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Amoebae are amphizoic, ubiquitous, and opportunistic protozoa that can affect different organs including skin, lungs, eyes, and the brain.1,2 In the central nervous system (CNS), two main, well defined disease entities have been described: primary amoebic meningoencephalitis, which is caused by Naegleria fowlery and is rapidly fatal, and granulomatous amoebic encephalitis, which is caused by Acanthamoeba spp and Balamuthia mandrillaris and is characterised by focal granulomatous lesions in the brain following a subacute or chronic course. CNS infections caused by free living amoebae are uncommon and, as of October 1996, for example, only 166 cases of granulomatous amoebic encephalitis have been reported from around the world.1 Identification of amoebae in cerebrospinal fluid (CSF) samples is a rare event.1,2
We have recently seen a patient with meningoencephalitis in whom amoebic forms were identified in the CSF. She was a 48 year old woman who was admitted to our neurological clinic because of vertigo, headache, bilateral hypacusia, ataxia, diplopia, dysphonia, dysphagia, anosmia, ageusia, tetraparesis, occurrence of transient clonic fits in the right upper limb, and vomiting episodes. The onset of her illness dated from about six weeks earlier, when she began to complain of a maddening noise, like a rustle, in the right ear, dizziness, difficulty in maintaining the erect posture, and vertigo. She was admitted to hospital. Computed tomography (CT) and magnetic resonance imaging (MRI) of the brain were normal, and analysis of CSF showed the following: cells, 60/mm3 (white blood cells, primarily mononuclear lymphocytes); glucose, 1.7 mmol/l; protein, 95 mg/dl; and the presence of giant, atypical cells. After tumour pathology was excluded, the patient was admitted to our clinic. Her blood pressure was 170/95 mm Hg, pulse 125 beats/min, body temperature 36.5°C, and respiratory rate 22 breaths/min. She had a Glasgow coma scale score of 13. The main abnormal haematological findings (given as the range observed during her hospital admission) were as follows: white blood cells, 19 100 to 22 000/mm3, with modest neutrophilia of 79–81%; lactate dehydrogenase, 500 to 506 IU/l (normal range 150 to 460 IU/l); Katz index, 70 to 75. Other blood indices and analyses were normal. These included immunological assessment tests with differentiation of lymphocytic subtypes and haematological cultures for infective agents, including cat scratch fever and HIV.
Culture studies were negative for bacteria, fungi, viruses, and acid-fast bacilli. Direct examination of the CSF by light microscopy showed the presence of free living amoebae. The motile forms presented the characteristic morphological features of spherical (round or ovoid) shape, abundant cytoplasm, and nuclei with prominent nucleoli (fig 1). A CSF specimen cultured at 32°C and 37°C on non-nutrient agar culture plates covered with a dense lawn of the Gram-negative bacterium Escherichia coli was negative.
Serial EEG records showed diffuse delta and theta activity with occasional prevalence of this activity in the left sided anterior regions. Cranial CT and MRI revealed diffuse oedema with hydrocephalus and dilatation of the cerebral aqueduct. No focal lesions were observed.
Treatment included dexamethasone (4 mg intravenously twice daily), doxycyclin (100 mg twice daily), rifampicin (600 mg daily), and amphotericin B, starting at doses of 20 mg intravenously per day with a progressive increase to 50 mg daily; simultaneous intrathecal amphotericin B was given at an initial daily dose of 0.012 mg, increasing progressively to a maximum daily dose of 0.250 mg.
Despite this treatment, the clinical situation rapidly worsened and the patient died after 11 days in hospital. A necropsy examination was denied.
Free living amoebae of the genus Acanthamoeba are the causative agents of several infections usually occurring in immunocompromised, debilitated individuals and almost always resulting in death. Most cases, therefore, are diagnosed only at necropsy. Our patient was apparently immunocompetent and without any of the usual predisposing factors, such as a history of aquatic activities, treatment with immunosuppressive, chemotherapeutic, or steroid agents or broad-spectrum antibiotics, and so on.1 The existence of extraneural infective foci in the skin, paranasal area, or lungs—a possible point of access for amoebae1—was also excluded. The most striking feature in our patient was that a firm diagnosis was made only through direct observation of the protozoon in the CSF. To our knowledge, this has not been described before in chronic amoebic meningoencephalitis and underlines the diagnostic value of CSF studies in this type of pathology. The main species reported as causing granulomatous amoebic encephalitis are A polyphaga, A castellanii, A culbertsoni, and Balamuthia mandrillaris.1 The diagnosis is usually made by microscopic examination of stained slices of brain specimens obtained at necropsy or biopsy and cultivation of the causal organism in an appropriate medium. This usually consists of non-nutrient agar covered with bacteria for Acanthamoeba spp or mammalian cell lines for B mandrillaris. Discrimination among species may not be easy and requires experience and the use of different methods.1 Morphological and ultrastructural analysis, for example, can help in discriminating acathamoeba from balamuthia, as some differences in shape, dimensions, nuclei, and cytoplasm have been described for both cystic forms and trophozoites.2,3 Physiological characteristics such as temperature tolerance, pH dependency, and others—although they have proved useful—are not of definitive value in differentiating pathogenic from non-pathogenic strains.2 More recently, molecular methods and the use of monoclonal antibodies for immunofluorescence microscopy have been developed; however, they have some limitations, such as their expense, laborious nature, and short lived reagents.2 Nonetheless, in spite of these various diagnostic procedures, accurate species determination may prove problematic.3 In our patient, we were unable to discriminate among the various species of amoebae that could have been involved in the infection. Culture tests on non-nutrient agar covered with bacteria were negative, and other cultures, serological tests, and CSF tests were not done because of the rapid progression of illness. Nonetheless, our case indicates that when an amoebic meningoencephalitis is suspected a careful search for the organisms in the CSF may be a decisive factor in the diagnosis.
In conclusion, this case report emphasises the importance of familiarising ourselves with this form of pathology and provides an example of how the identification of amoebae in the CSF may aid in making a firm diagnosis of this uncommon, underdiagnosed, life threatening, and difficult to treat CNS infection. As a successful therapeutic result may sometimes be achieved,4 a timely diagnosis together with prompt and adequate treatment are essential.