Primary Amebic Meningoencephalitis

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 some 30 species of Naegleria, only Naegleria fowleri has been shown to be pathogenic for humans.

Acanthamoeba exists in cyst and motile 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 into contact with contaminated water during swimming or use 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, chronic liver disease, renal failure, 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 are not immunocompromised.

Other free-living amebae can also cause infection, as illustrated by a case report of Sappinia pedata 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, vegetables, brackish water, and seawater, as well as most of the freshwater sources for Naegleria. It can also be found in tap water, because 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 except Antarctica. Most of the cases occur during the summer months in previously healthy individuals who have a history of swimming in or contact with freshwater lakes and rivers before their illness. Between 1962 and 2017, 143 cases of primary amebic meningoencephalitis (PAM) were reported in the United States. Most of the reports have come from the southern and southwestern states, particularly Florida and Texas, but infections have occurred in Kansas, Indiana, and even Minnesota. Of note, two cases from Louisiana in 2011 were linked to sinus irrigation with neti pots, which contained contaminated tap water. In 2013, a boy also from Louisiana developed PAM from an exposure to a lawn water slide, which derived its tap water from a treated public drinking water system. In 2015, a 21 yr old female developed N. fowleri meningoencephalitis possibly from a swimming pool supplied by an overland water pipe.