**1. Introduction**

328 Non-Flavivirus Encephalitis

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Free-living amoebae (FLA) belonging to *Acanthamoeba* and *Sappinia* genera as well as *Balamuthia mandrillaris* and *Naegleria fowleri* species are aerobic, mitochondriate, eukaryotic protists that occur worldwide and can potentially cause infections in humans and other animals (Visvesvara and Maguire, 2006; Visvesvara et al., 2007). Due to the fact that these amoebae have the ability to exist as free-living organisms in nature and only occasionally invade a host and live as parasites within host tissue, they have also been called amphizoic amoebae (Page, 1988).

All four amoebae are known so far to cause infections of the central nervous system (CNS). Several species of *Acanthamoeba* (i.e. *A. castellanii*, *A. culbertsoni*, *A. hatchetti*, *A. polyphaga*, *A. rhysodes*), the only known species of *Balamuthia*, *B. mandrillaris*, two species of *Sappinia* genus, *S. diploidea* and *S.pedata*, and only one species of *Naegleria*, *N. fowleri*, are known to cause disease in humans and other animals (Khan, 2006; Visvesvara et al., 2007).

Fig. 1. Trophozoite (right) and Cyst stages (left) in *Acanthamoeba* sp.

Encephalitis Due to Free Living Amoebae: An Emerging Issue in Human Health 331

individuals undergoing organ transplants or cancer chemotherapy, and drug abusers (Khan,

Therefore, the onset of AGE is slow and subtle and develops as a chronic disease from several weeks to months (Visvesvara & Maguire, 2006, Visvesvara et al. 2007). The usual features of AGE consist of headache, stiff neck, and mental-state abnormalities, as well as nausea, vomiting, low-grade fever, lethargy, cerebellar ataxia, visual disturbances, hemiparesis, seizures and coma. Facial palsy with numbness resulting in facial asymmetry is often seen. Cerebral hemispheres are usually the most heavily affected CNS tissue. They are often edematous, with extensive hemorrhagic necrosis involving the temporal, parietal, and occipital lobes. Computerized tomography (CT) scans of the brain show large, low-density abnormalities mimicking a single or multiple space-occupying mass. Magnetic resonance imaging (MRI) with enhancements shows multiple, ring-enhancing lesions in the brain

Acanthamoebae infecting the CNS are not readily found in the cerebrospinal fluid (CSF), although they have been isolated from the CSF in a few cases. *Acanthamoeba* that had apparently entered from the nasopharynx through a fistula have been detected in the CSF of a patient without CNS disease (Petry et al., 2006). CSF examination in general reveals lymphocytic pleocytosis with mild elevation of protein and normal or slightly depressed glucose. Examination of the autopsied brain reveals cerebral edema, areas of cortical and basal ganglia softening, and multiple necrotic and hemorrhagic areas of CNS tissues. The brainstem, cerebral hemispheres and cerebellum may show areas of hemorrhagic infarcts. Histological examination reveals the presence of multinucleated giant cells in the cerebral hemispheres, brain stem, mid-brain, cerebellum, and basal ganglion. Necrotic tissue with lipid-containing macrophages and neovascularization suggesting a tumour is often seen. Trophozoites and cysts of acanthamoebae are usually spread all over the infected tissue. Many blood vessels are thrombotic with fibrinoid necrosis and cuffed by polymorphonuclear leukocytes, amoebic trophozoites, and cysts. Multinucleated giant cells forming granulomas may be seen in immunocompetent patients but less often in

Some infected individuals, mostly with HIV/AIDS, develop chronic ulcerative skin lesions, abscesses, or erythematous nodules (Seijo-Martínez et al., 2000; Visvesvara & Maguire, 2006, Visvesvara et al., 2007), especially of the chest and limbs. These nodules are usually solid but sometimes they become ulcerated and purulent. The prodromal period is unknown and several weeks or months may elapse following infection before the disease becomes apparent. Because of the time delay, the precise portal of entry is not clearly known, but the wide dissemination of these amoebae in the environment allows for many possible contacts and modes of infection. Trophic amoebae and/or cysts of *Acanthamoeba* have been isolated from the nasal mucosa of healthy individuals, suggesting a nasopharyngeal route as one means of invasion. Amoebae may also enter the body through ulcers in the skin, resulting in hematogenous dissemination to the lungs and brain, or by inhalation of amoebic cysts

In addition to causing CNS infections, *Acanthamoeba* also causes a vision-threatening disease, *Acanthamoeba* **keratitis** (AK) which mostly affects contact lens wearers although many cases have been reported worldwide in non contact lens users mostly related to a previous corneal trauma [6, 7, 8]. The number of affected individuals is increasing worldwide. Morever, recent outbreaks of *Acanthamoeba* keratitis have been recently reported in the United States and Australia (Verani et al., 2009; Tu and Joslin, 2010; Patel et al., 2010;

2006; Visvesvara et al., 2007; da Rocha-Azevedo et al., 2009).

immunocompromised patients.

(Visvesvara & Maguire, 2006; Visvesvara et al 2007).

(Seijo-Martínez et al., 2000; Shirwadkar et al. 2006; Visvesvara et al., 2007).
