Amebic keratitis is a rare infection of the cornea (the clear layer in front of the iris and pupil of the eye) caused by the free-living ameba species Acanthamoeba. 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 tested.
Eye doctors remove infected and damaged cells if sores are superficial and treat the infection with antibiotic eye drops.
To help prevent this infection, people should keep their contact lenses in a sterile solution and should not wear their lenses while swimming, in hot tubs, or when bathing.
Free-living amebas are protozoa, which is a type of parasite. They make up a diverse group of microscopic, one-celled organisms.
The free-living ameba Acanthamoeba is further categorized as an extraintestinal protozoan (see also Introduction to Extraintestinal Protozoa), which means it causes infections only in areas outside of the intestines such as the eyes.
Acanthamoeba species are present worldwide in water, soil, sewage, and dust. They can also be present in tap water, showers, hot tubs, and air conditioning units. They do not need a human or animal host to live and rarely cause infection, but they can enter a body through the eyes, the nose, or through broken skin and multiply. Some infections develop after the cornea (the clear layer in front of the iris and pupil of the eye) is accidentally scratched. When Acanthamoeba enter the eye, they can cause a severe infection of the cornea (amebic keratitis).
The main risk factor for amebic keratitis in most cases is contact lens use, particularly if lenses are worn while swimming or bathing in contaminated water or if the lenses are cleaned or stored in contaminated lens solution. Soft contact lenses (except daily disposable ones), multi-purpose or recalled contact lens solutions, and poor contact lens hygiene are other risk factors.
(See also Overview of Parasitic Infections.)
Symptoms of Amebic Keratitis
Amebic keratitis can be progressively destructive.
Typically, a very painful sore or sores develop on the cornea and cause eye redness, excess tear production, sensation of a foreign body, and pain when the eyes are exposed to bright light (photosensitivity). Vision is usually impaired.
The sores typically affect only 1 eye. Rarely, they affect both eyes.
Diagnosis of Amebic Keratitis
Examination and culture of a sample taken from the cornea
Polymerase chain reaction (PCR) testing to check for the genetic material of the ameba
To diagnose amebic keratitis, doctors take a sample of tissue from the cornea to be examined under a microscope and cultured. A culture allows doctors to grow the ameba in a laboratory until there are enough to identify.
Doctors also do PCR tests on the tissue sample to look for the genetic material (DNA) of the ameba.
Treatment of Amebic Keratitis
Antibiotics
An eye doctor (ophthalmologist) should promptly start treatment. Early, superficial sores are easier to treat. If sores are superficial, doctors use a cotton-tipped applicator to remove infected and damaged cells. Antibiotics are started right away.
Doctors treat amebic keratitis with the antibiotics chlorhexidine, polyhexamethylene biguanide, or both applied as eye drops. These eye drops are applied every 1 to 2 hours for the first 3 days of treatment. Propamidine and hexamidine are other antibiotics that may be given.Doctors treat amebic keratitis with the antibiotics chlorhexidine, polyhexamethylene biguanide, or both applied as eye drops. These eye drops are applied every 1 to 2 hours for the first 3 days of treatment. Propamidine and hexamidine are other antibiotics that may be given.
Treatment is intensive the first month and gradually decreased as the eye heals. Treatment often lasts 6 to 12 months. If treatment is stopped too soon, the infection is likely to return. Steroids (also called corticosteroids or glucocorticoids) eye drops should not be used.
Surgery to replace the damaged cornea with a healthy one (corneal transplant) is rarely needed unless the antibiotics are ineffective.
Prevention of Amebic Keratitis
To help prevent amebic keratitis, people who wear contact lenses should do the following:
Clean and store their contact lenses following the recommendations of eye care providers and manufacturers
Wash their hands thoroughly before handling contacts
Keep storage solution fresh, not reused, and not topped off
To help prevent amebic keratitis, people who wear contact lenses should not use a homemade solution or tap water to clean or store their lenses or wear their lenses while swimming (the bleach in chlorine that is added to pools does not kill the amebas), in a hot tub, or when bathing.
Wearing contact lenses overnight while awake and sleeping with contact lenses in can increase the risk of developing amebic keratitis.
More Information
The following English-language resource may be useful. Please note that The Manual is not responsible for the content of this resource.
Centers for Disease Control and Prevention (CDC): About Acanthamoeba Infections
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