Amoebas, Free-Living

Key Features

Essentials of Diagnosis

• Acute meningoencephalitis: fever, headache, meningism, acute mental deterioration

• Chronic granulomatous encephalitis: insidious onset of focal neurologic deficits

• Keratitis: pain, photophobia, conjunctivitis, blurred vision

General Considerations

• Acute meningoencephalitis

  • Occurs mostly in children and young adults

  • Caused by Naegleria fowleri

    • Found in warm fresh water and moist soil

    • Infection is associated with swimming in warm freshwater lakes and using contaminated tap water for nasal irrigation

  • CNS invasion occurs after nasal inoculation of N fowleri which travel along the olfactory nerves via the cribiform plate to the brain

• Chronic granulomatous encephalitis

  • Caused by Acanthamoeba or Balamuthia

  • Can occur in immunocompotent patients

  • Occurs more commonly in immunocompromised patients

  • No association with freshwater swimming

• Acanthamoeba keratitis is a corneal infection associated with minor trauma or use of soft contact lenses in otherwise healthy persons

Clinical Findings

• Acute meningoencephalitis

  • Abrupt fever

  • Headache

  • Nausea and vomiting

  • Disturbances in smell and taste

  • Meningismus

  • Decreased mental status a few days to 2 weeks after exposure

• Chronic granulomatous encephalitis

  • Insidious onset of focal neurologic deficits

  • Headache is presenting symptom in about 50% of patients

  • Skin, sinus, or lung infections with Acanthamoeba precede many of the CNS infections and may still be present at the onset of neurologic disease

• Acanthamoeba keratitis

  • Usually follows an indolent course

  • May resemble herpes simplex or bacterial keratitis initially

  • Delay in diagnosis is associated with worse outcomes

Diagnosis

• Acute meningoencephalitis

  • Cerebrospinal fluid (CSF) is usually hemorrhagic, with leukocyte counts that may be normal early but later range from 400 to 2600/mL with neutrophil predominance, low to normal glucose, and elevated protein

  • Diagnosis relies on finding trophozoites on a wet mount of the CSF

  • Immunofluorescent and polymerase chain reaction (PCR)–based diagnostic assays are available through the Centers for Disease Control and Prevention (CDC)

• Granulomatous encephalitis

  • Brain biopsy of CT-identified nonenhancing lucent areas is diagnostic

  • CSF is usually nondiagnostic with a lymphocytic pleocytosis, mild to severe elevation of protein (> 1000 mg/dL), and normal or low glucose

  • Acanthamoeba and Balamuthia amoebas have only rarely been found in the CSF; however, they can be visualized in brain biopsies or grown from brain or other infected tissues

  • Immunofluorescent and PCR-based diagnostic assays are available through the CDC

• Acanthamoeba keratitis

  • Trophozoites found in corneal scrapings

  • Parasite is isolated from corneal specimens or contact lens cultures

Treatment

• Acute amebic meningoencephalitis caused by N fowleri

  • High-dose intravenous amphotericin B with the possible addition of miconazole and rifampin

  • Azithromycin may be tried as an adjuvant

• Balamuthia encephalitis: combination of flucytosine, pentamidine, fluconazole, sulfadiazine, and a macrolide has been reported successful

• Acanthamoeba encephalitis should be treated with a combination of pentamidine, an azole, flucytosine, and sulfadiazine

• Miltefosine is an investigational new drug with in vitro activity against free-living ameba which has become available for treatment through ...