Chapter 348

Amebiasis and infections due to Entamoeba species are discussed in Chapter 341. The ubiquitous, free-living amoebae of the genera Naegleria, Balamuthia,Acanthamoeba, and Sappinia are the etiologic agents of rare infections of the central nervous system and eyes.1Naegleria fowleri is the agent of primary amebic meningoencephalitis; Acanthamoeba and Balamuthia cause granulomatous amebic meningoencephalitis; and Acanthamoeba also can infect the eye, resulting in amebic keratitis.2-4 Recently, Sappinia diploidea, another free-living amoeba that is normally found in soil contaminated with herbivore feces, has been identified as causing encephalitis in a healthy young man.5Naegleria, Balamuthia,Acanthamoeba, and Sappinia have trophozoite and cyst stages; in addition, Naegleria has a flagellate stage. For Naegleria, only trophozoites are found in tissues. Naegleria trophozoites are 10 to 30 μm in diameter and have a clear nucleus with a prominent central dense nucleolus and cytoplasmic pseudopodia. Acanthamoeba and Balamuthia trophozoites are of similar size and appearance to Naegleria, but the cyst form of these parasites may also be observed in tissue.1,6Sappinia trophozoites are 40 to 80 μm in diameter and have an ovoid or oblong shape. They appear to be flattened with occasional wrinkles on the surface, and food vacuoles are seen in cytoplasm. The mature cysts are 15 to 30 μm in diameter and are round. They have two adjoining nuclei.1 Unequivocal identification of these amoebae is not routine, and specimens should be referred to a qualified protozoologist for confirmation.

Almost every example of acute primary amebic meningoencephalitis reveals a recent patient history of swimming in fresh or brackish water. The organisms probably gain access to neural tissue via the nasal mucosa and the cribriform plate. There have been several epidemics in which the same swimming facility was the focus of infection. This disease has been reported in England, the Czech Republic, Australia, Virginia, Texas, and Florida. Naegleria fowleri has been isolated from soil and the bottom sediment of lakes and pools from all parts of the world. It also has been reported in thermally polluted water, where it can reproduce at temperatures up to 46°C (114.8°F). Seroepidemiologic studies demonstrate that most young adults in the southern United States have agglutinating antibodies against Naegleria.7,8

Granulomatous amebic encephalitis caused by Acanthamoeba affects those who are immunocompromised and debilitated, whereas Balamuthia causes a subacute to chronic infection in both immunocompetent and immunocompromised individuals. These infections have incubation periods that exceed 1 week, and the clinical course, which usually is fatal, can last for weeks to months. The one known case of granulomatous amebic encephalitis caused by Sappinia diploidea occurred in a 38-year-old healthy white male. The patient’s prior medical history showed a recent frontal sinus infection.5

Keratitis caused by Acanthamoeba is most common in individuals who wear contact lenses. It is also seen as a complication of other corneal injuries. Corneal infection is associated with wearing the lenses while ...

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