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. The most common diseases caused by free-living amebas are primary amebic meningoencephalitis, granulomatous amebic encephalitis, and amebic keratitis.

Primary Amebic Meningoencephalitis

Primary amebic meningoencephalitis is a rare, usually fatal infection of the central nervous system (brain and spinal cord) caused by Naegleria fowleri.

The amebas that cause this infection live in fresh, often stagnant water throughout the world. When people, usually children or young adults, swim in contaminated water, the amebas can enter the central nervous system through the mucous membranes of the nose. When they reach the brain, they cause inflammation, tissue death, and bleeding.

Symptoms begin within 1 to 2 weeks. Sometimes the first symptom is a change in smell or taste. Later, people have a headache, a stiff neck, sensitivity to light, nausea, and vomiting. They may become confused and sleepy and may have seizures. The infection can progress rapidly, causing death within 10 days.

Doctors suspect the infection in people who have symptoms and have been swimming recently in fresh water, but the diagnosis is difficult to confirm. A spinal tap (lumbar puncture) is done to obtain a sample of cerebrospinal fluid. This test can exclude other possible causes of meningitis and brain infection, but doctors are not always able to find the amebas in the sample.

Because few people survive, determining the best treatment is difficult. A few antifungal drugs and antibiotics may help. Amphotericin BSome Trade Names
FUNGIZONE
may be injected into a vein (intravenously) or into the space around the spinal canal (intrathecally). Sometimes miconazoleSome Trade Names
MONISTAT
, rifampinSome Trade Names
RIFADIN RIMACTANE
, or sulfisoxazoleSome Trade Names
LIPO GANTRISIN
, taken by mouth, is also given. MiconazoleSome Trade Names
MONISTAT
may be given intrathecally.

Granulomatous Amebic Encephalitis

Granulomatous amebic encephalitis is a rare, usually fatal infection of the central nervous system caused by Acanthamoeba species or Balamuthia mandrillaris. It usually occurs in people with a weakened immune system or generally poor health.

The amebas that cause this infection live in water, soil, and dust throughout the world. Many people are exposed, but few are infected. It usually occurs in people whose immune system is weakened or whose general health is poor. Amebas probably enter through the skin or lungs and spread to the brain through the bloodstream.

Symptoms begin gradually. People may have a low-grade fever, blurred vision, changes in personality, and problems with speaking, coordination, or vision. One side of the body or face may become paralyzed. Sores may develop on the skin. Headache and seizures are common. Most infected people die, usually 7 to 120 days after symptoms begin.

Computed tomography (CT) and a spinal tap are usually done. These tests help exclude other possible causes but usually cannot confirm the diagnosis. Sores typically contain amebas and, if present, are biopsied. The diagnosis is often made after death.

Some antifungal drugs and antibiotics may be used. Dibromopropamidine, pentamidineSome Trade Names
NEBUPENT
, or propamidine seems most useful. Others may include amphotericin BSome Trade Names
FUNGIZONE
, fluorocytosine, itraconazoleSome Trade Names
SPORANOX
, ketoconazoleSome Trade Names
NIZORAL
, miconazoleSome Trade Names
MONISTAT
, neomycinSome Trade Names
NEO-RX
, paromomycin, or trimethoprim-sulfamethoxazoleSome Trade Names
.

Amebic Keratitis

Amebic keratitis is infection of the cornea caused by Acanthamoeba species. It usually occurs in people who wear contact lenses.

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 scraped.

Typically, painful sores develop on the cornea. Symptoms 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.

For diagnosis, doctors take a sample of tissue from the cornea.

Early, superficial infection can be treated more easily. If sores are superficial, doctors use a cotton-tipped applicator to remove infected and damaged cells. A combination of two or more antimicrobial drugs, such as polyhexamethylene biguanide (used to disinfect contact lenses) plus propamidine (applied topically), works best. They are applied hourly for the first 3 days. Other drugs applied topically (such as the antifungal drugs clotrimazoleSome Trade Names
LOTRIMIN AF MYCELEX
or fluconazoleSome Trade Names
DIFULCAN
or the antibiotic chlorhexidineSome Trade Names
HIBICLENS
) are sometimes also used. FluconazoleSome Trade Names
DIFULCAN
or itraconazoleSome Trade Names
SPORANOX
may be taken by mouth, particularly if the infection is severe. 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. Surgery to repair the cornea (keratoplasty) is rarely needed unless diagnosis and treatment are delayed.

Last full review/revision March 2007 by Richard D. Pearson, MD