Acanthamoeba and Naegleria - Primary Amebic Meningoencephalitis (Brain-Eating Amoeba) and Acanthamoeba Keratitis

Acanthamoeba and Naegleria are the most common free-living amoebae associated with human disease. Balamuthia mandrillaris and Hartmannella species are also free-living amoebae but less commonly cause clinically significant infections. Pathogenic species may cause fatal central nervous system (CNS) disease.


Indications for Testing

  • Acanthamoeba
    • Eye pain, redness, sensitivity to light, change in vision, sensation of foreign body in eye, and/or excessive tearing, particularly in contact lens wearer who has had ocular exposure to warm water at risk for contamination with Acanthamoeba parasite
    • Consider acanthamoebic keratitis after ruling out other causes of acute keratitis
  • Naegleria
    • In individual who has had nasal passage exposure to warm water (via swimming, hot tub, nasal rinses)
      • Severe headache, fever, nausea, and vomiting symptoms that progress to stiff neck, seizures, confusion, hallucinations, and coma (CDC, 2008)
      • Absence of routine organisms in cerebrospinal fluid (CSF)
      • Lack of response to conventional antibiotics

Laboratory Testing

  • CDC – Acanthamoeba testing recommendations
  • CDC – Naegleria testing recommendations
  • Initial testing for meningoencephalitis
    • Lumbar puncture with CSF analysis
      • Typically demonstrates elevated protein and elevated white blood cell (WBC) count with lymphocyte predominance
        • Elevated red blood cells (RBCs) with disease progression in primary amoebic meningoencephalitis
      • Culture
      • CSF examination for trophozoites with Wright or Giemsa stain
      • Consider viral panel testing to rule out most common viral etiologies (including herpes simplex virus [HSV])
    • CBC – usually without leukocytosis
  • Initial testing for keratitis – culture of eye specimen


  • Acanthamoeba – biopsy and pathologist examination of involved sites useful for diagnosis of disseminated disease
  • Naegleria – biopsy and pathologist examination can be useful for diagnosis
  • Useful stains include calcofluor stain and Giemsa stain

Differential Diagnosis

  • Central nervous system (CNS) disease
  • Corneal disease
  • Skin disease
    • Dimorphic fungal infections
    • Mycobacterial infections



  • Incidence
    • Acanthamoeba keratitis – 0.3-1/100,000
    • Naegleria, Balamuthia, Sappinia – 4/million
  • Transmission – contaminated water or soil


  • Acanthamoeba
    • Pathogenic species that infect humans
      • A. culbertsoni
      • A. castellanii
      • A. polyphaga
      • A. astronyxis
    • The life cycle of Acanthamoeba includes both a trophozoite stage and a dormant cyst stage that is highly resistant to antimicrobial agents
  • Naegleria pathogenic species – N. fowleri is the only species known to infect humans
  • Other pathogenic free-living amoebae include Balamuthia mandrillaris and Sappinia pedata

Risk Factors

  • Acanthamoeba infection
    • Keratitis
      • Soft contact lens wearers with poor hygienic practices
      • Patients exposed to contaminated water
    • Meningitis
      • Almost exclusively in immunocompromised patients
  • Naegleria infection
    • History of swimming in lakes or brackish water
    • Aspiration of contaminated water, inhalation of contaminated dust
  • Immunosuppression is a risk factor for infection from all free-living amoebae
    • N. fowleri and B. mandrillaris can also infect immunocompetent patients

Clinical Presentation

  • Acanthamoeba infection
    • Chronic granulomatous amoebic encephalitis
      • Fever, nausea, headache, vomiting, stiff neck, cranial nerve involvement, hemiparesis, ataxia
      • Fatality ratio of >90%
    • Ocular disorders and symptoms
      • Corneal ulcers, punctuate keratitis, anterior uveitis
      • Complications
        • Secondary glaucoma
        • Cataract
        • Iris atrophy
    • Chronic granulomatous skin lesions
  • N. fowleri infection
    • Primary amoebic meningoencephalitis
      • Acute onset of fever, nausea, vomiting, headache, stiff neck, cranial nerve involvement
      • Almost always fatal within 4-6 days
  • Other pathogenic free-living amoebae-caused infections
    • B. mandrillaris infection
      • Amoebic encephalitis
      • Chronic granulomatous skin lesions – papulonodular, erythematosus, possible ulceration
      • Oral cavity lesions – palate deformity
    • S. pedata infection
      • Amoebic encephalitis

ARUP Laboratory Tests

Primary Tests

Detect Acanthamoeba spp, Naegleria spp, and other free-living amoebae

Cerebrospinal fluid (CSF) specimens examined by calcofluor white and Giemsa stains upon receipt in addition to setup for culture

Culture does not detect Balamuthia mandrillaris – detected only by stain

Use to rule out most common viral etiologies

Molecular testing is preferred for patients presenting with meningitis/encephalitis; refer to meningitis/encephalitis panel by polymerase chain reaction (PCR)

Panel includes measles, mumps, varicella-zoster virus, herpes simplex virus (HSV) types 1 and 2, and West Nile virus antibodies

Related Tests

Use to rule out most common viral etiologies

Not a preferred test; refer to relevant test for the specific pathogen suspected

Panel includes measles, mumps, varicella-zoster virus, herpes simplex virus (HSV) types 1 and 2, and West Nile antibodies

Detect Acanthamoeba spp, Naegleria fowleri, and other free-living amoebae

Detect Acanthamoeba spp and N. fowleri in various specimen types

Medical Experts



Marc Roger Couturier, PhD, D(ABMM)
Professor of Pathology (Clinical), University of Utah
Medical Director, Parasitology/Fecal Testing, Infectious Disease Antigen Testing, Bacteriology, and Molecular Amplified Detection, ARUP Laboratories


Additional Resources