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Free-living amebas are protozoa that live independently in soil or water and do not require a human or animal host. They rarely cause disease, in contrast to the parasitic ameba Entamoeba histolytica, which is a common cause of intestinal infection (see Intestinal Protozoa: Amebiasis). Pathogenic free-living amebas are of the genera Naegleria, Acanthamoeba, and Balamuthia.
Three major syndromes occur:
Acanthamoeba can also cause skin lesions.
Primary Amebic Meningoencephalitis
Primary amebic meningoencephalitis is a generally fatal, acute CNS infection caused by Naegleria fowleri.
Naegleria fowleri inhabit bodies of warm fresh water worldwide. Swimming in contaminated water exposes nasal mucosa to the organism, which can enter the CNS via olfactory neuroepithelium and the cribriform plate. Most patients are healthy children or young adults.
Symptoms begin within 1 to 2 wk of exposure, sometimes with alteration of smell and taste. Fulminant meningoencephalitis ensues, with headache, meningismus, and mental status change, progressing to death within 10 days, usually due to cerebral herniation. Only a few patients survive.
Diagnosis is suspected based on history of swimming in fresh water, but confirmation is difficult because CT and routine CSF tests, although necessary to exclude other causes, are nonspecific. Wet mount of CSF should be done; it may demonstrate motile amebic trophozoites (which are destroyed by Gram stain techniques).
Optimal treatment is unclear. A reasonable regimen would include amphotericin B (given intravenously, intrathecally, or both) plus azithromycin or clarithromycin, azole antimicrobials (eg, ketoconazole, fluconazole, miconazole), and perhaps rifampin.
Granulomatous Amebic Encephalitis
Granulomatous amebic encephalitis is a generally fatal subacute CNS infection caused by Acanthamoeba sp in immunocompromised or debilitated hosts or by Balamuthia mandrillaris.
Acanthamoeba sp and Balamuthia mandrillaris are present worldwide in water, soil, and dust. Human exposure is common, but infection is rare. Acanthamoeba infection of the CNS occurs almost entirely in immunocompromised or otherwise debilitated patients, but B. mandrillaris may also infect healthy hosts. The entry portal is thought to be the skin or lower respiratory tract, with subsequent hematogenous dissemination to the CNS.
Onset is insidious, often with focal neurologic manifestations. Mental status change, seizures, and headache are common. B. mandrillaris may also cause skin lesions. Survival is highly unlikely (and only in immunocompetent patients); death occurs between 7 and 120 days after onset (average, 39 days).
Diagnosis is often postmortem. CT and routine CSF tests are obtained but are nonspecific. CT may show multiple nonenhancing lucent areas; in CSF, WBC count (predominantly lymphocytes) is elevated, but trophozoites are rarely seen. Visible skin lesions often contain amebas and should be biopsied; if detected, amebas may be cultured and tested for drug sensitivity. Brain biopsy is often positive.
Some patients with Acanthamoeba granulomatous encephalitis have responded to drug combinations, which may include pentamidine; sulfadiazine or trimethoprim/sulfamethoxazole; flucytosine; fluconazole, ketoconazole, itraconazole, or voriconazole; amphotericin B; and other drugs. Skin infections caused by Acanthamoeba sp are usually treated with the same drugs and surgical debridement.
B. mandrillaris has been treated with a combination of pentamidine, flucytosine, fluconazole, and sulfadiazine plus either azithromycin or clarithromycin combined with surgical resection.
Amebic Keratitis
Amebic keratitis is corneal infection with Acanthamoeba sp, typically occurring in contact lens wearers.
Acanthamoeba sp can cause chronic and progressively destructive keratitis in normal hosts. The main risk factor (85% of cases) is contact lens use, particularly if lenses are worn while swimming or if unsterile lens cleaning solution is used. Some infections follow corneal abrasion.
Lesions are typically very painful and produce a foreign body sensation. Initially, lesions have a dendriform appearance resembling herpes simplex keratitis. Later, there are patchy stromal infiltrates and sometimes a characteristic ring-shaped lesion. Anterior uveitis is usually also present. Vision is diminished.
Diagnosis is confirmed by examination of Giemsa- or trichrome-stained corneal scrapings and by culture on special media. Viral culture is done if herpes is considered.
Treatment
Early, superficial infection responds better to treatment. The encysted stage of the life cycle appears to cause most problems.
Epithelial lesions are debrided, and intensive drug therapy is applied. Topical propamidine isethionate 0.1% plus either polyhexamethylene biguanide or biguanide chlorhexidine drops is frequently the initial choice; for the first 3 days, drugs are given every 1 to 2 h. Other topical drugs that can be used include aminoglycosides, hexamidine diisethionate, miconazole, and neosporin.
Systemic treatment with fluconazole or itraconazole has also been used, particularly in patients with anterior uveitis or involvement of the sclera. Early recognition and treatment have eliminated the need for therapeutic keratoplasty in most instances, but it remains an option when pharmacologic therapy fails. Intensive treatment is required for the first month; it is tapered per clinical response but often continued for 6 to 12 mo. Recurrence is common if treatment is stopped prematurely.
Prevention
Contact lens solution should be kept clean. Nonsterile homemade contact lens solutions should not be used. Wearing contact lenses while swimming or showering should be avoided.
Last full review/revision December 2009 by Richard D. Pearson, MD
Content last modified February 2012
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