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BBB - Acanthamoeba spp., Naegleria fowleri and other amobae

Bad Bug Book:
Foodborne Pathogenic Microorganisms and Natural Toxins Handbook
Acanthamoeba spp., Naegleria fowleri and other amobae

A new version of the Bad Bug Book was released in 2012, below is a previous version.


1. Name of the Organism:

Acanthamoeba spp., Naegleria fowleri and other amobae

Members of the two genera named above are the principal examples of protozoa commonly referred to as pathogenic free-living amoebae.

2. Nature of Acute Disease:

Primary amoebic meningoencephalitis (PAM), Naegleria fowleri and granulomatious amoebic encephalitis (GAE), acanthamoebic keratitis or acanthamoebic uveitis.

These organisms are ubiquitous in the environment, in soil, water, and air. Infections in humans are rare and are acquired through water entering the nasal passages (usually during swimming) and by inhalation. They are discussed here because the FDA receives inquiries about them.

3. Nature of Disease:

PAM occurs in persons who are generally healthy prior to infection. Central nervous system involvement arises from organisms that penetrate the nasal passages and enter the brain through the cribriform plate. The organisms can multiply in the tissues of the central nervous system and may be isolated from spinal fluid. In untreated cases death occurs within 1 week of the onset of symptoms. Amphotercin B is effective in the treatment of PAM. At least four patients have recovered when treated with Amphotercin B alone or in combination with micronazole administered both intravenously and intrathecally or intraventrically.

GAE occurs in persons who are immunodeficient in some way; the organisms cause a granulomatous encephalitis that leads to death in several weeks to a year after the appearance of symptoms. The primary infection site is thought to be the lungs, and the organisms in the brain are generally associated with blood vessels, suggesting vascular dissemination. Treatment with sulfamethazine may be effective in controling the amobae.

Prior to 1985 amoebae had been reported isolated from diseased eyes only rarely; cases were associated with trauma to the eye. In 1985-1986, 24 eye cases were reported to CDC and most of these occurred in wearers of contact lenses. It has been demonstrated that many of these infections resulted from the use of home-made saline solutions with the contact lenses. Some of the lenses had been heat treated and others had been chemically disinfected. The failure of the heat treatment was attributed to faulty equipment, since the amoebae are killed by 65°C (149°F) for 30 minutes. The failure of the chemical disinfection resulted from insufficient treatment or rinsing the lenses in contaminated saline after disinfection. The following agents have been used to successfully eliminate the amoebic infection in the eye: ketoconazole , microconazole, and propamidine isothionate; however, penetrating keratoplasty has been necessary to restore useful vision.

4. Diagnosis of Human Illness:

PAM is diagnosed by the presence of amoebae in the spinal fluid. GAE is diagnosed by biopsy of the lesion. Ocular amoebic keratitis may be diagnosed by culturing corneal scrapings on nonnutrient agar overlaid with viable Escherichia coli; amoebae from PAM and GAE may be cultured by the same method. Clinical diagnosis by experienced practitioners is based on the characteristic stromal infiltrate.

5. Associated Foods:

Transmission is through water based fluids or the air.

6. Relative Frequency of Disease:

PAM and GAE are rare in occurrence; fewer than 100 cases have been reported in the United States in the 25 years since these diseases were recognized.

7. Course of Disease and Complications:

PAM and GAE both lead to death in most cases. Eye infections may lead to blindness.

8. Target Populations:

Immunodeficients, especially those infected with HIV, may be at risk for atypical infections. PAM, GAE, and eye infections have occurred in otherwise healthy individuals.

9. Food Analysis:

Foods are not analyzed for these amoebae since foods are not implicated in the infection of individuals.

10. Selected Outbreaks:

For more information on recent outbreaks see the Morbidity and Mortality Weekly Reports from CDC.

11. Education and Background Resources:

Literature references can be found at the links below.

Loci index for genome Acanthamoeba

Available from the GenBank Taxonomy database, which contains the names of all organisms that are represented in the genetic databases with at least one nucleotide or protein sequence.

12. Molecular Structural Data:

None currently available.

13. FDA Regulations or Activity:

Since infection is not known to be by way of the digestive tract, the FDA has no regulations concerning these organisms. Eye infections are indirectly regulated by FDA's Center for Medical Devices and Radiological Health; FDA's Center for Drug Evaluation and Research regulates heat sterilization units and saline solutions for ophthalmological use. FDA has published a paper documenting the presence of amoebae in eye wash stations, and warning about the potential danger of such contamination.