct 2019
Click here for the Professional Version

Amebic keratitis is a rare infection of the cornea (the clear layer in front of the iris and pupil of the eye) caused by Acanthamoeba species, free-living amebas. It usually occurs in people who wear contact lenses.

  • Amebic keratitis causes painful sores on the cornea, and vision is usually impaired.

  • Doctors take a sample of tissue from the cornea to be examined and cultured.

  • To help prevent this infection, people should keep their contact lens in a sterile solution and should not wear contact lenses while swimming, in hot tubs, or when taking a shower.

  • Eye doctors remove infected and damaged cells if sores are superficial and treat the infection with biguanide-chlorhexidine or polyhexamethylene biguanide (used to disinfect contact lenses) eye drops plus either propamidine or hexamidine eye drops.

Free-living amebas are protozoa that live in soil or water and do not need to live in people or animals. Although they rarely cause human infection, certain types of these amebas can cause serious, life-threatening diseases.

Amebic keratitis may be progressively destructive. Most (85%) infected people wear contact lenses. Infection is more likely if lenses are worn during swimming or if the lens cleaning solution used is unsterile. Some infections develop after the cornea is accidentally scraped.


Typically, painful sores develop on the cornea. Symptoms of amebic keratitis include eye redness, excess tear production, sensation of a foreign body, and pain when the eyes are exposed to bright light. Vision is usually impaired.


  • Examination and culture of a sample taken from the cornea

To diagnose amebic keratitis, doctors take a sample of tissue from the cornea to be examined and cultured.


To help prevent amebic keratitis, people should keep their contact lenses in a sterile solution. They should not use a homemade solution. Also, people should not wear contact lenses while swimming, in hot tubs, or when taking a shower.


  • Antimicrobial drugs

An eye doctor (ophthalmologist) should promptly start treatment of amebic keratitis. Early, superficial infection can be treated more easily. If sores are superficial, doctors use a cotton-tipped applicator to remove infected and damaged cells.

Doctors treat amebic keratitis with the following antimicrobial drugs applied as eye drops for 6 months to a year:

  • Chlorhexidine (a disinfectant) or polyhexamethylene biguanide (a disinfectant which is used in some cleaning products for contact lenses) with or without

  • Propamidine or hexamidine

These drugs are applied every hour or two when treatment is started.

Treatment is intensive the first month, then gradually decreased as healing occurs. Treatment often lasts 6 to 12 months. If treatment is stopped too soon, the infection is likely to recur. Corticosteroid eye drops should not be used.

In severe cases, treatment with oral itraconazole has been used along with the eye drop therapy, but itraconazole can have severe side effects and drug interactions.

Surgery to repair the cornea (keratoplasty) is rarely needed unless diagnosis and treatment are delayed or drug treatment is ineffective.

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