Chapter 272 Primary Amebic Meningoencephalitis
Naegleria, Acanthamoeba, and Balamuthia are small, free-living amebae that cause human amebic meningoencephalitis, which has 2 distinct clinical presentations. The more common is an acute, usually fatal amebic meningitis that is caused by Naegleria and occurs in previously healthy children and young adults. Granulomatous amebic meningoencephalitis is caused by Acanthamoeba, Balamuthia, and Sappinia and is a more indolent infection that is more likely to occur in immunocompromised individuals.
Etiology
Naegleria is an ameboflagellate that can exist as cysts, trophozoites, and transient flagellate forms. Temperature and environmental nutrient and ion concentrations are the major factors that determine the stage of the ameba. Trophozoites are the only stages that are invasive, although cysts are potentially infective, because they can convert to the vegetative form very quickly under the proper environmental stimuli. Although there are several species of Naegleria, only N. fowleri has been shown to be pathogenic for humans.
Acanthamoeba exists in cyst and trophozoite forms; only the trophozoite form is invasive. Cases of Acanthamoeba keratitis usually follow incidents of trivial corneal trauma followed by flushing with contaminated tap water. Infections can also occur among contact lens wearers who come in contact with contaminated water during swimming or using contact lenses cleaned or stored in contaminated tap water. Granulomatous amebic encephalitis from Acanthamoeba occurs worldwide and is associated with an immunocompromising condition such as HIV infection, diabetes mellitus, alcoholism, immunosuppressive therapy, or radiation therapy.
Balamuthia mandrillaris has been implicated as an etiology of granulomatous amebic encephalitis. Although the clinical presentation is similar to infection with Acanthamoeba, most patients have no known immunocompromising condition.
Other free-living amebae can also cause infection, as illustrated by case reports of Sappinia diploidea granulomatous encephalitis.
Epidemiology
The free-living amebae have a worldwide distribution. Naegleria species have been isolated from a variety of freshwater sources, including ponds and lakes, domestic water supplies, hot springs and spas, thermal discharge of power plants, groundwater, and occasionally from the nasal passages of healthy children. Acanthamoeba species have been isolated from soil, mushrooms and vegetables, brackish water, and seawater, as well as most of the freshwater sources for Naegleria. It can also be found in tap water, since chlorination does not kill Acanthamoeba. Balamuthia is present in soil and may be transmitted by inhalation or contamination of preexisting skin lesions.
Naegleria meningoencephalitis has been reported from every continent. Most of the cases occur during the summer months in previously healthy individuals who have a history of swimming in or contact with freshwater before their illness. Only 1-2 cases are reported in the USA per year, but 8 cases were reported in 2001-2002, and 6 cases in 2007. Most of the reports have come from the southern and southwestern states, with occasional infections occurring in the Midwest and East.
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