Organisms of the genera Naegleria, Acanthamoeba, and Balamuthia, also known as “free-living” amebae, are known to cause meningoencephalitis in humans. Another free-living ameba genus, Sappinia, has been isolated from animals but is rarely isolated from humans. Naegleria fowleri typically causes primary amebic meningoencephalitis (PAM), which is usually fulminant in nature, whereas infections caused by Acanthamoeba and Balamuthia mandrillaris tend to be more indolent. Like Naegleria, they primarily affect the central nervous system (CNS) causing granulomatous amebic encephalitis (GAE). Several species of Acanthamoeba abound and, besides GAE, are also known to cause amebic keratitis.
PRIMARY AMEBIC MENINGOENCEPHALITIS
PATHOGENESIS AND EPIDEMIOLOGY
N fowleri exists in 3 forms: trophozoite (infectious form), flagellate, and cysts. Once inhaled into the nasal cavity, this organism makes its way through the nasal mucosa and the cribriform plate and into the CNS via the olfactory nerves. Certain water-related recreational activities such as swimming, diving, and water skiing have been particularly associated with this infection. It is believed that the rapid and sometimes forceful propulsion of water into the nasal cavity propels the organisms through the nose and into the brain. Several factors contribute to the pathogenicity of the organism. These include an intense immune response as well as the release of several cytolytic molecules such as phospholipases, acid hydrolases, and neuraminidases. These induce rapid and fulminant brain swelling, hemorrhage, and necrosis that predominantly affect the frontal lobes leading to necrosis of brain tissue and most often death (Fig. 343-1). Thus far, there have only been 4 reported cases in the literature of individuals who have survived the infection.
Extensive hemorrhage and necrosis is present in the brain in primary amebic meningoencephalitis, mainly in the frontal cortex. (Reproduced with permission from the Centers for Disease Control and Prevention [CDC].)
Infections have been reported worldwide, predominantly in warmer climates. In the United States, most infections have been reported from the southern states, predominantly Florida and Texas, although in recent years, there have been cases reported from Minnesota, Indiana, and Kansas, suggesting perhaps a change in epidemiology (Fig. 343-2). N fowleri are ubiquitous, thermophilic amebae typically found in moist soil as well as warm, freshwater lakes, rivers, streams, swimming pools, tap water, and heating and air conditioning units. Hot tubs, thermal swimming pools, spas, rinsing of nostrils with tap water including irrigation of nostrils as part of management of sinus infections, and the Muslim ritual of ablution have all been associated with infections in different parts of the world. A recent case report linked N fowleri infection to swimming pool water supplied from an overland pipe. Most infections have been reported in children and young adults, and there does seem to be a male predominance.