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

Diagnosis

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)
    • 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

Histology

• 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
  • Viral encephalitis
  • Bacterial leptomeningitis
  • Fungal meningitis
  • Mycobacterium tuberculosis meningitis
• Corneal disease
  • HSV
  • Atypical mycobacteria
  • Adenovirus
  • Varicella-zoster virus
  • Ophthalmic manifestations of autoimmune disease
  • Ophthalmic rosacea
• Skin disease
  • Dimorphic fungal infections
  • Mycobacterial infections

Background

​Epidemiology

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

Organisms

• 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