Primary amoebic meningoencephalitis overview

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Primary amoebic meningoencephalitis is a disease of the central nervous system caused by infection from Naegleria fowleri. Naegleria fowleri is a free-living ameboflagellate that invades the brain and meninges via the nasal mucosa and olfactory nerve to cause acute, fulminant hemorrhagic meningoencephalitis (primary amebic meningoencephalitis – PAM), primarily in healthy children and young adults with a recent history of exposure to warm fresh water. Initial signs and symptoms of PAM begin 1 to 14 days after infection and include sudden onset of headache, fever, nausea, vomiting, and stiff neck accompanied by positive Kernig’s sign and Brudzinski’s sign. In some cases, abnormalities in taste or smell, nasal obstruction and nasal discharge might be seen. Other symptoms might include photophobia, mental-state abnormalities, lethargy, dizziness, loss of balance, other visual disturbances, hallucinations, delirium, seizures, and coma. After the onset of symptoms, the disease progresses rapidly and usually results in death within 3 to 7 days. Although a variety of treatments have been shown to be active against amoebae in vitro and have been used to treat infected persons, most infections have still been fatal.

Historical Perspective

Primary amoebic meningoencephalitis was first documented in Australia in 1965. In 1966, four cases were reported in the USA.

Pathophysiology

Naegleria fowleri is a heat-loving (thermophilic), free-living ameba (single-celled microbe), commonly found around the world in warm fresh water (like lakes, rivers, and hot springs) and soil that causes acute, fulminant hemorrhagic meningoencephalitis (primary amebic meningoencephalitis – PAM). Naegleria fowleri is the only species of Naegleria known to infect people. Most of the time, Naegleria fowleri lives in freshwater habitats by feeding on bacteria. However, in rare instances, the ameba can infect humans by entering the nose during water-related activities. Once in the nose, the ameba travels to the brain and causes a severe brain infection, primary amoebic meningoencephalitis (PAM), which is usually fatal.

Causes

Naegleria fowleri is the causative agent for primary amoebic meningoencephalitis, which is normally found in the natural environment and is well adapted to surviving in various habitats, particularly warm-water environments.

Differentiating Primary Amoebic Meningoencephalitis from other Diseases

Primary amoebic meningoencephalitis resembles acute bacterial meningitis and needs to be differentiated from that.

Epidemiology and Demographics

Primary amoebic meningoencephalitis is both exceptionally rare and highly lethal. In the 10 years from 2002 to 2011, 32 infections were reported in the U.S. Of those cases, 28 people were infected by contaminated recreational water, two people were infected by water from a contaminated, geothermal (naturally hot), untreated drinking water supply, and two people were infected after performing nasal irrigation using contaminated tap water.

Risk Factors

Swimming behaviors associated with the increased risk of primary amoebic meningoencephalitis include diving or jumping into the water, submerging the head under water, or engaging in other water-related activities that cause water to go up the nose.

Diagnosis

History and Symptoms

Patients with primary amoebic meningoencephalitis may have a history of swimming, diving, jumping, bathing, or playing in warm, generally stagnant, fresh water or irrigating sinuses (nose) using contaminated tap water during the previous few days to 2 weeks. Symptoms start 1-7 days (median 5 days) after swimming exposure and usually include high fever, headache, nuchal rigidity, photophobia, nausea, and vomiting.

Physical Examination

Initial signs of primary amoebic meningoencephalitis begin 1 to 14 days after infection and include nuchal rigidity, positive Kernig’s sign and Brudzinski’s sign, cerebellar ataxia, palsies of the third, fourth and sixth cranial nerves, and increased intracranial pressure.

Laboratory Findings

Primary amoebic meningoencephalitis is diagnosed using specific laboratory tests available in only a few laboratories in the United States. Because of the rarity of the infection and difficulty in initial detection, about 75% of diagnoses are made after the death of the patient. It can be diagnosed in the laboratory by detecting Naegleria fowleri organisms, nucleic acid, or antigen in cerebrospinal fluid (CSF), biopsy, or tissue specimens.

Treatment

Medical Therapy

Considering the the high mortality rate of primary amoebic meningoencephalitis, unusually non-suggestive symptomology of the early-stage disease, and necessity of microbial culture of the cerebrospinal fluid to effect a positive diagnosis, it has been suggested that physicians should give an array of antimicrobial drugs, including the drugs used to treat amoebic encephalitis, before the disease is actually confirmed in order to increase the number of survivors. However, administering several of those drugs at once (or even some of them known to treat the condition) is often very dangerous and unpleasant for the patient.

Primary Prevention

Personal actions to reduce the risk of primary amoebic meningoencephalitis should focus on limiting the amount of water going up the nose and lowering the chances that Naegleria fowleri may be in the water. Routine, enhanced education of the public in advance of the summer swim season might be helpful.

Future or Investigational Therapies

Recently, an investigational drug, miltefosine, a breast cancer and anti-leishmania drug, has shown some promise against the free-living amoebae in combination with some other drugs. Miltefosine has shown in vitro and mouse model amebicidal activity against Balamuthia, Naegleria fowleri, and Acanthamoeba.