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Brain and Skin Biopsies
Brain or skin biopsies may be diagnostic; amebic trophozoites and cysts, if present, are easily identified by light microscopic examination of tissue sections. However, in the majority of cases the diagnosis of GAE has been made at autopsy. In general, Acanthamoeba spp. and B. mandrillaris are difficult to differentiate in tissue sections by light microscopy because of their similar morphology (Martinez and Visvesvara, 1997). They can be differentiated by immunofluorescence analysis of tissue sections using rabbit anti-Acanthamoeba or anti-B. mandrillaris sera. Alternatively, biopsy or autopsy tissues fixed in formalin can be deparaffinized, rehydrated, post-fixed in Karnovsky’s, dehydrated and embedded in plastic (EPON) for electron microscopic studies. The morphology of the cysts is particularly useful in the identification of
B. mandrillaris. To identify the species of Acanthamoeba, one of following can be performed: immunoperoxidase or immunofluorescence tests, the modified indirect Staphylococcus protein A co-agglutination test, or culture. Acanthamoeba spp. can be easily grown on non-nutrient agar plates seeded with bacteria. Specimens for culture should be processed as soon as possible (Gordon et al., 1992). Balamuthia in contrast does not grow on bacteria-coated agar plates. Hence, biopsy specimens should also be inoculated on mono-layers of mammalian cells, e.g. human lung fibroblasts (HLF) or monkey kidney cells (EG).
PATHOGENIC AND OPPORTUNISTIC FREE-LIVING AMEBAS
CT and MRI of the head are important radiologic tests. Single or multiple heterogenous, hypodense, non-enhancing ‘space-occupying lesions’ involving the basal ganglia, cerebral cortex, subcortical white matter, cerebellum or pons may be encountered. These features may mimic a brain abscess, tumor or intracerebral hematoma (Lowichik et al., 1995; Schumacher et al, 1995).
Acanthamoeba trophozoites and especially cysts can be recovered from corneal scrapings or biopsy. Attempts should be made to isolate the organism by culture so that the species can be identified by isoenzyme electrophoresis and in vitro sensitivity testing performed with various chemotherapeutic agents (Berger et al., 1990; Illingworth et al., 1995; Larkin et al., 1992; Rabinovitch et al., 1990; Stehr-Green et al., 1989).
Acanthamoeba Keratitis (AK) Clinical
Acanthamoeba keratitis (AK) is a subacuute or chronic inflammatory reaction of the cornea, usually arising from an area of trauma to the corneal epithelium and resulting in ulceration and a 360° stromal ring infiltrate. AK usually produces severe ocular pain and congestion of the conjunctiva. In general, AK afflicts healthy individuals who wear contact lenses or who have a history of a small traumatic injury or abrasion to the cornea and exposure to contaminated water or other products.
Histopathologically, AK is characterized by chronic inflammation, with the presence of amebic trophozoites and cysts. Recurrent ulceration with a waxing and waning course that is refractory to medications used for bacterial, viral, or fungal infections is characteristic.
In the early stages of AK, the anterior cornea is destroyed by the invading Acanthamoeba trophozoites. Amebic trophozoites and cysts are seen infiltrated between the lamellae of the cornea. Infiltration, primarily of polymorphonuclear leukocytes, is commonly seen into the superficial and middle layers of the corneal stroma. During later stages of the disease, AK is characterized by ulceration, descemetocele formation and perforation of the cornea.
CLINICAL MANAGEMENT Primary Amebic Meningoencephalitis
Only a few patients have survived this disease. Amphotericin B, miconazole and rifampin may be effective. Amphotericin B and miconozole have been administered intrathecally or intravenously, alone or in combination, with rifampin given orally (Rowen et al., 1995).
Granulomatous Amebic Encephalitis
To date no effective treatment for GAE due to Acanthamoeba species has been identified although a few patients have survived (Martinez and Visvesvara, 1997; Seijo Martinez et al., 2000). The prognosis is uniformly poor, probably because of the inadequacy of the host’s immune system (Schuster and Visvesvara, 1996). In vitro experiments suggest that diamidine derivatives, such as pentamidine, propamidine, or dibromopropami-dine; paramomycin; neomycin; ketoconazole and miconazole; 5-fluorocytosine; and magainins may have activity against Acanthamoeba species (John, 1993; Martinez and Visvesvara, 1997; Schuster and Jacob, 1992). The prognosis of patients with disseminated skin infections without CNS involvement is, however, good (Hunt et al., 1995; Schuster and Visvesvara, 1996). Recent studies indicate that Balamuthia mandrillaris is sensitive to pentamidine isethionate in vitro and treatment with this drug may be beneficial to patients with Balamuthia GAE (Schuster and Visvesvara, 1996).