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Amebic Brain Infection: Primary Amebic Meningoencephalitis


Richard D. Pearson

, MD, University of Virginia School of Medicine

Last full review/revision Oct 2019| Content last modified Oct 2019
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Primary amebic meningoencephalitis is a rare, usually fatal infection of the central nervous system (brain and spinal cord) caused by Naegleria fowleri, a type of free-living ameba.

  • The amebas can enter the brain through the nose when people swim in contaminated warm, fresh water.

  • Primary amebic meningoencephalitis can progress rapidly from changes in smell or taste, headaches, a stiff neck, nausea, and vomiting to confusion and death.

  • To check for amebas, doctors do a spinal tap to obtain a sample of cerebrospinal fluid and sometimes remove a small piece of brain tissue (biopsy), then examine and analyse the sample obtained.

  • Determining the best treatment is difficult, but doctors typically use a combination of drugs including miltefosine.

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.

Naegleria fowleri, also known as a "brain eating ameba" is present worldwide. In the United States, the majority of infections have occurred in southern states following exposure to warm, fresh water in lakes and rivers in the summer. Naegleria fowleri can also live in hot springs or warm water discharged from industrial plants, poorly maintained swimming pools with minimal or no chlorination, and soil.  It can even grow in water heaters at temperatures up to 115°F (46°C) and survive for short periods at higher temperatures. Naegleria does not live in salt water.. When people, usually children or young adults, swim in contaminated water, the amebas can enter the central nervous system through the nose. When they reach the brain, they cause inflammation and tissue destruction, which usually progresses rapidly to death.

Primary amebic meningoencephalitis differs from granulomatous amebic encephalitis, which is another very rare, usually fatal infection of the central nervous system caused by different free-living amebas, Acanthamoeba species or Balamuthia mandrillaris. Granulomatous amebic encephalitis usually occurs in people with a weakened immune system or generally poor health.


Symptoms of primary amebic meningoencephalitis 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.


  • A spinal tap and analysis of cerebrospinal fluid

  • Brain biopsy

Doctors suspect primary amebic meningoencephalitis in people who have symptoms and have been swimming recently in fresh water, but the diagnosis is difficult to confirm. Imaging tests, such as computed tomography (CT) or magnetic resonance imaging (MRI), are done to rule out other possible causes of the brain infection, but they cannot confirm that amebas are the cause.

A spinal tap (lumbar puncture) is done to obtain a sample of cerebrospinal fluid (the fluid that surrounds the brain and spinal cord). This test can exclude some other possible causes of meningitis and brain infection, but doctors are not always able to find the amebas in the sample.

Other techniques are available in specialized laboratories and are more likely to detect the amebas. They include the following:

  • Culture (growing microorganisms in a laboratory until there are enough to identify)

  • Polymerase chain reaction (PCR) to check for the genetic material of the ameba

  • A biopsy of brain tissue that is stained and examined under a microscope or analyzed using PCR


  • A combination of drugs

Because few people survive, determining the best treatment for primary amebic meningoencephalitis is difficult.

Doctors typically use a combination of several drugs including

  • Miltefosine

and one or more of the following:

  • Amphotericin B (typically used to treat fungal infections—an antifungal drug)

  • Rifampin (an antibiotic)

  • Fluconazole or the related drugs voriconazole, ketoconazole, or itraconazole (antifungal drugs)

  • Azithromycin (an antibiotic)

Although miltefosine can cause birth defects in pregnant women, doctors often use it anyway because primary amebic encephalitis is usually fatal without treatment, so the potential benefits of therapy outweigh the risk. Women of childbearing age who are taking this drug must use effective birth control measures.

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