**1. Introduction**

306 Non-Flavivirus Encephalitis

Weenink, J.J., Weenink, A.G., Geerlings, S.E., et al. (2009). Severe cerebral toxoplasma infection cannot be excluded by a normal CT scan. *Neth J Med*. Vol.67, pp. 150-152. Yazdanpanah, Y., Chêne, G., Losina, E. et al. (2001). Incidence of primary opportunistic

Zeller, V., Truffot, C., Agher, R., et al. (2002). Discontinuation of secondary prophylaxis

*Int J Epidemiol*. Vol.30, pp. 864-871.

encephalitis. *Clin Infect Dis*. Vol. 34, pp. 662-667.

infections in two human immunodeficiency virus-infected French clinical cohorts.

against disseminated Mycobacterium avium complex infection and toxoplasmic

*Acanthamoeba* spp. are free-living amoebae that are ubiquitous in the environment. Most healthy individuals carry *Acanthamoeba*-reactive antibodies, suggesting constant exposure to amoebae. In spite of the high prevalence of the amoebae, the incidence of diseases caused by *Acanthamoeba* is very low. Non-opportunistically, Acanthamoebae can induce keratitis in healthy humans, but as an opportunistic pathogen, the amoebae can cause fatal encephalitis especially in immunocompromised individuals and treatments are often ineffective.

Amoebic encephalitis is a life-threatening disease of the central nervous system (CNS) caused by free-living amoebae belonging to the genera *Acanthamoeba*, *Balamuthia* and *Naegleria*. Because they lack host-specificity, the ubiquitous amoebae can infect a wide range of species (Marciano-Cabral & Cabral, 2003, Schuster & Visvesvara, 2004). The diseases caused by *Acanthamoeba* spp. and *Balamuthia* spp. are generally termed "granulomatous amoebic encephalitis" (GAE), whereas those caused by *Naegleria* spp. are called 'primary amoebic meningioencephalitis (PAM)'(Marciano-Cabral & Cabral, 2003, Schuster & Visvesvara, 2004, Khan, 2006, da Rocha-Azevedo*, et al.*, 2009). While Acanthamoebae induce illness mostly in immunocompromised individuals, *Balamuthia* spp. and *Naegleria* spp. can cause diseases in both immune-sufficient and immune-deficient individuals (Martinez & Visvesvara, 2001, Marciano-Cabral & Cabral, 2003, Schuster & Visvesvara, 2004, Khan, 2006, da Rocha-Azevedo*, et al.*, 2009). Nevertheless, all of them can induce keratitis in healthy individuals, often in contact lens-wearers (Jones*, et al.*, 1975, Martinez & Visvesvara, 1997, Marciano-Cabral & Cabral, 2003, da Rocha-Azevedo*, et al.*, 2009). We recently discovered that *A. castellanii* contains mimicry sequence for immunodominant epitope of CNS myelin proteolipid protein (PLP), suggesting that exposure to *A. castellanii* can lead to the generation of autoimmune responses by antigenic mimicry. In this review, we discuss our understanding of the pathophysiology of *Acanthamoeba*-induced encephalitis, with a special emphasis on autoimmunity in mediation of the disease, and implications for therapy.

#### **2. Characteristics of** *Acanthamoeba* **infections**

Based on morphological characteristics, such as shape and size of amoebic cysts, and growth conditions, the genus *Acanthamoeba* was initially classified into groups I, II, and III,

Autoimmunity in the Mediation of Granulomatous Amoebic Encephalitis: Implications for Therapy 309

malnutrition (Martinez & Janitschke, 1985, Sell*, et al.*, 1997, Marciano-Cabral & Cabral, 2003, Khan, 2006). Exacerbation of GAE lesions was reported in one patient undergoing treatment for cryoglobulinemia with a monoclonal antibody directed against CD20 which selectively depletes mature B cells (Meersseman*, et al.*, 2007). Likewise, GAE can occur in patients with systemic lupus erythematosus, further emphasizing the importance of a compromised immune system for disease-predisposition (Koide*, et al.*, 1998, Uschuplich*, et al.*, 2004, Cha*, et al.*, 2006). Since amoebic encephalitis is not a reportable disease, and diagnosis is often made postmortem, the number of cases documented in the literature does not reflect actual disease-incidence. One study has reported to have documented upto 500 cases of amoebic encephalitis worldwide (Sarica*, et al.*, 2009). However, the recent availability of PCR-based detection of *Acanthamoeba* is greatly facilitating diagnosis (Schroeder*, et al.*, 2001, Khan, 2006, da Rocha-Azevedo*, et al.*, 2009, Maritschnegg*, et al.*, 2011) and as a result, the number of

Recently, it is proposed that Acanthamoebae might play a role in the increased incidence of nosocomial infections (Michel*, et al.*, 1995, Marciano-Cabral & Cabral, 2003). It is well documented that Acanthamoebae act as natural vectors or reservoirs for a variety of microbes, such as *Escherichia coli, Klebsiella, Bacillus spp., Mycoplasma, Legionella pneumophila, Mycobacterium avium, Mycobacterium leprae, Clostridium frigidicarnis, Porphyromonas gingivalis, Prevotella intermedia, Burkholderia pseudomallei, Afipia felis, Vibrio cholerae, Mobiluncus curtissi, Campylobacter spp., Helicobacter pylori*, *Cryptococcus neoformans, Candida spp., Coxiella burnetti, Chlamydia, Rickettsia,* and Coxsackievirus among others (Marciano-Cabral & Cabral, 2003, Waldner*, et al.*, 2004, Khan, 2006, Mattana*, et al.*, 2006, Thomas*, et al.*, 2009). Bacteria grown in *Acanthamoeba* show resistance to bactericides and biocides; their survival and virulence are enhanced; and they mechanically transport disease-producing agents to various target organs, thus increasing the risk of multiple infections in the affected patients (King*, et al.*, 1988, Barker*, et al.*, 1995, Turner*, et al.*, 2000, Lloyd*, et al.*, 2001, Marciano-Cabral & Cabral, 2003). The amoebae shed waste through vesicles of 2.1 to 6.4 µm diameter, and they can potentially contain pathogenic microbes. For example, *A. polyphaga* can release up to 20 to 200 bacteria per vesicle and the vesicles can become aerosolized leading to their dispersal to

cases reported in recent years show an increasing trend.

**2.3 Importance of** *Acanthamoeba* **in nosocomial infections** 

wide-range of geographical locations (Rowbotham, 1980, Berk*, et al.*, 1998).

The amoebae can gain entry into the CNS through two routes: migration via the olfactory neuroepithelium and/or blood (Fig. 1). *Naegleria* spp. tend to follow the former route. After penetrating the nasal mucosa, the amoebae pass through the cribriform plate and travel along the nerve fibers to the olfactory bulb in the cerebrum (Khan, 2007, Elsheikha & Khan). Alternatively, the amoebae that enter through nasal exposure go to the lungs, enter the blood stream, and reach the CNS possibly by disrupting the blood brain barrier (BBB). The hematogenous route also is a choice for amoeba that enter through the skin (Khan, 2007). However, the mechanism by which amoebae actually enter the CNS is not clearly elucidated. Although it is postulated that they enter through the cerebral capillary endothelium or choroid plexus, the former being the more widely accepted mechanism (Khan, 2003, Marciano-Cabral & Cabral, 2003, Khan, 2005b, Khan, 2006, da Rocha-Azevedo*,* 

**3. Pathogenesis** 

containing 4, 11, and 5 species, respectively (De Jonckheere, 1987). Recently, reclassification was made to group various species into 15 genotypes (T1 to T15) based on 18S rRNA gene sequences, which can distinguish different genotypes showing a variation of as low as 5% (Gast*, et al.*, 1996, Schuster & Visvesvara, 2004). The genotypes T1, T4, T10, and T12 generally cause encephalitis, and *A. castellanii* and *A. polyphaga* that belong to T4 genotype are most frequently implicated as causes of GAE (Garate*, et al.*, 2006). The current trend is to classify Acanthamoebae based on genotype rather than morphology.
